1. SEX-LINKED TRAITS

2. SEX-LINKED TRAITS
Male and female genotypes…
XY XX

3. SEX-LINKED TRAITS
How is sex determined?
X
Ya
In this case,
Y
female child will
result
XX X
Y
X

4. SEX-LINKED TRAITS
How is sex determined?
X
Ya
In this case,
Y
male child will
result
XY X
Y
X

5. SEX-LINKED TRAITS
The X-chromosome may contain alleles for the following:
-Hemophilia
-Colour blindness
-Baldness
-Duchenne muscular dystrophy
These X-linked recessive “disorders” predominantly
affect males.

6. SEX-LINKED TRAITS
Females have two X chromosomes, so the X-linked
recessive diseases will not affect them.
XX XX H
XX C
XX B
Phenotype:
Normal Normal Normal Normal

7. SEX-LINKED TRAITS
However, males only have one X-chromosome…
XY X HY XC Y X BY

8. SEX-LINKED TRAITS
However, males only have one X-chromosome…
XY X HY XC Y X BY
Phenotype:
Normal Hemophiliac Colour blind Bald

9. SEX-LINKED TRAITS
So how is baldness inherited?
XXB XY
X Y
X XX XY
X B XXB XBY
F1 Genotypes:
25% XX, 25% XXB, 25% XY, 25% XBY
F1 Phenotypes:
50% of males are bald. Females are “normal” (wild type)

10. SEX-LINKED TRAITS
The female with the genotype XXB will
not become bald, BUT she is a carrier
She can pass
the baldness
allele to her XXB XXB
offspring

11. SEX-LINKED TRAITS
It is possible for women to be bald, too!

12. SEX-LINKED TRAITS
How is this possible?!
If the father is bald, and the mother is a carrier, then half of
her children will become bald
XXB XBY XB Y
X XXB XY
X B XBXB XB Y

13. SEX-LINKED TRAITS
How is this possible?!
If the father is bald, and the mother is a carrier, then half of
her children will become bald
It is very rare
for a women to be
bald. Why did I
It’s not fair! have to be one of me
Don’t get
them?!!
started…
XXB XB XB XY XBY
F1 Genotypes:
25% XXB, 25% XBXB, 25% XY, 25% XBY
F1 Phenotypes:
50% of male and female offspring are bald

14. SEX-LINKED TRAITS
So how is hemophilia inherited?
In the same way as baldness, hemophilia is inherited usually
because the mother is a carrier. Once again, usually male
children are affected
X Y
X XX XY
XX H XY X H XXH XH Y

15. SEX-LINKED TRAITS
Sample question:
If a hemophiliac man has children with a “normal” woman (who is NOT a carrier),
a) What is the probability of having kids that will develop hemophilia
b) What is the probability of female kids being carriers?
XH Y
X XXH XY
a) _______________________
0% chance of kids having X XXH XY
hemophilia
_________________________
100% of females will be carriers
b) ______________________________________________________