BIO 100_CH. 10.Mendelian Genetics

DominanceMonohybrid CrossDihybrid Cross

Incomplete DominanceCodominance

Quantitative GeneticsAdditive InheritanceImportance of Environment

Terms: Mendelian Genetics, blending hypothesis vs.particulate hypothesis, pure lines, qualitative traits, law ofsegregation, law of independent assortment, allele, dominance,dominant, recessive, homozygous, heterozygous, genotype,phenotype, incomplete dominance, codominance, monohybridcross, Punnet square, dihybrid cross, additive inheritance,polygenic inheritance, quantitative trait, environment, PKU,Huntington’s disease-

Questions:1. How many kinds of gametes are possible with each of the following

genotypes: a. Aa b. AaBB c. AaBb2. What is the probability of getting the gamete ab from each of the

following genotypes: a. aabb b. Aabb c. AaBb3. If an offspring has the genotype Aa, what possible combinations of

parental genotypes can exist? List them all.4. In certain pea plants, the allele T for tallness is dominant over t for

shortness. If a homozygous tall and homozygous short plant arecrossed, what will be the phenotypic and genotypic ratios of theoffspring? Use a Punnet’s square to aid you.

5. In certain pea plants, the allele T for tallness is dominant over t forshortness. If both individuals are heterozygous, what will be thephenotypic and genotypic ratios? Use a Punnet’s square.

6. Discuss the importance of genotype and environment in theexpression of a phenotype.

Do 1 - 5 as homework for second class of this lecture. Show yourwork.

Competing Hypotheses of Heredity (Pre-Mendel):

Blending Hypothesis: Theory suggests that geneticmaterial from parents is blended in offspring(Mixing of two paints as an analogy).

Particulate Hypothesis: Parents pass on discreteparticulate heritable units that retain their separateidentities.

Gregor Mendel: Provided evidence for the particulatehypothesis. Mendel formulated some of the basiclaws of genetics.

Mendelian genetic analysis:(1) traits affected by single gene of major effect (2)present only in one of two alleles: dominant orrecessive.

Alleles: alternative forms of a gene

Dominant: an allele thatexpresses itself and masks theeffects of the allele(s) for thetrait.

recessive: an allele that does notexpress itself in the phenotypewhen it is paired with a dominantallele.

Mendel worked with GardenPea:

1. Plants can self-pollinate,so can make pure lines(homogeneous) for agiven trait.

2. Many traits of peas arequalitativetraits—expressed as eitherone phenotype or asecond phenotype.

3. Can make crosses betweenpure lines.

Genotype refers to anindividual’s genes

Phenotype refers to theexpression of the trait--anindividual’s appearance.

homozygous dominant:having two dominantalleles:AA

homozygous recessive:having two recessivealleles: aa

heterozygous: having onedominant and onerecessive allele, Aa

Law of Segregation: each organism contains two alleles foreach trait (diploid), and the alleles segregate during theformation of gametes. Each gamete contains only one allele.Traits retain their individuality—not blended.

Mendel: reproductionbetween heterozygousindividuals (Aa) resultedin dominant and recessivephenotypes, even thoughboth parents’ phenotypesexpressed the dominantphenotype.

Monohybrid Cross: geneticcross that tracks theinheritance of a singlecharacter

P generation: ParentalGeneration.

F1 generation: First filialgeneration (i.e. firstgeneration after parents).

F2 generation: Generationproduced from self-pollination of F1

generation.

Punnet square:1. Assign a symbol for each allele (will depend which allele is

dominant.2. Determine the genotype of each parent.3. Determine the two possible kinds of gametes each parent

can make.4. Determine the gene combinations.5. Determine the phenotypes of each potential offspring.6. Calculate the genotypic and phenotypic probabilities.

Parent 1 (female - GG)

Parent 1 (male - gg)

G G

g

g

Imaging crossing two pure lines - one that makes greenpea pods and one that makes yellow pea pods.

What will the genotypes of the offspring be? What willthe phenotypes be?

Note: We could have eitherparent be the dominant andrecessive parent, but the greenpod is dominant to the yellowpod.G = green (dominant)G = yellow (recessive)

(Law of segregation)

GgGg

GgGg

Parent (female - GG)

Parent (male - gg)

G G

g

g

Genotypes: 4 GgPhenotypes: 100% green pea pods.

Punnet square:Calculate thegenotypic andphenotypicprobabilities.

