Genes and Patterns of Inheritance


History of Genetics

  • Austrian monk Gregor Mendel studied inheritence of characteristics
  • By carrying out a series of breeding investigations
  • Used the garden pea as:
    • can be grown outdoors in large numbers
    • easy to self and cross pollinate
    • had many 'heritable traits' with distinct features
      • Height: short/tall
      • Flower colour: red/white
      • Pea/Seed shape: smooth/wrinkled
  • What is a gene?
    • a short length of DNA which codes for a polypeptide.
  • Homologous chromosomes contain the same genes in the same loci, but not necessarily the same allele
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  • Genetics: study of inherited characteristics
  • Gene: a short section of DNA which codes for the production of a polypeptide
  • Allele: an alternative form of a gene
  • Dominant: the allele is always expressed if present
  • Recessive: trait will only show in phenotype if there are 2 recessive alleles
  • Homozygous: the two alleles of a gene are the same
  • Heterozygous: the two alleles of a gene are different
  • Genotype: an individual's genetic makeup
  • Phenotype: an individual's outward physical appearance
  • Monohybrid cross: a cross which considers the inheritance of only one pair of alleles at one gene locus/ one characteristic
  • Gene Locus: the position on a chromosome where a gene is located
  • Fand F2: the first and second filial generations
  • Test cross: method of determining an unknown genotype by crossing the unknown individual with a known homozygous recessive
  • Multiple Alleles: 3 or more possible alleles at one locus are known
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Glossary (Continued)

  • Complete Dominance: the heterozygote has the same phenotype as the homozygous dominant
  • Co-dominance: the 2 alleles at a particular gene locus are different, but neither has a masking effect on the other. The phenotype of the heterozygote is different to that of both homozygotes eg. flower colour pink
  • Dihybrid cross: a cross where the inheritance of alleles at two gene loci are considered
  • Autosomes: c'somes not involved in sex determination
  • Sex Linkage: a characteristic whose inheritance is linked with the inheritance of sex as the gene controlling it is located on a sex c'some eg. red-green colour blinkness and haemophilia in males only
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