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Concepts

Incomplete dominance is exhibited when the heterozygote has a phenotype intermediate between the phenotypes of the two homozygotes. When a trait exhibits incomplete dominance, a cross between two heterozygotes produces a 1:2:1 phenotypic ratio in the progeny.

Genetic Symbols

As we have seen, genetic crosses are usually depicted with the use of symbols to designate the different alleles. Lowercase letters are traditionally used to designate recessive alleles, and uppercase letters are for dominant alleles. Two or three letters may be used for a single allele: the recessive allele for heart-shaped leaves in cucumbers is designated hi, and the recessive allele for abnormal sperm head shape in mice is designated azh.

The normal allele for a character—called the wild type because it is the allele most often found in the wild — is of

|Table

3.2| Phenotypic ratios for simple genetic crosses (crosses for a single locus)

Ratio

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