Synonyms
More meanings
Tip: select a single word for meaning & synonyms.
Select multiple words normally to copy text.
Heredity: Class 10 Science answers, notes
Get summaries, questions, answers, solutions, notes, extras, PDF and guides for Chapter 8 Heredity: Class 10 Science textbook, which is part of the syllabus for students studying under SEBA (Assam Board), NBSE (Nagaland Board), TBSE (Tripura Board), CBSE (Central Board), MBOSE (Meghalaya Board), BSEM (Manipur Board), WBBSE (West Bengal Board), and all other boards following the NCERT books. These solutions, however, should only be treated as references and can be modified/changed.
If you notice any errors in the notes, please mention them in the comments
Summary
Reproduction creates new living things. These new beings are like their parents but also a little different. Even when only one parent is involved, small changes, called variations, happen. When two parents are involved, like in humans, these variations are much more noticeable. This chapter explains how these variations happen and how they are passed down.
When living things reproduce, the new generation gets a basic body plan from its parents. It also gets small changes. This new generation will then reproduce and pass on its own mix of old and new changes. If a single germ, like a bacterium, divides, its children will be very similar. This is because the main way changes happen is through tiny mistakes when their information code, DNA, is copied. But if two parents are involved, much more variety is created. Not all these changes help the new being survive. Some changes might be helpful in certain surroundings, and these help the living thing to live better.
Parents pass down their features, or traits, to their children. A child has basic human features but is not an exact copy of its parents. People show many differences. Both the father and mother give almost equal amounts of their genetic information to the child. This means every trait a child has can be shaped by information from both parents. So, for each trait, a child has two versions of instructions.
A scientist named Mendel studied how traits are passed on using pea plants. He found that if he mixed a tall plant and a short plant, all their children in the first group were tall. There were no medium plants. This meant the “tall” instruction was stronger. When these tall children plants made their own seeds, some of their offspring were short. This showed that the “short” instruction was still there, just hidden in the first group. Mendel suggested that there are two “factors” (now called genes) for each trait. One factor can be stronger, or dominant, and hide the effect of the weaker, or recessive, factor. For a plant to be short, it needed two “short” factors.
Mendel also studied plants with two different traits, like tall plants with round seeds and short plants with wrinkled seeds. He found that these traits were passed down independently. A plant could be tall with wrinkled seeds, or short with round seeds, showing new combinations. This happens because the information for traits is carried on separate units called chromosomes. Each parent gives one chromosome from each pair to its offspring. When the parents’ cells combine, the offspring gets the usual number of chromosomes.
The information for making us who we are is in our DNA. A part of DNA that holds instructions for one protein is a gene. Proteins control our traits. For example, plant height is controlled by hormones, which are made by enzymes. If a gene makes an efficient enzyme, the plant grows tall. If the gene is changed and the enzyme is less efficient, the plant is short.
In humans, sex is decided by genes. Most of our chromosomes come in pairs. We have 22 such pairs. One special pair, the sex chromosomes, decides if we are male or female. Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY). A mother always passes an X chromosome to her child. If the father passes an X chromosome, the child is a girl (XX). If the father passes a Y chromosome, the child is a boy (XY).
Textbook solutions
Intext Questions and Answers I
1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer: Trait B is likely to have arisen earlier. This is because variations are created and inherited, leading to an accumulation of these variations over succeeding generations within a population. A trait that is present in a larger percentage of the population, such as trait B at 60%, suggests it has been inherited and has accumulated through more generations compared to trait A, which exists in only 10% of the population.
2. How does the creation of variations in a species promote survival?
Answer: The creation of variations in a species promotes survival because, depending on the nature of these variations, different individuals would have different kinds of advantages in the environment in which they find themselves. For instance, bacteria that can withstand heat, which is a type of variation, will survive better in a heat wave. The selection of such advantageous variants by environmental factors forms the basis for evolutionary processes.
Intext Questions and Answers II
1. How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: Mendel’s experiments show that traits may be dominant or recessive through observations in pea plants. In the first-generation, or F1 progeny, from a cross of, for example, tall and short plants, there were no ‘medium-height’ plants; all plants were tall. This meant that only one of the parental traits was seen. When these F1 tall plants were allowed to self-pollinate, the second-generation, or F2, progeny were not all tall; instead, one quarter of them were short. This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. This led Mendel to propose that two copies of a factor (now called genes) controlling traits are present in a sexually reproducing organism. For instance, both TT and Tt genotypes result in tall plants, while only tt results in a short plant. In other words, a single copy of ‘T’ is enough to make the plant tall, making ‘T’ a dominant trait, while both copies have to be ‘t’ for the plant to be short, making ‘t’ a recessive trait.
