Chapter Summary

• Physical features of organisms, or characters, can exist in different forms, or traits. A heritable trait is one that can be passed from parent to offspring. A phenotype is the physical appearance or other detectable characteristic of an organism; a genotype is the genetic constitution of the organism.

• The different forms of a gene are called alleles. Diploid organisms that have two identical alleles for a trait are called homozygous; organisms that have two different alleles for a trait are called heterozygous. A gene resides at a particular site on a chromosome called a locus.

• Mendel’s experiments included reciprocal crosses and monohybrid crosses between true-breeding pea plants. Analysis of his meticulously tabulated data led Mendel to propose a particulate theory of inheritance stating that discrete units (now called genes) are responsible for the inheritance of specific traits, to which both parents contribute equally.

• 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. Thus every individual in the F₁ inherits one copy from each parent. Review Figure 12.1, Focus: Key Figure 12.2

• Mendel used a test cross to find out whether an individual showing a dominant phenotype was homozygous or heterozygous. Review Figure 12.3, Activity 12.1

• 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 chromosomes in meiosis leads to new combinations of phenotypes in the offspring of a dihybrid cross. Review Figure 12.4, Focus: Key Figure 12.5, Animation 12.1

• Probability calculations and pedigrees help geneticists trace Mendelian inheritance patterns. Review Figures 12.6, 12.7, Activity 12.2

• 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. Review Figure 12.8

• In incomplete dominance, neither of two alleles is dominant. The heterozygous phenotype is intermediate between the homozygous phenotypes. Review Figure 12.9

• Codominance exists when two alleles at a locus produce two different phenotypes that both appear in heterozygotes. Review Figure 12.10

• An allele that affects more than one trait is said to be pleiotropic.

• In epistasis, one gene affects the expression of another. Review Figure 12.11

• Environmental conditions can affect the expression of a genotype.

• Penetrance is the proportion of individuals in a group with a given genotype that show the expected phenotype. Expressivity is the degree to which a genotype is expressed in an individual.

• Variations in phenotypes can be qualitative (discrete) or quantitative (graduated, continuous). Most quantitative traits result from the effects of several genes and the environment. Genes that together determine quantitative characters are called quantitative trait loci.

• Each chromosome carries many genes.

• Genes on the same chromosome can recombine by crossing over. The resulting recombinant chromosomes have new combinations of alleles. Review Figures 12.16, 12.17

• Sex chromosomes are a pair, one of which often determines whether the organism will produce male or female gametes. All other chromosomes are called autosomes. The specific functions of sex chromosomes differ among different groups of organisms.

• Primary sex determination in mammals is usually a function of the presence or absence of the SRY gene. Secondary sex characteristics are the outward manifestations of maleness and femaleness.

• 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 carriers of such alleles. Review Figure 12.18

• Cytoplasmic organelles such as plastids and mitochondria contain small numbers of genes. 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 12.20

• Prokaryotes reproduce primarily asexually but can exchange genes in a sexual process called bacterial conjugation. Review Figure 12.21

• Plasmids are small, extra chromosomes in bacteria that carry genes involved in important metabolic processes and that can be transmitted from one cell to another. Review Figure 12.22