Problems and variation

In problems we write linked genes as AB/ab: the letters over the line lie on the same chromosome. With complete linkage (no crossing over) AB/ab gives only two kinds of gametes, AB and ab, so AB/ab × ab/ab gives offspring 1:1 (AB/ab and ab/ab). If crossing over is x%, then x% of the offspring belong to the new (crossover) classes, split equally in two, and the remaining (100 − x)% belong to the parent-like classes, split equally. In a sex-linked problem write the genotypes with X and Y: XᴬXᵃ, XᴬY; the Y chromosome is taken to carry no allele, so in a male a recessive X-linked gene shows in the phenotype at once. Daughters get an X from both parents, sons get X only from the mother and Y from the father; so the trait passes from mother to son (“criss-cross” inheritance). A carrier is a woman who carries a recessive X-linked allele without showing it.