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| 5.1 When Gene Expression Appears to Alter Mendelian Ratios 1. A number of factors can appear to disrupt Mendelian ratios. Lethal Allele Combinations 1. Homozygous recessive lethal alleles eliminate a progeny class. Multiple Alleles 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. Different Dominance Relationships 1. Incomplete dominance of an allele produces a phenotype in the heterozygote that is intermediate between that of either homozygote. 2. Codominant alleles are both expressed in a heterozygote. Epistasis-When One Gene Affects Expression of Another 1. In epistasis, one gene masks the effect of another. An example of epistasis in humans is the Bombay phenotype. 2. The Bombay phenotype results in O type blood for individuals homozygous recessive for the recessive "h" allele. Penetrance and Expressivity 1. Genotypes vary in penetrance (percent of individuals affected) and expressivity (severity of symptoms). 2. Penetrance and variable expression are not well understood biochemically and are probably due to the complex biochemical environment all genes function in. Pleiotropy-One Gene, Many Effects 1. A gene with more than one phenotypic effect is pleiotropic. Phenocopies-When It's Not in the Genes 1. A trait caused by the environment but resembling a genetic trait or occurring in certain family members is a phenocopy. Genetic Heterogeneity-More than One Way to Inherit a Trait 1. Genetic heterogeneity occurs when different genes (or alleles) cause the same phenotype. 5.2 Maternal Inheritance and Mitochondrial Genes 1. Only females transmit mitochondrial genes, in the oocyte cytoplasm. In maternal inheritance, a trait passes from females to offspring of both sexes, but not from males. 2. Mitochondrial DNA does not cross over, mutates faster than nuclear DNA, and is present in many copies per cell. Mitochondrial Disorders 1. A variety of disorders, many causing muscle weakness and fatigue, result from mutations in mitochondrial genes. 2. Ooplasmic transfer has been used to over come mitochondrial disease in in vitro conceived children. Heteroplasmy Complicates Mitochondrial Inheritance 1. In heteroplasmy, a mutation is present in some mitochondria only. Mitochondrial DNA Studies Clarify the Past 1. Mitochondrial DNA can be used for forensic pedigree and phylogenetic analysis. 5.3 LinkageLinkage Was Discovered In Pea Plants 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. Linkage Maps 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. Examples of Linked Genes in Humans 1. Linkage data from pooled human pedigrees can be used to develop a human linkage map. 2. Linkage idsequilibrium (LD) in the human genome reveals non-randomly distributed sequences where recombination occurs infrequently. The Evolution of Gene Mapping 1. Computer analysis of linkage data allows the computation of a LOD score (logarithm of the odds). A LOD score of 3 or greater signifies linkage between markers. 2. Haplotypes are a tightly linked series of markers that appear to be inherited as a single unit. 3. Markers may be protein coding alleles, variable restriction sites (RFLPs), variable short repeated sequences (VNTRs), or single nucleotide polymorphisms (SNPs). |