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.
All the chromosomes in a single human body cell (includes the autosomes and sex chromosomes)
2) How many chromosomes are found in a human cell? 46 How many pairs of chromosomes? 23

3. 3) Which pairs are autosomes (pairs 1-22), and which pair is the sex chromosomes (pair 23)? 4) What are the two types of sex chromosomes? X and Y chromosomes
LTA

4. LTB 5) Sex chromosomes found in females (XX)? Males (XY)?
6) Traits found on autosomes? Traits found on sex chromosomes?
Autosomes contain genes for body traits that are found in both males AND females
Sex chromosomes contain genes for primary or secondary sex characteristics that are only found in males or females

5. Sex-Linked Traits LTC LTC
9) What does “linked” mean? Synonyms: Related, Connected, To Bring Together In this case, sex-linked traits are traits that are “linked” or connected to genes on the sex chromosomes. In other words, these traits are controlled by genes on the sex chromosomes. 10) X-Linked Traits vs. Y-Linked Traits (see notes) 11-12) Traits on autosomes and X-linked traits have different alleles… what are alleles? Different forms of a gene (ex: purple flower allele vs. white flower allele…. These are different alleles for the flower color gene)
LTC

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

7. Hemophilia: Possible Genotypes and Phenotypes for Males and Females
LTD
14)
Genotype
Sex
Phenotype
XH XH
Female
Normal blood clotting
XH Xh
Xh Xh
Hemophilia
XHY
Male
XhY

8. 15) X-linked recessive traits will be more common in which sex? Why?
It is more common in males because…
-Males only need to receive one copy of the recessive hemophilia allele (Xh) to have the disease hemophilia
-Females must receive two copies of the allele to have the disease… this does not happen a lot
LTE

9. 16) Women with the genotype XHXh are called carriers? (see notes)

10. 17) Punnett square reminders… -Why do we use a Punnett square
17) Punnett square reminders… -Why do we use a Punnett square? To predict genotype and phenotype frequencies in the offspring based on the parent genotypes Genotype = combination of two alleles that an organism has for a trait (ex: Aa) Phenotype = physical trait that results from the genotype (ex: Aa  tall) -Letters on top and left are alleles found in gametes. What are gametes? Sex cells (ex: eggs and sperm)

11. XH Y XHXH XHY Xh XHXh XhY LTsF-I
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? XHY and XHXh Set-up and fill in the Punnett square in the space to the right
LTsF-I XH Y
XHXH
XHY Xh
XHXh
XhY
Rules: X comes before Y, and H comes before h
Sex and phenotype frequencies in the offspring? (written as percentages)
50% Normal Females
25% Normal Males
25% Hemophilia Males

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

13. 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)

14. FOIL Method LTL F = first, O = outer, I = inner, L = last
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)
F = first, O = outer, I = inner, L = last
Let’s say we wanted to FOIL the parent genotype GgPp
First G and P = GP
Outer G and P = Gp
Inner G and P = gP
Last G and P = gp

15. LTM LTN 25) Let’s set up and fill in our dihybrid Punnett square. GP
GGPP
GGPp
GgPP
GgPp
GGpp
Ggpp
ggPP
ggPp
ggpp
26) Methods for counting offspring phenotype frequencies: color coding, symbol coding, or crossing out box
27) Offspring phenotype frequencies
Green, Purple = 9/16
Green, White = 3/16
Yellow, Purple = 3/16
Yellow, White = 1/16
LTN

16. 27) Expressing the offspring phenotype frequencies from the previous slide as ratios… 9 Green, Purple : 3 Green, White : 3 Yellow, Purple : 1 Yellow, White

17. 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? 9/16 with both dominant traits 3/16 with the first trait dominant and the second trait recessive 3/16 with the first trait recessive and the second trait dominant 1/16 with both recessive traits
LTO

18. 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?
Parent 1: BbRR
Parent 2: bbrr

19. 31) FOIL the parent genotypes to find the gamete alleles that go on the top and left sides of the Punnett square
Parent 1: BbRR (top of square)
F = BR
O = BR
I = bR
L = bR
Parent 2: bbrr (left side of square)
F = br
O = br
I = br
L = br

20. 32) Let’s set up and fill in our dihybrid Punnett square
BbRr
bbRr
33) Offspring phenotype frequencies as fractions
Black, Running = 1/2
Brown, Running = 1/2
33) Offspring phenotype frequencies as a ratio:
1 Black, Running : 1 Brown, Running
(This ratio is reduced from a 2:2 ratio)