1. Unit 7, Part 3 Notes: Sex-Linked Traits and Dihybrid Crosses
Pre-AP Biology, Mrs. Krouse

2. Types of Chromosomes in a Human Cell
Describe what is shown in a karyotype.
How many chromosomes are found in a human cell? How many pairs of chromosomes?

3. 3) Which pairs are autosomes, and which pair is the sex chromosomes
3) Which pairs are autosomes, and which pair is the sex chromosomes? 4) What are the two types of sex chromosomes?

4. 5) Sex chromosomes found in females? Males?
6) Traits found on autosomes? Traits found on sex chromosomes?

5. Sex-Linked Traits
9) What does “linked” mean? 10) X-Linked Traits vs. Y-Linked Traits 11-12) X-Linked Traits have different alleles… what are alleles?

6. Example of an X-Linked Trait (hemophilia)
12) What is hemophilia?
13) Xh = hemophilia allele
XH = allele for normal blood clotting

7. Hemophilia: Possible Genotypes and Phenotypes for Males and Females
15) X-linked recessive traits will be more common in which sex? Why?
Genotype
Sex
Phenotype
XH XH
XH Xh
Xh Xh
XHY
XhY

8. Sample X-Linked Trait Punnett Square
16) Women with the genotype XHXh are called carriers? Why?
17) Punnett square reminders…
-Why do we use a Punnett square?
-Letters on top and left are alleles found in gametes. What are gametes?

9. 17) A man who does not have hemophilia marries a woman who is a carrier of the hemophilia allele. What are the genotypes of the parents? Set-up and fill in the Punnett square in the space to the right
Sex and phenotype frequencies in the offspring? (written as percentages)

10. Tracking the Inheritance of Two Traits
18-19) Monohybrid vs. dihybrid crosses 19) 16 boxes in a dihybrid Punnett square 20) We usually just count the offspring phenotype frequencies

11. Example Dihybrid Cross Problem
21)
Trait 1: G = green leaves and g = yellow leaves
Trait 2: P = purple flowers and p = white flowers
Parent Genotypes: GgPp x GgPp
Rules for Writing Dihybrid Genotypes:
-Keep the two alleles for a trait together (keep G’s together and P’s together)
-Within a trait, write dominant alleles first (ex: G before g)
-Write the alleles in alphabetical order (G before P)

12. FOIL Method
22-24)
Use FOIL on the parent genotypes to determine the alleles that go into each gamete on the top and left side of the Punnett square
Each gamete should have one allele for each trait (ex: one G and one P)
Let’s use the white board to FOIL the parent genotypes

13. 25) Let’s set up and fill in our dihybrid Punnett square in the space below

14. 26) Counting offspring phenotype frequencies Possible methods = symbol coding, color coding, or crossing out boxes 27) On the white board, let’s count the offspring phenotypes frequencies for the Punnett square on the previous slide. We will write these frequencies as fractions and ratios.

15. 28) These are offspring phenotype frequencies that we ALWAYS see when doing a dihybrid cross of two parents that are both heterozygous for both traits (ex: GgPp x GgPp) How do we write these frequencies in more general terms?

16. Another Example Dihybrid Cross Problem
29)
Trait 1: R = running mice and r = waltzing mice
Trait 2: B = black hair and b = brown hair
Parent Information: Let’s cross a homozygous running, heterozygous black mouse with a waltzing brown mouse.
30) What are the genotypes of the two parent mice?

17. 31) FOIL the parent genotypes to find the gamete alleles that go on the top and left sides of the Punnett square

18. 32) Let’s set up and fill in our dihybrid Punnett square in the space below

19. 33) On the white board, let’s count the offspring phenotypes frequencies for the Punnett square on the previous slide. We will write these frequencies as fractions and ratios.