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Summary

Mendel’s experiments on pea plants supported the particulate theory of inheritance stating that discrete units (now called genes) are responsible for the inheritance of specific traits. Review Figure 8.1
Mendel’s first law, the law of segregation, states that when any individual produces gametes, the two copies of a gene separate, so that each gamete receives only one member of the pair. Review Figures 8.2 and 8.3
Mendel used a test cross to find out if an individual with a dominant phenotype was homozygous or heterozygous for that phenotype. Review Figure 8.4 and ACTIVITY 8.1
Mendel’s use of dihybrid crosses to study the inheritance of two characters led to his second law: the law of independent assortment. The independent assortment of genes in meiosis leads to novel combinations of phenotypes. Review Figures 8.5 and 8.6 and ANIMATED TUTORIAL 8.1
Pedigree analysis can determine whether an allele is dominant or recessive. Review Figure 8.8 and ANIMATED TUTORIAL 8.2

New alleles arise by random mutation. Many genes have multiple alleles. A wild-type allele gives rise to the predominant form of a trait. When the wild-type allele is present at a locus less than 99 percent of the time, the locus is said to be polymorphic.
In incomplete dominance, neither of two alleles is dominant. The heterozygous phenotype is intermediate between the homozygous phenotypes. Review Figure 8.10
Codominance exists when two alleles at a locus produce two different phenotypes that both appear in heterozygotes. Review Figure 8.11
In epistasis, one gene affects the expression of another. Review Figure 8.12
Environmental conditions can affect the expression of a genotype.
Heritability is the relative contribution of genetic versus environmental factors to the variation in a character in a population.

Each chromosome carries many genes, and the genes on a single chromosome show genetic linkage. Review Figure 8.13 and ANIMATED TUTORIAL 8.3
Genes on the same chromosome can recombine by crossing over. The resulting recombinant chromosomes have new combinations of alleles. Review Figure 8.14
Recombination frequencies can be used to generate a genetic map of a chromosome. Review Figure 8.15
In fruit flies and mammals, the X chromosome carries many genes, but the Y chromosome has only a few. Males have only one allele (are hemizygous) for X-linked genes, so recessive sex-linked mutations are expressed phenotypically more often in males than in females. Females may be unaffected heterozygous carriers of such alleles. Review Figure 8.16
Some genes are present on the chromosomes of organelles such as plastids and mitochondria. In many organisms, cytoplasmic genes are inherited only from the mother because the male gamete contributes only its nucleus (i.e., no cytoplasm) to the zygote at fertilization. Review Figure 8.18

Prokaryotes reproduce asexually but can transfer genes from one cell to another in a process called bacterial conjugation. Review Figure 8.19
Plasmids are small, extra DNA molecules in bacteria that carry genes involved in important metabolic processes. Plasmids can be transmitted from one cell to another. Review Figure 8.20

See ACTIVITIES 8.2 and 8.3 for a concept review of this chapter.

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