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Unformatted text preview: 0 Extensions to Mendel
Complexities in relating genotype to phenotype
Chapter 3 Chapter Outline
I. 0 Extensions to Mendel for singlegene inheritance A. Dominance is not always complete 1. In incomplete dominance, the F1 hybrid resembles neither purebred parent 2. In codominance, alternative traits are both visible in the F1 hybrid 3. Variations on complete dominance do not negate Mendel's laws of segragation B. A gene may have more than 2 alleles, and mutations are the source of new alleles C. One gene may contribute to several visible characteristics. Some alleles may cause lethality D. A comprehensive example: sicklecell anemia illustrates many extensions to Mendel's analysis of singlegene inheritance 1. Multiple alleles 2. Pleitropy 3. Recessive lethality 4. Different dominance relations II. Extensions to Mendel for multifactorial inheritance A. Two genes can interact to determine one trait 1. Novel phenotypes can emerge from the combined action of the alleles of two genes 2. In epistasis, one gene's alleles mask effects of another gene's alleles 3. For some traits, homozygosity for a mutant allele at one of two or more genes produces the phenotype B. Breeding studies help decide how a trait is inherited with humans, pedigree analysis replaces breeding experiments C. The same genotype does not always produce the same phenotype 1. Phenotype often depends on penetrance and expressivity 2. Modifer genes produce secondary effects on phenotype 3. The environment can effect the phenotypic expression of a genotype 4. Chance can effect penetrance and expressivity D. Continuous variation can be explained by extensions to Mendelian analysis E. Mouse coat and tail: example of multiple genes & multifactorial traits 1. Gene 1: agouti or other color patterns 2. Gene 2: black or brown 3. Gene 3: albino or pigmented 4. Short or long tail Extensions to Mendel
Complexities in Relating Genotype to Phenotype 4 Outline of extensions to Mendel's analysis Singlegene inheritance In which pairs of alleles show deviations from complete dominance and recessiveness In which different forms of the gene are not limited to two alleles Where one gene may determine more than one trait Multifactorial inheritance in which the phenotype arises from the interaction of one or more genes with the environment, chance, and each other. 3-5 Dominance is not always complete Crosses between truebreeding strains can produce hybrids with phenotypes different from both parents. Incomplete dominance Codominance F1 hybrids that differ from both parents express an intermediate phenotype. Neither allele is dominant or recessive to the other. Phenotypic ratios are same as genotypic ratios F1hybrids express phenotype of both parents equally. Phenotypic ratios are same as genotypic ratios. 3-6 Single Gene Inheritance Models
COMPLETE DOMINANCE (pea plant traits) Crosses between truebreeding strains produces offspring completely like one of the parents INCOMPLETE DOMINANCE (plant flower colors) F1 hybrids that differ from both parents express an intermediate phenotype. Neither allele is dominant or recessive to the other Phenotypic ratios are same as genotypic ratios CODOMINANCE (human blood types, lentil coat patten) F1 hybrids express phenotype of both parents equally Phenotypic ratios are same as genotypic ratios Summary of dominance relationships Fig. 3.2 COMPLETE DOMINANCE (F2 cross) R R r r ________ genotype ratio ________ phenotype ratio Incomplete dominance in snapdragons Fig. 3.3 INCOMPLETE DOMINANCE (F2 cross) R R r r ________ genotype ratio ________ phenotype ratio Codominant blood group alleles Fig. 3.4b Codominant lentil coat patterns Fig. 3.4a CO-DOMINANCE (F2 cross) A B B A _____ genotype ratio (all ___ genotype) _____ phenotype ratio (all type ___ blood) Do variations on dominance relations negate Mendel's law of segregation? Dominance relations affect phenotype and have no bearing on the segregation of alleles. Alleles still segregate randomly. Gene products control expression of phenotypes differently. Mendel's law of segregation still applies. Interpretation of phenotype/genotype relation is more complex. A gene can have more than two alleles Genes may have multiple alleles that segregate in populations. Although there may be many alleles in a population, each individual carries only 2 of the alternatives. ABO blood group Dominance relations are unique to a pair of alleles. 3 alleles 6 possible ABO genotypes: IAIA, IBIB, IAIB, IAi, IBi, or ii Dominance or recessiveness is always relative to a second allele. ABO blood group
IA is completely dominant to i but codominant to IB. 6 genotypes generate 4 phenotypes. (AA) (AO) (BB) (BO) (AB) (OO) A gene can have more than two alleles (cont'd.) Lentils offer another example of multiple alleles 5 alleles present that determine coat pattern Marbled1 (m1), marbled2 (m2), spotted (s), dotted (d), clear (c) Basically, some alleles were more dominant than others A priority ranking could be determined m1 > m2> s =d > c Notice that spotted and dotted were equal in dominance A DOMINANCE SERIES was revealed 0 Perform reciprocal crosses between pure breeding lines of all phenotypes Another DOMINANCE SERIES example
0 Genotype Phenotype A_ atat aa agouti Black/ yellow black 0 A > a > a
t A gene can have more than two alleles (cont'd.) 0 Mutations are the source of new alleles Some are good, some are bad. Mutation rates vary tremendously, depending on many factors Allele frequency: total # of gene copies, expresses as a % Wildtype allele: allele frequency > 1% Mutant allele: allele frequency < 1% Monomorphic: gene with only one wildtype allele Every organism carries 2 copies of every allele Plant INCOMPATIBILITY example Alleles determine acceptance or rejection of pollen Idea is that selfpollination in some plants is avoided Again, multiple alleles are used in the pollen and ovules Separate male and female parts on the plant However, each plant can still only carry a maximum of 2 different allele types on one particular plant There are >90 alleles for the incompatibility complex Overall, this promotes variability within the One Gene May Contribute to Several Visible Characteristics Pleiotropy single gene determines more than one distinct and seemingly unrelated characteristics Some alleles may cause lethality (death).