2. How do Mendel’s experiments show that traits are inherited independently?
Answer: Mendel’s experiments show that traits are inherited independently by breeding pea plants showing two different characteristics. For example, when a tall plant with round seeds is bred with a short plant with wrinkled-seeds, the F1 progeny are all tall and have round seeds, indicating tallness and round seeds are dominant. When these F1 progeny are used to generate F2 progeny by self-pollination, the experiment reveals that some F2 progeny are tall plants with round seeds, and some are short plants with wrinkled seeds. However, there would also be F2 progeny that showed new combinations: some would be tall but have wrinkled seeds, while others would be short but have round seeds. The formation of these new combinations of traits in F2 offspring, when factors controlling for different traits like seed shape and seed colour recombine to form the zygote, demonstrates that the tall/short trait and the round seed/wrinkled seed trait are independently inherited.
3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?
Answer: Yes, this information is enough to tell which trait is dominant. Since the daughter has blood group O, she must have inherited the factor for blood group O from both parents. The mother has blood group O, so her genetic makeup for this trait means she could only pass on the factor for O. Because the daughter has blood group O, she must have also received a factor for O from her father. The father has blood group A, but he passed on a factor for O to his daughter; this means his genetic makeup includes both A and O. Since his blood group is A, the trait A is expressed even when the factor for O is present. Traits that are expressed when only one copy of their gene is present are dominant, while traits that require two copies to be expressed are recessive. Therefore, blood group A is dominant, and blood group O is recessive.
4. How is the sex of the child determined in human beings?
Answer: In human beings, the sex of the individual is largely genetically determined by the genes inherited from the parents. All human chromosomes are not paired perfectly; while most human chromosomes have a maternal and a paternal copy (22 such pairs), one pair, called the sex chromosomes, differs between males and females. Women have a perfect pair of sex chromosomes, both called X (XX). Men have a mismatched pair, with one normal-sized X chromosome and a short one called Y (XY). All children will inherit an X chromosome from their mother. Thus, the sex of the children will be determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl (XX), and one who inherits a Y chromosome from him will be a boy (XY).
Exercise Questions and Answers
1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer: (c) TtWW
Explanation:
This is because for the progeny to have almost half of them short (genotype tt), when crossed with a short parent (ttww), the tall parent must be heterozygous for height (Tt). For all progeny to bear violet flowers when crossed with a white-flowered parent (ww), the tall parent must be homozygous dominant for violet flower color (WW), ensuring all offspring receive at least one dominant allele for violet color (Ww). Thus, the genetic make-up of the tall parent is TtWW.
2. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Answer: No, on this basis alone, we cannot definitively say whether the light eye colour trait is dominant or recessive. The determination of whether a trait is dominant or recessive, such as in Mendel’s experiments, involves controlled breeding of individuals with contrasting traits and observing the characteristics of subsequent generations, like the F1 and F2 progeny, to see which trait is expressed or reappears. The observation that children with light-coloured eyes are likely to have parents with light-coloured eyes is a correlation from a population study, not the result of such controlled experimental crosses. Without information from specific crosses (for example, the outcome if parents with different eye colours have children, or if two dark-eyed parents have a light-eyed child), a conclusion cannot be reliably made.
3. Outline a project which aims to find the dominant coat colour in dogs.
Answer: A project to find the dominant coat colour in dogs, based on the principles of Mendelian inheritance involve the following steps:
- Select dogs from true-breeding lines with two contrasting, clearly distinguishable coat colours. For example, one line of dogs that consistently produces black coat colour and another line that consistently produces a different distinct coat colour, such as yellow.
- Perform a cross between these two types of dogs: mate a true-breeding black-coated dog with a true-breeding yellow-coated dog. This is the parental (P) generation.
- Observe the coat colour of all the puppies born from this cross (the F1 generation). If all the F1 puppies exhibit only one of the coat colours (e.g., all are black), then that coat colour (black, in this example) would be considered the dominant trait, and the other (yellow) would be considered recessive.
- To further confirm this, interbreed the F1 generation individuals (e.g., mate male and female F1 dogs that all show the dominant coat colour with each other).
- Observe the coat colours of the offspring from this F1 cross (the F2 generation). If the coat colour that was not visible in the F1 generation (e.g., yellow) reappears in the F2 generation, typically in a predictable ratio (such as approximately 3:1, where three-quarters of the F2 dogs show the dominant coat colour and one-quarter show the recessive coat colour), this would confirm the dominance of the first coat colour and the recessiveness of the coat colour that reappeared.
