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Mendel's Law of Segregation

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Genotype and Phenotype
From Mendel's law of segregation we see that the alleles for a trait separate when gametes are formed (through a type of cell division called meiosis). These allele pairs are then randomly united at fertilization. If a pair of alleles for a trait are the same they are called homozygous. If they are different they are called heterozygous. In the first example (Figure A), the F1 plants were all heterozygous for the pod color trait. Their genetic makeup or genotype was (Gg). Their phenotype or expressed physical trait was green pod color.

The F2 generation pea plants (Figure B) showed two different phenotypes (green or yellow) and three different genotypes (GG, Gg, or gg). The genotype determines the phenotype that is expressed. The F2 plants that had a genotype of either (GG) or (Gg) were green. The F2 plants that had a genotype of (gg) were yellow.

The phenotypic ratio that Mendel observed was 3:1, 3/4 green plants to 1/4 yellow plants. The genotypic ratio however was 1:2:1. The genotypes for the F2 plants were 1/4 homozygous (GG), 2/4 heterozygous (Gg), and 1/4 homozygous (gg).
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