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Lecture+6+Topic+IV+Mendelian+Extensions - BIS101-001 Genes...

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Unformatted text preview: BIS101-001 Genes and Gene Expression Lecture 6 Topic IV Mendelian Extensions January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 1 Last Lecture: III. Chromosomal Basis of Inheritance Part 2 Sex linkage Sex Sex-linked traits Sex Sex chromosomes Sex Sex linkage in Drosophila Sex Drosophila Sex determination Sex Sex-linked traits in humans Sex Summary Summary January 6, 2010 BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ 2 IV. This Lecture: Mendelian Extensions Multiple alleles Multiple Modifications of dominance relationships Modifications Gene interactions Gene Essential genes and lethal alleles Essential Penetrance and expressivity Penetrance BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions A. Multiple alleles Two examples: 1. Drosophila eye color Drosophila eye 2. ABO blood groups BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions A. Multiple alleles: an allele is a form of a gene. There can be more than two alternate forms of a given gene: these are called multiple alleles. 1. Drosophila eye color w+ = red (wild-type eye) w = white we = eosin (reddish orange) eosin wch = cherry cherry wa = apricot apricot There are at least twelve different alleles for eye color For multiple alleles we can set up a dominance relationship: w+ > we > w (where > = dominant) BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions Futhermore: Any given individual can only inherit two alleles from the pool of twelve alleles. Look at the female because this trait is sex-linked (X). i.e.: w+/w+ = red (w+ > we > w) w+/we = red red we/we = eosin eosin we/w = eosin w+/w = red w/w = white BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions A. Multiple alleles 2. ABO blood groups The ABO blood system has three different alleles of one gene. The gene can produce no proteins in the blood, antigens (which is the O phenotype), or it can produce A antigens which produces the A phenotype, or it can produce B antigens which produces the B phenotype. Both A and B are dominant (codominance) to the O phenotype, which means that the A genotype can be AA or AO, and the B phenotype can be BB or BO. If a person has both the A and B alleles, they have the AB phenotype. A person can pass down either the A allele, B allele, or O allele, and the resulting combination from the other parent makes the genotype of the child. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions 2. ABO blood groups in humans: 3 alleles: IA, IB, i Dominance: IA & IB > I and IA & IB are codominant. Genotype: IA IA or IB IB or IA IB ii Phenotype: A B AB O IA i IB i So 3 different alleles control 4 blood groups. One can test for phenotype by doing a blood test. Antibodies will “attack” what seems foreign causing agglutination (coagulation). BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions B. Modifications of dominance relationships 1. Incomplete dominance a. Plumage color in chickens b. Flower color in snapdragons and in four-o’clocks 2. Codominance (already discussed) a. Antigens alleles (AB) of ABO blood group BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ B. Modifications of dominance relationships 1. Incomplete dominance: a situation in which a heterozygote shows a phenotype quantitatively (but not exactly) intermediate between the corresponding homozygote phenotypes Example: plumage color in chickens: CB CB Cw Cw CB Cw = black = white = bluish grey incomplete dominance Cross: Parents: F1 progeny: Cross F1: F2 progeny: Black CB CB x White x Cw Cw CB Cw - bluish grey CB Cw x CB Cw 1CB CB : 2CB Cw : 1 Cw Cw 1 black : 2 bluish grey* : 1 white * grey is never true bleeding so incomplete dominance. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Incomplete dominance Thus the clue to look for incomplete dominance is when two grey birds are crossed you get a 1:2:1 ratio with three phenotypes. This violates Mendel’s laws that some one factor that is dominant will govern the phenotype. You can’t tell the difference between incomplete dominance and codominance by looking at phenotype alone. The difference between the two is determined at the molecular level. Text example: sickle cell anaemia BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Incomplete dominance in four-o’clock plants BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Incomplete dominance of flower color b. Another example is four-o’clock flower color: Alleles: c+/c+ = red flower c+/c = pink (intermediate phenotype between red and white) c/c = white flower F1 pink flowers do not breed true. F2 generation follows a normal heterozygous cross with a 1: 2: 1 ratio of red: pink: white flower color due to incomplete dominance. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Incomplete dominance of flower color b. Snapdragon flower color: Alleles: C+/C+ = red flower C+/C = pink (intermediate phenotype between red and white) C/C = white flower F1 pink flowers do not breed true. F2 generation follows a normal heterozygous cross with a 1: 2: 1 ratio of red: pink: white flower color due to incomplete dominance. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Incomplete dominance of flower color BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Incomplete dominance of flower color BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions B. Modifications of dominance relationships 1. Incomplete dominance a. Flower color 2. Codominance (done) a. ABO blood group antigens alleles (AB) BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions 2. Codominance The situation in which a heterozygote show the phenotypic effects of both alleles equally. Example: Human: MN (Blood group) -phenotype: antigen on surface of red blood cell. Alleles: LM LN Genotype Phenotype LM L M M antigen LN L N N antigen LM L N M N: both antigens (codominant) BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions C. Gene interactions 1. New phenotypes Ex. comb type in chickens (p. 257) 2. Epistasis a. Recessive epistasis b. Duplicate recessive epistasis -complementary gene action BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ What does gene interactions mean? C. Gene interactions (modified Mendelian ratios) -A given phenotype is governed by more than one gene. There is a phenotypic ratio of 9:3:3:1 in a dihybrid cross of two unrelated phenotypes. But the 9:3:3:1 ratio can also exist in two genes that affect one phenotype, such as the shape of the comb on a chicken’s head. