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If a colourblind woman marries a normal visioned man, their sons will be -
A
All normal visioned
B
One-half colourblind and one-half normal
C
Three-fourths colourblind and one-fourth normal
D
All colourblind
Detailed Solution
Colour blindness is a recessive X-linked character. Let $X^c$ be the X chromosome carrying the gene for colour blindness.
A colourblind woman must be homozygous: $X^cX^c$.
A normal visioned man is $XY$.
The mother produces only $X^c$ eggs; the father produces $X$ sperms and $Y$ sperms.
Sons receive the Y chromosome from the father and their only X chromosome from the mother, so all sons are $X^cY$, i.e. colourblind.
Daughters are $X^cX$: normal visioned but carriers.
This passing of the character from mother to sons is called criss-cross inheritance.
So all their sons will be colourblind.
A colourblind woman must be homozygous: $X^cX^c$.
A normal visioned man is $XY$.
The mother produces only $X^c$ eggs; the father produces $X$ sperms and $Y$ sperms.
Sons receive the Y chromosome from the father and their only X chromosome from the mother, so all sons are $X^cY$, i.e. colourblind.
Daughters are $X^cX$: normal visioned but carriers.
This passing of the character from mother to sons is called criss-cross inheritance.
So all their sons will be colourblind.