1. Genetics: Karyotypes and X-linked traits
March 2012

2. Section 12.2 Summary – pages 315 - 322
Sex determination
If you are female, your 23rd pair of chromosomes are homologous, XX. X X
Female
If you are male, your 23rd pair of chromosomes XY, look different. X Y
Male
Section 12.2 Summary – pages

3. Section 12.2 Summary – pages 315 - 322
XY Male
Sex determination X Y X
XX Female
XY Male
XX Female X
XX Female
XY Male
Section 12.2 Summary – pages

4. What is a karyotype?
Karyotypes are a chart of chromosomes pairs in metaphase. They are arranged according to length and location of the centromere.

5. What are karyotypes used for?
They are used to determine the gender of an offspring and also can be used to pinpoint unusual chromosome numbers in cells.

6. What do they look like?
Normal Female
Normal Male

7. Remember . . . Males have an X and Y at chromosome pair 23.
Females have two X’s.

8. Abnormalities - Trisomy

9. Abnormalities: Why do they happen?
Nondisjunction!
Nondisjunction is when sister chromosomes fail to separate properly during meiosis; this results in too many or too few chromosomes in the egg and sperm.

10. Section 12.2 Summary – pages 315 - 322
Sex-linked inheritance
Traits controlled by genes located on sex chromosomes are called sex-linked traits.
The alleles for sex-linked traits are written as superscripts of the X or Y chromosomes.
Because the X and Y chromosomes are not homologous, the Y chromosome has no corresponding allele to one on the X chromosome and no superscript is used.
Section 12.2 Summary – pages

11. Example: Male-pattern baldness

12. Genotype and Phenotype Ratios