4. How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: The equal genetic contribution of male and female parents in the progeny is ensured through the mechanism of sexual reproduction involving chromosomes and germ cells. Both the father and the mother contribute practically equal amounts of genetic material to the child. Each body cell in an organism has two copies of each chromosome, one inherited from the male parent and one from the female parent. This means that for each trait, there will be two versions of the gene in the child, one from each parent.
This equal contribution is specifically ensured because:
- Each gene set is present as separate independent pieces called chromosomes.
- During the formation of germ cells (sperm in males and eggs in females), each germ cell receives only one chromosome from each pair. Thus, each germ cell contains only one set of genes.
- When a male germ cell (sperm) and a female germ cell (egg) combine during fertilisation, they restore the normal number of chromosomes in the progeny. The resulting zygote, and subsequently the new individual, will have two copies of each chromosome, one contributed by the male parent (via the sperm) and one by the female parent (via the egg). This ensures that the progeny inherits one complete set of genes from the father and one complete set from the mother, leading to an equal genetic contribution from both parents.
Extras
Additional MCQs (Knowledge Based)
1. In human beings, how many pairs of autosomes (non-sex chromosomes) are typically found?
A. 21
B. 22
C. 23
D. 46
Answer: B. 22
2. Who is credited with conducting experiments on pea plants to understand the principles of inheritance?
A. Darwin
B. Lamarck
C. Mendel
D. Watson
Answer: C. Mendel
3. In certain reptile species, the sex of the offspring can be determined by the _________ at which the fertilised eggs are incubated.
A. Humidity
B. Light intensity
C. Temperature
D. Pressure
Answer: C. Temperature
4. A specific segment of DNA that carries the instructions for producing a particular protein is known as a ________.
A. Chromosome
B. Trait
C. Gene
D. Gamete
Answer: C. Gene
5. This Austrian monk, through his work with garden peas, laid the foundation for the science of genetics by meticulously tracking inherited characteristics.
A. Charles Darwin
B. Gregor Mendel
C. Jean-Baptiste Lamarck
D. James Watson
Answer: B. Gregor Mendel
6. If a pea plant inherits two ‘t’ alleles for height (where ‘T’ is for tallness), its height characteristic is considered:
A. Dominant
B. Co-dominant
C. Recessive
D. Incompletely dominant
Answer: C. Recessive
7. Regarding sex determination in human beings, identify the correct statements:
P. The mother contributes an X chromosome to all her offspring.
Q. The father’s sperm determines the sex of the child.
R. An offspring receiving a Y chromosome from the father will be female.
S. Both parents contribute a Y chromosome to a male child.
A. P and Q
B. Q and R
C. P and S
D. R and S
Answer: A. P and Q
8. Complete the analogy for human sex chromosomes: Female : XX :: ________ : XY.
A. Child
B. Male
C. Gamete
D. Zygote
Answer: B. Male
9. Which of the following is NOT a primary reason for the greater variation observed in organisms that reproduce sexually compared to those that reproduce asexually?
A. The combination of genetic material from two different parents.
B. The occurrence of mutations at a significantly higher rate during sexual reproduction.
C. The independent assortment of chromosomes during gamete formation.
D. The creation of new combinations of existing genes.
Answer: B. The occurrence of mutations at a significantly higher rate during sexual reproduction.
10. If a pure-breeding tall pea plant (TT) is crossed with a pure-breeding short pea plant (tt), and the resulting F1 generation is allowed to self-pollinate, what fraction of the F2 generation will phenotypically express the short trait?
A. 0
B. 1/4
C. 1/2
D. 3/4
Answer: B. 1/4
11. In which of the following organisms is sex determination known to be flexible, with individuals capable of changing sex, rather than being strictly genetically determined at conception?
A. Humans
B. Fruit flies
C. Snails
D. Pea plants
Answer: C. Snails
12. When Gregor Mendel crossed a tall pea plant with a short pea plant, what was the characteristic height of the plants in the first filial (F1) generation?
A. All short
B. All medium
C. All tall
D. Half tall, half short
Answer: C. All tall
13. In the body cells of a sexually reproducing organism like a pea plant, how many copies of the gene for a specific trait are usually present?
A. One
B. Two
C. Three
D. Four
Answer: B. Two
14. The evolutionary process by which individuals with traits better suited to their environment tend to survive and reproduce more successfully is known as:
A. Genetic drift
B. Artificial insemination
C. Selection
D. Random mating
Answer: C. Selection
15. If a newly arisen trait ‘X’ is found in 5% of an asexually reproducing population, and an older trait ‘Y’ is found in 70% of the same population, which trait is generally considered to have appeared more recently?
