Section 2

Genetic Crosses

• The genotype is the genetic makeup of an organism. The phenotype is the appearance of an organism.

• Probability is the likelihood that a specific event will occur. A probability may be expressed as a decimal, a percentage, or a fraction.

• A Punnett square can be used to predict the outcome of genetic crosses.

• A cross in which one characteristic is tracked is a monohybrid cross. The offspring of a monohybrid cross are called monohybrids.

• A testcross, in which an individual of unknown genotype is crossed with a homozygous recessive individual, can be used to determine the genotype of an individual whose phenotype expresses the dominant trait.

• Complete dominance occurs when heterozygous individuals and dominant homozygous individuals are indistinguishable in phenotype.

• Incomplete dominance occurs when two or more alleles influence the phenotype and results in a phenotype intermediate between the dominant trait and the recessive trait.

• Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring. Neither allele is dominant or recessive, nor do the alleles blend in the phenotype as they do in incomplete dominance.

• A cross in which two characteristics are tracked is a dihybrid cross. The offspring of a dihybrid cross are called dihybrids.

Vocabulary genotype (p. 180) phenotype (p. 180) homozygous (p. 181) heterozygous (p.181)

probability (p. 181) monohybrid cross (p. 182) Punnett square (p. 182) genotypic ratio (p. 183)

phenotypic ratio (p. 183) testcross (p. 183) complete dominance

incomplete dominance

(p. 184) codominance (p. 184) dihybrid cross (p. 185)

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