NCERT Solutions For Class 10 Biology: Heredity

Intext Questions (Page 129)

Question 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. In an asexually reproducing population, variations arise due to small inaccuracies in DNA copying. Since there is no mixing of genetic material, the variation that arises earliest will have had the most time to be replicated and inherited by subsequent generations, leading to a higher percentage of the population exhibiting that trait.

Question 2: How does the creation of variations in a species promote survival?

Answer-
The creation of variations in a species promotes survival because niches can change drastically due to environmental factors (like heat waves). Depending on the nature of the variations, different individuals would have different kinds of advantages. For example, bacteria that can withstand heat will survive better in a heat wave. The selection of variants by environmental factors forms the basis for evolutionary processes, ensuring that some members of the species survive niche alterations and the species is not wiped out.

Intext Questions (Page 133)

Question 1: How do Mendel’s experiments show that traits may be dominant or recessive?

Answer-
Mendel’s experiments showed that traits may be dominant or recessive by crossing plants with contrasting visible characters, such as tall and short pea plants.
1.F1 Generation: When Mendel crossed a tall plant with a short plant, the resulting first-generation ($\text{F}_1$) progeny were all tall. This meant that only one of the parental traits was seen, and the trait for shortness was not expressed.

2.F2 Generation: When these $\text{F}_1$ tall plants were self-pollinated, the second-generation ($\text{F}_2$) progeny were not all tall; instead, one quarter of them were short.

This indicated that the shortness trait was inherited in the $\text{F}_1$ generation but only the tallness trait was expressed. The trait that is expressed (like 'T' for tallness) is called the dominant trait, while the unexpressed trait (like 't' for shortness) is called the recessive trait.

Question 2: How do Mendel’s experiments show that traits are inherited independently?

Answer-
Mendel's experiments showed that traits are inherited independently by breeding pea plants that showed two different contrasting characteristics simultaneously (e.g., a tall plant with round seeds crossed with a short plant with wrinkled seeds).
1.F1 Generation: The $\text{F}_1$ progeny all looked alike: tall and having round seeds, confirming that tallness and round seeds are dominant traits.

2.F2 Generation: When these $\text{F}_1$ progeny were self-pollinated to generate $\text{F}_2$ progeny, the $\text{F}_2$ offspring showed new combinations. Some progeny were tall with wrinkled seeds and others were short with round seeds.

This result demonstrated that the tall/short trait and the round seed/wrinkled seed trait are independently inherited, meaning the inheritance of one trait (like colour) is not linked to the inheritance of the other trait (like shape).

Question 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 suggest that blood group O is the recessive trait.
The daughter inherited two versions of the gene, one from the father (A) and one from the mother (O). Since the daughter expresses blood group O, both copies must be 'O' for the trait to be seen (O/O). If the father (blood group A) passed on a recessive copy (O) and the mother (blood group O) passed on a recessive copy (O), the resultant phenotype (O) is recessive. This implies that the Blood Group A trait is dominant because the father must have carried both versions (A and O, or A/O), but only the A trait was expressed in him, following Mendel's rule for dominant traits.

Question 4: How is the sex of the child determined in human beings?

Answer-
The sex of the child in human beings is largely genetically determined by the sex chromosomes inherited from the parents.
1.Parental Chromosomes: Women have a perfect pair of sex chromosomes ($\text{XX}$), while men have a mismatched pair ($\text{XY}$).

2.Inheritance: All children inherit an X chromosome from their mother.

3.Determination: The sex is determined by what the child inherits from the father. If the child inherits an X chromosome from the father, the child will be a girl ($\text{XX}$). If the child inherits a Y chromosome from the father, the child will be a boy ($\text{XY}$). Half the children will be boys and half will be girls.

Exercise Questions (Page 133-134)

Question 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-
The correct option is (c) TtWW.
*All progeny bore violet flowers: This means the violet trait (W) is dominant, and since all offspring are violet, the tall parent must have carried two copies of the dominant violet trait (WW).

*Almost half the progeny were short: This means the tall parent must have been heterozygous for height (carrying both tall and short factors, Tt). When crossed with a short plant (tt), approximately half the offspring will be Tt (tall) and half will be tt (short).

Question 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-
On this basis alone, we cannot definitively say whether the light eye colour trait is dominant or recessive.
1.Possible Recessive Trait: If light eye colour is a recessive trait, then both parents must carry two copies of the gene for light eye colour (e.g., $\text{ll}$) for the trait to be expressed in them. Since the trait is seen in both parents, all their children would also inherit two recessive copies ($\text{ll}$) and express light eye colour, fitting the study's observation.

2.Possible Dominant Trait: If light eye colour is a dominant trait, then the parents could be homozygous dominant ($\text{LL}$) or heterozygous ($\text{Ll}$). If both parents are homozygous dominant, all their children would inherit the dominant trait ($\text{L}$), resulting in light eye colour. If both parents are heterozygous, most of their children (three quarters) would still express the dominant light eye colour trait.

Therefore, the observation that the trait passes from parents to children does not distinguish between dominant and recessive traits without further information on the inheritance pattern (like Mendel's $\text{F}_1$ or $\text{F}_2$ generation analysis).

Question 3: Outline a project which aims to find the dominant coat colour in dogs.

Answer-
A project aiming to find the dominant coat colour in dogs would follow Mendelian experimental rules:
1.Selection of Parents (P Generation): Select purebred dogs showing contrasting coat colours (e.g., two dogs: one black coat and one white coat). The dogs must be sexually reproducing organisms.

2.First Generation (F1): Cross the two purebred parents (P generation) and record the coat colour of all the progeny ($\text{F}_1$). The colour that is seen in all the $\text{F}_1$ progeny will be the dominant trait.

3.Second Generation (F2): Allow the $\text{F}_1$ progeny (who all express the dominant trait) to reproduce among themselves (or cross two $\text{F}_1$ individuals).

4.Observation and Calculation: Observe and calculate the percentage and ratio of coat colours in the $\text{F}_2$ generation. If the dominant colour appears in approximately 75% (3/4) of the $\text{F}_2$ progeny, and the recessive colour appears in 25% (1/4), this confirms the dominant nature of the expressed colour.

Question 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 is ensured in the progeny during sexual reproduction by the mechanism of germ-cell formation and fusion:
1.Equal Contribution: Both the father and the mother contribute practically equal amounts of genetic material to the child.

2.Two Gene Sets: All other cells in the body have two copies (sets) of all genes. For the inheritance mechanism to work, the progeny must inherit one set from each parent.

3.Germ Cell Formation: To achieve this, each germ cell (sperm and egg) is formed with only a single set of genes, meaning it contains only one copy of each chromosome.

4.Restoration of Chromosome Number: When the two germ cells (one male, one female) combine (fertilisation), they restore the normal number of chromosomes (two copies of each) in the progeny. This mechanism ensures the stability of the DNA of the species and that each trait is influenced by both paternal and maternal DNA.