A. Trait Y
B. Trait X
C. Both appeared together
D. Insufficient information
Answer: B. Trait X
16. What biological mechanism ensures the maintenance of a consistent number of chromosomes, and thus DNA stability, from one generation to the next in sexually reproducing species?
A. Frequent mutations in somatic cells.
B. The fusion of gametes, each containing half the normal number of chromosomes.
C. The direct inheritance of all parental chromosomes by the offspring.
D. Asexual reproduction methods like budding.
Answer: B. The fusion of gametes, each containing half the normal number of chromosomes.
17. The inheritance of genetic material from parents to offspring ensures a fundamental ________, along with minor variations, for the subsequent generation.
A. Body design
B. Mutation rate
C. Environmental adaptation
D. Acquired skill
Answer: A. Body design
18. In Mendelian genetics, if ‘R’ represents the allele for round seeds (dominant) and ‘r’ for wrinkled seeds (recessive), a pea plant with the genotype ‘Rr’ is described as:
A. Homozygous dominant
B. Heterozygous
C. Homozygous recessive
D. Pure-breeding
Answer: B. Heterozygous
19. The principle that alleles of different genes assort independently during gamete formation, as demonstrated by crosses involving two different traits (e.g., seed shape and seed color), was a significant discovery made by:
A. Watson and Crick
B. Charles Lyell
C. Gregor Mendel
D. Thomas Hunt Morgan
Answer: C. Gregor Mendel
20. Which of the following was NOT explicitly mentioned as one of the contrasting characteristics studied by Mendel in his experiments with garden peas?
A. Seed shape (round/wrinkled)
B. Plant height (tall/short)
C. Flower color (violet/white)
D. Leaf texture (smooth/hairy)
Answer: D. Leaf texture (smooth/hairy)
21. A woman with blood group O marries a man with blood group A. Their first child has blood group O. This outcome indicates that blood group O is:
A. Dominant over A
B. Recessive to A
C. Co-dominant with A
D. Linked to sex chromosomes
Answer: B. Recessive to A
22. Which statement inaccurately describes the role of DNA and genes in heredity?
A. DNA in cells serves as the blueprint for protein synthesis.
B. A gene typically codes for a single, specific protein.
C. Genes exclusively determine an organism’s physical structure, not its physiological processes.
D. Changes in the DNA sequence of a gene can alter an organism’s traits.
Answer: C. Genes exclusively determine an organism’s physical structure, not its physiological processes.
23. Consider a Mendelian cross where pure-breeding tall pea plants (TT) are crossed with pure-breeding short pea plants (tt). Which statements accurately describe the F1 and F2 generations?
P. All F1 individuals will be phenotypically tall.
Q. The F1 individuals will have a genotype of Tt.
R. Phenotypically short individuals will reappear in the F2 generation.
S. The F2 generation will exhibit a 1:1 phenotypic ratio of tall to short plants.
A. P, Q, and R
B. Q, R, and S
C. P, R, and S
D. P, Q, and S
Answer: A. P, Q, and R
24. In the chromosomal basis of sex determination in humans, the Y chromosome is characteristically found in:
A. Females only
B. Males only
C. Both males and females equally
D. Neither males nor females
Answer: B. Males only
25. Which mode of reproduction is primarily responsible for maximizing the generation of successful variations within a species?
A. Asexual reproduction
B. Vegetative propagation
C. Sexual reproduction
D. Fragmentation
Answer: C. Sexual reproduction
26. For the mechanisms of Mendelian inheritance to operate as described, each gamete (sperm or egg cell) must possess:
A. One complete set of genes
B. Two complete sets of genes
C. A variable number of gene sets
D. No gene sets
Answer: A. One complete set of genes
27. When a single bacterium undergoes two successive divisions (binary fission), the resulting four bacteria are generally:
A. Genetically identical to each other
B. Very similar, with only minor variations due to DNA copying inaccuracies
C. Significantly different due to high mutation rates
D. Half identical to the original, half different
Answer: B. Very similar, with only minor variations due to DNA copying inaccuracies
28. A trait that manifests in an individual’s appearance or physiology even when only one allele for it is inherited from a parent is termed:
A. Recessive
B. Dominant
C. Polygenic
D. Sex-linked
Answer: B. Dominant
29. In a dihybrid cross involving a pea plant heterozygous for both round seeds (Rr) and yellow seeds (Yy), how many distinct genetic combinations of these alleles can be found in its gametes?
