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.
4 Determining GenderWhich parent determines the sex of an offspring – DADWhy?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 karyotypeMost are caused by nondisjuction during meiosisNondisjunction – failure of chromosomes to separate during meiosisDisorders:Down Syndrome (Trisomy 21) – individual has 3 of the 21st chromosome instead of 2Turner Syndrome – female has only one X for her sex chromosomeKlinefelter’s Syndrome – male has or more extra X chromosomeAutosomalSex-Linked
6 Sex-Linked TraitsSEX-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:HemopheliaColorblindness
7 ExampleIn 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 XYGenotype Ratio:Phenotype Ratio:XHXY
8 Example Cross a carrier female with a male with hemophilia. XHX x XHY Genotype Ratio:Phenotype Ratio:XHXY
9 ExampleIn 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 XYGenotype Ratio:Phenotype Ratio:XCXY
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.
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