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Human genetics follows the same rules as general genetics, but studying humans is more complex due to factors such as late sexual maturity and limited offspring. Notably, X-linked genes are expressed differently in males and females, which is crucial in conditions like hemophilia—a recessive disorder on the X chromosome. Males inherit hemophilia from their mothers, while females have varying chances of passing it to their children. For instance, a heterozygous female marrying a non-hemophiliac male shows through a Punnett square that none of the daughters will have hemophilia, while half of the sons could inherit the disorder.
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HUMAN GENETICS All the rules for genetics also apply to human beings. However, humans are harder to study for genetics • Humans are not sexually mature for 12-18 years • Humans only have 1-3 offspring at a time, and most humans only have 1-5 offspring • At any one time, only 2 or 3 generations are alive.
Special circumstances • X linked genes Most genes are found on the autosomes (chromosomes 1-22). Some genes are found on the X chromosome. In these cases, they are expressed differently in men than in women.
HEMOPHILIA Hemophilia is a disease where a person is unable to stop bleeding from a cut. It is a recessive gene on the X chromosome. Males inherit hemophilia differently from females.
MALES XH Y this is a male without hemophilia (dominant) Xh Y this is a male with hemophilia (recessive) A man inherits hemophilia from his mother (why?)
FEMALES XHXH this is a female without hemophilia. She cannot pass hemophilia to her children. XHXh this is a female without hemophilia. She can pass hemophilia to her children. XhXh this is a female with hemophilia.
Assume a woman heterozygous for hemophilia marries a man without hemophilia? Write parental genotypes: woman: XHXh man: XHY
PUNNETT SQUARE XH Xh XHXH XH XHXh XHY Y XhY
None of the daughters have hemophilia. Half of the sons have hemophilia.
