Heredity

Intext Questions 1

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

Asexually reproducing species show very few variations which are seen usually due to mutation. Any such variation in an individual takes time to become considerable part of the population as it shows up only in individual's progeny. Therefore, if a trait A exists in 10% of a population of an asexually reproducing species, it must be a recent variation and if a trait B exists in 60% of the same population, it is likely to have arisen earlier.

Question 2

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

Answer

Any species is adapted to survive in a particular niche. In case of any sudden change in that niche the species may not survive if all the individuals are alike. But if few individuals are different due to variations they can adapt to the changed conditions and thus species can avoid being wiped out. Therefore, the creation of variations in a species promote its evolution and hence its survival.

Intext Questions 2

Question 1

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

Answer

Mendel used a number of contrasting visible characters of garden peas – round/wrinkled seeds, tall/short plants, white/violet flowers and so on. When he crossed plants with contrasting characters, there were no halfway characteristics in the first generation. For example, if a tall plant was crossed with a short plant, all plants were either tall or short and no plant was found with medium height. In F₂ generation cross between two tall plants gave short plant as well. This proved that there are two contrasting characters out of which one is dominant which masks the effect of other which is recessive.

Question 2

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

Answer

Mendel crossed a tall pea plant with round seeds with a short pea plant with wrinkled seeds. The F₁ progeny were all tall with round seeds. Thus, tallness and round seeds are the dominant traits.
Mendel then used F₁ progeny to generate F₂ progeny by self-pollination. He observed that some F₂ progeny were tall plants with round seeds, and some were short plants with wrinkled seeds. However, he also found that some F₂ progeny showed new combinations like tall but with wrinkled seeds, while others would be short but have round seeds. Thus, Mendel’s experiments showed that traits are inherited independently.

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

No, the given information is not sufficient to tell which of the traits is dominant because we don't know all possible blood groups of all the progeny.

The father's blood group is A, which means he can be homozygous (AA) or heterozygous (AO). Similarly, mother can also be homozygous or heterozygous.

Question 4

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

Answer

In humans, the males have 'XY' chromosomes and females have 'XX' chromosome as sex chromosomes. Therefore, 50% male gametes have X chromosome and other 50% have Y chromosome. The females have all gametes with X chromosome. The sex of the child is determined at the time of fertilisation. If male gamete with X chromosome fertilizes with female gamete, it leads to a female child whereas if a gamete with Y chromosome fertilizes with female gamete, it leads to a male child.

Exercises

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

1. TTWW
2. TTww
3. TtWW
4. TtWw

Answer

TtWW

Reason — Possibility of genotype for parent plants:

1. Tall pea plants bearing violet flowers — TTWw or TtWw or TTWW or TtWW
2. Short pea plants bearing white flowers — ttww or ttWw

Since, the progeny all bore violet flowers, the tall parent must be homozygous for violet flower colour i.e. WW. Also, almost half of the progeny were short shows that the tall parent must be heterozygous for tallness i.e. Tt. Therefore, the genetic make-up of the tall parent can be depicted as TtWW.

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

No, we cannot say anything about whether the light eye colour trait is dominant or recessive based on above given fact because:

1. It is not clear whether parents are heterozygous or homozygous.
2. It requires to study inheritance pattern upto three or more generations.

Question 3

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

Answer

Following are the steps that should be followed in order to find the dominant coat colour in dogs:

1. Find the homozygous male for coat colour.
2. Find the homozygous female for contrasting coat colour.
3. Both are mated to get F₁ generation.
4. The colour of coat of progeny will be the dominant colour.

Question 4

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

Answer

During gamete formation the cell divides by meiosis giving four haploid (n) cells. The normal number of chromosomes in human a cell is 46 i.e. 23 pairs. The gamete cells, therefore, formed after meiosis have 23 chromosomes each. When fertilization occurs, the male gamete and female gamete fuse together and the original number of chromosomes is restored. This means that the zygote has 46 (23+23) chromosomes — 23 from male gamete and 23 from female gamete. This is how the equal genetic contribution of male and female parents is ensured in the progeny.