How Inherited Traits are Transmitted
10.1 Tracing the Inheritance of One Gene
1. Using pea plant crosses, Mendel followed transmission of one or two traits at a time. The genes for the traits he studied were carried on different chromosomes, and each trait had two forms, or alleles.
2. Mendel’s law of segregation states that inherited “elementen” (genes) separate in meiosis. Each individual receives one copy of each gene from each parent. Single genes cause Mendelian traits.
3. An allele whose expression masks another is dominant; an allele whose expression is masked by a dominant allele is recessive.
4. A heterozygote has two different alleles of a gene. A homozygous recessive individual has two recessive alleles. A homozygous dominant individual has two dominant alleles.
5. The combination of alleles is the genotype, and the expression of a genotype is its phenotype. A wild type allele is the most common in a population. A change in a gene is a mutation and may result in a mutant phenotype.
6. The parental generation is designated P1, the next generation is the first filial generation, or F1, and the next is the second filial generation, or F2.
7. A monohybrid cross yields a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1. A test cross breeds an individual of unknown genotype to a homozygous recessive individual.
8. Punnett squares, based on the principles of probability, can be used to predict the outcomes of genetic crosses.
9. An autosomal recessive trait can appear in either sex, is passed from parents who are carriers or are affected, and can skip generations. An autosomal dominant trait affects both sexes and is inherited from one affected parent. Pedigrees can trace traits in families and reveal mode of inheritance.
10.2 Tracing the Inheritance of Two Genes
10. Mendel’s law of independent assortment was derived by observing transmission of two or more characters whose genes are on different chromosomes. Because maternally and paternally derived chromosomes (and the genes they carry) assort randomly in meiosis, different gametes receive different combinations of genes.
11. A dihybrid cross yields a 9:3:3:1 phenotypic ratio.
12. The product rule can be used as an alternative to Punnett squares for following inheritance of more than one trait at a time.
10.3 How Can Gene Expression Appear to Alter Mendelian Ratios?
13. In some crosses, the ratio of progeny phenotypes does not seem to follow Mendel’s laws. Lethal allele combinations cease development, eliminating a progeny class. A gene can have multiple alleles because its DNA sequence may be altered in many ways.
14. In an incompletely penetrant genotype, phenotype is not expressed in all individuals that inherit it. Phenotypes that vary in intensity are variably expressive. The environment can influence expression of conditional mutations.
15. A pleiotropic gene has several expressions.
16. In epistasis, one gene masks the effect of another.
17. Different dominance relationships influence phenotypic ratios. Heterozygotes of incompletely dominant alleles have phenotypes intermediate between those of the two homozygotes. Codominant alleles are both expressed.
18. A phenocopy appears to be inherited because it occurs repeatedly within a family, but is environmentally caused.
19. Genetic heterogeneity occurs when different genes are associated with the same phenotype.
10.4 Complex Traits
20. Complex traits do not follow Mendel’s laws but have an inherited component. A polygenic trait is determined by more than one gene and varies continuously in its expression. The environment and genes cause multifactorial traits.
21. Empiric risk measures the likelihood of a multifactorial trait recurring based on its prevalence. The risk rises with genetic closeness to an affected individual, severity of the phenotype, and number of affected relatives.
22. Heritability estimates the proportion of variation in a multifactorial trait due to genetics. It is calculated by comparing observed to expected frequencies of a trait among pairs of individuals who are related in a certain way.
23. Characteristics adopted people share with their biological parents are mostly inherited, whereas similarities between adopted people and their adopted parents mostly reflect environmental influences.
24. Concordance measures the expression of a trait in monozygotic or dizygotic twins. It equals the percentage of twin pairs in which both members express the trait. A high concordance value indicates that genes predominantly cause a trait.