Agriculture relies on the principles of genetics, especially when using artificial selection to improve the quality of plants and animals. Let’s explore these principles to understand how traits are passed on to offspring. Almost all organisms receive two copies of every gene, one from the female parent and one from the male parent.
The actual genes an organism has for a specific trait are called the organism’s genotype. A genotype is represented by two letters. A dominant gene is expressed by an uppercase letter. A recessive gene is generally shown with a lowercase letter.
For example, in pea plants, green color is dominant over yellow color. The dominant green gene would be expressed by an uppercase G and the recessive yellow gene would be expressed by a lowercase G.
A genotype is homozygous if the genes the organism has from both parents are the same for a specific trait. A genotype is heterozygous if the genesthe organism has from both parents are different for a specific trait. An organism’s phenotype is the outward expression, or observable characteristics, of the genes the organism has for a specific trait.
Dominant genes will be expressed if they are in the organism’s genetic makeup. In the pea plant example, plants with the genotype double uppercase G or uppercase G/lowercase G will both appear to be green. Recessive genes are those that are expressed only when no dominant gene is found in the genetic makeup.
Pea plants that have a yellow phenotype would have the genotype of two lowercase G’s. The odds of an organism receiving different genotypes and phenotypes from their parents can be calculated using a Punnett Square.
To use a Punnett Square, you place the two genes from the male parent above the columns of the square and the two genes from the female parent next to the rows of the square. By combining the genes from each column and row, you can see the chances that each offspring has for receiving a specific genotype or phenotype.
This Punnett Square shows a cross between two pea plants. Both pea plants have a heterozygous genotype for color, because the genes for the trait are different, as indicated by the uppercase and lowercase G’s.
In this example, there is a one in four chance that the offspring in the cross will be homozygous and green (two uppercase G’s). There is a two in four chance that the offspring will be heterozygous (uppercase G/lowercase G). There is a three in four chance that the phenotype of the offspring will be green and a one in four chance that the phenotype will be yellow.