Imagine you are a cattle farmer and you are performing a dihybrid cross to determine if the alleles of gene a and gene b assort independently. you mate a bull with the aabb genotype with a cow that has the aabb genotype (a dominant over a; b dominant over b), cross the f1’s with themselves, and produce the following ratio of phenotypes in 160 offspring from the f2 generation:

character a trait/character b trait

89 dominant/dominant

28 dominant/recessive

32 recessive/dominant

11 recessive/recessive

remembering that mendel’s expected phenotype ratio is based on the assumption that the two genes’ alleles do assort independently, perform a chi square analysis to determine if these two genes assort independently. use 7.815 as your critical value of chi squared.

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.