Transmission Genetics Chapters 14-15 in the textbook

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  • 1. Transmission Genetics
    • Chapters 14-15 in the textbook
    • Focus on Section 14.4-human traits
  • 2. Disambiguation
    • Historically, heredity regarded as a blending process of parental traits
    • Heredity first studied scientifically in 1860s
    • Experimental organism-garden pea
    • Blending does not really occur
    • Fruit fly studies extend knowledge
  • 3. Transmission Genetics-some definitions
    • Genotype-genetic composition of an organism for trait under study
    • Phenotype-appearance of organism for trait under study
    • Allele-variant form of a gene
  • 4. Transmission Genetics-some definitions
    • Homozygous-two copies of same allele
    • Heterozygous-different alleles
    • Dominant-allele that is expressed at the phenotypic level being examined
    • Recessive-allele that is not expressed at the phenotypic level being examined
  • 5. Transmission Genetics- a medical example
    • Blood transfusion-a haphazard process
    • Many fatalities in early trials
    • Systematic basis in early 20 th century
    • Landsteiner and others
    • Genetic basis understood by 1920s
    • Case studies and family histories
  • 6. Transmission Genetics-ABO blood groups
    • ABO blood groups based on cell surface glycoproteins
    • Type A blood has Type A sugars
    • Type O has neither A nor B
  • 7. Transmission Genetics-ABO blood group biochemistry
    • CHO chain named H substance attached to lipid named sphingosine
    • Blood group A-GalNAc attached to H
    • Blood group B-Gal attached to H
    • Enzyme is ABO blood group transferase
    • ER to plasma membrane
  • 8. Transmission Genetics-ABO blood group genomics Genomic context chromosome : 9; Location: 9q34.1-q34.2 One gene, or “locus”, aka “I” or “I locus” Many variant forms (alleles) exist Subtle variations in blood cell surface properties
  • 9. Transmission Genetics-ABO blood group genetics
    • With this background-a hypothetical case
    • Population of true-breeding A individuals
    • (type A phenotype)
    • Second population of true-breeding O individuals (type O phenotype)
    • Genetic outcomes if A marries O?
  • 10. Transmission Genetics-ABO blood group genetics
    • What is the genotype for true-breeding A?
    • What is the genotype for true-breeding O?
  • 11. Transmission Genetics-ABO blood group genetics
    • Reproduction of A x O involves meiosis and fertilization (sexual reproduction)
    • What are the gamete genotypes for true-breeding A?
    • What are the gamete genotypes for true-breeding O?
  • 12. Transmission Genetics-ABO blood group genetics I A I A I O I O Mom’s gametes Dad’s gametes
  • 13. Transmission Genetics-ABO blood group genetics
    • IA IO individuals are heterozygous
    • What cell surface CHO’s are present?
    • What is the phenotype of an IA IO heterozygote?
    • Which allele is the dominant allele?
    • Recessive allele-lower case, therefore IO is “i”
    • Heterozygote is I A i
    • What happens when I A i x I A i?
  • 14. Transmission Genetics-ABO blood group genetics
    • Types AB and O always correlate with one genotype
    • Types A and B may be homozygous or heterozygous
    • Antibodies made against non-self
  • 15. Transmission Genetics-ABO blood group genetics
    • I A , I B and i are three alleles of the same gene
    • A “multiple allele” system
    • Very common in humans due to the many variant forms that exist for all genes
    • CFTR gene-exists in over 1000 forms
    • In homozygous state causes cystic fibrosis but symptoms may vary depending upon alleles inherited
  • 16. Transmission Genetics-ABO blood group genetics
    • I A > i, I B > I
    • But I A = I B
    • I A and I B are codominant
    • How many genotypes/phenotypes can exist with a multiple allele series with 3 alleles such as above?
  • 17. Transmission Genetics-quantitative inheritance
    • Phenotypic trait controlled by more than one gene
    • Each gene may have many alleles
    • Dominance relationships may be complex
    • This is the usual situation for most human traits
    • Human hair color controlled by at least two gene pairs: brown > blond and not-red > red
    • “ quantitative inheritance” or “polygenic inheritance”
  • 18. Transmission Genetics-quantitative inheritance
    • Model for skin color inheritance with three alleles
    • Bell curve or normal distribution
  • 19. Transmission Genetics-penetrance
    • The proportion or % of individuals carrying a gene who show expression of the gene
    • For unknown reasons-genes may be present but not expressed at the phenotypic level.
  • 20. Transmission Genetics-penetrance in Huntington’s Disease
    • Neurodegenerative disorder
    • Caused by dominant gene at 4p16.3
    • Penetrance is incomplete-95%
    • Penentrance is delayed until late adulthood
  • 21. Transmission Genetics-Huntington’s Disease
  • 22. Transmission Genetics-Tay-Sachs
    • Lysosomal storage disorder due to hexoseaminidase A deficiency
    • Ganglioside lipid accumulates
    • Disease gene frequent (3%?) in Ashkenazi populations
    • Penetrance delayed usually
  • 23. Transmission Genetics-Tay-Sachs
    • Recessive gene “HEXA”
    • Cytogenetic Location: 15q23-q24, base pairs 70,422,832 to 70,455,392
    • 25% chance of disease in children of carriers
    • Why is the gene present at high frequencies in some populations?
    • Heterozygote advantage? (selection)
    • or
    • Accidental?
    • (founder effect)