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Lynn has red hair but her brother, Jim, does not—and neither do their parents. How did Lynn end up with red hair? These outcomes can be explained by patterns of genetic inheritance, how the sets of genes from each parent interact. As we have discussed, each person has 23 pairs of chromosomes, one pair inherited from the mother and one from the father. The genes within each chromosome can be expressed in different forms, or alleles, that influence a variety of physical characteristics. When alleles of the pair of chromosomes are alike with regard to a specific characteristic, such as hair color, the person is said to be homozygous for the characteristic and will display the inherited trait. If they are different, the person is heterozygous, and the trait expressed will depend on the relations among the genes (Lewis, 2017). Some genes are passed through dominant–recessive inheritance in which some genes are dominant and are always expressed regardless of the gene they are paired with. Other genes are recessive and will be expressed only if paired with another recessive gene. Lynn and Jim’s parents are heterozygous for red hair; both have dark hair, but they each carry a recessive gene for red hair.

When an individual is heterozygous for a particular trait, the dominant gene is expressed, and the person becomes a carrier of the recessive gene. For example, consider Figure 2.3. Both parents have nonred hair. People with nonred hair may have homozygous or heterozygous genes for hair color because the gene for nonred hair (symbolized by N in Figure 2.3) is dominant over the gene for red hair (r). In other words, both a child who inherits a homozygous pair of dominant genes (NN) and one who inherits a heterozygous pair consisting of both a dominant and recessive gene (Nr) will have nonred hair, even though the two genotypes are different. Both parents are heterozygous for red hair (Nr). They each carry the gene for red hair and can pass it on to their offspring. Red hair can result only from having two recessive genes (rr); both parents must carry the recessive gene for red hair. Therefore, a child with red hair can be born to parents who have nonred hair if they both carry heterozygous genes for hair color. As shown in Table 2.1, several characteristics are passed through dominant–recessive inheritance.

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Figure 2.3 Dominant–Recessive Inheritance

Table 2.1

Source: McKusick (1998) and McKusick-Nathans Institute of Genetic Medicine (2019).