Gregor Mendels Discoveries PreMendel Blending Theory of Heredity

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 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 segregates into gametes independently

Fig. 14 -16 Parents Normal Aa Sperm A a A AA Normal Aa Normal (carrier) aa Albino Eggs

Fig. 14 -9 Rr Rr Segregation of alleles into sperm Segregation of alleles into eggs Multiplication rule 1. Compute probability for each event 2. Multiply to obtain overall probability of these events occurring together Sperm 1/ R 2 R 1/ 2 R R Eggs 4 r 2 R 1/ 1/ 1/ r 1/ 4 r r R r 1/ 4 Addition rule *When there are 2 ways gametes can combine to produce same result Add them

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 – When a gene produces multiple phenotypes

Sex-Linked Traits

A= normal clotting factor a-=Hemophilia What is the probability of normal homozygous mother and affected father having a son with hemophilia? A daughter?

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

Fig. 14 -10 -1 P Generation Red CRCR Gametes White CW CW CR CW

Fig. 14 -10 -2 P Generation Red CRCR Gametes White CW CW CR CW Pink CRCW F 1 Generation In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties Gametes 1/2 CR 1/ 2 CW

Fig. 14 -10 -3 P Generation Red CRCR White CW CW CR Gametes CW Pink CRCW F 1 Generation Gametes 1/2 CR 1/ CW 2 Sperm 1/ 2 CR 1/ 2 CW F 2 Generation 1/ 2 CR Eggs 1/ 2 CRCR CRCW CW

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: IA, IB, and i. Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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

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

Fig. 14 -12 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

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

Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • For example, in mice and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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

Nature and Nurture: The Environmental Impact on Phenotype • The norm of reaction is the phenotypic range of a genotype influenced by the environment • For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

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

Multifactorial Disorders • Genetic predisposition or susceptibility and an environmental trigger. • Often the genetic predisposition is polygenic or due to several genes. • Multifactorial conditions tend to run in families, but the pattern of inheritance is not as predictable as with single gene disorders. • Many are late onset Cancer • Examples: Diabetes, cleft lip/palate, spina bifida, schizophrenia

Fig. 14 -15 b 1 st generation (grandparents) 2 nd generation (parents, aunts, and uncles) Ww ww ww Ww Ww Ww ww 3 rd generation (two sisters) WW or Ww Widow’s peak ww No widow’s peak (a) Is a widow’s peak a dominant or recessive trait?

Fig. 14 -15 c 1 st generation (grandparents) Ff 2 nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff Ff ff ff FF or Ff 3 rd generation (two sisters) Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

Dominantly Inherited Disorders • Some human disorders are caused by dominant alleles • Dominant alleles that cause a lethal disease are rare and arise by mutation • Achondroplasia is a form of dwarfism caused by a rare dominant allele Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 14 -17 Parents Dwarf Dd Normal dd Sperm D d d Dd Dwarf dd Normal Eggs

Huntington’s Disease • Huntington’s disease is a degenerative disease of the nervous system • The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

• The frequency of recombination between two points on a chromosome varies directly with the distance between the two points • 1 map unit = 1% crossing over • Crossing-over can result in a recombination, or creation of combinations of alleles in chromosomes not present in either parent (linkage between red hair and light complexion can be broken if the chromosome breakage occurs between the genes for these traits) Linked Genes • The further apart the genes are from each other in a chromosome, the greater the likelihood that they will be unlinked as a result of crossing -over. • Linked genes appear to not follow the Mendel’s Law of independent assortment

X - Inactivation

Genomic Imprinting • Prader Willi Syndrome Angleman syndrome Deletions on chromosome 15 given by the mother • severe congenital mental retardation • unusual facial appearance • muscular abnormalities -jerky movements

Fig. 14 -UN 2 Degree of dominance Complete dominance of one allele Example Description Heterozygous phenotype PP same as that of homozygous dominant Pp Incomplete dominance Heterozygous phenotype intermediate between of either allele the two homozygous phenotypes CR CR CR CW CW CW Codominance Heterozygotes: Both phenotypes expressed Multiple alleles In the whole population, ABO blood group alleles some genes have more IA , I B , i than two alleles Pleiotropy One gene is able to affect multiple phenotypic characters IA IB Sickle-cell disease

Fig. 14 -UN 3 Relationship among genes Epistasis Example Description One gene affects the expression of another Bb. Cc BC b. C Bc bc 9 Polygenic inheritance A single phenotypic Aa. Bb. Cc character is affected by two or more genes : 3 : 4 Aa. Bb. Cc