Studying HeredityStudying HeredityChapter 8 Section 8-3Chapter 8 Section 8-3

Objectives for section 8-3Objectives for section 8-3 Predict results of genetic Predict results of genetic

crosses w/Punnett Squarecrosses w/Punnett Square Describe how a testcross can Describe how a testcross can

be used to determine be used to determine genotypegenotype

Define & analyze a simple Define & analyze a simple pedigreepedigree

PG. 170PG. 170

Punnett SquarePunnett Square• predicts possible predicts possible

outcomes of outcomes of genetic crosses genetic crosses

Inventor– Reginald Inventor– Reginald Punnett Punnett

Monohybrid Cross Monohybrid Cross

1. Assign letters for traits1. Assign letters for traits

2. Determine parents’ genotypes

4. Fill in each box of the Punnett square

3. Write genotypes across top & side

5. State genotype and phenotype

A heterozygous dimpled female A heterozygous dimpled female marries a heterozygous male. marries a heterozygous male. What are the possible outcomes of What are the possible outcomes of their children?their children?

D

Test cross: Test cross: dominant phenotype/unknown genotype (x) w/ homozygous recessive

Breeders need to Breeders need to know if know if organism- organism- heterozygous/ heterozygous/ homozygous homozygous dominant for a dominant for a charactercharacter

Why?Why?

EX: A plant w/ yellow seeds & EX: A plant w/ yellow seeds & unknown genotype (Y?) is test unknown genotype (Y?) is test crossed with a plant that has green crossed with a plant that has green seeds (yy).seeds (yy).

V. V. DiDihybrid Crosseshybrid Crosses = =22 contrasting traits contrasting traits

Cross 2 pea plants heterozygous for Cross 2 pea plants heterozygous for seed shape AND seed colorseed shape AND seed color1. R=round r=wrinkled AND 1. R=round r=wrinkled AND

Y=yellow y=green Y=yellow y=green2. Parents’ genotypes2. Parents’ genotypes

a. RrYy x RrYya. RrYy x RrYy3. Possible Gametes??3. Possible Gametes??

a. RY, Ry, rY, rya. RY, Ry, rY, ry

Typical Dihybrid CrossTypical Dihybrid Cross

4. Phenotypic ratio9 round, yellow

3 round, green

3 wrinkled, yellow

1 wrinkled green

Typical dihybrid cross ratio: 9:3:3:1

Monohybrid vs. Dihybrid Monohybrid vs. Dihybrid CrossCross

MonoMonohybrid: involves hybrid: involves oneone trait trait with a phenotypic ratio for the Fwith a phenotypic ratio for the F22 generation of 3:1generation of 3:1

DiDihybrid: involves hybrid: involves twotwo traits with a traits with a phenotypic ratio for the F phenotypic ratio for the F22 generation of 9:3:3:1generation of 9:3:3:1

Why use Punnett Squares?Why use Punnett Squares?

A. Improve cropsA. Improve crops

B. Improve domestic animalsB. Improve domestic animals

C. Determine risk for genetic C. Determine risk for genetic disordersdisorders

D. Predict expected results of D. Predict expected results of genotypes or phenotypesgenotypes or phenotypes

ProbabilityProbability –likelihood that a –likelihood that a specific event will occurspecific event will occur

A. Expressed in words, decimals, A. Expressed in words, decimals, fractions, or percentagesfractions, or percentages

B. Probability= # of 1 kind of possible B. Probability= # of 1 kind of possible outcome/total # of all possible outcome/total # of all possible outcomesoutcomes

C. C. BothBoth parents must be considered parents must be considered when calculating probability when calculating probability

D. To find probability, D. To find probability, multiply multiply the the separate probabilities of the two eventsseparate probabilities of the two events

Pedigree:Pedigree: family history-shows a family history-shows a trait inherited over several trait inherited over several generationsgenerations

Helps in tracking genetic disordersHelps in tracking genetic disorders

♀ ♂

Colorblindness Pedigree

What is determined from a What is determined from a pedigree?pedigree?

1.1. AutosomalAutosomal: gene will appear in both : gene will appear in both sexes equally sexes equally

2.2. Sex-linkedSex-linked: allele located only on the : allele located only on the X chromosome & are recessive X chromosome & are recessive

a. Males carrying recessive allele on X a. Males carrying recessive allele on X chromosome exhibit sex-linkedchromosome exhibit sex-linked character…Why??character…Why??

b. Females-less chance of exhibiting b. Females-less chance of exhibiting sex-linked condition…Why??sex-linked condition…Why??

Pedigree for the Albino Trait

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Pedigree for Hemophilia

Sex linked!! Only ♂ affected

autosomal

4. 4. RecessiveRecessive: individual w/ : individual w/ condition can have 1, 2, or neither condition can have 1, 2, or neither parent exhibit the conditionparent exhibit the condition

3. 3. Autosomal DominantAutosomal Dominant: every : every individual w/condition has a parent individual w/condition has a parent

w/conditionw/condition

66. . Homozygous recessiveHomozygous recessive: : phenotype shows the recessive allelephenotype shows the recessive allele

Can 2 people who are heterozygousCan 2 people who are heterozygouscarriers of a recessive mutation carriers of a recessive mutation have children with the mutation??have children with the mutation??

5. 5. Homozygous dominant Homozygous dominant or or heterozygousheterozygous: phenotype will show: phenotype will show

the dominant allele the dominant allele

Hemophilia-the Hemophilia-the Royal diseaseRoyal disease

1 in 10,000 ♂ 1 in 100,000,000 ♀