In Chapter 3, we discussed the simplest principles of transmission genetics. In this chapter we will restrict our initial discussion to the inheritance of traits that are under the control of only one set of genes. In diploid organisms, which have homologous pairs of chromosomes, two copies of each gene influence such traits. The copies need not be identical because alternative forms of genes (alleles) occur within populations. How alleles influence phenotypes is our primary focus. We will then consider how a single phenotype can be controlled by more than one set of genes, a situation sometimes described as gene interaction, and explore numerous examples.
- 4.1 Alleles Alter Phenotypes in Different Ways
- 4.2 Geneticists Use a Variety of Symbols for Alleles
- 4.3 Neither Allele Is Dominant in Incomplete, or Partial, Dominance
- 4.4 In Codominance, the Influence of Both Alleles in a Heterozygote Is Clearly Evident
- 4.5 Multiple Alleles of a Gene May Exist in a Population
- The ABO Blood Groups
- The Bombay Phenotype
- The white Locus in Drosophila
- 4.6 Lethal Alleles Represent Essential Genes
- 4.7 Combinations of Two Gene Pairs Involving Two Modes of Inheritance Modify the 9:3:3:1 Ratio
- 4.8 Phenotypes Are Often Affected by More Than One Gene
- Epistasis
- Novel Phenotypes
- Other Modified Dihybrid Ratios
- 4.9 Complementation Analysis Can Determine if Two Mutations Causing a Similar Phenotype Are Alleles
- 4.10 X-Linkage Describes Genes on the X Chromosome
- X-Linkage in Drosophila
- X-Linkage in Humans
- 4.11 In Sex-Limited and Sex-Influenced Inheritance, an Individual's Sex Influences the Phenotype
- 4.12 Phenotypic Expression Is Not Always a Direct Reflection of the Genotype
- Penetrance and Expressivity
- Temperature Effects
- Onset of Genetic Expression
- Genetic Anticipation
- Genomic Imprinting
- 4.13 Extranuclear Inheritance Modifies Mendelian Patterns
- Organelle Heredity: DNA in Chloroplasts and Mitochondria
- Chloroplasts: Variegation in Four O'Clock Plants
- Mitochondrial Mutations: poky in Neurospora and petites in Saccharomyces
- Mitochondrial Mutations: Human Genetic Disorders
- Maternal Effect: Limnaea Coiling
