Gregor Mendel’s Discoveries Pre-Mendel  Blending Theory of Heredity –Hereditary material from each parent mixes in the offspring 2 problems Individuals of a population should reach a uniform appearance after many generations Once traits are blended, they can’t be separated Gregor Mendel  Particulate Theory of Heredity –Traits are inherited as separate factors

Mendel used quantitative approach Studied peas for 3 reasons: –Many varieties –Self pollinating/cross pollinating –Each variety had 2 alternative forms Used true breeding varieties Used large sample sizes and accurate observations Used math to develop probabilities and perform statistical analyses Used terms to define generations as: P, F 1, F 2

Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: –When alleles are on the sex chromosomes –When alleles are not completely dominant or recessive –When a gene has more than two alleles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Sex linked traits *Hemophilia *Muscular dystrophy *Red/green colorblindness *Rickets

In codominance, phenotypes of both alleles are exhibited in the heterozygote

Fig Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW

Fig Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

Fig Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2

Incomplete Dominance

Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A, I B, and i. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig IAIA IBIB i A B none (a) The three alleles for the ABO blood groups and their associated carbohydrates Allele Carbohydrate Genotype Red blood cell appearance Phenotype (blood group) I A I A or I A i A B I B I B or I B i IAIBIAIB AB iiO (b) Blood group genotypes and phenotypes

Polygenic Inheritance Additive effect of 2 or more genes on a single phenotypic character

Fig b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Widow’s peakNo widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Wwww Ww ww Ww wwWW Ww or

Fig c Attached earlobe 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Free earlobe (b) Is an attached earlobe a dominant or recessive trait? Ff ffFf ff FFor FF Ff