1. DO NOW
GRAB YOUR GREEN FOLDER.
•In golden retrievers, the allele for brown hair is
dominant to the allele for yellow hair. If a yellow-
haired golden retriever mated with a heterozygous
brown-haired retriever, what is the probability that
their puppies will be brown haired?
You have 5 minutes.
3. I CAN…
• I CAN make and interpret a Punnett
square to predict the genotypes and
phenotypes of offspring for sex linked,
incomplete, and co-dominant traits
• I CAN interpret a dihybrid cross Punnett
square
4. Can you predict the outcome of 2
traits at the same time?
• YES! This is called a dihybrid cross. Just
follow the same Punnett square rules.
• On the EOC, you will NOT have to make a
dihybrid cross Punnett Square; you will only
have to interpret it correctly.
1.Just figure out who wins the battle in each
square, as usual, to determine phenotype
IT’S SO
5. Dihybrid Punnett Square Cross
• Cross between 2
guinea pigs
heterozygous for
hair color and
length
• B=black
• b=white
• S=short
• s=long
7. Q: What fraction of the offspring are
black and long tailed?
• Cross between 2
A: guinea pigs
heterozygous for
hair color and
length
• B=black
• b=white
• S=short
• s=long
8. Q: What fraction of the offspring are
white and short tailed?
• Cross between 2
A: guinea pigs
heterozygous for
hair color and
length
• B=black
• b=white
• S=short
• s=long
9. Mr. Im lied to me…
• There are exceptions to the dominant allele
always winning the battle against the recessive
allele that might show up on the EOC
1. Incomplete dominance: when two alleles
mix to create a new phenotype
2. Co-dominance: when alleles are both
dominant and will both express their
phenotypes
• The EOC will always let you know in the
question if the trait is incomplete or co-
dominant.
10. Incomplete Dominance:
• The alleles for red flower and white flower petals
show incomplete dominance (two alleles mix to
create a new phenotype)
▫ What is the new phenotype? PINK!
11. Check for Understanding
Q: In roses the allele for red flowers shows
incomplete dominance with the allele for white
flowers. One pink rose and one white rose are
crossed. What is the probability of producing a
white rose?
12. Co-dominance
• The alleles for white petals and red petals show
co-dominance (alleles are both dominant and
will both express their phenotypes)
▫ Both capital letters.
• What is the phenotype of the flower? Spotted red
& white!
13. Check for Understanding
Q: A homozygous brown-coated dog is crossed
with a brown-and-white-coated dog. Brown and
white coats are co-dominant. What is the
probability of producing a brown-and-white
coated puppy?
15. Why are more males colorblind?
• Some genes are carried on sex
chromosomes.
▫ Sex chromosomes are the X and Y
chromosomes
Example: The allele for colorblindness is
recessive and carried on the X
chromosome
E=normal eyes
▫ XEXE: healthy female e=color blind
▫ XeXe: colorblind female
▫ XEXe: female carrier
▫ XeY: color blind male
▫ XEY: healthy male
• It is easier for males to be colorblind,
because they only have one X
16. Check for Understanding
Q: Hemophilia (not being able to clot your blood) is a sex-
linked recessive disease. A heterozygous female and a male
who has hemophilia are worried about having a baby with
the disease. What is the probability that they will have a
child with hemophilia?
