Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 13

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.
Mendel's first law, segregation, states that inherited "elementen" separate in meiosis. Each individual receives one copy of each elementen from each parent. Single genes cause Mendelian traits.
An allele whose expression masks another is dominant; an allele whose expression is masked by a dominant allele is recessive.
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.
The combination of alleles constitutes a genotype, and the expression of a particular genotype is its phenotype. A wild-type allele is the most common one in a population. A change in a gene is a mutation and may result in a mutant allele and possibly phenotype.
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.
Punnett squares, based on the principles of probability, can be used to predict the outcomes of genetic crosses.
A monohybrid cross is between two heterozygotes and 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.
Geneticists use pedigrees to trace traits in families. Squares denote males, and circles, females. Horizontal lines represent either parents or siblings, and vertical lines depict generations. Pedigrees can reveal mode of inheritance. 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.
Mendel's second law, independent assortment, follows transmission of two or more characters on different chromosomes. Because maternally and paternally derived chromosomes (and the genes they carry) sort randomly in meiosis, different gametes have different combinations of genes.
A dihybrid cross breeds two heterozygotes, yielding a 9:3:3:1 phenotypic ratio.
The product rule can be used as an alternative to Punnett squares.
In some crosses, the ratio of progeny phenotypes does not seem to follow Mendel's laws. Individuals who have lethal allele combinations cease developing before they can be detected among progeny. A gene can have multiple alleles because its DNA sequence may be altered in many ways. Different types of dominance relationships influence phenotypic ratios among offspring. Heterozygotes of incompletely dominant alleles have phenotypes intermediate between those of the two homozygotes. Codominant alleles are both expressed. In epistasis, one gene masks the effect of another.
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. Pleiotropic genes have several expressions. A phenocopy appears to be inherited because it occurs repeatedly, but is environmentally caused.
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.
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.
Heritability estimates the proportion of variation in a multifactorial trait due to genetics. It equals twice the difference of the measurable variability for that trait between two groups of individuals. Heritability is not a gene characteristic; it varies among populations.
The coefficient of relationship describes the proportion of genes different relatives share.
Characteristics adopted people share with their biological parents are mostly inherited, whereas similarities between adopted people and their adopted parents reflect environmental influences.
Concordance measures the expression of a trait in MZ or DZ 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.