Genotype And Phenotype

An organism's genetic makeup is its genotype (JEEN-uh-TlEP). The genotype consists of the alleles that the organism inherits from its parents. For example, the genotype of the white-flowering pea plant in Figure 9-6 consists of two recessive alleles for white flower color, represented as pp. The genotype of a purple-flowering pea plant may be either PP or Pp. Either of these two genotypes would result in a pea plant that has purple flowers because the P allele is dominant.

An organism's appearance is its phenotype (FEE-noh-TlEP). The phenotype of a PP or a Pp pea plant is purple flowers, whereas the phenotype of a pp pea plant is white flowers. As this example shows, a phenotype does not always indicate genotype. In addition to recessive alleles, certain environmental factors can affect phe-notype. For example, lack of proper nutrition can cause a genetically tall plant to remain short.

When both alleles of a pair are alike, the organism is said to be homozygous (HOH-moh-ZIE-guhs) for that characteristic. An organism may be homozygous dominant or homozygous recessive. For example, a pea plant that is homozygous dominant for flower color has the genotype PP. A pea plant that is homozygous recessive for flower color has the genotype pp. When the two alleles in the pair are different, the organism is heterozygous (HET-uhr-OH-ZIE-guhs) for that characteristic. A pea plant that is heterozygous for flower color has the genotype Pp.

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