1. Genetic Crosses Review
Do Now: Do the dihybrid cross on your notes sheet.

2. Dihybrid cross: RW Rw rW rw RW Rw rW rw RR WW RR Ww Rr WW Rr Ww RR ww
Phenotype Ratio: 9:3:3:1
RW Rw rW rw RW Rw
rW rw
RR WW
RR Ww
Rr WW
Rr Ww
RR ww
Rr ww
rr WW
rr Ww

3. 5 Types of Genetic Crosses
The dihybrid cross is one of the 5 types of genetic crosses we have studied.
In a dihybrid cross, the inheritance of 2 traits is predicted at the same time
Since each parent has 4 possible combinations of alleles, dihybrid crosses are 4 x 4 squares.

4. Complete dominance Complete dominance is the first type we looked at.
One allele hides the effect of the other.
AA = dominant phenotype
Aa = dominant phenotype
aa = recessive phenotype
Possible crosses include:
Heterozygous x heterozygous (Aa x Aa)
3:1 phenotype ratio, 1:2:1 genotype ratio
Homozygous x heterozygous (aa x Aa and AA x aa)
1:1 phenotype and genotype ratios
Homozygous x homozygous (AA x AA, aa x aa, and AA x aa)
100% have the same genotype and phenotype

5. Complete Dominance Example
In humans, albinism (a) is recessive to normal pigmentation (A). If an albino man has a child with a woman who is heterozygous for the trait, what is the probability their child will be an albino?

6. Codominance
For codominant traits, BOTH alleles show up in the phenotype
AA = dominant phenotype
Aa = both phenotypes (i.e. red + white fur)
aa = recessive phenotype
Possible crosses include:
Heterozygous x heterozygous (Aa x Aa)
1:2:1 phenotype ratio, 1:2:1 genotype ratio
Homozygous x heterozygous (aa x Aa and AA x aa)
1:1 phenotype and genotype ratios
Homozygous x homozygous (AA x AA, aa x aa, and AA x aa)
100% have the same genotype and phenotype

7. Codominance Example
In a species of tropical frog, there are frogs that have blue skin, frogs that have black skin, and frogs that have both black and blue skin.
If two black and blue frogs mate, what will the phenotype ratio of their offspring be?

8. InComplete dominance
Traits with incomplete dominance produce a heterozygous phenotype that is a blend of the two allele’s effect
AA = dominant phenotype
Aa = blended phenotype (pink flowers
aa = recessive phenotype
Possible crosses include:
Heterozygous x heterozygous (Aa x Aa)
1:2:1 phenotype ratio, 1:2:1 genotype ratio
Homozygous x heterozygous (aa x Aa and AA x aa)
1:1 phenotype and genotype ratios
Homozygous x homozygous (AA x AA, aa x aa, and AA x aa)
100% have the same genotype and phenotype

9. Incomplete dominance example
A type of pepper comes in three colors, red, orange, and yellow. If a gardener crosses two orange pepper plants, what will the phenotype ratio of the offspring be?

10. Sex-linked Traits
The alleles for sex linked traits are located on the sex chromosomes (X chromosome in humans)
Recessive sex-linked traits are much more common in males than females, since they have only 1 copy

11. X+ Xo X+X+ X+Xo Y X+Y XoY Sex-Linked traits
In a Punnett square for a sex-linked trait, the sex chromosomes are used instead of letters.
X+X+ normal f
X+Xo carrier f
XoXo color-blind f
X+Y normal m
XoY color-blind m X+ Xo
X+X+
X+Xo Y
X+Y
XoY

12. Sex-Linked Example
Because she has a brother with hemophilia, a biology student wanted to figure out the probability that she was a carrier for the X-linked disorder. Neither of her parents has hemophila. What is the probability she is a carrier of the trait?

13. QUIZ Monday!
There will be a quiz on genetic crosses this Monday! Each of the 5 types will show up on it.
Do your homework for practice!