Terminology and notation

The form of genetic notation that we still use for describing genetic alleles comes from Mendel’s original paper. Mendel used a capital letter (A) to describe a dominant trait, lowercase (a) to describe a recessive trait, and both together (Aa) to describe a hybrid that did not breed true. Mendel’s F2 ratio of 1:2:1 could be more described by genotype as 1AA:2Aa:1aa.

Although Mendel’s work focused on pea plants, his work is applicable for all diploid organisms. When an organism has two of the same allele (AA or aa, in Mendel’s notation), the organism is said to be homozygous for that gene. When an organism has two different alleles (Aa in Mendel’s notation), the organism is said to be heterozygous. For a trait controlled by dominant and recessive alleles of a single gene, heterozygous individuals will always show the dominant trait. The notation can also be used to describe alleles of more than one gene at a time. If so, each gene is indicated by a different letter. “AABb” would describe the genotype of an individual that is homozygous dominant for gene A and heterozygous for gene B.

Some additional terminology:

Homozygous individuals are often described as true-breeding, since a cross between two homozygous individuals with the same phenotype will always give offspring with that same phenotype. Heterozygotes – individuals who are heterozygous – are often referred to as hybrid. A monohybrid is heterozygous for a single gene (Aa); a dihybrid is heterozygous for two genes (AaBb); a trihybrid is heterozygous for three genes (AaBbCc), etc. Note that these terms are used to highlight the genes we are tracking in an experiment: a dihybrid individual actually has tens of thousands of genes!  But we’re just paying attention to two of them.

We can describe crosses by the types of parents involved. Some specific examples are listed below.

  • A monohybrid cross is between two individuals both heterozygous for one gene: Aa x Aa.
  • A dihybrid cross is between two individuals both heterozygous for two genes. AaBb x AaBb
  • A self-cross is when the same individual contributes both gametes. This is not possible for sexually dimorphic organisms like humans, but it is possible for organisms like plants which can self-pollinate.
  • A back-cross is when an individual is crossed “back” with a parent. In addition to happening in controlled lab settings (like crossing pea plants, fruit flies, or mice), it is also common in agricultural settings where plants or animals are being selected for desirable traits.

We will use this terminology – and add to this list – in later modules of this text.

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