1. Prevalence of dominance
Because an allele is dominant does not mean…
it is better
it is more common
Polydactyly:
dominant allele

2. Polydactyly recessive allele far more common than dominant
individuals are born with extra fingers or toes
dominant to the recessive allele for 5 digits
recessive allele far more common than dominant
 399 individuals out of have only 5 digits
 most people are homozygous recessive (aa)

3. Hound Dog Taylor

4. Sex-linked traits Hh x HH XHXh XHY XHXh XH Xh XH Y XHXH XHXH XHY XHY
sex-linked recessive
XHXh
XHY
Hh x HH
XHXh XH Xh XH Y
male / sperm
XHXH
XHXH
XHY
XHY XH Xh
female / eggs
XHY Y XH
XHXh
XHXh
XhY
XhY

5. Sex-linked traits summary
follow the X chromosomes
males get their X from their mother
trait is never passed from father to son
Y-linked
very few traits
only 26 genes
trait is only passed from father to son
females cannot inherit trait

6. Cells We have 2 types of cells 1. Somatic Cells (2n) – body cells
2. Sex Cells (n) – sperm and egg

7. Chromosomes We have 2 types of Chromosomes:
1. Autosomal chromosomes – pairs 1 through 22 in humans (non-sex chromososmes)
2. Sex chromosomes – XX or XY

8. Sex-Linked Traits

9. Definition Traits that are controlled by genes on the sex chromosomes
The X and Y chromosomes

10. Are they controlled by both sex chromosomes?
Most sex-linked traits are controlled by genes on the X chromosome.
This is because an X chromosome is much larger than a Y chromosome.
A few traits are suspected to be controlled by genes on the Y chromosome, however there is less research about Y-linked traits.

11. Different Forms of Sex-Linked Inheritance
There are three different forms of sex-linked inheritance that we will be examining:
X-linked recessive inheritance
X-linked dominant inheritance
Y-linked inheritance

12. X-linked Recessive Inheritance
X-linked recessive traits are traits resulting from a recessive allele on the X chromosome.
There are over 100 different human conditions that are caused by recessive alleles found on the X chromosomes.
X-linked recessive alleles are represented by a X , superscript lower case letter or a plus/minus
(+/-)

13. X-linked Recessive Inheritance
These traits tend to show up in males more than females.
Why?
Because males only carry one X chromosome! ( they have one x chromosome, and one Y chromosome that make up their sex chromosomes).

14. X-linked Recessive Inheritance
Example #1: Colorblindness
The allele that controls colorblindness is found on the X chromosome and is recessive .
What genotype must a female have to be diagnosed as colorblind?
Homozygous recessive (X-X-)

15. Are you colorblind? Normal Color Vision: A: 29, B: 45, C: --, D: 26
Red-Green Color-Blind: A: 70, B: --, C: 5, D: -- 3.
Red Color-blind: A: 70, B: --, C: 5, D: 6 4.
Green Color-Blind: A: 70, B: --, C: 5, D: 2

16. Try this problem!
What is the probability that the sons of a homozygous recessive mother would be colorblind?
Represent colorblindness with a small “c”
Mothers Genotype Xc X c
Each son would receive an X chromosome from their mother which means…
Each son would have an X chromosome with a recessive allele for colorblindness and would express the trait!

17. X-linked Recessive Inheritance
Example #2: Hemophilia Hemophilia
Hemophilia is a serious ailment in which the blood lacks a clotting factor, and therefore when an individual is injured, they cannot stop bleeding.
Hemophilia is also controlled by recessive alleles on the X chromosome.

18. Try this problem
A woman who is heterozygous (a carrier) for hemophilia marries a normal man:
a. What are the genotypes of the parents?
b. Make a Punnett square for the above cross.
c. What is the probability that a male offspring will have hemophilia? __________
d. What is the probability of having a hemophiliac female offspring? _________

19. X-linked Dominant Inheritance
X-linked dominant traits are traits that result from the presence of a dominant allele on the X chromosome.
Unlike X-linked recessive traits, females and males both require only ONE dominant allele in order to express the trait.
X-linked dominant traits are represented by an X, superscript capital letter or a (+)

20. X-linked Dominant Inheritance
Example: Faulty Tooth Enamel and Dental Discoloration
Individuals who have an X chromosome that carries a dominant allele for this trait will have dental discoloration.

21. Try these problems!
Would a heterozygous woman for dental discoloration display the trait?
YES! Because this is a X-linked dominant trait, you only need to carry one dominant allele in order to express the trait.

22. Try these problems!
What percentage of the children from a heterozygous mother and an affected father would have dental discoloration?
Represent dental discoloration with a “D”
All of the daughters will display the trait as they each carry at least one dominant allele.
One son will demonstrate the trait while the other won’t.

23. Y-linked Inheritance
Y-linked traits are controlled by alleles on the Y chromosome
Another word for Y-linked traits is holandric traits as they are “completely male”.

24. Y-linked Inheritance
Do the words homozygous or heterozygous apply to Y-linked traits?
Are females affected by Y-linked traits?
No, because there is only ever one Y chromosome present at a time!
No, because females don’t carry Y chromosomes!

25. What is this?

26. Y-linked Inheritance
An example of a Y-linked trait is HAIRY EARS!

27. Y-linked Inheritance
All of the sons of an affected male will display this Y-linked trait.

28. A final example of Sex-linked Traits
Eye color in fruit flies ( Drosophilia melanogaster )
Eye color is controlled by the X chromosome.
Red eyes are dominant to white.
White eyes are most common in males.
Females only display white eyes if they are homozygous recessive for the trait.

29. Fruit Flies Continued What type of X-linked inheritance is this?
Try your new knowledge out on the handout you are about to receive!
X-linked recessive Inheritance because the males show the trait more often that the females. Also, the females must have two X chromosomes, both of which carry the recessive alleles for white eye color in order to have white eyes!