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

Developed terms such as: –Alleles (factor) –Dominant/Recessive –Homozygous/Heterozygous –Phenotype/Genotype –Testcross Derived 2 principles: –Law of segregation – two alleles for a character separate when gametes are formed

Law of Independent assortment – each pair of alleles segregate into gametes independently

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

Fig Rr  Segregation of alleles into eggs Sperm R R R R R R r r r r r r 1/21/2 1/21/2 1/21/2 1/21/2 Segregation of alleles into sperm Eggs 1/41/4 1/41/4 1/41/4 1/41/4

Fig Parents Normal Sperm Eggs Normal (carrier) Normal (carrier) Albino Aa A A AA Aa a aa a 

Fig Eggs Parents Dwarf Normal Dwarf Sperm Dd  dd d D Dd dd Dd d d

Dominant condition Fatal Only one Huntington’s allele needed Produces abnormal protein that clumps up in cell nuclei – especially nerve cells in the brain

If an allele for tall plants (T) is dominant to short plants (t), what offspring would you expect from a TT x Tt cross? A. ½ tall; ½ short B. ¾ tall; ¼ short C. All tall Concept Quiz

Identify vocab term: This allele gets masked in the phenotype Genetic Makup Alternate forms of a gene The allele that gets fully expressed Two identical alleles for a trait Physical appearance

If two heterozygotes are crossed for dimpled chin. What will be the expected genotypic ratio? Phenotypic ratio? 1:2:1 3:1

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 –When a gene produces multiple phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

Affects 1 in 2500 individuals in European populations

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

Concept Quiz A red carnation and a white carnation produce offspring that are all pink. The type of inheritance pattern occurring is: A. Complete dominance B. Incomplete dominance C. Codominance

Red is dominant to white in flower petal color. If a homozygous dominant is crossed with a homozygous recessive and this inheritance is incomplete dominance. What will be the phenotypic ratio of this cross? 1:2:1

Fur color in rabbits shows incomplete dominance. F B F B individuals are brown, F B F W individuals are cream, F W F W individuals are white. What is the expected ratio of a F B F W x F W F W cross? A. 3 white : 1 brown B. 3 white : 1 cream C. 2 white : 2 cream Concept Quiz

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

Who is the universal recipient? Who is the universal donor?

What blood types would the offspring have if one parent Is type O and one parent is AB? Both parents AB?

Pleiotropy The ability of a gene to affect an organism in many ways

Fig BbCc Sperm Eggs BCbC Bcbc BC bC Bc bc BBCC 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 BbCC BBCc BbCc BbCC bbCC BbCc bbCc BBCcBbCc bbCc BBccBbcc bbcc 9: 3 : 4 

Epistasis B = Black b = Brown C = Pigment c = nonpig A gene at one locus Alters a gene at another locus

Polygenic Inheritance Additive effect of 2 or more genes on a single phenotypic character Eye color, skin color

Identify each type of inheritance? Both alleles are expressed in heterozygote More than two alleles are possible for a trait The dominant allele masks the recessive allele Additive effect of two or more genes Intermediate phenotype in heterozygotes A gene at one locus controls a gene at another locus

Quantitative Genetics The environment plays a role – traits such as height, weight, musical ability, susceptibility to cancer,and intelligence Quantitative traits show continuous variation; we can see a large range of phenotypes in the population The amount of variation in a population is called variance

Genetically the same but Phenotypically different Nutrition, exercise, and exposure to sun can cause differences in phenotypes

Diversity in Offspring Mutation, independent assortment, crossing over, and random fertilization result in unique combinations of alleles These processes produce the diversity of individuals found in humans and all other sexually reproducing biological populations You are one out of 64 trillion genetically different children that your parents could produce

Fraternal (non- identical) –dizygotic: two separate fertilized eggs –not genetically the same

Identical –monozygotic: one single fertilized egg that separates –genetically the same

Sex Determination and Sex Linkage Some genes are on the X chromosome and are inherited in a specific manner In humans, sex determination involves the X and Y chromosomes

Sex Linkage The genes on the X or Y chromosomes are called sex- linked genes Genes on X are called “X-linked,” while those on Y are called “Y-linked” The X chromosome is much larger and carries far more genetic information

X-Linked Genes Since males only have one X chromosome, they are more likely to suffer from X-linked diseases –Hemophilia –red-green color blindness –muscular dystrophy Since females get one X chromosome from each parent, and have two copies, they are less likely to suffer from X-linked diseases

X inactivation allows some female organisms to shut off their X chromosomes

7.3 Pedigrees A pedigree is a chart showing inheritance patterns in a family Pedigrees can be used to identify different types of inheritance patterns