Question

An achondroplastic male dwarf with normal vision marries a achondroplastic female that is color-blind. Both their fathers were six-feet tall. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive. What proportion of their sons would be color-blind and of normal height?

a.

1/2

b.

3/4

c.

all

d.

none

e.

1/4

Answer 1

Answer d. none

Explanation:

Symbol: D-dwarf d-normal B-normal b-colorblind (dominant is capitalized)

Father ("the man") : D? BB

Mother ("the woman") : D? bb (D? for achondroplastic and bb for recessive color blindness)

Grandfather--Father ("the man")'s father--dd
Grandfather—Father ("the woman")'s father --dd

As shown above, the father of man i.e. grandfather has normal height (dd). He will contribute one allele to his offspring (man), so one allele of man must be d. Hence, the man genotype will be Dd BB

Same rationale is applicable for the woman, so her genotype will be Dd bb.

Now, the cross between man and woman will result in following offspring:

Dd BB X Dd bb

	Db	db
DB	DDBb	DdBb
dB	DdBb	ddBb

Proportion of sons with color-blindness and normal height (ddbb) is zero.