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Genetics: Inheritance - Simple Mendelian genetics Punnett squares - Dihybrid crosses gene linkage - Other patterns of inheritance Refer to chapter 11 in text A sample of simple human traits widow’s peak have ____ don’t ____ dimples ____ ____ unattached ear lobes ____ ____ straight thumbs (vs. hitchhiker’s thumb)____ ____ tongue rolling ____ ____ polydactyl ____ ____ PTC tasting ____ ____ ↓Gregor Mendel in the garden. How he assured the parentage of his pea plants→ P (parental) generation - true breeding genotype = homozygous - Includes two phenotypes - Each can only make one gamete type (P, p) F1 (first filial) generation - phenotypically uniform showing dominant trait (recessive trait is masked) - genotype = heterozygous - Gametes could be p or P from any plant represented. F2 (second filial) generation - phenotypes show a 3:1 ratio dominant : recessive - 1:2:1 genotypic ratio Punnett squares You have a purple-flowered and a white-flowered pea plant: What are their genotypes? How do you test that? p p P P P p P p p Pp p P p P p Pp pp Pp pp phenotypes? phenotypes? If you have a dominant-phenotype expressed and cross with the recessive type, it is called a test cross. Dihybrid cross …looking at two traits at once. Assuming the traits sort independently (they are unlinked), and they are autosomal (not sex-linked, i.e. on a sex chromosome)… Any pairing of genes in is equally likely. (This is a Mendelian cross, starting with true-breeding P) What is the resultant phenotypic ratio? 9:3:3:1 What is the phenotype ratio for each trait, separately? If genes are in a linkage group (near each other on a chromosome) they won’t sort as expected. Purple flower (P) dominant over red (p): Long pollen (L) dominant over short (l): Resulting F2 from self-cross → PL pL pl PL PPLL PPLl PpLL PpLl Pl PPLl pL PpLL PpLl ppLL ppLl pl PpLl (Could you set up the 4x4 Punnett Square?) example lifted from Allott Pl PPll Ppll PpLl ppLl Ppll ppll Expected phenotypic ratio (9:3:3:1) in 6952 plants= 3910.5 : 1303.5 : 1303.5 : 434.5 observed 4831 : 390 : 393 : 1338 NB - Genotypes for linked autosomes PL or pL have a special way of being written: pl Pl (What determines which it is?.... “=“ can be seen as chromosomes. Any that are not like one of the parental types are recombinant. How does this occur, if they are “linked”? How often does recombination occur? Hey… is this like that gene mapping thing? Other patterns of inheritance Mendel worked with dominant/recessive traits… What if the heterozygous phenotype is intermediate to the homozygous phenotypes (a black chicken crossed with a white makes speckled?) Codominance the heterozygote shares the phenotypes; both forms are seen together. What if a red flower crossed with a white one produces pink? Incomplete dominance (There is some argument that incomplete dominance is just codominance on a local level…) What if there are more than two choices for a given trait, like rabbit fur? Multiple Alleles: more than two versions, or two alleles, for a given gene. What if the trait shows a wide range of phenotypes, like human height or skin color? Polygenic Inheritance More than one gene controls that trait What if a trait shows a gender bias? Queen Victoria was a carrier ← for hemophilia (XhXH) Sex-linked - the allele is carried on the X (usually)or Y chromosome. This also demonstrates the use of a pedigree to track a condition: Note symbols (□, o, half and totally filled, Mader p. 198 has a good walk through). Hemizygous: X?Y What is the chance of Prince William having a child with hemophilia? Human X chromosome color blindness is also X-linked (Xb vs. XB) Human Y chromosome “Real human Y chromosome” http://www.life.uiuc.edu/bio100/lectures/s07lects/12s07-chromo.html pleiotropy: some genes have more than one affect. Sickle-cell is cited in text as an example: The point mutation impacting Hb has many ramifications. epistasis: one gene can specifically impact the expression of another. Albinism is an example of this: The genes for various colors might be there, but homozygous recessive for a color-deposition gene (cc) at a different locus, prevents the colors from being seen. the ABO blood group is an example of both multiple alleles and codominance: The alleles code for blood proteins A, or B, or neither (denoted IA, IB, and i, for immunoglobulin A, etc.). A and B are codominant, both are dominant over O. If you have protein A (only), you make anti-B antigens, and vice-versa… Hence the concept of universal donor (O) or recipient (AB). Rh factor is at another locus. Other considerations: Internal environment: A gene might not be activated until puberty, may only be expressed in combination with sex-inked genes (bird plumage), or when other compensating genes shut down with age (gray hair). External environment: A gene might only be expressed at a given temperature, or might be activated by chemicals or viruses. Multifactorial: Genes can be polygenic and subject to environmental influence Probabilities: - If a man has type A blood, and his wife type B, what types could their kids have? What if his father was type O? What if her parents were both AB? - If your grandfather has Huntington’s disease (dominant), what is the likelihood you will have it? - If your sister is colorblind, what is the likelihood your daughter also will be? What is meant by linked genes? How would you test for it? How might environment affect gene expression? Discuss two distinct aspects. What is a test cross? What are you testing? Give the names and examples of 3 non-traditional inheritance patterns. One plant is homozygous for yellow seeds, which are wrinkled; Another is heterozygous for both color and seed shape. Determine the resulting phenotypic ratio. Determine your parents’ blood types: What were the possible types in their kids? What was the probability of you getting yours? genotype dihybrid cross polygenic inheritance P autosomal sex linked homozygous sex chromosome pedigree phenotype 9:3:3:1 hemizygous F1 3 to 1 carrier dominant 1 to 2 to 1 pleiotropy recessive linkage group epistasis heterozygous Punnett Square ABO blood group F2 recombinant internal environment test cross codominance external environment unlinked incomplete dominance multifactorial multiple alleles