A. One
B. Two
C. Three
D. Four
Answer: D. Four
30. The fleshy, lower external part of the human ear, which exhibits variation in its attachment (free or attached) among individuals, is known as the:
A. Helix
B. Tragus
C. Earlobe
D. Auditory canal
Answer: C. Earlobe
Additional MCQs (Competency Based)
1. Assertion (A): In a monohybrid cross between a tall pea plant (TT) and a short pea plant (tt), all F1 progeny are tall.
Reason (R): The allele for tallness (T) is dominant over the allele for shortness (t).
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
2. Assertion (A): Mendel chose pea plants for his experiments because they showed several contrasting visible characters.
Reason (R): Mendel was educated in a monastery and later studied science and mathematics at the University of Vienna.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (b) Both A and R are true but R is not the correct explanation of A.
3. Assertion (A): A man with XY chromosomes and a woman with XX chromosomes can produce offspring with XX or XY chromosomal combinations.
Reason (R): The Y chromosome is exclusively inherited from the mother.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (c) A is true but R is false.
4. Assertion (A): In humans, a child’s sex is solely determined by the X chromosome inherited from the mother.
Reason (R): Males produce sperm cells, half of which carry an X chromosome and half carry a Y chromosome.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (d) A is false but R is true.
5. Assertion (A): Sexually reproducing organisms show greater diversity in offspring compared to asexually reproducing organisms.
Reason (R): Sexual reproduction involves the fusion of gametes from two parents, leading to new combinations of genes.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
6. Assertion (A): Each human germ cell (sperm or egg) contains 23 chromosomes.
Reason (R): During gamete formation, homologous chromosomes pair up and then segregate so that each gamete receives only one chromosome from each pair.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
7. Assertion (A): The stability of DNA of a species is ensured because offspring inherit the exact same number of chromosomes as their parents.
Reason (R): Germ cells combine during fertilization, restoring the normal diploid number of chromosomes in the progeny.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
8. A child has blood group O. Her mother has blood group O. What can be definitively concluded about the father’s possible blood group genotype, assuming standard ABO inheritance where A and B are codominant and dominant over O (recessive)?
(a) He must have blood group O (genotype OO).
(b) He can have blood group A (genotype AO).
(c) He can have blood group B (genotype BO).
(d) He can have blood group A (AO), B (BO), or O (OO).
Answer: (d) He can have blood group A (AO), B (BO), or O (OO).
9. In a certain species of reptile, the temperature at which eggs are incubated determines the sex of the offspring. Eggs incubated at 25°C produce males, while eggs incubated at 30°C produce females. What type of sex determination is this?
(a) Genetically determined, involving sex chromosomes.
(b) Environmentally determined.
(c) Determined by the mother’s genotype.
(d) Random, with no specific determining factor.
Answer: (b) Environmentally determined.
10. A gardener has a field of sugarcane plants. He observes very little variation among the individual plants. Another farmer has a herd of cattle (which reproduce sexually) and observes many distinct variations among the animals. What is the primary reason for this difference in observed variation?
(a) Sugarcane reproduces asexually, while cattle reproduce sexually.
(b) Sugarcane is a plant, while cattle are animals.
(c) Sugarcane experiences more environmental pressures leading to uniformity.
(d) Cattle have a much shorter generation time than sugarcane.
Answer: (a) Sugarcane reproduces asexually, while cattle reproduce sexually.
11. Mendel crossed tall pea plants with short pea plants. He observed that all F1 progeny were tall, and there were no ‘medium-height’ plants. This observation led him to conclude that:
(a) Both tallness and shortness traits are equally expressed in the F1 generation.
(b) The trait for shortness was lost in the F1 generation.
(c) One of the parental traits was dominant over the other.
(d) Environmental factors determined the height of F1 plants.
Answer: (c) One of the parental traits was dominant over the other.
12. If a single bacterium undergoes binary fission three times, and assuming no mutations occur, how many genetically identical bacteria will be produced from the original one?
(a) 3
(b) 4
(c) 6
(d) 8
Answer: (d) 8
13. Stimulus: “In human beings, the sex of the individual is largely genetically determined. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y.”
Based on this information, what combination of sex chromosomes will result in a male offspring?
(a) XX from mother, Y from father
(b) X from mother, Y from father
(c) Y from mother, X from father
(d) X from mother, X from father
Answer: (b) X from mother, Y from father
14. Stimulus: “If one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar. There would be only very minor differences between them, generated due to small inaccuracies in DNA copying.”
This passage primarily describes variation arising during which type of reproduction?
