Question

A woman with “hitch-hiker’s thumb” marries a man who lacks this trait (but whose mother had it). What is the probability that a child born to this couple will have a hitch-hiker’s thumb?

Answer 1

The Hitch-hiker's thumb is the hyper-flexible thumbs that can be able to bend backward beyond the normal range. It is thought to be regulated by a recessive allele.  The allele which codes for the normal finger can be denoted by the letter H and the allele which codes for the hitch-hikers finger can be denoted by the letter h. Since it is a recessive character, the homozygous recessive organism will only have this character. In the question, it is given that the mother has a hitchhikers thumb, hence her genotype will be hh. Even though the father has normal fingers, her mother had the hitchhikers thumb, hence his genotype will be either HH or Hh. Let's look at the crosses

Condition 1

If the father is homozygous to this gene (HH) and the affected mother (hh)

Father X Mother

Genotype of parents HH X hh

Gametes H, H X h, h

Offsprings   

	h	h
H	Hh Normal	Hh Normal
H	Hh Normal	Hh Normal

In this condition, all the offsprings will have normal fingers. Since they are heterozygous to this gene, they all are carriers.

Condition 2

If the father is heterozygous to this gene (Hh) and the affected mother (hh)

Father X Mother

Genotype of parents Hh X hh

Gametes H, h X h, h

Offsprings

	h	h
H	Hh Normal	Hh Normal
h	hh Hitchhikers thumb	hh Hitchhikers thumb

In this condition, half of the offsprings will have normal thumbs, but they have a heterozygous genotype. Half of the offsprings have homozygous recessive genotype and they will have the hitchhikers thumb