7.4

• Pedigree


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


  • Make it easier to visualise relationships within families, particularly large extended families
  • Are often 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 and 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 so that they can easily be referred to i.e., II 2
• Shading in pedigrees


  Shade the individuals that are affected by the trait that is being studied<|>If an individual is a carrier of the trait (i.e., is heterozygous) the the circle/square is only half shaded
• Sex-linked inheritance


  A gene is said to be sex-linked when it is located on the X or Y chromosome
• 1. linked recessive inheritance


  • Female carriers are heterozygous, carrying one haemophiliac allele and one normal allele
  • Carrier females are phenotypically normal
  • Females will produce eggs that contain the haemophiliac or normal alleles with equal frequency, so sons of carrier females have 50% chance of having haemophilia
  • X-linked recessive disorders are usually found at a much higher frequencies in males than females as females need to inherit a copy of the mutant gene from both parents
• 1. linked dominant inheritance


  • The x-linked disorder shows a dominant phenotype
  • Example: vitamin D resistant rickets
• 1. linked inheritance


  • Will only affect males
  • The Y-chromosome is smaller than the X-chromosome and contains less genes, mostly related to male sex determination and fertility, so very few disorders are found on the Y-chromosome
• When pedigrees are used

  To follow inheritance of traits through a family over several generations<|>To determine the type of inheritance pattern<|>When experimental 'crosses' cannot be set up<|>When the environment in which humans live cannot be controlled experimentally<|>Due to strict legal and ethical laws concerning human experiment<|>Humans tend not to have large numbers of offspring
• Autosomal recessive inheritance


  • Likely if neither parent has the phenotype, but one of their offspring does
  • Can 'skip' generation but can still appear in every generation
• Autosomal dominant inheritance


  • Likely if both parents display the phenotype, but their offspring don't
  • May also be seen in all generations and individuals with the trait will also have at least one parent with the trait
• Determining if a trait is inherited autosomally or sex-linked


  1. Look at the difference in incidence of the trait between males and females

  2. Look at the frequency of the trait across generations
• 1. linked recessive inheritance


  • Males are affected more than females because they only inherit 1 X chromosome
  • The second X chromosome in females can mask the trait, so they require both X chromosomes to be affected to express the trait
• 1. linked dominant inheritance


  • These traits are rare
  • Affect more females than males because females inherit 2 X chromosomes and therefore have twice the chance of inheriting the trait
  • The daughters of affected males will always be affected, however, their sons won't be
  • The daughters affected will have a 50% chance of inheriting the trait
• 1. linked inheritance


  • Relatively rare
  • Only males are affected
  • All male offspring are affected
  • The trait is observed in every generation in which males are born
• Ruling out sex-linked inheritance

  1. linked recessive traits mean that affected mothers must have affected sons
• Ruling out X-linked dominant inheritance

  In X-linked dominant inheritance every daughter of an affected male must also be affected
• Ruling out Y-linked inheritance

  If a trait is Y-linked then only males are affected and all affected fathers pass the trait on to their sons