(a) Sexual reproduction
(b) Asexual reproduction
(c) Budding
(d) Fragmentation
Answer: (b) Asexual reproduction
15. Stimulus: “Inheritance from the previous generation provides both a common basic body design, and subtle changes in it, for the next generation. The second generation will have differences that they inherit from the first generation, as well as newly created differences.”
This accumulation of variations over generations is fundamental to which biological process?
(a) Homeostasis
(b) Photosynthesis
(c) Evolution
(d) Osmosis
Answer: (c) Evolution
16. Stimulus: “Genes control characteristics, or traits. If the gene for an enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short.”
This statement illustrates that a change in a gene (an alteration) can directly lead to a change in:
(a) The number of chromosomes in the plant.
(b) The plant’s ability to perform photosynthesis.
(c) The structure or function of a protein, affecting a trait.
(d) The type of soil the plant prefers.
Answer: (c) The structure or function of a protein, affecting a trait.
17. Match the genetic term with its correct description.
| Column A | Column B |
| (i) Gene | 1. The observable characteristic of an organism. |
| (ii) Allele | 2. The genetic makeup of an organism for a particular trait. |
| (iii) Genotype | 3. A segment of DNA that codes for a protein. |
| (iv) Phenotype | 4. An alternative form of a gene. |
Codes:
(a) (i)–3, (ii)–4, (iii)–2, (iv)–1
(b) (i)–4, (ii)–3, (iii)–1, (iv)–2
(c) (i)–3, (ii)–2, (iii)–4, (iv)–1
(d) (i)–1, (ii)–2, (iii)–3, (iv)–4
Answer: (a) (i)–3, (ii)–4, (iii)–2, (iv)–1
18. Match the type of organism/cell with its characteristic chromosome composition related to sex determination in humans.
| Column A | Column B |
| (i) Female somatic cell | 1. Contains either an X or a Y chromosome (plus autosomes) |
| (ii) Male somatic cell | 2. Contains one X chromosome (plus autosomes) |
| (iii) Egg cell (ovum) | 3. Contains XX sex chromosomes (plus autosomes) |
| (iv) Sperm cell | 4. Contains XY sex chromosomes (plus autosomes) |
Codes:
(a) (i)–3, (ii)–4, (iii)–2, (iv)–1
(b) (i)–4, (ii)–3, (iii)–1, (iv)–2
(c) (i)–3, (ii)–1, (iii)–2, (iv)–4
(d) (i)–2, (ii)–4, (iii)–3, (iv)–1
Answer: (a) (i)–3, (ii)–4, (iii)–2, (iv)–1
19. Match the Mendelian generation with its typical characteristic in a monohybrid cross (e.g., Tall x Short pea plants).
| Column A | Column B |
| (i) P generation | 1. Exhibits a 3:1 ratio of dominant to recessive traits. |
| (ii) F1 generation | 2. Pure-breeding parents with contrasting traits. |
| (iii) F2 generation (phenotype) | 3. All individuals heterozygous, expressing the dominant trait. |
| (iv) F2 generation (genotype for a Tt x Tt cross) | 4. Exhibits a 1:2:1 ratio (e.g., TT:Tt:tt). |
Codes:
(a) (i)–2, (ii)–3, (iii)–1, (iv)–4
(b) (i)–3, (ii)–2, (iii)–4, (iv)–1
(c) (i)–2, (ii)–1, (iii)–3, (iv)–4
(d) (i)–4, (ii)–3, (iii)–1, (iv)–2
Answer: (a) (i)–2, (ii)–3, (iii)–1, (iv)–4
20. Arrange the following events in Mendel’s typical monohybrid cross experiment in the correct sequence, starting from the parental generation:
(i) F1 generation plants are self-pollinated.
(ii) Pure-breeding tall plants are crossed with pure-breeding short plants.
(iii) F2 generation shows both tall and short plants.
(iv) All F1 generation plants are tall.
(a) (ii) → (iv) → (i) → (iii)
(b) (i) → (ii) → (iii) → (iv)
(c) (ii) → (i) → (iv) → (iii)
(d) (iv) → (ii) → (i) → (iii)
Answer: (a) (ii) → (iv) → (i) → (iii)
21. Consider the expression of a gene for plant height. Arrange the following steps in the correct order from gene to observable trait:
(i) The plant exhibits a tall phenotype.
(ii) The gene for height provides information for a specific enzyme.
(iii) The enzyme’s activity influences the amount of growth hormone produced.
(iv) Sufficient growth hormone is produced.
