Mendelian GeneticsMendelian Genetics

Why the garden pea?Why the garden pea?

““Model Organism” Model Organism” Easy to growEasy to grow Easy to self or cross-fertilizeEasy to self or cross-fertilize Cross-fertilization may be prevented by Cross-fertilization may be prevented by

bagging the flowerbagging the flower Grows to maturity in one seasonGrows to maturity in one season

Genetic termsGenetic terms

HomozygousHomozygous (true breeding) (true breeding) 2 identical alleles for a gene2 identical alleles for a gene

HeterozygousHeterozygous (hybrid) (hybrid) 2 different alleles for a gene2 different alleles for a gene

Dominant Dominant allele that is expressed in a heterozygoteallele that is expressed in a heterozygote

Recessive Recessive allele that is masked in a heterozygote allele that is masked in a heterozygote

Chapter 13Chapter 13

Genetic Terms:Genetic Terms: P (parental)P (parental)

parental generation of a crossparental generation of a cross F1 (first filial)F1 (first filial)

the first generation after the parental (the the first generation after the parental (the results of the first cross)results of the first cross)

F2 (second filial)F2 (second filial) a cross between F1 individuals yields F2a cross between F1 individuals yields F2

Mendel’s Law of Mendel’s Law of SegregationSegregation AllelesAlleles

alternative forms of genesalternative forms of genes 2 alleles for the same gene2 alleles for the same gene Each parent passes one allele for each Each parent passes one allele for each

gene to his/her offspringgene to his/her offspring What process does this law represent?What process does this law represent?

MeiosisMeiosis What stage of meiosis?What stage of meiosis?

Anaphase IAnaphase I

The test crossThe test cross

How can you determine the genotype of How can you determine the genotype of a dominant individual?a dominant individual?

Sheep - white is dominant to blackSheep - white is dominant to black W = white and w = black (use W = white and w = black (use

dominance to assign letter)dominance to assign letter) How can you tell if white sheep is How can you tell if white sheep is

homozygous (WW) or heterozygous homozygous (WW) or heterozygous (Ww)?(Ww)?

Conduct a test cross, where the unknown Conduct a test cross, where the unknown dominant individual is crossed with the dominant individual is crossed with the known recessive .known recessive .

W ___ x wwW ___ x ww

W ?W ?

w w

ww

Punnett SquaresPunnett Squares

Monohybrid cross (Monohybrid cross (Ss Ss x x SsSs))

RATIO?

RatiosRatios

Genetic crosses occur in predictable Genetic crosses occur in predictable ratios.ratios.

A homoz. dominant and a homoz. A homoz. dominant and a homoz. recessive always produce 100% recessive always produce 100% heterozygotesheterozygotes

A heterozygote cross always produces A heterozygote cross always produces a a 1:2:1 GENOTYPIC1:2:1 GENOTYPIC ratio, ratio, 3:1 3:1 PHENOTYPICPHENOTYPIC ratio ratio

Probability & Product RuleProbability & Product Rule

Probability Probability In a heterozygous cross for flower color In a heterozygous cross for flower color

(Pp), what is the probability of the (Pp), what is the probability of the offspring being heterozygous? offspring being heterozygous? Homozygous dominant? Homo recess?Homozygous dominant? Homo recess?

FOIL FOIL Pp x PpPp x Pp

¼ ¼

¼ + ¼ = ½

Law of Independent Law of Independent AssortmentAssortment The odds of two different traits occurring The odds of two different traits occurring

together in the same offspring is the product together in the same offspring is the product of each independent trait occurring in of each independent trait occurring in gametogenesis gametogenesis

In which phase of meiosis does this event In which phase of meiosis does this event occur?occur?

Metaphase IMetaphase I What assumption did Mendel make for this What assumption did Mendel make for this

law to work?law to work? The traits occur on different chromosomes!The traits occur on different chromosomes!

Dihybrid CrossDihybrid Cross

R = redR = red r = whiter = white T = tallT = tall t = shortt = short What is the FWhat is the F22 phenotypic ratio of phenotypic ratio of

offspring?offspring? Punnett Square ApproachPunnett Square Approach Fork Lined ApproachFork Lined Approach

What is the probability that a cross What is the probability that a cross between 2 organisms with genotypes between 2 organisms with genotypes AaBbCc and AaBbCc will produce an AaBbCc and AaBbCc will produce an offspring with aabbcc?offspring with aabbcc?

Take each trait’s individual probabilityTake each trait’s individual probability 1/4 x 1/4 x 1/4 = 1/641/4 x 1/4 x 1/4 = 1/64

Incomplete DominanceIncomplete Dominance

Not all traits show the typical Not all traits show the typical “dominant/recessive” patterns of “dominant/recessive” patterns of allelesalleles

Incomplete dominance – heterozygotes Incomplete dominance – heterozygotes show a distinct intermediate phenotype show a distinct intermediate phenotype different from homozygous genotypesdifferent from homozygous genotypes

In snapdragons, red snapdragons In snapdragons, red snapdragons crossed with white and produce all pink crossed with white and produce all pink flowers. flowers.

Flower color of snapdragonsFlower color of snapdragons

A cross between a CA cross between a CWWCCWW (white) plant (white) plant and a Cand a CRRCCRR (red) plant will produce all (red) plant will produce all pink Fpink F11 offspring offspring

Self-pollination of the FSelf-pollination of the F11 offspring offspring produces 25% Cproduces 25% CWWCCWW, 25% C, 25% CRRCCRR, and , and 50% pink offspring50% pink offspring

Dominant AllelesDominant Alleles

Organismal levelOrganismal level Biochemial levelBiochemial level Molecular levelMolecular level Complete dominance Complete dominance Incomplete Incomplete

Dominance Dominance Codominance Codominance

CodominanceCodominance

Codominance – 2 alleles affect the phenotype Codominance – 2 alleles affect the phenotype in separate, distinguishable waysin separate, distinguishable ways

The "recessive" & "dominant" traits appear The "recessive" & "dominant" traits appear togethertogether in the phenotype of hybrid in the phenotype of hybrid organismsorganisms

Curly hair = homozygous for curly hair allelesCurly hair = homozygous for curly hair alleles Straight hair = homozygous for straight hair Straight hair = homozygous for straight hair

allelesalleles Heterozygous individuals have wavy hairHeterozygous individuals have wavy hair

Multiple AllelesMultiple Alleles

TermsTerms

Plietropy = multiple effects from a Plietropy = multiple effects from a single genesingle gene

Epistasis = one locus alters the Epistasis = one locus alters the phenotypic expression of a second phenotypic expression of a second genegene

Polygenic inheritance = additive efffect Polygenic inheritance = additive efffect of 2 or more genes on a single traitof 2 or more genes on a single trait

PleiotropyPleiotropy

EpistasisEpistasis

Polygenic InheritancePolygenic Inheritance

EnvironmentEnvironment

Genetic ScreeningGenetic Screening

Carrier recognitionCarrier recognition Fetal testing:Fetal testing:

AmniocentesisAmniocentesis CVSCVS

UltrasoundUltrasound FetoscopyFetoscopy

Genetic DisordersGenetic Disorders

Tay SachsTay Sachs Cystic FibrosisCystic Fibrosis Huntington’s DiseaseHuntington’s Disease Sickle Cell Anemia Sickle Cell Anemia

1/400 African Americans1/400 African Americans Cause? Cause?

Single sub of AA in hemoglobinSingle sub of AA in hemoglobin