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Gene interactions and comb shape (Fig. on p. 257) Let’s begin with true breeding lines: rose shape (♀) Parents: RRpp x F1: pea shape (♂) rrPP RrPp walnut shape (new phenotype = gene interaction) RrPp 9 R-P3 R-pp 3 rrP1 rrpp x RrPp walnut rose pea single (another new phenotype) Cross F1: F2: Two genes interact to give 2 additional phenotypes but still has 9:3:3:1 ratio! BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions C. Gene interactions (modified Mendelian ratios) 1. New phenotypes Ex. comb type in chickens 2. Epistasis a. Recessive epistasis b. Duplicate recessive epistasis -complementary gene action BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Epistasis definition Epistasis - a form of gene interaction when one gene interferes or masks the expression of another gene -two genes involved. We will discuss “recessive epistasis”: -the recessive allele masks the expression of another gene. Ex: aa is epistatic to B or b BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ A model for recessive epistasis BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ b. Complementary gene action Example: flower color Remember: purple dominant to white In the F2: see 3 purple :1 white, and purple will always breed true. Puzzler: pure breeding white lines from different parts of U. S. P:white x white F1: all purple (self F1): F2: 9 purple : 7 white This is different from what Mendel observed. The 9 : 7 ratio is a modification of the 9 : 3 : 3 : 1 ratio (dihybrid) 9 A-B=> purple 3 A-bb 3 aaBthese 7 genotypes result in white flowers 1 aabb -therefore both dominant alleles (A and B) must be present for the expression of purple color BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ This is an example of “duplicate recessive epistasis” aa epistasis to B or b bb epistasis to A or a “Complementary gene action” Example: P: aaBB x AAbb F1: all dihybrids AaBb (2 dom. genes so purple) Two genes (A and B) are involved in color production!!! Biochemical explanantion underlying this phenomenon is described in the text. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Molecular basis of genetic complementation BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ F1 has one gene with two mutant alleles BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Complementation or not BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Summary BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions D. Essential genes and lethal alleles 1. Dominant lethal allele 2. Recessive lethal allele 3. Pleiotropic genes BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions Definitions: lethal allele: a specific form of gene (allele) whose expression results in the death of the individual expressing it autosomal or sex linked dominant or recessive the time at which the allele can be lethal varies (it could occur anytime at various stages of development) essential genes - a gene that is defined by the character of life or death. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions D. Essential genes and lethal alleles 1. Dominant lethal allele: -lethal phenotype in heterozygote (the presence of one copy causes death). Example: Huntington’s Disease. 2. Recessive lethal allele: -lethal phenotype in a homozygote (the presence of two copies causes death). Example: Yellow coat color in mice. 3. Pleiotropic genes: -gene that affects more than one phenotype. Example: Yellow coat color in mice. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions D. Essential genes and lethal alleles 1. Dominant lethal allele 2. Recessive lethal allele Ex. yellow coat color in mice 3. Pleiotropic genes BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ A case of a recessive lethal allele, yellow coat in mice BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ A case of a recessive lethal allele, yellow coat in mice Example: yellow coat color in mice (see next figure) Y = dominant yellow coat color Y+ = dark color (wild type) Y is dom to Y+ Genotype Phenotype Lethal YY yellow yes* YY+ yellow no Y+Y+ dark no *notice that the lethal phenotype is YY. Only lethal if 2 copies are present. -the mice with lethal phenotype die before birth -thus yellow coat color mice never breed true (only the YY+ are left) -all yellow adult mice are heterozygous P: Yellow x Yellow F1: 1YY: 2YY+: 1Y+Y+ Progeny: 1 dies: 2 yellow: 1 dark BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ A case of a recessive lethal allele, yellow coat in mice BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions D. Essential genes and lethal alleles 1. Dominant lethal allele 2. Recessive lethal allele 3. Pleiotropic genes Ex. yellow coat color in mice BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions E. Penetrance and expressivity Genotype ---“potential” Phenotype affected by other genes & environment Two individuals might have the same genotype but they might not express the same phenotype. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions Penetrance: The frequency with which a dominant or homozygous recessive gene manifests itself in individuals in a population. -basically when the expected phenotype form a genotype does not show through with the expected ratios. a.) complete penetrance -all individuals with the same genotype (e.g. AA) will have the exactly same phenotype; gene is 100% penetrant -all heterozygotes look alike and the homozygotes look alike All individuals in population with the same genotype express the same phenotype. -100% have hair on their head BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions Incomplete penetrance: If all individuals with the AA genotype are expected to have a certain phenotype but only 75% do and 25% do not, then the gene is said to be 75% penetrant: BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions Expressivity: The degree/extent to which a phenotype is expressed in an individual in a population. -describes the fact that humans have a genotype that might express that phenotype, but the extent of expression varies in individuals BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Variable expressivity: piebald spotting in beagles BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ Penetrance and expressivity considered together BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ FINAL POINT: Inheritance is not always so clear cut as in Mendelian genetics. There are extensions to Mendel’s Laws because of gene interactions. BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ IV. Mendelian Extensions : Lecture Summary Lecture Multiple alleles Multiple Modifications of dominance relationships Modifications Gene interactions Gene Essential genes and lethal alleles Essential Penetrance and expressivity Penetrance BIS101-001, Winter 2010—Genes and Gene Expression, S.D. O’Neill ©2010 BIS1012010— O’ ...
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