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Sex-linked and Polygenic Inheritance

Sex-linked and Polygenic Inheritance. Sex Chromosomes. Female sex chromosomes XX. Male sex chromosomes XY. Only one pair of these (chromosome pair 23. The X chromosome is much larger than the Y chromosome and can carry more genes. These genes are said to be SEX LINKED.

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Sex-linked and Polygenic Inheritance

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  1. Sex-linked and Polygenic Inheritance

  2. Sex Chromosomes • Female sex chromosomes XX. • Male sex chromosomes XY. • Only one pair of these (chromosome pair 23. • The X chromosome is much larger than the Y chromosome and can carry more genes. These genes are said to be SEX LINKED.

  3. If the X and Y chromosome meet during fertilisation the genes on the X chromosome are always expressed as the smaller Y chromosome does not possess the alleles so cannot be dominant over them. • The alleles of the sex-linked genes are represented by superscripts. • XCXc or XcY

  4. Colour Blindness Carrier mother XCXc Normal Father XCY Normal daughter XCXC Carrier daughter XCXc Normal son XCY Colour-blind son XcY

  5. Haemophilia • This is a disease caused by a recessive allele carried on the X chromosome. • The blood clotting allele is defective and the blood takes a long time to clot or even fails to do so. • XH for normal blood clotting agent. • Xh for haemophilia. • Woman can be carriers.

  6. Carrier mother XHXh Haemophiliac father XhY Carrier daughter XHXh Haemophiliac daughter XhXh Normal son XHY Haemophiliac son XhY

  7. Discontinuous Variation • These differences divide the members of a species into distinct groups e.g. eye colour, ability to roll tongue, fingerprint etc… • Information is usually presented as a bar chart.

  8. Continuous Variation • The differences vary amongst the members of the species in a smooth continuous way e.g. height, hand span, foot size etc… • Information is usually presented as a histogram. • See Torrance pg 89

  9. Polygenic Inheritance • Characteristic showing continuous variation that is controlled by the alleles of more than one gene e.g. height, weight.

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