Mendelian Genetics Notes
Mendelian Genetics Notes BIOL 1406 02
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This 3 page Class Notes was uploaded by locnaschek on Thursday October 13, 2016. The Class Notes belongs to BIOL 1406 02 at Lamar University taught by Dr. Randall Terry in Fall 2016. Since its upload, it has received 13 views. For similar materials see General Biology I (Majors) in Biology at Lamar University.
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Date Created: 10/13/16
Mendelian Genetics Dealing with Parentsoffspring (traits passed) Gregor Mendel worked out the basic principles using plants o Experimented on pea plants by crossing purple-flowered plants with white-flowered o All of the traits he looked at had one gene with two alleles Mendel studied 7 traits, we are going to focus on flower color and plant height o P generation (true breed) crossed the plants 1 o F g1neration only showed purple phenotype o F g2neration had about a 3:1 ratio of purple to white Showed Mendel that purple dominated white Law of Dominance o In some cases, one allele (dominant) may mask the expression of another (recessive) Terms o Monohybrid Cross- cross between two parents looking at one trait o Alleles- different forms of genes o Phenotype- physical appearance o Genotype- alleles of the organism o Phenotypic/Genotypic ratio- how many of each phenotype or genotype there is compared to how many of the other(s) o If something breeds true, it is homozygous (AA or aa) and if it is segregated it is heterozygous (Aa) o A test-cross is a cross with a homozygous recessive to determine whether the parent was homozygous dominant or heterozygous Alleles are located on homologs (at the flower color locus) Assumptions o 1 gene and 2 alleles per trait o One allele is dominant to the other o Genes are never “linked” (not on the same chromosome) Law of Segregation o Alleles segregate (separate) during gamete formation and randomly unite during fertilization Exceptions to Simple Mendelian Genetics o Quantitative Traits Show a bell curve in large populations Controlled by many genes each with a small effect on the trait An example he gave was if each parent had a deck of cards (52, 26 red suits representing a tall gene and 26 black suits representing a short gene), shuffled them, and gave 26 at random to their child (Jr.), Jr. would be expected to end up with about half red and half black causing an average height. However, it is entirely possible that Jr. could end up with only, say, 4 black cards and the rest red or vice versa and end up very short or very tall respectively. Also, after going through this each generation, each parent will end up with a changing amount of “red and black cards” so the likelihood of having the same suit increases (if they get more black than red and give more black than red, then the child will be more likely to be shorter). o Incomplete Dominance If an enzyme is present (given by dominant allele only), then it can make, for example, a purple flower If only recessive is present, then it will stay white because the reaction is not catalyzed In snapdragons, RR=red, WW=white, and RW=pink because neither are dominant so one enzyme is not enough to completely catalyze the reaction and it stops in the middle somewhere The colors mix because neither dominates the other o Multiple Alleles and Codominance 3 or more alleles per gene An example is blood type (A, B, and O) A dominates O, B dominates O, and A and B are equal AA or AO= blood type A, BB or BO= blood type B, AB= blood type AB, and OO= blood type O Each blood type makes a particular protein (antigen) o A and B each have their own, AB has both, and O doesn’t have one o Each antigen has a complementary antibody (A is comp. to B and vice versa) so AB does not have one and O has both o O will never be rejected by the body so it is the universal donor (AB is universal acceptor because it has neither antibody and both antigen) o Epistasis One gene influences the expression of another (part of a common biosynthetic pathway) Must have the enzyme to go from point X point Y and then from point Y point Z in order to get the full expression of the gene Example: enzyme that comes from A will catalyze XY and B will catalyze YZ. Without A, it doesn’t matter if you have B because A happens first. Even if you do have A, however, without B the trait will not be expressed.
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