The most basic type of genetics looking at one gene with 2 alleles; one dominant and one recessive found on an autosomal chromosome. As stated in the previous lab, we typically use letters to represent the alleles with capital letters indicating dominant alleles and lower-case letters indicating recessive alleles. We might use (A) to represent the dominant allele and (a) to represent the recessive allele. Each organism has two copies of a gene (one from each parent), so there are 3 possible genotypes for a trait: AA (homozygous dominant), Aa (heterozygous) and aa (homozygous recessive).
Since an organism needs only one of the dominant alleles to show the dominant condition, this means that there are two possible phenotypes, the dominant condition which would occur if the genotype is AA or Aa, and the recessive condition which would occur only if the genotype is aa. In order to reproduce, the parents must form gametes through meiosis, which separates their alleles for each gene into different haploid cells. So a parent that is AA can only produce gametes that have A, a parent that is Aa can produce gametes that are either A or a, and a parent that is aa can only produce gametes that are a.
Given that there are only 3 possible genotypes for each parent, this means that there are nine possible crosses of two parents, starting with a cross between two homozygous dominant individuals. What are the 9 possible crosses?
These are all the possible crosses. Notice that 3 are repeated cases in which the genotypes of the parents are just reversed. This leaves only six possible crosses. List them below.