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Studying Heredity
Chapter 8 Section 8-3
Objectives for section 8-3
Predict results of genetic crosses
w/Punnett Square
 Describe how a testcross can be
used to determine genotype
 Define & analyze a simple
pedigree
PG. 170

Punnett Square
•

predicts possible
outcomes of genetic
crosses
Inventor– Reginald
Punnett
Monohybrid Cross
1. Assign letters for traits
2. Determine parents’ genotypes
3. Write genotypes across top & side
4. Fill in each box of the Punnett square
5. State genotype and phenotype
A heterozygous dimpled female marries a
heterozygous male. What are the
possible outcomes of their children?
D
Test cross: dominant
phenotype/unknown genotype (x) w/
homozygous recessive


Breeders need to
know if organismheterozygous/
homozygous
dominant for a
character
Why?
EX: A plant w/ yellow seeds & unknown
genotype (Y?) is test crossed with a
plant that has green seeds (yy).
V. Dihybrid Crosses =2
contrasting traits

Cross 2 pea plants heterozygous for seed
shape AND seed color
1. R=round r=wrinkled AND
Y=yellow y=green
2. Parents’ genotypes
a. RrYy x RrYy
3. Possible Gametes??
a. RY, Ry, rY, ry
Typical Dihybrid Cross
4. Phenotypic ratio
9 round, yellow
3 round, green
3 wrinkled,
yellow
1 wrinkled green
Typical dihybrid cross ratio: 9:3:3:1
Monohybrid vs. Dihybrid Cross
Monohybrid: involves one trait with a
phenotypic ratio for the F2 generation
of 3:1
Dihybrid: involves two traits with a
phenotypic ratio for the F2 generation
of 9:3:3:1
Why use Punnett Squares?
A. Improve crops
B. Improve domestic animals
C. Determine risk for genetic disorders
D. Predict expected results of
genotypes or phenotypes
Probability –likelihood that a
specific event will occur
A. Expressed in words, decimals, fractions,
or percentages
B. Probability= # of 1 kind of possible
outcome/total # of all possible outcomes
C. Both parents must be considered when
calculating probability
D. To find probability, multiply the separate
probabilities of the two events
Pedigree: family history-shows a trait
inherited over several generations

Helps in tracking genetic disorders
♀
Colorblindness Pedigree
♂
What is determined from a pedigree?
1.
Autosomal: gene will appear in both sexes
equally
2.
Sex-linked: allele located only on the X
chromosome & are recessive
a. Males carrying recessive allele on X
chromosome exhibit sex-linked
character…Why??
b. Females-less chance of exhibiting sexlinked condition…Why??
Pedigree for the Albino Trait
♀
♂ autosomal
Pedigree for Hemophilia
Sex linked!! Only
♂ affected
3. Autosomal Dominant: every
individual w/condition has a parent
w/condition
4. Recessive: individual w/ condition
can have 1, 2, or neither parent exhibit
the condition
5. Homozygous dominant or
heterozygous: phenotype will show
the dominant allele
6. Homozygous recessive: phenotype
shows the recessive allele
Can 2 people who are heterozygous
carriers of a recessive mutation
have children with the mutation??
Hemophilia-the
Royal disease
1 in 10,000 ♂
1 in 100,000,000 ♀