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Pedigree analysis
 

Pedigree analysis

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    Pedigree analysis Pedigree analysis Presentation Transcript

    • The Inheritance of Single-Gene Differences Human pedigree analysis
    • Pedigree analysis
      • Analysis of inheritance in human families
      • A very important tool for studying human inherited diseases
        • Allow inferences concerning genotypes in a family or population
        • Allows predictions concerning phenotypes of offspring inheriting a genetic disease (genetic counseling)
      • Typically small number of offspring
        • Mendelian ratios rarely observed
        • this means the normal 3:1 dominant to recessive ratio doesn’t usually occur.
    • Most common symbols used in creating a pedigree unaffected male unaffected female affected male affected female
    • Other common signs and symbols used in pedigree analysis
    • Sample pedigree - cystic fibrosis The roman numerals indicate the generation (I, II or III) and the numbers show birth order of children (1, 2, 3, 4 etc.) female male affected individuals
      • Describe the pedigree on the previous slide for the following questions:
      • What disease or trait is being shown?
      • Is person I-1 male or female?
      • Has person II-3 mated?
      • How many children do I-1 and I-2 have?
      • Which individuals in this family have cystic fibrosis?
      • Compared to the individuals in generation I, are the individuals in generation III: children, parents, grandchildren or siblings?
    • Based on the same pedigree, we can determine a reasonable guess as to what the genotypes of the parents and offspring are:
      • Assuming this is a recessive gene, we know that II-3 and III-4 must be “aa.”
      • Therefore both parents of these offspring must have given an “a” allele.
      • So, I-1, I-2, II-4 and II-5 must each have one “a” in their genotypes.
      • Since I-1, I-2, II-4 and II-5 don’t have the disease, their other allele must be “A.”
      • All the rest of the people could be either AA or Aa.
    • I-1 I-2 II-1 II-2 II-3 II-4 II-5
      • What are the genotypes or possible genotypes for each individual in this family?
      • Use the letter B to represent dominant alleles and b to represent recessive alleles.
    • Categories of inheritance
      • Autosomal means inherited on chromosome 1-22 while sex-linked means inherited on either X or Y chromosome.
      • Autosomal recessive
        • e.g., PKU, Tay-Sachs, albinism
      • Autosomal dominant
        • e.g., Huntington’s Disease
      • X-linked recessive (meaning this allele is found on only the X chromosome: can be in males or females)
        • e.g., color-blindness, hemophilia
      • X-linked dominant (meaning this allele is found on X chromosomes; can be in males or females)
        • e.g., hypophosphatemia
      • Y-linked (meaning the allele is found on the Y chromosome and can only be in males )
    • Autosomal recessive traits
      • Trait is rare in the pedigree
      • Trait often skips generations (hidden in heterozygous carriers)
      • Trait affects males and females equally
      • Possible diseases include: Cystic fibrosis, Sickle cell anemia, Phenylketonuria (PKU), Tay-Sachs disease
    • Autosomal dominant pedigrees
      • Trait is common in the pedigree
      • Trait is found in every generation
      • Affected individuals transmit the trait to about 1/2 of their children (regardless of sex)
    • Autosomal dominant traits
      • There are few autosomal dominant human diseases (why?), but some rare traits have this inheritance pattern
      • For example: achondroplasia (a sketelal disorder causing dwarfism)
    • - Half the people in the Venezuelan village of Barranquitas are affected - A large-scale pedigree analysis was conducted including 10,000 people - Example for one particular family: Huntington’s disease: an example of AD disorder
    • X-linked recessive pedigrees
      • Trait is rare in pedigree
      • Trait skips generations
      • Affected fathers DO NOT pass to their sons
      • Males are more often affected than females
      • Females are carriers (passed from mom to son)
    • Hemophilia: an example of X-linked recessive disorder
    • X-linked recessive traits ex. Hemophilia in European royalty
    • X-linked recessive traits
      • ex. Glucose-6-Phosphate Dehydrogenase deficiency
      • hemolytic disorder causes jaundice in infants and (often fatal) sensitivity to fava beans in adults
      • the most common enzyme disorder worldwide, especially in those of Mediterranean ancestry
      • may give the individual resistance to malaria
    • X-linked dominant pedigrees
      • Trait is common in pedigree
      • Affected fathers pass to ALL of their daughters
      • Males and females are equally likely to be affected
    • X-linked dominant diseases
      • X-linked dominant diseases are extremely unusual
      • Often, they are lethal (before birth) in males and only seen in females
      • ex. incontinentia pigmenti (skin lesions)
      • ex. X-linked rickets (bone lesions)
    • Pedigree Analysis in real life
      • Remember:
        • dominant traits may be rare in a population
        • recessive traits may be common in a population
        • alleles may come into the pedigree from 2 sources
        • mutation happens
        • often traits are more complex
            • affected by environment & other genes