Karyotypes and Sex-Linked Traits

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. 

Male of Female? Female: XX Male: XY

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

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

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

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:   XH X Y

Example Cross a carrier female with a male with hemophilia. XHX x XHY Genotype Ratio: Phenotype Ratio:   XH X Y

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. XCXC x XY Genotype Ratio: Phenotype Ratio:   XC X Y

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.