Question

- A couple with the curly-haired husband and straight-haired wife have all their children curly-haired because
A) Both are heterozygous
B) Husband is homozygous and wife is heterozygous
C) Husband is heterozygous and wife is homozygous
D) Both are homozygous

Answer 1

Curly hairs and straight hairs are both independent traits which are expressive both in their homozygous conditions. These are known as phenotypes. These are expressive according to their dominant or recessive condition.

Complete answer:
The trait for hair type is expressive I.e. couples have contrasting traits for hair type- curly and straight. From the question, we get to know that the children are curly-haired therefore indicating that curly hair is dominant over straight hair. Let us assume here the trait for hair is 'C'. So here the genotype of the father could be either 'CC' or 'Cc'.

The mother having straight hair has a recessive genotype for hair I.e. 'cc'. In this case, assuming all the children who are born to the given couple are curly-haired, the father's genotype should be the dominant condition I.e. Father is having the genotype 'CC'. Father is homozygous dominant and the mother is homozygous recessive 'cc'. Therefore the children born to a homozygous father with dominant traits and dominant recessive traits of the mother would be heterozygous 'Cc'.

If taking the second condition of the father being heterozygous one of the children of the couple would be a straight hair child. Therefore recessive phenotype of straight hair. This is the best example of Mendel's first law of dominance which is the Law of dominance which best explains that in one pair of factors one member dominates the other character which is recessive. So, in this case, both the parents are homozygous.

| Father| Mother
---|---|---
Parent generation| CC| cc
Gametes| C| c
F1 generation| Cc| Cc

In, F1 generation all are heterozygous with curly haired traits being dominant over straight hair and therefore all their children are curly haired.

Hence, the correct answer is option "D".

Note: Traits in progeny are controlled by genes in parents which are inherited from one generation to another generation via gametes. Mendel best described the laws of inheritance which changed the definition of genetics for one and all.