NonMendelian Genetics Mendel says One allele is dominant

Non-Mendelian Genetics Mendel says… One allele is dominant; one allele is recessive Expected ratios unrelated to sex Independent assortment of alleles Single gene controlling trait Two possible alleles for every gene One allele comes from each parent Traits are determined by heritable factors But these ideas are not true for MANY traits!

Degrees of Dominance • Complete dominance =

Degrees of Dominance • Incomplete dominance =

Degrees of Dominance • Codominance =

Degree of Dominance Depends On the Biological Level • Example: Tay-Sachs Disease Organismal Level: Biochemical Level: Molecular Level:

Changes in Observed Phenotypic Ratio!

Multiple Alleles • Multiple Alleles = • Example: Blood Type

Fig. 14 -11 Allele IA IB Carbohydrate A B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) IAIA or IA i A IBIB or IB i B IA IB AB ii O (b) Blood group genotypes and phenotypes

Changes in Observed Phenotypic Ratio!

Pleiotropy

Pleiotropy • Many genes have multiple phenotypic effects • Example: Phenylketonuria (PKU) • • • Recessive allele Can’t break down amino acid phenylalanine Builds up and harms the central nervous system Altered skin pigmentation, delayed mental skills, hyperactivity, seizures, skin rashes, jerking movement

Epistasis • A gene at one locus alters the expression of a gene at a second locus • Example: Labrador Retrievers

Epistasis • More than one gene controls a single trait (a gene at one locus alters the expression of a gene at a second locus) • Example: Labrador Retrievers

Fig. 14 -12 Changes in Observed Phenotypic Ratio! Bb. Cc Sperm 1/ 4 BC 1/ 4 b. C Bb. Cc 1/ 4 Bc 1/ 4 bc Eggs 1/ 1/ 4 BC BBCC Bb. CC BBCc Bb. CC bb. CC Bb. Cc bb. Cc BBCc Bb. Cc BBcc Bb. Cc bb. Cc Bbcc bbcc 4 b. C 4 Bc 4 bc 9 : 3 : 4

Polygenic Inheritance • Pleiotropy Polygenic Inheritance

Fig. 14 -13 Changes in Observed Phenotypic Ratio! Aa. Bb. Cc Sperm 1/ Eggs Aa. Bb. Cc 1/ 8 1/ 1/ 8 8 1/ 64 15/ 8 1/ 1/ 8 8 8 1/ 8 8 1/ Phenotypes: 64 Number of dark-skin alleles: 0 6/ 64 1 15/ 64 2 20/ 3 64 4 6/ 64 5 1/ 64 6

Sex-Linked Genes • Usually on the X chromosome. Why? • Examples: – Color-Blindness – Muscular Dystrophy – Hemophilia Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Sex-Linked Genes: Changes in Observed Phenotypic Ratios • Example: Red-Green Color-Blindness Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

y or x-linked?

Linked Genes (Linkage) • Genes located on the same chromosome that tend to be inherited together are called linked genes

Fig. 15 -9 -4 EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) bb w w B B W W F 1 dihybrid (wild type) B b W Double mutant TESTCROSS w Testcross offspring bb w w BW bw Wild type (gray-normal) Blackvestigial If genes are located on different chromosomes: 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 0 965 : 944 : 206 : 185 Eggs B w b W Blacknormal Grayvestigial bw Sperm PREDICTED RATIOS RESULTS

Fig. 15 -10 b Recombinant chromosomes Eggs Testcross offspring b+ vg+ 965 Wild type (gray-normal) b vg 944 Blackvestigial b+ vg 206 Grayvestigial b vg+ 185 Blacknormal b+ vg+ b vg b+ vg b vg+ b vg Parental-type offspring Recombination frequency = Recombinant offspring 391 recombinants 2, 300 total offspring 100 = 17% b vg Sperm

• A linkage map is a genetic map of a chromosome based on recombination frequencies • Distances between genes can be expressed as map units; one map unit, or centimorgan (c. M), represents a 1% recombination frequency • Map units indicate relative distance and order, not precise locations of genes Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 15 -11 RESULTS: GENETIC MAP Recombination frequencies 9% Chromosome 9. 5% 17% b cn vg

• Genes that are far apart on the same chromosome can have a recombination frequency near 50% • Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Time to Practice!