CBSE Class X Science Chapter 8 Heredity Questions and Answers

In-Text Questions (Page 129)

Q1: 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?

Trait B is likely to have arisen earlier. In asexual reproduction, DNA copying carries over existing traits to offspring with only minor inaccuracies. Therefore, traits that emerged earlier have been replicated through more generations and are present in a larger percentage (60%) of the population compared to newly arisen traits (10%).

Q2: How does the creation of variations in a species promote survival?

Creation of variations promotes survival by giving different individuals unique advantages depending on the nature of the variation. When environmental conditions change drastically (for instance, a heat wave), variant individuals capable of withstanding the new conditions (such as heat-resistant bacteria) will survive while others perish, preventing the entire species from becoming extinct.

In-Text Questions (Page 133)

Q1: How do Mendel's experiments show that traits may be dominant or recessive?

Mendel crossed a tall pea plant ($TT$) with a short pea plant ($tt$). In the first generation ($F_1$), all offspring were tall ($Tt$), showing that only the tallness trait was expressed while the shortness trait remained hidden. When the $F_1$ tall plants were self-pollinated, the $F_2$ generation produced both tall and short plants in a 3:1 ratio. This showed that both traits were inherited, but the trait expressed in the presence of its alternative (tallness) is dominant, whereas the trait expressed only when two copies are present (shortness) is recessive.

Figure 8.3: Inheritance of traits over two generations

Q2: How do Mendel's experiments show that traits are inherited independently?

Mendel crossed pea plants having two contrasting traits: tall plants with round seeds and short plants with wrinkled seeds. The $F_1$ offspring were all tall with round seeds. When $F_1$ plants self-pollinated to produce $F_2$ progeny, new combinations appeared along with the parental combinations—such as tall plants with wrinkled seeds and short plants with round seeds. The formation of these new combinations demonstrates that plant height and seed shape are inherited independently of each other.

Figure 8.5: Independent inheritance of two separate traits, shape and colour of seeds

Q3: 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?

No, this information is not sufficient to determine which trait is dominant. Blood group traits are governed by two copies of genes inherited from parents. Without knowing whether the father carries two copies of the 'A' allele or one 'A' and one 'O' allele, or whether 'A' or 'O' expresses in a heterozygous state, both possibilities can account for a daughter with blood group O. Therefore, a single family outcome without allele details is not enough evidence.

Q4: How is the sex of the child determined in human beings?

In human beings, sex is genetically determined by sex chromosomes:

• Females have a matching pair of sex chromosomes ($XX$).
• Males have a mismatched pair of sex chromosomes ($XY$).

All children inherit an $X$ chromosome from their mother. The sex of the child depends on the chromosome inherited from the father: an egg fertilised by a sperm carrying an $X$ chromosome results in a female child ($XX$), while an egg fertilised by a sperm carrying a $Y$ chromosome results in a male child ($XY$).

Figure 8.6: Sex determination in human beings

Chapter End Exercises (Page 133)

Q1: 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: Since all progeny bore violet flowers, the parent must be homozygous dominant ($WW$) for flower colour. Since almost half of the offspring were short, the tall parent must carry a recessive allele for height ($Tt$).

Q2: 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?

No, we cannot conclusively say whether the light eye colour trait is dominant or recessive based on this information alone. The observation simply shows that the trait is inherited from parents to children. To determine dominance or recessiveness, data regarding the traits shown in $F_1$ and $F_2$ generations from crosses of contrasting eye-colour traits (or genetic ratios) are required.

Q3: Outline a project which aims to find the dominant coat colour in dogs.

To determine the dominant coat colour in dogs, a project can be designed as follows:

• Selection: Select pure-bred dogs with two distinct contrasting coat colours (for example, pure black coat and pure white coat).
• First Generation ($F_1$ Cross): Cross a pure-bred black-coated dog with a pure-bred white-coated dog and observe the coat colour of all $F_1$ puppies. The coat colour expressed in all or majority $F_1$ puppies represents the dominant trait.
• Second Generation ($F_2$ Cross): Cross the $F_1$ generation dogs among themselves and record the ratio of coat colours in the $F_2$ puppies to confirm Mendelian inheritance.

Q4: How is the equal genetic contribution of male and female parents ensured in the progeny?

Equal genetic contribution is ensured through chromosomes during sexual reproduction:

• Human cells carry 22 paired chromosomes and 1 pair of sex chromosomes.
• During germ-cell (gamete) formation, special cell division reduces the gene sets so that each germ cell contains only one chromosome from each pair.
• When a male germ-cell fuses with a female germ-cell during fertilization, the resulting zygote receives one copy of each chromosome from the mother and one from the father, restoring the normal double set of chromosomes.