*Observable Patterns of InheritanceChapter 10

*Earlobe Variation Whether a person is born with attached or detached earlobes depends on a single gene

Gene has two molecular forms (alleles)

* Earlobe VariationYou inherited one allele for this gene from each parent

Dominant allele specifies detached earlobes

Recessive allele specifies attached lobes

*Dominant & Recessive AllelesIf you have attached earlobes, you inherited two copies of the recessive allele

If you have detached earlobes, you may have either one or two copies of the dominant allele

*Early Ideas About Heredity People knew that sperm and eggs transmitted information about traitsBlending theoryProblem:Would expect variation to disappearVariation in traits persists

* Gregor MendelStrong background in plant breeding and mathematicsUsing pea plants, found indirect but observable evidence of how parents transmit genes to offspring

*The Garden Pea PlantSelf-pollinatingTrue breeding (different alleles not normally introduced)Can be experimentally cross-pollinated

* GenesUnits of information about specific traits

Passed from parents to offspring

Each has a specific location (locus) on a chromosome

*AllelesDifferent molecular forms of a gene Arise by mutationDominant allele masks a recessive allele that is paired with it

*Allele CombinationsHomozygous having two identical alleles at a locusAA or aaHeterozygous having two different alleles at a locusAa

*Genetic TermsA pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes

*Genotype & PhenotypeGenotype refers to particular genes an individual carries

Phenotype refers to an individuals observable traits

Cannot always determine genotype by observing phenotype

*Tracking GenerationsParental generation Pmates to produce

First-generation offspring F1mate to produce

Second-generation offspring F2

* F1 Results of One Monohybrid Cross

*F2 Results of Monohybrid Cross

*Mendels Monohybrid Cross Results787 tall277 dwarf651 long stem207 at tip705 purple224 white152 yellow428 green299 wrinkled882 inflated6,022 yellow2,001 green5,474 round1,850 wrinkledF2 plants showed dominant-to-recessive ratio that averaged 3:1

* Mendels Theory of Segregation

An individual inherits a unit of information (allele) about a trait from each parent

During gamete formation, the alleles segregate from each other

*ProbabilityThe chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached

*Punnett Square of a Monohybrid CrossDominant phenotype canarise 3 ways,recessive only one

*Test CrossIndividual that shows dominant phenotype is crossed with individual with recessive phenotype

Examining offspring allows you to determine the genotype of the dominant individual

*Punnett Squares of Test Crosses Two phenotypesAll dominant phenotype

*Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits

*A Dihybrid Cross - F1 ResultsAABBaabbxAaBbABABababTRUE-BREEDING PARENTS:GAMETES:F1 HYBRID OFFSPRING:purple flowers, tallwhite flowers,dwarfAll purple-flowered, tall

* F1 Results of Mendels Dihybrid CrossesAll plants displayed the dominant form of both traits We now know:All plants inherited one allele for each trait from each parentAll plants were heterozygous (AaBb)

*Phenotypic Ratios in F2Four Phenotypes:Tall, purple-flowered (9/16)Tall, white-flowered (3/16)Dwarf, purple-flowered (3/16)Dwarf, white-flowered (1/16)AaBb XAaBb

*Explanation of Mendels Dihybrid ResultsIf the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible1/41/41/41/41/41/41/41/4AaBbaabbAabbaaBbAABBAABbAaBBAaBbAABbAAbbAaBbAabbAaBbaaBBaaBbAaBB1/161/161/161/161/161/161/161/161/161/161/161/161/161/161/161/16

*16 Allele Combinations in F2 1/41/41/41/41/41/41/41/4AaBbaabbAabbaaBbAABBAABbAaBBAaBbAABbAAbbAaBbAabbAaBbaaBBaaBbAaBB1/161/161/161/161/161/161/161/161/161/161/161/161/161/161/161/16

*Independent AssortmentMendel concluded that the two units for the first trait were to be assorted into gametes independently of the two units for the other trait

Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

*Independent AssortmentMetaphase I

Metaphase II:

Gametes:1/4 AB1/4 ab1/4 Ab1/4 aBAAAAAAAAAAAABBBBBBBBBBBBaaaaaaaaaaaabbbbbbbbbbbbOR

*Tremendous VariationNumber of genotypes possible in offspring as a result of independent assortment and hybrid crossing is 3n(n is the number of gene loci at which the parents differ)

*Impact of Mendels WorkMendel presented his results in 1865Paper received little noticeMendel discontinued his experiments in 1871Paper rediscovered in 1900 and finally appreciated

*Dominance Relations Complete dominance Incomplete dominanceHeterozygote phenotype is somewhere between that of two homozyotesCodominanceNon-identical alleles specify two phenotypes that are both expressed in heterozygotes

* Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant

Pink-flowered F1 plants(homozygote)(homozygote)(heterozygotes)

*Flower Color in Snapdragons: Incomplete Dominance

Pink-flowered plant X Pink-flowered plant

White-, pink-, and red-flowered plants in a 1:2:1 ratio(heterozygote)(heterozygote)

*Flower Color in Snapdragons: Incomplete DominanceRed flowers - two alleles allow them to make a red pigmentWhite flowers - two mutant alleles; cant make red pigmentPink flowers have one normal and one mutant allele; make a smaller amount of red pigment

*Genetics of ABO Blood Types: Three AllelesGene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells

Two alleles (IA and IB) are codominant when paired

Third allele (i) is recessive to others

*ABO Blood Type:Allele CombinationsType A - IAIA or IAiType B - IBIB or IBiType AB - IAIB Type O - ii

*ABO Blood Type: Glycolipids on Red CellsType A - Glycolipid A on cell surfaceType B - Glycolipid B on cell surfaceType AB - Both glyocolipids A & BType O - Neither glyocolipid A nor B

*ABO and TransfusionsRecipients immune system will attack blood cells that have an unfamiliar glycolipid on surfaceType O is universal donor because it has neither type A nor type B glycolipid

*Pleitropy Alleles at a single locus may have effects on two or more traits

Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

* Genetics of Sickle-Cell AnemiaTwo alleles1) HbA Encodes normal beta hemoglobin chain2) HbSMutant allele encodes defective chainHbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

*Pleiotrophic Effects of HbS/HbSAt low oxygen levels, cells with only HbS hemoglobin sickle and stick together

This impedes oxygen delivery and blood flow

Over time, it causes damage throughout the body

*Epistasis

Interaction between the products of gene pairs

Common among genes for hair color in mammals

* Genetics of Coat Color in Labrador RetrieversTwo genes involved- One gene influences melanin productionTwo alleles - B (black) is dominant over b (brown)- Other gene influences melanin depositionTwo alleles - E promotes pigment deposition and is dominant over e

*Allele Combinations and Coat Color Black coat - Must have at least one dominant allele at both lociBBEE, BbEe, BBEe, or BbEEBrown coat - bbEE, bbEeYellow coat - Bbee, BBee, bbee

*AlbinismPhenotype results when pathway for melanin production is completely blockedGenotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

*Comb Shape in PoultryAlleles at two loci (R and P) interact Walnut comb - RRPP, RRPp, RrPP, RrPpRose comb - RRpp, RrppPea comb - rrPP, rrPpSingle comb - rrpp