1. Sex-linked Inheritance

2. Some Review: Autosomes:
Non-sex chromosomes (#1-22 homologous pairs, 44 total chromosomes)
Sex Chromosomes:
Chromosome pair #23
Female XX
Male XY

3. Sex Inheritance Sex Chromosomes carry Sex Traits and…
…the Inheritance can be followed in a…
…Punnett Square
Gametes

4. Sex-linked traits Trait is carried on the X chromosome.
The Y chromosome is too small to carry other traits besides the one determining the male sex characteristics.

5. Ex. In Drosophila (fruit flies):
P: White-eyed male x Red-eyed female
F1:
All Red eyes, &
Then for the F1 x F1
F2:
½ Red eyes, female
¼ Red eyes, male
¼ white eyes, males

7. **How do you know?
**you know you are dealing with a sex-linked trait when mostly MALES exhibit the trait!!!**

8. XRXr = “carrier” female
Carrier: Only females have two X chromosomes
a carrier is heterozygous and carries a recessive trait but shows the dominant trait
Therefore the recessive trait is masked and not exhibited but the gene is still there to be passed on.
XRXr = “carrier” female

9. We can also use Punnett squares in questions concerning sex-linked traits

10. We can also use Punnett squares in questions concerning sex-linked traits
Instead of using a lone letter to represent each allele, we connect it to the sex chromosome.

11. We can also use Punnett squares in questions concerning sex-linked traits
Instead of using a lone letter to represent each allele, we connect it to the sex chromosome.
Ex. XAXa or XAY

12. We can also use Punnett squares in questions concerning sex-linked traits
Instead of using a lone letter to represent each allele, we connect it to the sex chromosome.
Ex. XAXa or XAY
The dominant allele is written as a capital letter and the recessive allele as a lowercase letter – just like in complete dominance.
Ex. XAXa or XAXA or XAY or XaY
The Y has no allele attached.

13. XA Xa
XAXA
XAXa
XAY
XaY XA Y
ONE EXAMPLE

14. Human examples (notice that all are recessive traits):
Color Blindness: Xc (XC = normal color vision)
Ex. Two normal-visioned parents produce a color blind son. What are the genotypes of the parents? What are the chances of their next child being a color-blind daughter?

15. XC Y

16. XC XC Y

17. XC
XcY XC Y

18. XC
XCY
XcY XC Y

19. XC
XCXC
XCY
XcY XC Y

20. XC Xc
XCXC
XCY
XcY XC Y

21. XC Xc
XCXC
XCXc
XCY
XcY XC Y

22. Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy?

23. Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy? Xc Y

24. Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy?
XC Xc Xc Y

25. XC Xc XCXc XcXc XCY XcY Xc Y
Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy?
XC Xc
XCXc
XcXc
XCY
XcY Xc Y

26. XC Xc XCXc XcXc XCY XcY Xc Y
Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy?
XC Xc
XCXc
XcXc
XCY
XcY Xc Y

27. XC Xc XCXc XcXc XCY XcY Xc Y
Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy?
XC Xc
XCXc
XcXc
XCY
XcY Xc Y

28. XC Xc XCXc XcXc XCY XcY Xc Y
Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy?
XC Xc
XCXc
XcXc
XCY
XcY Xc Y
Color Blind Boy = 1/4

29. Hemophilia: Xh (XH = normal blood clotting)
Ex. a man, whose father was a hemophiliac but who’s clotting time was normal, marries a woman with no record of hemophilia in her ancestry. What are the chances of hemophilia in their children?

30. XH XH XH Y Hemophilia: Xh (XH = normal blood clotting)
Ex. a man, whose father was a hemophiliac but who’s clotting time was normal, marries a woman with no record of hemophilia in her ancestry. What are the chances of hemophilia in their children? XH XH XH
0% chance Y

31. Ex. A woman whose father was a hemophiliac, but who is not herself a “bleeder,” marries a normal man. What is the chance of hemophilia in their children?

32. Ex. A woman whose father was a hemophiliac, but who is not herself a “bleeder,” marries a normal man. What is the chance of hemophilia in their children?
Hemophilia Children = 1/4

33. Ex. If a boy’s father has hemophilia and his mother’s cells have one gene for the disease, what is the chance the boy will inherit the disease?

34. Ex. If a boy’s father has hemophilia and his mother’s cells have one gene for the disease, what is the chance the boy will inherit the disease? XH Xh
½ or 50% chance XH Y

35. Ex. If a woman who is not a carrier of genes for hemophilia is married to a man who is a hemophiliac, what percentage of their male offspring could be expected to be hemophiliac?

36. Ex. If a woman who is not a carrier of genes for hemophilia is married to a man who is a hemophiliac, what percentage of their male offspring could be expected to be hemophiliac?
0% chance

37. Muscular Dystrophy: Xm
(XM = normal) Mother is a carrier, father is normal.
What is the chance for a MD child?

38. Muscular Dystrophy: Xm
(XM = normal) Mother is a carrier, father is normal.
XM Xm
What is the chance for a MD child? XM Y

39. Muscular Dystrophy: Xm
(XM = normal) Mother is a carrier, father is normal. M
XM Xm
XMXm
XMY
XmY
What is the chance for a MD child? Xm Y

40. Muscular Dystrophy: Xm
(XM = normal) Mother is a carrier, father is normal.
One our of four children…
…but one out of two sons.
XM Xm
XMXm
XMY
XmY XM Y