1. Sex Linkage

2. X –Linked Traits Definitions: Autosomes - Chromosomes #1 – 22
Sex chromosomes - #23
In males, the 23rd pair of chromosomes is NOT homologous
Females have a homologous 23rd pair.
Traits located on the 23rd pair are sex-linked or X-linked.
Males: XY Females: XX

3. X –Linked Traits
Genes for sex-linked (X-linked) traits are carried on the X chromosome but not on the Y chromosome
Therefore in a male, the gene on the X chromosome is expressed whether it is dominant or recessive
In a female, she must have two recessive alleles to have the recessive phenotype

4. Thomas H. Morgan: Gender and Inheritance
Breakthrough work done by Thomas Hunt Morgan – American Geneticist
1908 – Work on fruit flies (Drosophila melanogaster) provided deeper understanding of heredity based on gender

5. Drosophila melanogaster
Ideal for genetic experiments for several reasons:
Rapid life cycle (10-15 days)
Hundreds of eggs at one time
Crossing experiments can be repeated many times
Offspring mature quickly
Males/females different
Small
Markedly contrasting traits
Only 8 chromosomes (4 pairs)

7. Female
Male
How are they
different??

8. Male or Female? Male (XY)
Non
Homologous
Chromosomes

9. Chromosomes in Fruit Flies
Morgan found the following:
Females  4 homologous pairs of chromosomes  3 autosomal pairs, one sex pair (XX)
Males  3 homologous pairs:
3 autosomal pairs
Sex chromosomes were not homologous
one X and one Y (XY)

10. Solving X-Linked Problems: Eye Color In Drosophila
Let R = red, r = white
Assume these alleles are found on the X chromosome only (they are sex-linked)
Cross 1. Cross a red eyed homozygous female with a white eyed male. r R R
Female:
Male:
X Y
X X
Now, put these gametes into a Punnett Square...

11. ALL RED-EYED OFFSPRING
First Cross XR XR
XR Xr
XR Xr Xr
XRY
XRY Y
ALL RED-EYED OFFSPRING

12. Second Generation (F1 Cross)XR Xr
3 RED OFFSPRING
- 1 WHITE MALE
XR XR
XR Xr XR
What is the probability of these flies having a red-eyed daughter?
XRY
XrY Y
0.50 or 50 % or ½

13. Solving X-Linked Problems: Eye Color In Drosophila
Let R = red, r = white
Assume these alleles are found on the X chromosome only (they are sex-linked)
Cross 2. Cross a red eyed heterozygous female with a white eyed male. r R r
Female:
Male:
X Y
X X
Now, put these gametes into a Punnett Square...

14. 1 White female, 1 White male
Cross 2 XR Xr
What is the probability of these flies having a red-eyed son?
XR Xr
Xr Xr Xr
0.25 or 25 % or ¼
XRY
XrY Y
1 Red female, 1 Red male,
1 White female, 1 White male

15. Morgan’s Conclusions Traits controlled by genes carried
Genes controlling eye color must be on X Chromosome
Traits coded for by genes on sex chromosomes are called SEX-LINKED TRAITS
Y does not carry gene for eye color
Males cannot be heterozygous for sex linked traits
Female
Male
XR XR
XR Xr
Xr Xr
Xr Y
XRY
Traits controlled
by genes carried
on Y chromosome
only affect males

16. Examples of Sex-Linkage in Humans
Color Blindness
genes for visual pigments responsible for perception of red and green are found on the X chromosome
Hemophilia A
caused by the lack of a blood protein, called Factor VIII, that is critical for blood clotting
Gene for Factor VIII on X Chromosome
Both known as X-linked recessive

17. Color Blindness Caused by gene carried on X only
Female Genotypes
XCXC
XCXc
XcXc
3 possible genotypes
2 phenotypes
Male Genotypes
XCY
XcY
only 2 possible phenotypes
2 possible genotypes

18. Link to Ishihara Test for Color Blindness

19. Other Gender Related Inherited Traits
Sex Limited
Eg. Milk production in cows is controlled by an autosomal gene
Bulls carry the genes...but they do not make milk!!!!!!
The gene is not expressed for bulls (because they have low levels of female hormones which switch on this gene)
Sex Influenced
Eg. Male only needs one allele for baldness – male hormones stimulate the expression of this allele
females need two recessive alleles to be bald
Gene is dominant in males and recessive in females
more common in males