Vitamin Dresistant rickets is an XIinked dominant disorder R

Vitamin D-resistant rickets is an X-Iinked dominant disorder. Red hair is an autosomal recessive trait. Mary has rickets, which she inherited from her father, her mother does not have the disorder. Mary also has red hair. Her husband Tom does not have rickets or red hair, but his mother had red hair. What is the chance that a child of Mary and Tom will be a boy that has both rickets and red hair? none 3/8 1.8 1/4 1/16

Solution

Rickets is X-linked dominant factor : so in Mary it will be : X\'X (where X\' denotes rickets chromosome)

Mary will not be X\'X\' as only her father had the disesase (X\'Y) not her mother (XX).

Red hair : autosomal recessive trait (rr) (only if both rr alleles are present, red hair colour will be exhibited)

So Mary is X\'X and rr

Tom : No rickets = XY

Tom : No red = Rr (as his mother had red hair so he is a carrier)

Now for a child boy of Mary and Tom, to have both rickets and red hair, he should be X\'Y and rr

Probability of having a boy = 1/2

Probability of child having red hair: (rr) = 2/4 = 1/2

Probability of child having rickets (X\'-) = 2/4 = 1/2

Overall probability of having a boy with both rickets and red hair is =

1/2 x 1/2 x 1/2

(c) 1/8

	Tom(X)	Tom(Y)
Mary (X)	child (XX)	child (XY)
MAry(X)	child (XX)	child (XY)