Beyond Mendalian Genetics What Mendel could not learn from pea plants

Mendelian principles have been extended Mendelian principles have been extended diverse organisms diverse organisms more complex patterns of inheritance than Mendel described. more complex patterns of inheritance than Mendel described. Mendel worked with a system that was relatively simple genetically. Mendel worked with a system that was relatively simple genetically. Each character (but one) is controlled by a single gene. Each character (but one) is controlled by a single gene. Each gene has only two alleles, one of which is completely dominant to the other. Each gene has only two alleles, one of which is completely dominant to the other. The relationship between genotype and phenotype is rarely simple

Some alleles show incomplete dominance Some alleles show incomplete dominance heterozygotes show a distinct intermediate phenotype, not seen in homozygotes. heterozygotes show a distinct intermediate phenotype, not seen in homozygotes. This is not blended inheritance This is not blended inheritance the traits are separable (particulate) in further crosses. the traits are separable (particulate) in further crosses. Offspring of a cross between heterozygotes show three phenotypes Offspring of a cross between heterozygotes show three phenotypes both parentals both parentals heterozygote. heterozygote. The phenotypic and genotypic ratios are identical, 1:2:1. The phenotypic and genotypic ratios are identical, 1:2:1. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Flower Color in Snapdragons: Incomplete Dominance Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant Pink-flowered F 1 plants (homozygote) (heterozygotes)

Incomplete dominance in snapdragon color

Incomplete dominance in carnations

Codominance in ABO Blood Types: Three Alleles Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells Two alleles (I A and I B ) are codominant when paired Two alleles (I A and I B ) are codominant when paired Third allele (i) is recessive to others Third allele (i) is recessive to others

Multiple alleles for the ABO blood groups

ABO blood types

Pleitropy Alleles at a single locus may have effects on two or more traits Alleles at a single locus may have effects on two or more traits Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

Genetics of Sickle-Cell Anemia Genetics of Sickle-Cell Anemia Two alleles Two alleles 1) Hb A Encodes normal beta hemoglobin chain 2) Hb S Mutant allele encodes defective chain Hb S homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia Hb S homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

Pleiotropic effects of the sickle-cell allele in a homozygote

Epistasis Interaction between the products of gene pairs Interaction between the products of gene pairs Common among genes for hair color in mammals Common among genes for hair color in mammals

Genetics of Coat Color in Labrador Retrievers Genetics of Coat Color in Labrador Retrievers Two genes involved Two genes involved - One gene influences melanin production Two alleles - B (black) is dominant over b (brown) Two alleles - B (black) is dominant over b (brown) - Other gene influences melanin deposition Two alleles - E promotes pigment deposition and is dominant over e Two alleles - E promotes pigment deposition and is dominant over e

Allele Combinations and Coat Color Black coat - Must have at least one dominant allele at both loci Black coat - Must have at least one dominant allele at both loci BBEE, BbEe, BBEe, or BbEE BBEE, BbEe, BBEe, or BbEE Brown coat - bbEE, bbEe Brown coat - bbEE, bbEe Yellow coat - Bbee, BbEE, bbee Yellow coat - Bbee, BbEE, bbee

An example of epistasis

Albinism Phenotype results when pathway for melanin production is completely blocked Phenotype results when pathway for melanin production is completely blocked Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin- synthesizing pathway Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin- synthesizing pathway

Comb Shape in Poultry Alleles at two loci (R and P) interact Walnut comb - RRPP, RRPp, RrPP, RrPp Walnut comb - RRPP, RRPp, RrPP, RrPp Rose comb - RRpp, Rrpp Rose comb - RRpp, Rrpp Pea comb - rrPP, rrPp Pea comb - rrPP, rrPp Single comb - rrpp Single comb - rrpp

Dominance relationships have four important points. from complete dominance, though various degrees of incomplete dominance, to codominance. reflect the mechanisms by which specific alleles are expressed in the phenotype. do not involve the ability of one allele to subdue another at the level of DNA. They are not a measure of the relative abundance of alleles in a population.

Campodactyly: Unexpected Phenotypes Effect of allele varies: Effect of allele varies: Bent fingers on both hands Bent fingers on both hands Bent fingers on one hand Bent fingers on one hand No effect No effect Many factors affect gene expression Many factors affect gene expression

Continuous Variation Continuous Variation A more or less continuous range of small differences in a given trait among individuals A more or less continuous range of small differences in a given trait among individuals The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait

Human Variation Some human traits occur as a few discrete types Some human traits occur as a few discrete types Attached or detached earlobes Attached or detached earlobes Many genetic disorders Many genetic disorders Other traits show continuous variation Other traits show continuous variation Height Height Weight Weight Eye color Eye color

A simplified model for polygenic inheritance of skin color

Temperature Effects on Phenotype Himalayan rabbits are Homozygous for an allele that specifies a heat- sensitive version of an enzyme in melanin- producing pathway Himalayan rabbits are Homozygous for an allele that specifies a heat- sensitive version of an enzyme in melanin- producing pathway Melanin is produced in cooler areas of body Melanin is produced in cooler areas of body

Environmental Effects on Plant Phenotype Environmental Effects on Plant Phenotype Hydrangea macrophylla Hydrangea macrophylla Action of gene responsible for floral color is influenced by soil acidity Action of gene responsible for floral color is influenced by soil acidity Flower color ranges from pink to blue Flower color ranges from pink to blue

The effect of environment of phenotype

Thomas Hunt Morgan Thomas Hunt Morgan the first to associate a specific gene with a specific chromosome the first to associate a specific gene with a specific chromosome early 20th century. early 20th century. Drosophila melanogaster Drosophila melanogaster Morgan’s experimental animal Morgan’s experimental animal fruit fly species that eats fungi on fruit. fruit fly species that eats fungi on fruit. prolific breeders and have a generation time of two weeks. prolific breeders and have a generation time of two weeks. three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males). three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males). Morgan traced a gene to a specific chromosome

Morgan’s first mutant

Sex-linked inheritance

Evidence for linked genes in Drosophila

Recombination due to crossing over

Using recombination frequencies to construct a genetic map

A partial genetic map of a Drosophila chromosome

The transmission of sex-linked recessive traits