Human Genetics: Concepts and Applications (Lewis), 9th Edition

Chapter 5: Beyond Mendel's Law

Chapter Outline

CHAPTER OVERVIEW

1. The traditional focus of genetics on traits determined by single genes is giving way to the realization that genes do not usually function alone.
2. Most genes interact with other genes and environmental factors.

1. A number of factors can appear to disrupt Mendelian ratios.

1. A gene can have more than two alleles, but a diploid individual only has one or two of them.
2. Different allele combinations can produce different phenotypes and different severities of symptoms.

1. Incomplete dominance of an allele produces a phenotype in the heterozygote that is intermediate between the homozygotes.
2. Codominant alleles are both expressed in a heterozygote.

1. In epistasis, one gene prevents another from contributing to the phenotype. An example of epistasis in humans is the Bombay phenotype.
2. The Bombay phenotype results in O type blood for individuals who are homozygous recessive for the recessive "h" allele.

1. Genotypes vary in penetrance (percent of individuals with an allele who are affected) and expressivity (severity of symptoms or expression).
2. Penetrance and variable expression are not well understood biochemically and are probably due to the complex biochemical environment all genes function in.

1. A gene with more than one phenotypic effect is called pleiotropic.

1. Genetic heterogeneity occurs when different genes (or alleles) cause the same phenotype.

1. A trait caused by the environment but resembling a genetic trait is termed a phenocopy.
2. It may occur in families and seem to be inherited.

1. Only females transmit mitochondrial genes via the oocyte.
2. Traits pass to offspring of both sexes from the mother but not the father.
3. Mitochondrial DNA does not cross over, mutates faster than nuclear DNA, and is present in many copies per cell.

1. A variety of disorders, many causing muscle weakness and fatigue, result from mutations in mitochondrial genes.
2. Ooplasmic transfer has been used to overcome mitochondrial disease in children conceived in vitro.

1. In heteroplasmy, a mutation is present in some mitochondria only.

1. Mitochondrial DNA can be used for forensic pedigree and phylogenetic analysis.

1. Linkage was discovered in peas when a dihybrid cross did not show a typical 9:3:3:1 ratio, but had an excess of parental types.
2. Genes on the same chromosome are linked, and have different patterns of inheritance than the unlinked genes Mendel observed.
3. The farther apart two genes are on a chromosome, the greater the probability that crossing over will occur between them.

1. Genetic linkage maps are based on the observation that the likelihood of a crossover between two linked genes is directly proportional to the distance between them.
2. Linkage maps are built by tracking transmission of two traits at a time, and then determining the percent recombination between them.

1. Linkage data from pooled human pedigrees can be used to develop a human linkage map.
2. Linkage disequilibrium (LD) in the human genome reveals non-randomly distributed sequences where recombination occurs infrequently.

1. Genome-wide association studies are yielding new genetic markers. Markers may be protein coding alleles, variable restriction sites, variable short repeated sequences, or single nucleotide polymorphisms (SNPs).
2. Computer analysis of linkage data allows the computation of a LOD score (logarithm of the odds). A LOD score of 3 or greater indicates linkage between markers.
3. Haplotypes are a tightly linked series of markers that appear to be inherited as a single unit.