Mendel’s Law of SegragationB. Applications of and exceptions toMendelian GeneticsGeneticsA. Gregor Mendel – Father of Genetics1. Conducted controlled, scientific experiments and had excellent quantitative approach2. Lucked out with peas:a. self pollinate control matings3. Mendelian Genetic Crossesa. cross pollinate true-breeding parents of the P generation to produce hybrids of the F1 generationb. then the F1 hybrids self- pollinate to produce the F2 4. 2 Laws:a. Law of Segregation-The 2 alleles (2 forms of a gene) for a character (trait) are packaged in separate gametesb. Law of Independent Assortment-Each pair of alleles segregates in gametes independently5. Mendel’s hypothesisa. alternative version of the gene (different alleles) account for variations in inherited characteristics.Different alleles vary in the sequence of nucleotides at a specific gene locusb. For each character, an organism inherits two alleles, one from each parent(1) in homozygous or ‘truebreeding’ organisms thehomologous alleles are identical.(2) in the heterozygous or‘hybrid’ the two allelesare diferrent.(a) one allele is dominant- always expressed in the organism’s phenotype,physical appearance(b) one allele is recessive- only expressedphenotypicallywhen it is in the homozygous condition,genotypically, eg. geneticallyc. The two alleles for each character segregate (separate) during gamete productioneg. meiosis Mendel’s Law of SegragationAccounts for the 3:1 ratioobserved in the F2 genration6. Punnett square- predicts the results of a genetic cross between individuals of known genotype.-Always use a capital letter for the dominant allele and the lowercase for the recessive.-Monohybrid cross for one trait-Dihybrid cross for two traitsa. test cross- breeding a homozygous recessive with a dominant phenotype, but unkown genotype can determine the identity of the unknown allele.b. Ex. dihybrid cross:Mendel crosses true-breeding plants that had yellow, round seeds (YYRR)With true-breeding plants that has green, wrinkled seeds (yyrr)(1) Phenotypic Ratio9:3:3:1for every 9 individuals with both dominant phenotypes (Yellow/Round), there are 3individuals with one dominant and one recessive phenotypes (Yellow/wrinkled), 3 individuals with one recessive and one dominant phenotypes (green/Round), and one individualwith both recessive phenotypes (green/wrinkled)-The above results are due to the independent assortment of each pair of alleles during gamete formation,  Law of Independent AssortmentB. Applications of and exceptions to Mendelian Genetics1. Incomplete Dominance- Heterozygotes show an intermediate phenotype.-offsprings of a cross betweenheterozygotes will show three phenotype Ex. Snapdragons(a) F1 are 100% pink(b) F2 are 25% white, 25% red, 25% pink-Incomplete dominance an intermediatephenotype is produced.2. Codominance- the two alleles affect the phenotype in separate, distinguishable ways-Ex. M & N blood groups(a) two types of RBC surface proteins: M & N- 3 phenotypes:M, N, &, MN3. Organismal vs. biochemical levels of dominance vs. recessive-Ex. Tay-Sachshumans with Tay-Sachs have a malfunctioning enzyme, a recessive trait. Therefore, homozygous recessive condition.-functioning enzymes are found in both homozygous dominant and heterozygous, therefore, normal.4. Dominant alleles are not more abundant in the populationEx. Polydactyly ‘many fingers’5. Multiple AllelesEx. ABO blood groups are RBC surface proteins determined by 3 alleles: IA, IB, I- IA and IB are codominant toeach other- both IA and IB are dominant to I there are 6 possible genotypes and 4 possible phenotypes (blood types):type A, type B, type AB, and type O-Matching compatible blood groups is necessary for blood transfusion or tissue compatibility because the body produces antibodies against the foreign ‘blood factors’ it doesn’t recognize. 6. Pleiotropy occurs when one geneaffects multiple phenotypesEx. Sickle-Cell Disease7. Epistasis- a gene at one locus alters the phenotypic expression of a gene at a second locus.a. in mammals, coat color depends on 2 genes(1) one, the epistatic gene, determines whether pigment will be deposited in hair or not.Presence (C) is dominant to absence (c).(2) The second determines whether the pigment to be deposited is black (B) dominant, or brown (b) recessive.Ex. Cross between two heterozygous black mice: BbCc x BbCc8. Polygenic Inheritance- an additive effect of two or more genes on a single phenotype.a. Quantitative characters- vary in thepopulation along a continuum. Ex. skin color- in humans controlled by 3 or more different genes inherited separately.(1) for each gene an allele for dark skin (A, B, C) is incompletely dominant to an allele for light skin (a, b, c). (2) An intermediate skin shade would be (Aa, Bb, Cc)-9. Nature Vs. NurtureThe ultimate phenotype depends on genes and the interaction of the environment10. Pedigree Analysis-Ex. Family Tree of widow’s peak (W) dominant to straight hairline (w) recessive. and Free earlobes (F) dominant to attached earlobes (f) recessive10. Human Recessive Disorders- only show up in individuals that are homozygous for the trait because they inherit one recessive gene from eachparent (usually heterozygous normal)a. Cyctic FibrosisThe defective gene codes for an abnormal cell membrane channel protein that cannot transport Cl¯ Therefore, Chloride levels build up on the outside of the cell, which causes mucus to build up, especially in the pancreas, lungs, digestive tract. This leads to bacterial infections and diminished organ function.11. Dominant Disorders- usually kills prior to reproduction  not passed on.An exception is Huntington’s Disease, which causes death at an age after whichreproduction has occurred  the gene is passed