Biology 11th Edition by Cecie Starr ,Ralph Taggart

Exercise 5

Certain alleles are so essential for normal development that an individual who is homozygous recessive for a mutant form cannot survive. Such recessive, lethal alleles can be perpetuated in the population by heterozygotes.
Consider the allele Manx (M L ) in cats. Homozygous cats ( M L M L ) die when they are still embryos inside the mother cat. In heterozygotes ( M L M ), the spine develops abnormally. The cats end up with no tail.
Two M L M cats mate. What is the probability that any one of their surviving kittens will be heterozygous