7.5

• Pedigree


  A diagram of family relationships that uses symbols to represent genetic relationships
• Pedigree


  • Makes it easier to visualise relationships within families, particularly large extended families
• Mendel


  Re-visited
• Monohybrid crosses


  Only one characteristic was examined at a time
• Dihybrid crosses


  Two characteristics were examined at a time
• Mendel's cross


  Round seeds were dominant to wrinkled seeds<|>Yellow seeds were dominant to green seeds
• Mendel's cross


  1. Crossed two pure bred plants

  2. Round - yellow seed x wrinkled green seed
• Dihybrid ratio


  The ratio 9:3:3:1
• Looking at each trait separately, Mendel's ratio holds true (dominant : recessive, 3:1)
• Mendel's second law - the law of independent assortment


  Each pair of factors can combine with either of another pair of factors
• Linked genes


  Two genes that are close on the same chromosome
• Linked genes do not follow Mendel's law
• First cross - purebred parents

  1. Phenotype: Purple-long x red-round

  2. Genotype: FFLL x ffll

  3. Gametes: FL x fl
• The result is no different to unlinked genes
• Actual cross


  1. Purple flowers, long pollen: 68

  2. Red flowers, short pollen: 18

  3. Purple flowers, short pollen: 7

  4. Red flowers, long pollen: 7
• Crossing over


  Produces new gametes called recombinant gametes, resulting in recombinants
• Crossover frequency


  The percentage of offspring that are recombinants, indicating the distance between genes on the chromosome
• Pedigrees


  • Used to determine the mode of inheritance (dominant, recessive, etc.) of genetic traits such as a disease or characteristic
• How to draw pedigrees

  1. Squares represent males, circles represent females

  2. Horizontal lines connecting a male and female represent mating

  3. Vertical lines extending downward from a couple represent their children

  4. An upside-down 'v' represents twins

  5. Use roman numerals to indicate the generation number, individuals can also be numbered
• Sex-linked inheritance


  A gene located on the X or Y chromosome
• 1. linked recessive inheritance


  Females are carriers, males are affected
• 1. linked dominant inheritance


  The X-linked disorder shows a dominant phenotype
• 1. linked inheritance


  The trait is located on the Y-chromosome, only affects males
• When to use pedigrees

  • Experimental 'crosses' cannot be set up
  • The environment in which humans live cannot be controlled experimentally
  • There are strict legal and ethical laws concerning human experiment
  • Humans tend not to have large numbers of offspring to score
  • Each generation of humans take many years to reach sexual maturity and produce offspring
• Autosomal recessive inheritance


  Likely if neither parent has the phenotype, but one of their offspring does
• Autosomal dominant inheritance


  Likely if both parents display the phenotype, but their offspring don't
• Determining if a trait is inherited autosomally or sex-linked


  Difference in incidence of a trait between males and females<|>Frequency of the trait across generations
• 1. linked recessive inheritance


  Males are affected more than females because they only inherit 1 X chromosome
• 1. linked dominant inheritance


  Affect more females than males because females inherit 2 X chromosomes
• 1. linked inheritance


  Only males are affected, all male offspring are affected, the trait is observed in every generation in which males are born
• 1. linked recessive traits mean that affected mothers must have affected sons
• In X-linked dominant inheritance every daughter of an affected male must also be affected
• If a trait is Y-linked then only males are affected and all affected fathers pass the trait on to their sons