Complex Inheritance Patterns Patterns of Inheritance 1 Complete

Complex Inheritance Patterns

Patterns of Inheritance 1. Complete dominance ◦ One allele is expressed over another in the heterozygote ◦ Mendel’s pea traits

Complete dominance Example: pure tall x pure short T = tall t = short TT x tt Genotype ratio T T 0 TT: 4 Tt: 0 tt t Tt Tt Phenotype ratio t Tt Tt 4 tall: 0 short

Patterns of Inheritance Incomplete dominance ◦ The heterozygous condition has an intermediate phenotype 2.

Incomplete Dominance Example: pure red x pure white R = red R’ = white RR x R’R’ Genotype ratio R R 0 RR: 4 RR’: 0 R’R’ R’ RR’ Phenotype ratio R’ RR’ 0 red: 4 pink: 0 white

Patterns of Heredity 3. Codominance ◦ Both alleles are equally expressed in the heterozygote

Codominance Example: pure black x pure white CB = black CW = white C BC B x C W Genotype ratio CB CB C W C BC W 0 CBCB : 4 CBCW : 0 CWCW Phenotype ratio CW CBCW 0 black: 4 checkered: 0 white

Patterns of Heredity 4. Multiple Alleles ◦ When one trait has more than 2 alleles ◦ Produces multiple phenotypes ◦ Example: human blood type has 3 alleles I A, I B, i

Multiple Alleles Genotype Phenotype IAIA or IAi A type blood IBIB or IBi B type blood I AI B AB type blood ii O type blood

Multiple Alleles The alleles indicate the type of marker proteins found on the red blood cells

Sex-linked Inheritance Thomas Hunt Morgan accidentally discovered sex chromosomes when studying fruit flies

Sex-linked Inheritance �Humans have 23 pairs of chromosomes � 22 pairs are autosomes and the 23 rd pair is sex chromosomes

Sex-linked Inheritance �Human males have sex chromosomes XY �Human females have sex chromosomes XX

Sex-linked Inheritance Human Sex-linked Disorders (caused by recessive alleles) 1. Red-green colorblindness 2. Hemophilia A 3. Duchenne’s muscular dystrophy

Important Things to Remember �males pass their X-linked allele to ALL their daughters (never to sons) �Heterozygous (carrier) females have a 50% chance of passing a recessive allele to all her children �males receiving the recessive X from Mom will ALWAYS express the trait

Sex-linked Inheritance Red-green colorblindness is a disorder caused by a sex-linked recessive gene. Normal vision is dominant. XB = normal vision Xb = colorblindness (Y chromosomes don’t carry the gene)

Example a. �Find the results of a cross between a colorblind man and a homozygous normal woman. X B x X b. Y

Example a. XB XB Xb X BX b Y X BY Genotypes M: all XBY F: all XBXb (carriers) Phenotypes M: all normal F: all normal

Example b. �Find the results of a cross between a normal woman, whose father was colorblind, and a normal man. X BX b x X BY

Example b. XB XB Y Xb X BX B X BX b X BY X b. Y Genotypes M: ½ XBY, ½ Xb. Y F: ½ XBXB, ½ XBXb Phenotypes M: ½ normal, ½ colorblind F: all normal

The Flintstones Wilma (mom) Fred (dad) Pebbles (daughter)

Patterns of Inheritance Polygenic Inheritance ◦ When one trait is controlled by multiple genes ◦ Produces a bell curve of phenotypes ◦ Ex. Skin color, hair color

Polygenic Inheritance �Each dominant allele contributes one unit of color to the phenotype