Mendelian Genetics Gregor Mendel is the father of modern genetics 1. Austrian monk who did genetic experiments with pea plants A. Why peas? 1. Peas are available in many varieties - purple and white flowers - round and wrinkled seeds - short and long stems - these characters are called traits 2. Peas can self-fertilize - each flower contains both male and female gametes 3. One pollination produces many seeds B. Mendel began with plants that were true-breeding 1. All offspring of the plants that self-fertilize had the same traits as the parents 2. These parents are called the P generation 3. The offspring of the P generation are called the F1 generation (F = filial) 4. When F1 plants are allowed to self-fertilize, they give rise to the F2 generation 5. Pea plants can also be cross-pollinated C. Mendel’s experiments revealed 2 fundamental laws of heredity 1. The law of segregation 2. The law of independent assortment D. What Mendel found when he crossed a plant with purple flowers with a plant with white flowers E. The law of segregation (with modern knowledge) 1. The ratios in the F2 generation suggested to Mendel that there were 2 alternative versions of each trait 2. Each trait was the consequence of a single gene 3. The different versions of a gene are called alleles - each allele has a locus on a chromosome - the purple flower allele, P, is dominant over the white flower allele, p - p is recessive to P - each individual has two alleles for a gene because each individual has two homologues for each chromosome 4. Each individual inherits one allele from each parent - that is, they inherit one homologue of each pair of chromosomes from the sperm and one homologue of each pair of chromosomes from the egg F. More vocabulary 1. Homozygous: - organism has 2 identical alleles for a particular trait 2. Heterozygous: - organism has 2 different alleles for a trait 3. Dominant: - this allele masks the trait which could be conferred by the other allele 4.Recessive: - this allele’s trait is masked by the other allele 5. An organism’s appearance is its phenotype 6. An organism’s genetic makeup is its genotype 7. Sometimes there is incomplete dominance - cross a red and a white flowered plant and get an F1 with pink flowers 8. Wild-type allele: - standard or normal allele 9. Mutant allele: - codes for traits that are infrequent in a population of organisms - usually recessive G. Why do genes have different alleles? 1. Genes are composed of DNA - DNA is subject to mutations 2. These mutations happen quite often 3. If a mutation produces a more “fit” individual or if the mutation has no harmful effects - the mutant allele can get “fixed” in the population 4. Over evolutionary time, this has led to numerous alleles for nearly every locus H. Test crosses 1. How to determine if an organism is homozygous or heterozygous at a particular locus 2. How do we know if a pea plant with purple flowers is PP or Pp? 3. Cross the purple phenotype with a white phenotype - the genotype of the white plant has to be pp since p is recessive - pp is the only genotype to produce white flowers 4. This is a test cross - homozygous recessive to an organism with an unknown genotype I. Law of segregation 1. Alleles are responsible for variations in inherited characters 2. For each trait, an organism inherits two alleles - one allele from each parent 3. Alleles can be dominant, recessive or partially dominant 4. The two alleles for each trait segregate during gamete production - progeny inherit one of a particular parent’s two alleles for each trait J. Law of independent assortment (with modern knowledge added) 1. Two traits on different chromosomes will segregate into gametes independently of each other 2. Mendel derived this law using dihybrid crosses - crosses examining two different traits at the same time 3. Cross round (RR, Rr) or wrinkled (rr) seeds and Yellow (YY, Yy) or green (yy) seeds 4. These two traits (seed shape and color) are on two different chromosomes 5. Let’s do the cross and test the hypothesis K. Underlying mechanism of dominance and recessiveness 1. The round allele R is dominant over the wrinkled allele r 2. R codes for an enzyme that converts sugar to starch in the seed 3. Wrinkled r codes for a defective enzyme - sugar is not converted to starch in homozygous recessive individuals 4. High sugar causes the recessive embryo to swell with water and when it dries it is wrinkled L. Blood types: an example of codominance 1. There are 4 possible blood phenotypes - A, B, AB, O 2. IA allele codes for A antigen 3. IB allele codes for B antigen 4. i allele codes for no antigen 5. i is recessive to both IA and IB 6. IA and IB are codominant - both antigens are produced 7. What are the blood types?
