1. Punnett squares for two traits
Dihybrid Crosses:
Punnett squares for two traits

2. Genes on Different Chromosomes
If two genes are on different chromosomes, all four possible alleles combinations for two different genes in a heterozygous cross are equally likely due to independent assortment. If parents are RrYy (heterozygous for both traits) Ry rY RY ry Gametes: 25% Ry, 25% rY, 25% RY, 25% ry R y r Y R r Y y
Equally likely to line up either way when dividing OR

3. Setting up a Dihybrid Cross: RrYy x RrYy
Each side of a Punnett Square represents all the possible allele combinations in a gamete from a parent.
Parent gametes always contain one allele for each gene.(One of each letter -R or r & Y or y in this case).
Four possible combinations of the alleles for the two genes are possible if heterozygous for both traits. (For example: RY, Ry, rY and ry)
Due to independent assortment, each possible combination is equally likely if genes are on separate chromosomes. Therefore Punnett squares indicate probabilities for each outcome.
Due to independent assortment, each possible combination is equally likely if genes are on separate chromosomes. Therefore Punnet squares represent probabilities for each outcome.

4. A. R r Y y
Which is correct for a dihybrid cross of two heterozygous parents RrYy x RrYy? C. RY Ry rY ry B. RR Rr rr rR YY Yy yy yY

5. Dihybrid Cross of 2 Heterozygotes9 3 1

6. Heterozygous Dihybrid Cross
Dominant
for both traits
Dominant 1st trait
Recessive 2nd trait
Recessive
1st trait
Dominant 2nd trait
Round
Yellow
Green
Wrinkled 9 3 1
¾ x ¾ =
9/16
¾ x ¼ =
3/16
¼ x ¾ =
¼ x ¼ =
1/16
Heterozygous cross 9 : 3 : 3 : 1 ratio if independent assortment

7. Mendel came up with the Law of Independent Assortment because he realized that the results for his dihybrid crosses matched the probability of the two genes being inherited independently.

8. Linked Genes
Studying fruit flies, Thomas Hunt Morgan identified dihybrid crosses that did not result in the phenotype ratios expected through independent assortment.
Recall that expected ratios are:
9:3:3:1 for heterozygous crosses
1:1:1:1 for cross of a heterozygote with a recessive

9. Four possible gametes from heterzygote.
Equally likely if independently assorting.
One possible gamete from homozygous recessive parent.
YyRr x yyrr yr YR
YyRr
Yellow Round Yr
Yyrr
Yellow Wrinkled yR
yyRr
Green Round
yyrr
Green Wrinkled
Test cross results if independently assorting: 1:1:1:1 (25% of each of 4 possible combinations.)

10. Review of Recombinants
Recombinants have new combinations of the genes. If the parental generation of cats are white furred with short tails (SS bb) and brown fur with long tails (ss BB), Then Sb and sB are the parental combinations for the gametes (egg or sperm) and SB and sb are the recombinant gametes.
Parental Genotypes
Parental
Combinations
Recombinant
gametes

11. Linked Genes
When two genes are linked (relatively close together on the same chromosome), it is more likely for the parental combinations to be passed down.
Recombinants do occur due to crossing over.
The further apart the genes are the more likely recombinants are because crossing over is more likely to occur.

12. Parental combinations: Gray with normal wings and
Black with vestigal wings
Cross of the heterozygous offspring with a double recessive.
Expect 1:1:1: 1 if independently assorting.
Results show more of parental combinations because genes are on the same chromosome.

13. Parental combinations: Gray with normal wings and
Black with vestigal wings
Cross of the heterozygous offspring with a double recessive.
Expect 1:1:1: 1 if independently assorting.
Results show more of parental combinations because genes are on the same chromosome.

14. Mythbuster:
Many people assume that dominant traits are always more common. It is true that if both parents are heterozygous, it is more likely that the offspring will have the dominant trait (75% probability). However, a recessive trait can be more common when the recessive allele is more prevalent in the population. For example, Achrondoplasia dwarfism is caused by a dominant allele, and yet the trait is very rare in the population.

15. Genetics and Natural Selection
Natural selection can change the allelic frequency. (Allelic frequency = how common an allele is in the gene pool.)
Initial allelic frequency of b = 50% (12/24 alleles)
If the white rabbits are more likely to survive, the frequency of b allele will increase in the gene pool.
Allelic frequency of b after natural selection= 96% (23/24)
Bb BB bb bb Bb Bb bb BB Bb Bb BB Bb
bb bb bb bb bb Bb bb bb bb bb bb bb bb bb

16. Natural Selection and Genetic
Natural selection acts upon phenotype (expression of trait) rather than genotype.
Most harmful genetic disorders are recessive. Carriers can have a hidden copy of the harmful gene that does not affect their own survival.