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Introduction to Mendelian Genetics

Author(s): Lisa M. Meffert, PhD

Crossing the Parental Lines: Female Contribution

Now write in each type of allele that the female can contribute in each box going across. In this case, every offspring gets a "t," again following Mendel's Law of Segregation. Each combination in the grid results in an offspring that is "Tt." A genotype with two different alleles (forms of a characteristic) is called a "heterozygote."

In pea plants, the "T" allele masks the effects of the "t" allele. The terms "dominant" and "recessive" are used for the masking and the covered allele, respectively. All offspring from this cross are heterozygotes in terms of their genotypes. They also are tall (because the allele for tall masks the allele for short) in terms of their "phenotype" (the observable feature produced by the genotype). These alleles thus follow Mendel's third law of inheritance: The Law of Dominance. Mendel had the  advantage of the simple inheritance pattern of pea plant height. Unlike human height, which is determined by multiple factors, plant height is determined by one major heritable factor. In addition, dominant alleles do not always cover up the recessive allele entirely. Sometimes, the dominant allele only partially masks the recessive allele. This is called "incomplete dominance."

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