Genotype ?

One human gene, which was one thought to be X-linked, controls the length of hair on men's ears. If a man with hairy ears has sons, all of them will have hairy ears, but if he has daughters none of them will have hairly ears.

Why these differences exist?

Genotype ?

Two reasons.

1. Hormones - females have less testosterone, which is the hormone which stimulates hair growth in males. Females with excess testosterone have more hair.

2. With each of the XX chromosomes, one of them is shut down in each cell (lionisation). This is a random thing within each cell so, on average, only half the X chromosomes are expressing the hairiness gene. In males, the X chromosome is fully expressed, so there is no suppression of the gene.

So, a combination of lionisation, and lack of hormone stimulation results in non-hairy ears for females - no doubt with the gratitude of women everywhere.

Genotype ?

X-linked recessive. XY for sons, so all express the trait. XX for daughters, so the dominant gene, hairlessness, is expressed, while the recessive, hairy, is not.

PS A below correction has brought to my attention that it could be Y-linked.

Genotype ?

this will also have to mean one of the mothers' parents carried the hairy-ear gene

Genotype ?

Then shouldn't it be Y-linked? But some sex linked genes only need one X, the other may cancel it out.

Genotype ?

Well it seems like it doesnt worked and it could be "y-linked" if that is possible, but since it states that it controls the length of hair on men's ears, maybe girls would not be affected

Genotype ?

girls have another x chromosome on which the gene for hairy ears isnt