(a) (ii) → (iii) → (iv) → (i)
(b) (iii) → (ii) → (i) → (iv)
(c) (ii) → (iv) → (iii) → (i)
(d) (i) → (iv) → (iii) → (ii)
Answer: (a) (ii) → (iii) → (iv) → (i)
22. Arrange the levels of genetic organization from the smallest functional unit to the largest structure carrying these units, as discussed in the context of inheritance.
(i) Chromosome
(ii) Gene
(iii) DNA (as the molecule making up genes)
(a) (iii) → (i) → (ii)
(b) (ii) → (iii) → (i)
(c) (ii) → (i) → (iii)
(d) (iii) → (ii) → (i)
Answer: (b) (ii) → (iii) → (i)
23. In a dihybrid cross, if an F1 individual with genotype RrYy (round, yellow seeds) is self-pollinated, and the genes for seed shape (R/r) and seed colour (Y/y) assort independently, the F2 generation shows 556 seeds. The observed numbers are: 315 round yellow, 108 round green, 101 wrinkled yellow, and 32 wrinkled green. Approximately what proportion of the F2 offspring are expected to be wrinkled and green?
(a) 9/16
(b) 3/16
(c) 1/16
(d) 4/16
Answer: (c) 1/16
24. In a population of asexually reproducing organisms, Trait X is found in 5% of individuals and Trait Y is found in 70% of individuals. Both traits arose from mutations and are neutral or beneficial. Which trait is likely to have arisen earlier in the population’s history, assuming both persist?
(a) Trait X
(b) Trait Y
(c) Both arose simultaneously
(d) Cannot be determined without knowing mutation rates.
Answer: (b) Trait Y
25. A student observes the earlobes of 100 classmates. 75 students have free earlobes, and 25 have attached earlobes. Assuming free earlobes (F) are dominant over attached earlobes (f). What percentage of students exhibit the recessive phenotype?
(a) 75%
(b) 50%
(c) 25%
(d) Cannot be determined
Answer: (c) 25%
26. In an experiment, crossing two heterozygous tall pea plants (Tt x Tt) resulted in 100 offspring. Based on Mendelian ratios, which of the following is the LEAST likely number of short (tt) offspring to be observed?
(a) 25
(b) 22
(c) 28
(d) 50
Answer: (d) 50
27. A cross between two pea plants produces 120 offspring. 90 of these are tall and 30 are short. What is the most likely genotype of the parent plants, if T (tall) is dominant to t (short)?
(a) TT x tt
(b) Tt x Tt
(c) TT x Tt
(d) Tt x tt
Answer: (b) Tt x Tt
Additional Questions and Answers
1. Which type of reproduction produces very similar offspring with only minor differences?
Answer: Asexual reproduction produces very similar offspring. For instance, if one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar. There would be only very minor differences between them, generated due to small inaccuracies in DNA copying.
2. Which type of reproduction generates greater diversity among offspring?
Answer: The number of successful variations are maximised by the process of sexual reproduction. If sexual reproduction is involved, even greater diversity will be generated among offspring.
3. What process forms the basis for evolutionary processes by favouring certain variants?
Answer: The selection of variants by environmental factors forms the basis for evolutionary processes.
4. Who formulated the main rules of heredity by studying garden-pea plants?
Answer: Gregor Johann Mendel worked out the main rules of inheritance. He used a number of contrasting visible characters of garden peas and was the first one to keep count of individuals exhibiting a particular trait in each generation, which helped him to arrive at the laws of inheritance.
5. What term describes a version of a gene that masks the expression of another version?
Answer: A trait that gets expressed when two gene copies are not identical is called the dominant trait. For example, a single copy of ‘T’ is enough to make a pea plant tall, so traits like ‘T’ are called dominant traits.
6. What term describes a trait that appears only when both gene versions are identical?
Answer: A trait that is not expressed when paired with a dominant trait is called a recessive trait. For example, both copies of a gene have to be ‘t’ for a pea plant to be short, so traits that behave like ‘t’ are called recessive traits.
7. What is the name for a section of DNA that provides information for one protein?
Answer: A section of DNA that provides information for one protein is called the gene for that protein.
8. What molecule serves as the information source for protein synthesis in cells?
Answer: Cellular DNA is the information source for making proteins in the cell.
9. What ratio of tall to short plants appears in the F₂ generation of a monohybrid cross?
Answer: In the F₂ generation progeny of F1 tall plants, one quarter of them are short. This results in a 3:1 ratio of tall to short plants.
