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  • Linkage
    • Alleles of different genes cannot assort independently if they are on the same chromosome
    • During meiosis linked genes are inherited together because they pass into the gamete and offspring together
    • During meiosis at least one chiasma forms between two homologous chromosomes but it may not form between a particular pair of genes
    • The further apart the genes are the more likely it is that crossing over will result in the formations of recombinants
    • Sex-Linked Inheritance
      • Genes that are located on one or the other of the sex chromosomes are 'sex-linked'
      • These genes determine body characters and have nothing to do with sex
      • Sex-linked genes may be on the X chromosome (X-linkage) or on the Y chromosome (Y-linkage)
      • X chromosomes carry many sex-linked genes whereas Y chromosomes carry very few
      • In humans features of Y chromosomes will only arise in males but features of X chromosomes arise in either sex
    • X-Linkage
      • Most sex-linked genes are located on the X chromosome
      • X chromosomes carry genes for characteristics such as the ability to see certain colours and to clot blood properly
      • Recessive alleles express themselves more often in the male phenotype
        • They are unpaired so there is no dominant allele present (Y chromosome)
        • Females have 2 X chromosomes and therefore 2 alleles for the gene
          • Females with a recessive allele on only one X chromosome won't be affected because of the dominant allele on the other X chromosome
            • 'Carriers'
      • Males cannot pass on X-linked characteristics to their sons because they must receive the Y chromosome to become male
        • However all his daughters must receive the recessive gene from him
      • Haemophilia
        • Sex-linked trait in humans
        • Blood doesn't clot normally
          • Causes intensive internal and external bleeding
        • Haemophilia A is a sex-linked characteristic caused by a recessive allele carried on the X chromosome
        • If a male inherits one allele for haemophilia A he has the disease because he doesn't have another dominant allele to mask it
          • A female only inherits the disease with 2 recessive alleles


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