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Human genetics

Human genetics. How to determine inheritance of a trait in humans Can’t (shouldn’t) mandate breeding partners Low numbers of offspring. Pedigrees Follow inheritance of trait in families Compare results to other families Draw conclusions. Key to pedigrees. Pedigree sample-1.

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Human genetics

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  1. Human genetics • How to determine inheritance of a trait in humans • Can’t (shouldn’t) mandate breeding partners • Low numbers of offspring. • Pedigrees • Follow inheritance of trait in families • Compare results to other families • Draw conclusions.

  2. Key to pedigrees

  3. Pedigree sample-1 • Look at inheritance of trait expressed by shaded individual. • You KNOW that it can’t be dominant because at least 1 of the parents would also have to show that phenotype. *Look for things you know must be true.

  4. Pedigree sample-2 • Beware of things that seem logical but might NOT be true. • The Shaded trait is dominant. • “A” dominant, “a” recessive • The mother must be aa. • The father, however, may or may not be homozygous: • If the father is AA, you would expect all offspring to be Aa (AA x aa = Aa); this is what appears to be true.

  5. continued BUT, if the father is Aa, the odds for each child showing the dominant phenotype is 50:50. Just like you can flip a coin 3 times and get heads each time, you could get 3 children that are all Aa, showing the dominant phenotype. The father COULD be Aa. Likely? No. Possible? Definitely.

  6. Pedigree problem A and a are alleles. Which is shaded? What are the genotypes? Find the sure things first. II 6 must have a recessive trait, being unlike both parents (who must be heterozygous).

  7. Genetic Notation -eukaryotes • Dominant: upper case; recessive: lower case. • From Plant studies • Based on dominant/recessive relationships • Letter describing trait: P p for Purple, white alleles • http://faculty.users.cnu.edu/rcheney/Genetic%20Notation.htm

  8. Genetic Notation –eukaryotes-2 • From animal studies; based on “wild type” concept • Wild type is most common allele, indicated by “+” • Example: e+/ e where e+ is wild type, slash separates alleles from homologs • Example: Wr+/ Wr shows mutant phenotype because Wr is a dominant mutant allele • Multiple alleles: R1 & R2; IA & IB; • Bacterial notation different

  9. Mutation and phenotype • Mutations are the source of new alleles • A new allele may result in a new phenotype because of changes in enzyme activity • Enzyme usually has decreased or no activity • Enzyme may have increased activity • usually, change in a regulatory gene • Enzyme may be unaltered despite change in DNA • Allele only at DNA level, no other phenotype

  10. Alterations to Mendel • Incomplete or partial dominance • Codominance • Multiple alleles • Lethal alleles • Gene interactions • Sex-linked, sex-limited, & sex-influenced • Effect of environment • Extranuclear inheritance

  11. Multiple genes • Sometimes a phenotype is controlled by more than one gene • Different from multiple alleles of same gene!! • Gene products don’t necessarily directly interact. • Genes may code for enzymes in a pathway • Cascade of gene during development • Epistasis: a gene (or gene pair) masks or modifies the expression of another gene (or gene pair).

  12. Epistasis: gene interaction that perturbs normal Mendelian ratios • Example: interaction between two genes in the pathway for pigment production, C and P • C, P = dominant; c, p = recessive. • Because both genes are needed, if individual is homozygous recessive for either gene, no color. • CcPp, all colored in F1 • CcPp x CcPp 9:7 phenotypic ratio • Do the Punnett square and see • Independent assortment still applies • Various interactions produce different ratios.

  13. Epistasis example: Bombay phenotype In the ABO blood groups, A and B are codominant. How can the person in this pedigree be type O (IO IO)?

  14. Epistasis example: explanation A second gene, Ih codes for the base sugar chain to which the A and B sugars are added. A rare mutation Ih Ih prevents proper formation so that the A and B sugars cannot be added even though the enzyme for doing that is being made. (Diagram next slide) A second gene is masking the normal phenotype.

  15. Molecular explanation

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