Mendelian Genetics AP Biology Unit 3

Mendel’s Experiments Crossbred Pea Plants P, F1, F2 generations

Alleles Alleles are different (alternate) forms of a gene How do alleles relate to homologous chromosomes? –Homologous chromosomes may contain different alleles

Yeast Complementation & Alleles How did alleles play a role in the yeast complementation experiment? –Strains of yeast have different alleles –When mated, the a normal allele could complement a mutated allele –HA1 = mutated ade 1 allele, normal trp allele –HBT = normal ade1 allele, mutated trp allele

Dominant vs. Recessive Alleles Dominant Allele –Determine phenotype if there is a copy present Recessive Allele –Need 2 copies to determine phenotype

Law of Segregation “Two alleles for a trait separate during gamete formation” –Gametes can have different alleles

Allow you to predict possible genotypes of offspring if parental genotypes are known Phenotypes can be deduced from the genotypes Punnett Squares

Phenotype Ratios Characteristic possible phenotype ratios always occur 2:2 (1:1) 3:1 4:0 (1:0)

Test Cross Suppose you have an organism with a dominant phenotype You want to determine its genotype– PP or Pp? Cross with a recessive phenotype to determine genotype = test cross

Law of Independent Assortment “Pairs of alleles for a given trait segregate independently of other pairs of alleles for another trait” –Ex. YyRr can result in YR, Yr, yR, or yr This only applies to two traits on different chromosomes (NOT LINKED)

Blood Type Alleles Codominance = both I A and I B alleles are dominant Multiple alleles (more than 2 alleles)

Practice Problem #1 The Huntington’s disease allele is dominant (represented as “H”). A husband’s genotype is Hh; a wife’s genotype is hh a)Which of them will develop Huntington’s disease b)What are the possible genotypes for their children? What are the chances their child will have Huntington’s Disease?

Answer The husband will develop Huntington’s Disease Children could be Hh (50% chance) or hh (50% chance) –So there is a 50% chance their child will have Huntington’s disease.