Question

Achondroplastic dwarfism is a dominant genetic trait that causes severe malformation of the skeleton. Homozygotes for this condition are spontaneously aborted (hence, the homozygous condition is lethal) but heterozygotes will develop to be dwarfed. Matthew has a family history of the condition, although he does not express the trait. Jane is an achondroplastic dwarf.
Matthew and Jane are planning a family of several children and want to know the chances of producing a child with achondroplastic dwarfism. If three children are born to Matthew and Jane, what are the chances that the first two children will not express the trait but that the third child will be an achondroplastic dwarf.

Answer 1

Explanation:

A punnet square can be described as a diagram which is used to illustrate the results of a cross. As the condition described, Mathew does not have Achondroplastic dwarfism hence he will have recessive alleles, aa. Jane has the condition so she will have the allele Aa ( as the child with both AA is aborted). The punnet square for a cross between hem can be drawn as follows:

        a                 a

A    Aa                Aa

a      aa               aa

There is 1/8 chance that the first two children will not express the trait but that the third child will be an achondroplastic dwarf.