1. General Genetics Lecturer Alshehri, Dokhnah Saeed dalshehri@ut.edu.sa
(Practical Session) BIO221
Lecturer
Alshehri, Dokhnah Saeed
Alshehri, Dikhnah Saeed

2. The 6th Lab Sex-Linked Genes
                                                                                                                         
Alshehri, Dikhnah Saeed

3. Outlines: Gender Determination. Sex-Linked Traits.
X-Linked Inheritance.
Sex-Linked Disorders.
Some practical Exercise
and class activates.
Alshehri, Dikhnah Saeed

4. Sex determination
Sex chromosomes – determines the sex of an individual ( Y, X )
* Males have X and Y
Two kinds of gametes
* Female have two X’s
Only one type of gamete
Alshehri, Dikhnah Saeed

5. Which parents determines the sex of the offspring?XY
male X Y XX
Female X X
It is the male that determines the sex of the offspring.
Alshehri, Dikhnah Saeed

6. Sex Linked Traits
1- Traits controlled by genes located on sex chromosomes (X or Y)
2- Most are X-linked: The X chromosome is larger and has more genes.
3- A Y has very few genes.
4- Show inheritance pattern that differs for autosomal traits.
Alshehri, Dikhnah Saeed

7. X-linked Inheritance
Alshehri, Dikhnah Saeed

8. * Males only have one copy of the X chromosome (hemizygous), plus a Y.
* Female have two X chromosomes, and may be homozygous or heterozygous for a trait
X-linked genes are never passed from father to son.
* The Y chromosome is the only sex chromosome that passes from father to son.
Alshehri, Dikhnah Saeed

9. Females with one copy of the normal gene and one copy of the mutated gene are called carriers. They don’t show the trait.
* Males are never carriers – if they have a mutated gene on the X chromosome, it will be expressed
Alshehri, Dikhnah Saeed

10. X-linked disorders Recessive Dominant Red-green color blindness
Hemophilia
Duchenne muscular dystrophy
Ichthyosis
Dominant
Hypertrichosis
Alshehri, Dikhnah Saeed

11. Red-green color blindness
Can not distinguish red from green.
Normal red-green
Red-green colorblind
Some 10 million American men—fully 7 percent of the male population—either cannot distinguish red from green, or see red and green differently from most people. This is the commonest form of color blindness, but it affects only .4 percent of women.
There are three basic variants of color blindness. Red/green color blindness (deuteranopia) is the most common deficiency, affecting 8% of Caucasian males and 0.5% of Caucasian females. The prevalence varies with culture.
Blue color blindness (protanopia) is an inability to distinguish both blue and yellow, which are seen as white or gray. Protanopia is quite rare and has equal prevalence in males and females.
Alshehri, Dikhnah Saeed

12. What a red-green colorblind person would see
Normal
Red-green colorblind
Alshehri, Dikhnah Saeed

13. Alshehri, Dikhnah Saeed

14. Hemophilia
Alshehri, Dikhnah Saeed

15. X Chromosome Inactivation
While females have two X chromosomes, only one is active.
The other is turned off.
This is random, in some cells the maternal X is turned off and in others the paternal X is turned off.
All descents of that cell will have same X turned off
It is condensed and known as a Barr Body.
Alshehri, Dikhnah Saeed

16. Exercise 1
Color blindness
A normal man married to a normal woman (her father affected with color blindness).
What is the probability of children (male and female) to be affected with the disease?
female
male
Xᴱ Xᴱ Normal
Xᴱ Y Normal
Xᴱ Xᵉ Carrier
Xᵉ Y Affected
Xᵉ Xᵉ Affected
Alshehri, Dikhnah Saeed

17. Exercise 2 Hemophilia Hemophilia is an X-linked recessive trait.
Write the genotype for the following individuals:
A normal man
An affected man
An affected woman
Unaffected and not carrier woman
Unaffected and carrier woman
Female
Male
Xᴴ Xᴴ Normal
Xᴴ Y Normal
Xᴴ Xʰ Carrier
Xʰ Y Affected
Xʰ Xʰ Affected
Alshehri, Dikhnah Saeed

18. Exercise 3 A woman carried hemophilia disease married to a normal man.
Write the genotype of the parents.
What is the probability of the male children to be affected with the disease?
Alshehri, Dikhnah Saeed

19. Assignment: How many males of their progeny will show the trait?
A woman “affected with red color blindness" which is a recessive, sex- linked trait, marries a normal male.
How many males of their progeny will show the trait?
A. All
B. 1/2
C. 1/4
D. 0
E. 3/4
Alshehri, Dikhnah Saeed

20. Exercise 4
White eyes in Drosophila are an X- linked recessive trait. Red eyes is the X-linked dominant form. If a Red eyed, heterozygous female fly crossed with a red eyed male fly, How many females of their offspring will have white eyes?
How many males of their offspring will have white eyes?
Alshehri, Dikhnah Saeed

21. Alshehri, Dikhnah Saeed