10. What phenomenon explains the appearance of new combinations of traits in the F₂ generation of a dihybrid cross?
Answer: The appearance of new combinations of traits in the F₂ generation is explained by the fact that the traits are independently inherited. For example, the tall/short trait and the round seed/wrinkled seed trait are inherited separately, giving rise to new combinations of traits in the offspring.
11. Which chromosomes determine human sex?
Answer: The pair of chromosomes called the sex chromosomes determines human sex. These are the X and Y chromosomes.
12. What combination of sex chromosomes denotes a female human?
Answer: Women have a perfect pair of sex chromosomes, both called X. The combination that denotes a female human is XX.
13. What combination of sex chromosomes denotes a male human?
Answer: Men have a mismatched pair of sex chromosomes in which one is a normal-sized X while the other is a short one called Y. The combination that denotes a male human is XY.
14. From which parent does a child inherit the chromosome that determines its sex?
Answer: The sex of the children is determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.
16. Explain how variations accumulate over successive generations in asexually reproducing organisms.
Answer: In asexually reproducing organisms, inheritance from the previous generation provides both a common basic body design and subtle changes in it for the next generation. The second generation will have differences that they inherit from the first generation, as well as newly created differences. The original organism gives rise to two individuals, similar in body design, but with subtle differences. Each of them, in turn, gives rise to two individuals in the next generation. Each of the four individuals in the bottom row will be different from each other. While some of these differences will be unique, others will be inherited from their respective parents, who were different from each other, leading to the creation of diversity over succeeding generations.
17. Describe how sexual reproduction amplifies diversity compared to asexual reproduction.
Answer: While some amount of variation is produced even during asexual reproduction, the number of successful variations are maximised by the process of sexual reproduction. In asexual reproduction, the generated individuals would be very similar, with only minor differences due to small inaccuracies in DNA copying. However, if sexual reproduction is involved, even greater diversity will be generated.
18. Explain the process by which genes control the expression of physical characteristics in organisms, using plant height as an example.
Answer: Genes control characteristics, or traits. Cellular DNA is the information source for making proteins, and a gene is a section of DNA that provides information for one protein. Using plant height as an example, plants have hormones that trigger growth, so plant height depends on the amount of a particular plant hormone. The amount of hormone made depends on the efficiency of an enzyme important for this process. If this enzyme works efficiently, a lot of hormone will be made, and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short.
19. Discuss Mendel’s monohybrid experiments and how they led to the concepts of dominant and recessive traits.
Answer: Mendel took pea plants with different characteristics, a tall plant and a short plant, and produced progeny by crossing them. In the first-generation, or F1 progeny, all plants were tall, meaning only one of the parental traits was seen. When these F1 tall plants were self-pollinated, the second-generation, or F2, progeny were not all tall; instead, one quarter of them were short. This indicated that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. This led Mendel to propose that two copies of a factor, now called a gene, control traits. A single copy of ‘T’ is enough to make the plant tall, while both copies have to be ‘t’ for the plant to be short. Traits like ‘T’ are called dominant traits, while those that behave like ‘t’ are called recessive traits.
20. Describe Mendel’s dihybrid cross and explain how it demonstrates the independent inheritance of two traits.
Answer: When pea plants showing two different characteristics, such as a tall plant with round seeds and a short plant with wrinkled-seeds, are bred, the F1 progeny are all tall and have round seeds. When these F1 progeny are used to generate F2 progeny by self-pollination, some F2 progeny are tall plants with round seeds, and some are short plants with wrinkled seeds. However, there are also F2 progeny that show new combinations, such as tall plants with wrinkled seeds and short plants with round seeds. The formation of these new combinations of traits in the F2 offspring demonstrates that the tall/short trait and the round seed/wrinkled seed trait are independently inherited.
21. Explain how germ cells ensure that each gamete contains only one set of chromosomes.
Answer: Each gene set is present as separate independent pieces, each called a chromosome. Each cell in an organism has two copies of each chromosome, one from the male and one from the female parent. To ensure each gamete contains only one set, every germ-cell takes one chromosome from each pair, which may be of either maternal or paternal origin. When two germ cells combine, they restore the normal number of chromosomes in the progeny.
22. Outline the genetic mechanism of sex determination in human beings, including the roles of X and Y chromosomes.
Answer: In human beings, the sex of the individual is largely genetically determined. Most human chromosomes exist in 22 pairs, with each pair having a maternal and a paternal copy. However, the sex chromosomes are not always a perfect pair. Women have a perfect pair of sex chromosomes, both called X (XX). Men have a mismatched pair with one normal-sized X chromosome and one short Y chromosome (XY). All children inherit an X chromosome from their mother. The sex of the children is determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.
