• Created by: Anon
  • Created on: 13-03-15 16:08
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  • Inheritance
    • Sex linkage
      • In humans gender is determined by chromosomes rather than genes
      • The X chromosome is much longer than the Y chromosome, which means that for most genes that appear on the X chromosome, there is no homologous pair
      • The recessive characteristics on the X chromosome with therefore appear in males more frequently than females
        • Because they only need a single recessive allele to express the characteristic
      • Any gene that appears on the X or Y chromosome is described as sex-linked
      • An example of a sex-linked disease is haemophilia
    • Co-dominance
      • Co-dominance occurs when both alleles are equally dominant
      • Both alleles are therefore expressed in the phenotype
      • When representing co-dominance we cannot use upper and lower case letters, as this implies one is dominant over the other, instead we use different letters
      • An example is the snapdragon plant, both red and white flowers are equally dominant
        • heterozygous plants will be pink
    • Multiple alleles
      • Multiple alleles occur when there are more than two alleles for the same gene
      • An example is human blood groups
        • Each blood type codes for an antigen
        • IA codes for antigen A
        • IB codes for antigen B
        • IO does not code for any antigen
        • A person can only have 2 of the 3 alleles
        • IA and IB are co-dominant and IO is recessive to both
    • Hardy-Weinburg
      • The Hardy-Weinburg principle predicts that the proportion of dominant and recessive alleles will be the same from one generation to the next, provided that 5 conditions are satisfied
        • 1. No mutations arise
        • 2. The population is isolated so there is no flow of alleles
        • 3. There is no selection of alleles
        • 4. The population is large
        • 5. Mating within the population is random
      • The equation is: p squared + q squared + 2pq = 1.0
        • p is the number of dominant alleles
        • q is the number of recessive alleles


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