G g

G

g

Imaging crossing the two heterozygotes - both makegreen pea pods, but both have alleles for yellow peapods.

What will the genotypes of the offspring be? What willthe phenotypes be?

ggGg

GgGGG g

G

g

Genotypes: 1 GG, 2 Gg, 1 gg - 1:2:1Phenotypes: 75% green pea pods;

25% yellow pea pods. - 3:1

G g

g

g

Imaging backcrossing the a heterozygote with therecessive pure line.

What will the genotypes of the offspring be? What willthe phenotypes be?

ggGg

ggGg

Parent 1 (female - Gg)

Parent 1 (male - gg)

G g

g

g

Genotypes: 2 Gg, 2 gg, 1:1Phenotypes: 50% green pea pods;

50% yellow pea pods. - 1:1

Parent (female)

Parent (male)

Imaging crossing the two heterozygotes - but for flowercolor.

What will the genotypes of the offspring be? What willthe phenotypes be?

Note: Purple flowers aredominant to whiteflowers.

ppPp

PpPP

Parent (female - Pp)

Parent (male - Pp)

P p

P

p

Genotypes: 1 PP, 2 Pp, 1 pp - 1:2:1Phenotypes: 75% purple flowers;

25% white flowers. - 3:1

Dihybrid Cross: genetic cross that tracks the inheritance oftwo characters simultaneously.

Imaging crossing two heterozygotes that produce greenpea pods and purple flowers. What is the genotype ofthe heterozygote? What will the genotypes of theoffspring be? What will the phenotypes be? At whatratios?

Law of Independent Assortment: members of anallelic pair segregate independently from membersof another allelic pair.

Heterozygote: GgPp Gametes can be:GP, Gp, gP, gp.

GpgP or

GgPp

GPGP or

GGPP

GP Gp gP gp

GP

Gp

gP

gp

Punnet Square:

ggppggPpGgppGgPp

ggPpggPPGgPpGgPP

GgppGgPpGGppGGPp

GgPpGgPPGGPpGGPP

GP Gp gP gp

GP

Gp

gP

gp

Phenotypes: 3:1 (12:4) and 3:1 (12:4) for each trait;

Combined: 9 green/purple; 3 green/white; 3 yellow/purple;1 yellow/white

Incomplete dominance:when heterozygouscondition has intermediatephenotype to thehomozygous conditions.

(Why is this not evidenceof blending?)

Alternative forms of inheritance:

FWFWFRFW

FRFWFRFR

FR FW

FR

FW

Genotypes: 1 FRFR, 2 FRFW, 1 FWFW - 1:2:1Phenotypes: 25% red flowers; 50% pink

flowers; 25% white flowers. - 1:2:1

Snapdragons:Incomplete Dominance

Alternative forms of inheritance:

Codominance: both alleles are expressed by the heterozygote.Blood groups - three different alleles.

IA: codes for enzyme that puts Acarbohydrate on RBC’s cellmembrane.

IB: codes for enzyme that puts Bcarbohydrate on RBC’s cellmembrane.

i: codes for neither carbohydrate(recessive to IA and IB).

also, Multiple alleles - three different alleles.

Alternative forms of inheritance:

Polygenic Inheritance: when anumber of different pairs ofalleles at several loci areimportant for expression of atrait. Such traits are typicallyquantitative in nature, notqualitative as seen indominance/recessiverelationships.

At least three different genes at three loci (locations onchromosomes) affect skin color. Each gene mayhave two alleles that affect melanin expression, withone allele (for melanin) dominant to the other allele.

Two types of Genetics:

Mendelian Genetics: qualitative traits; single genes of majoreffect.

Quantitative Genetics: quantitative traits; polygenicinheritance.

Student examples of each type of trait.

Environment: can have a dramatic effect phenotype.Pigmentation (freckles or skin color) can increase in thepresence of sunlight.

(Some medications—for malaria—prevent tanning, sochemical environment can also affect expression.).

For quantitative genetic traits:Phenotype = Genotype + Environment

Human diseases and disorders: recessive PKU: Phenylketonuria; Maternal PKU; Treatment of PKU

Human diseases and disorders: dominant

Huntington’s disease:

Progressive disorder involving degeneration of nerve cellsin the brain.

It is inherited as a single faulty gene on chromosome #4.

Symptoms do not usually appear until adulthood, typicallybetween ages 35 and 50 years old.

Loss of mental function and loss of cognitive functions.

THE END.