Dominant vs. Recessive Alleles The concept of dominant and recessive alleles was first described in the 1860s by a monk named Gregor Mendel who studied the relationship between genotype and phenotype in pea plants. Therefore, traits or diseases which are due to single genes are sometimes called Mendelian. * Remember that alleles function in pairs… A dominant allele is one that influences the phenotype even if it is present in only one copy. Two recessive alleles are required in order to influence the phenotype.

A homozygous individual would either be SS or ss. An organism is homozygous if it has the same allele on both of its homologous chromosomes. A homozygous individual would either be SS or ss. Possessing two different forms of a particular allele, one inherited from each parent. A person who is heterozygous is called a heterozygote or a gene carrier;  Ss Definition: An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation. Examples: The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), Wrinkled: (rr).

A Punnett square is a chart which shows/predicts all possible gene (allele) combinations (SS, Ss, or ss) in a cross of parents (whose genes are known). In other words, to determine the possible combination of alleles for a certain gene. Punnett squares are named for an English geneticist, Reginald Punnett. He used this method to breed desirable traits in chickens.

Here’s how the Punnett square works: Dominant alleles (genes) = S (capitalized) Recessive alleles (genes) = s (lower case)