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.
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.
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.
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.
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).
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
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
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
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
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).
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.
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)?
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.