Describing genetic crosses

The F2 ratio of 1 true-breeding round : 2 hybrid round : 1 true-breeding wrinkled is consistent with each individual having 2 copies of the gene for seed shape. The two gene copies can be the same or different. Different versions of a gene are called alleles. The combination of alleles carried by an individual is its genotype. For example, let’s say R (read “big R”) is the dominant round allele, and r (read “little r”) is the recessive wrinkled allele. True-breeding round peas would have two copies of the R allele and a genotype of RR. True-breeding wrinkled peas would have two copies of the r allele and a genotype of rr. Hybrid round peas would have one copy of the R allele and one copy of the r allele, and a genotype of Rr.

P     RR x rr    
F1     Rr    
F2 1 RR : 2 Rr : 1 rr
  arrow   arrow   arrow
F3 RR   3 R- : 1 rr   rr

An individual who carries two copies of the same allele (i.e. RR or rr) is called a homozygote (homo = same, zygote = embryo). Homozygotes produce genoctypially identical offspring when allowed to self-fertilize. There are dominant homozygotes (i.e. RR individuals) and recessive homozygotes (i.e. rr individuals). An individual who carries two different alleles (i.e. Rr) is a heterozygote (hetero = different).

A dash (-) is used to represent an unknown allele or a mix of alleles. For example, a round pea randomly picked from Mendel’s garden would have the genotype R-, since it could be either homozygous dominant or heterozygous. The ratio of 3 round : 1 wrinkled could be depicted as 3 R- : 1 rr because the round peas include both heterozygotes and homozygotes.

The dominant allele is often assigned a capital letter, and the recessive allele is assigned the corresponding lowercase letter. The letter chosen is often the first letter of the dominant trait. For example, Y might be the allele for yellow seeds, while y is the allele for green seeds. P might be the allele for purple flowers, while p is the allele for white flowers. Also notice that the alleles are italicized (or underlined when written by hand). If you go on to study more advanced genetics, you will learn more complicated systems of notation. After all, there are only 26 letters in the English alphabet and thousands of traits! For the time being, it’s good to practice using this basic system of notation, so you get in the habit of clearly conveying the relevant information with a few symbols.