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Introduction to Mendelian Genetics

Author(s): Lisa M. Meffert, PhD

Working Out Mendelian Genetics

The Punnet square is a bookkeeping tool that allows us to work out all possible compositions of male or female gametes for a particular cross. For the sake of simplicity, we will refer to these male and female gametes as sperm and egg, respectively. The specific form of the "particle" that is passed on to the offspring is called the "allele." The boxes in the Punnet square help us to keep track of all possible offspring that can be produced by this particular cross.

As with Mendel's experiment on tall versus short pea plants, let's start with the characteristic of "plant height." Each individual has two "particles," called alleles, that correspond to height. One allele was inherited from each parent. "T" represents the allele that confers tallness and "t" represents the allele that is related to short plant height. Recall that Mendel started with pure breeding lines of tall and short plants. Thus, all individuals of our "tall" line have only "T" alleles, and all of the individuals of our "short" line have "t" alleles. The allelic combinations ("genotypes") of the tall and short individuals are "TT" and "tt," respectively. Individuals or genotypes that have two copies of the same allele are called "homozygotes."

In this demonstration, we will cross a tall male with a short female. (We would get the same results if we crossed a tall female with a short male. However, in more complicated crosses, the direction of the cross, with regard to sex, can make a difference.) During fertilization, each parent contributes only one allele for plant height-even though each parent has two alleles, which can be the same or different. In our example, both alleles from the male parent are the same (T's, written as "TT"). Similarly, both alleles that could be contributed by the female are the same (t's, written as "tt"). Here, we are writing the potential male contributions on the top and potential female contributions on the left-hand side. By putting one "T" in each sperm, the alleles are following Mendel's first law of inheritance: The Law of Segregation. The two alleles that made the male tall are now segregating to produce reproductive cells (gametes) that can combine with contributions from the female to form the next generation. The same is true for the female.