We are trying to determine the possible modes of inheritance

We are trying to determine the possible modes of inheritance for some particular human disorder. We have a very small pedigree for this disorder. Assuming the pedigree is accurate, indicate which of the six modes of inheritance are consistent with this pedigree:

autosomal recessive

x-linked recessive

autosomal dominant

x-linked dominant

Y linked

cytoplasmic

Solution

x-;linked recessive

This can be explained with an example

Colour blindness

Particular traits in human beings renders them unable to differentiate between red colour and green colour. The gene for this red green colour blindness is located on X-chromosome. Colour blindness is recessive to normal vision so that if colour blind man marries a girl who is normal (homozygous) for this character, sons will be normal, but daughters will be heterozygous (normal phenotype) which means that these daughters would be carriers of this trait. If such a carrier girl marries a colour blind man, 50% of female progeny and 50% of male progeny would colour blind.