Question

A red-haired woman marries a brown-haired man and all the children are brown-haired. Explain this genetically.

Answer 1

Red hair and brown hair both are independent traits and are defined as phenotypes. The traits are usually expressed based on a pair of alleles in which depending on the condition of dominant or recessive one trait is expressed resulting in a particular phenotype.

Complete answer:
In this given situation, all the children are brown-haired proving that allele for brown-haired should be a dominant trait. So here the presence of red hair is a recessive trait and is expressed only in the dominant condition which is written in the form of small 'r' and the condition is 'rr' in dominant recessive state.

Whereas the presence of brown hair is a dominant trait and is represented by capital 'R'. Presence of this allele in the dominant condition as 'RR' and its heterozygous condition along with recessive allele as 'Rr' the results will be the same as the progeny in the first generation will be brown-haired.

Hence if the allele for brown hair will be present in homozygous condition or if the allele for brown hair is present along with the allele for red hair then only the brown hair condition is expressed and all the children will be brown-haired.

This explains the First Law of Mendel which is the Law of Dominance which states that alleles always occur in pairs and a pair of alleles, one is always dominant and another trait is always recessive.

| Red-haired woman| Brown-haired man
---|---|---
Parent generation| rr| RR
Gametes | r| R
F1 generation| Rr| Rr

In this condition in F generation, all children having a genotype of Rr thereby have brown hair. Because the R is dominant over r.

Note: Due to the work of Mendel in the field of genetics there is a great advancement knowing the laws of inheritance and how traits are inherited from parents to the progeny. It was also possible to find about the genes and how they control various traits. With the advancement in the field of genetics, there is a great improvement and research is always going on to find how these traits are inherited, expressed and controlled generation after generation.