Q.There is a marriage between a normal man and a carrier woman of Haemophilia. Their Doctor informs that there is a possibility of a Haemophilic child born to them.
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Start your 14-day free trial to unlock the full solution →Haemophilia is X-linked recessive; crossing a normal man (X^H Y) with a carrier woman (X^H X^h) gives four equally likely offspring types, so haemophilic boys and homozygous-normal girls each make up 1/4 (25%) of the total progeny.
Setting up the cross:
Haemophilia is caused by a recessive allele (h) on the X chromosome; the normal, dominant allele is H.
- Normal man's genotype: X^H Y
- Carrier woman's genotype (carries the recessive allele but is phenotypically normal): X^H X^h
Gametes:
- Father (X^H Y) produces two types of gametes: X^H and Y (each with probability 1/2).
- Mother (X^H X^h) produces two types of gametes: X^H and X^h (each with probability 1/2).
(ii) Punnett Square:
| X^H (from father) | Y (from father) | |
|---|---|---|
| X^H (from mother) | X^H X^H (normal girl) | X^H Y (normal boy) |
| X^h (from mother) | X^H X^h (carrier girl) | X^h Y (haemophilic boy) |
This gives four possible offspring types in a 1:1:1:1 ratio:
- X^H X^H — normal (non-carrier) girl
- X^H X^h — carrier girl (phenotypically normal, but carries the h allele)
- X^H Y — normal boy
- X^h Y — haemophilic boy
(i) Percentage of haemophilic boys among the progeny:
Out of the 4 equally likely combinations, only 1 (X^h Y) is a haemophilic boy.
So the percentage of haemophilic boys among the total progeny = 1/4 × 100 = 25%.
(Note: among sons alone, the chance is 1/2 or 50%, but as a fraction of ALL the progeny it is 25%.)
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