Question

Excessive growth of hair on the pinna is a feature found only in males because
(a) The female sex hormone estrogen suppresses the character in females
(b) The gene responsible for the character is present on the Y-chromosome only
(c) The gene responsible for the character is recessive in females and dominant only in males
(d) The character is induced in males as males produced testosterone

Answer 1

A human has 23 pairs of chromosomes, out of which, the 23rd pair of the chromosomes are known as the sex chromosomes. The character of excessive growth of hair on the pinna is determined by the differences in males and females at the level of genetic composition.

Complete answer:
Hypertrichosis is the condition of excessive hair growth in a particular area which otherwise lacks hair growth. It is not counted among the normal hair growth patterns irrespective of the age, sex, or ethnicity of an individual. Hypertrichosis pinnae Auris is one such type of abnormal hair growth seen particularly in older men. It is commonly known as hairy ears due to the excessive growth of hairs on the outer ridge of pinna i.e helix of the ear. This is a Y-linked chromosomal character and their pattern of inheritance is from a father to his son.

Additional Information: -In females, there is a pair of identical sex chromosomes called the X-chromosomes. Thus, they are homomorphic. In males, the 23rd pair of chromosomes are non-identical, consisting of one X-chromosome and Y-chromosome. Thus, they are heteromorphic. Morphologically, the Y-chromosome is shorter than the X-chromosome.
-The sex of humans is determined by its sex chromosomes. The primary sex determination is based on the presence of the Y-chromosomes. It is known as the XY- chromosomal system of sex-determination.
-The action of some genes is influenced by sex hormones. For males, testosterone is the sex hormone required for their normal functioning of the reproductive system and sexual characteristics. For females, estrogen and progesterone play a vital role in their sexual development and reproduction.
So, the correct answer is ‘The gene responsible for the character is present on the Y-chromosome only.’

Note: -Cytogeneticists have divided the X and Y chromosome in some species into homologous and non-homologous or differential regions.
-The genes in the non-homologous regions do not have any counterpart on the other sex chromosome. Thus, they are said to be homozygous or half zygous.