GED Science: Mendel’s Law of Segregation and Alleles

Mendel’s data suggested that a parent plant had two discrete factors for a trait, and that only one of those factors was passed on to its offspring. Today, we use the term alleles to refer to these different versions of the same gene.  For example, for flower color in pea plants, there are two alleles, one for purple color and one for white color.  Each parent had two alleles for flower color. Mendel developed a special notation for pea plant traits, using capital and lowercase letters to represent dominant and recessive traits, respectively (for example, P would represent the dominant purple flower color trait and p would represent the recessive white color trait). This notation allowed Mendel to model the segregation of alleles from parent to offspring.  Today, this type of notation is still used to represent different alleles of a single gene – the capital letter is the dominant allele, and the lowercase letter is the recessive allele.

The combination of different alleles that an organism inherits from its two parents is called its genotype. In the sample flower color crosses above, there are three genotypes: homozygous dominant (PP), heterozygous (Pp), and homozygous recessive (pp).  An organism’s genotype determines what kind of physical characteristics can be directly observed. These directly-observed physical characteristics are called the organism’s phenotype.  In the flower color example, notice that there are two different genotypes (PP and Pp) that could be responsible for one given phenotype (purple flowers).

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