Karyotypes and Sex-Linked Traits

1. Karyotypes and Sex-Linked Traits

2. Karyotype • Karyotype – a picture of chromosomes • Autosomes: the first 22 homologous pairs of chromosomes. • Autosomes are the same for both males and females. •  Sex chromosomes: determines the sex of the individual. • The sex chromosomes are the 23rd pair of chromosomes. 

3. Male of Female? Female: XX Male: XY

4. Determining Gender • Which parent determines the sex of an offspring – DAD • Why? • All moms have the genotype XX. When egg cells are made, they will all carry a single X chromosome. • All dads have the genotype XY. When sperm cells are made, 50% will have an X chromosome and 50% will have a Y chromosome. • Therefore, males and females are born in roughly a 50:50 ratio

5. Diagnosing Disorders from Karyotypes • Some disorders can be diagnosed by looking at a person’s karyotype • Most are caused by nondisjuction during meiosis • Nondisjunction – failure of chromosomes to separate during meiosis • Disorders: • Down Syndrome (Trisomy 21) – individual has 3 of the 21st chromosome instead of 2 • Turner Syndrome – female has only one X for her sex chromosome • Klinefelter’s Syndrome – male has or more extra X chromosome Autosomal Sex-Linked

6. Sex-Linked Traits • SEX-LINKED TRAITS: those traits that are controlled by genes on the X or Y chromosomes. • Most sex-linked traits are on the X chromosome. • The Y chromosome is much smaller than the X chromosome and only contains a few genes. • Ex: • Hemophelia • Colorblindness

7. Example • In humans, hemophilia is a sex-linked trait.  Having hemophilia is recessive (XH) to being normal (X). The heterozygous female is called a carrier.  Cross a carrier female with a normal male. XHX x XY Genotype Ratio: Phenotype Ratio:

8. Example • Cross a carrier female with a male with hemophilia. XHX x XHY Genotype Ratio: Phenotype Ratio:

9. Example • In humans, red-green colorblindness is a sex-linked trait.  People with red-green colorblindness can not tell the difference between red and green.  Colorblindness is the result of a recessive allele. Cross a female with colorblindness with a male with normal vision. XCXCx XY Genotype Ratio: Phenotype Ratio:

10. Why are sex-linked traits more common in males than in females? • Because a male only has to inherit ONE recessive allele in order to get a sex-linked trait and a female has to inherit TWO recessive alleles in order to acquire the sex-linked trait.It is easier to inherit one recessive allele than two. If the female only inherits one recessive allele, then they are a carrier but have the normal phenotype. • X’s are dominant to Y’s. • If female gets a “bad” from one parent, she could still get a “good” X, and become a carrier (has the trait but isn’t expressed). The only way for the female to express the trait is to inherit two “bad” X’s. • If a male gets a “bad” X, the only other sex chromosome is a Y. The “bad” X is dominant to the Y, therefore, the male will express the trait.