transmission genetics chapters 14-15 in the textbook

Download Transmission Genetics Chapters 14-15 in the textbook

Post on 24-May-2015




1 download

Embed Size (px)


  • 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 thcentury
  • 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 Ior I locus Many variant forms (alleles) exist Subtle variations in blood cell surface properties 9. TransmissionGenetics-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. TransmissionGenetics-ABO blood group genetics

  • What is the genotype for true-breeding A?
  • What is the genotype for true-breeding O?

11. TransmissionGenetics-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. TransmissionGenetics-ABO blood group genetics I A I A I O I O Moms gametes Dads gametes 13. TransmissionGenetics-ABO blood group genetics

  • IAIO individuals are heterozygous
  • What cell surface CHOs 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. TransmissionGenetics-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. TransmissionGenetics-ABO blood group genetics

  • I A, I Band 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. TransmissionGenetics-ABO blood group genetics

  • I A> i, I B> I
  • But I A= I B
  • I Aand I Bare 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 Huntingtons Disease

  • Neurodegenerative disorder
  • Caused by dominant gene at 4p16.3
  • Penetrance is incomplete-95%
  • Penentrance is delayed until late adulthood

21. Transmission Genetics-Huntingtons 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)

View more