1. DO NOW WRITE THE DEFINTION FOR EACH OF THE FOLLOWING USE YOUR NOTES!!!
Allele
Dominant and Recessive
Homozygous and Heterozygous
Incomplete Dominance
Co-Dominance

2. Punnett Squares
for
Sex-linked Traits
and Genetic Diseases

3. 22 pairs of autosomal chromosomes
What is an autosomal chromosome?
A chromosome that is not involved in determining a person’s sex (gender)

4. 22 pairs of autosomal chromosomes

5. 1 pair of sex chromosomes
What do you notice about the y chromosome?

6. The “Basic” Punnett Square for Sex-Linked Chromosomes
Boy or Girl?
The “Basic” Punnett Square for Sex-Linked Chromosomes
The Basic Key
XX = Female
XY = Male

7. Now you do the Punnett Square for Sex…
What is the probability that the couple will have a girl?
What is the probability that the couple will have a boy?

8. The Punnett Square for Sex
Mom
50% chance for a girl (XX)
50% chance for a boy (XY) X X X XX XX D a d XY XY Y

9. Does Mom or Dad determine the sex of the baby?
WHY?

10. Color-Blindness is Sex-Linked
Are you color-blind?

11. A woman can be a carrier of the gene for color-blindness.
A carrier is a heterozygous individual. They have one copy of the allele for a disease, but they don’t have the disease because the good copy of the allele produces enough of the good protein.
What is a carrier?

12. Key for Color Blindness
Normal vision is dominant (XB) over color-blindness (Xb). A woman can be a carrier of the gene for color-blindness.
XBXB = female, normal vision
XBXb = female, normal vision, carrier
XbXb = female, color-blind
XBY = male, normal vision
XbY = male, color blind

13. But Wait!
Why isn’t there a superscript on the Y?

14. Why isn’t there a superscript on the Y?
In most case, the y chromosome doesn’t carry genetic info
This is why many diseases are more common among boys
The disease may be recessive, but if a boy inherits one “diseased” allele from mom, he will have the disease

15. Color-Blindness is Sex-Linked
Normal vision is dominant (XB) over color-blindness (Xb). A woman is a carrier of the gene for color-blindness. Her husband has normal vision.
Draw a Punnet square to determine what percentage of her sons will be color-blind.

16. ½ or 50% of her sons will be color-blind
Solve our problem….
Normal vision is dominant (XB) over color-blindness (Xb). A woman is a carrier of the gene for color-blindness. Her husband has normal vision.
Mom
Answer
½ or 50% of her sons will be color-blind XB XB Xb XB
XBXB
XBXb D a d Y
XBY
XbY