Recessive lethal allele: in certain combinations, some alleles may negatively affect survival of individuals Coat color of mice 0 All yellow mice are heterozygotes: Cross between inbred agouti and yellow mice reveals a 1:1 ratio Yellow mice are therefore AYA heterozygotes 0 Yellow mice do not breed true:
Cross between two yellow mice reveals a 2:1 ratio of yellow to agouti The AY allele (when homozygous) is LETHAL Traits Determined by One Gene: Extensions to Mendel's Ideas Explain Variations on the 3:1 Monohybrid Ratio
0 Table 3.1 Sickle Cell Anemia: Extensions to Single Gene Inheritance Multiple alleles Pleitropy Recessive Lethality Different Dominance Relations 1. MULTIPLE ALLELES Hemoglobin is composed of two polypeptide chains Alpha globin Beta globin The Beta globin gene Normal wildtype allele called "A" One type of common mutant allele called "S" because it causes sickling of RBC 2. PLEIOTROPY The mutant "S" allele affects more than one trait SS individuals RBCs clog after releasing oxygen Low oxygen causes tissues to cramp Anemia also results However, these individuals resistant to malaria SA individuals (consider later slide) AA individuals (consider later slide) 3. RECESSIVE LETHALITY SS Individuals Develop heart failure b/c of stress on circulatory system Lethality of "S" allele obvious here when homozygous Normal circulatory system Normal circulatory system SA individuals AA individuals 4. DIFFERENT DOMINANCE RELATIONS Codominance SS individuals SA individuals AA individuals Complete dominance & recessiveness SS individuals SA individuals AA individuals 4. DIFFERENT DOMINANCE RELATIONS (cont'd.) Complete dominance & recessiveness SS individuals SA individuals AA individuals Incomplete dominance PLEIOTROPY SUMMARY OF SICKLE CELL ANEMIA (know this table...) Part Deux: 0 Extensions to Mendel for Multifactorial Inheritance Two Genes can Interact to Determine One Trait
0 Novel phenotypes can emerge from the combined action of alleles of two genes Let's consider the traditional dihybrid cross in pea plants for seed color Start with two purebreeding parental lines Aabb x aaBB (tan x gray) All F1 offspring are AaBb (brown) F2 offspring generate 9:3:3:1 of A_B_ : A_bb : aaB_ : aabb (brown : tan : gray : green) 0 Two Genes can Interact to Determine One Trait (cont'd.)
0 The twogene hypothesis explains why there is: Only one green phenotype: purebreeding aabb Two types of tans: purebreeding Aabb; tan and green producing Aabb Two types of grays: purebreeding aaBB; gray and green producing aaBb Four types of brown: purebreeding AABB; brown and tan producing AABb; brown and gray producing AaBB; and AaBb dihybrids that give rise to all four colors Four color phenotypes arise from four genotypic classes (9:3:3:1 indicates dihybrid cross of this Two Genes can Interact to Determine One Trait (cont'd.)
0 Also the case of complementary gene action A modified 9:3:3:1 ratio is observed Instead of four phenotypic classes, only two emerge. WHY??? The 9:3:3:1 ratio gets "lumped" 9:3:3:1 = 9:7 0 So, both genes must be turned "ON" in order for purple to be expressed In Epistasis, One Gene's Alleles Mask the Effects of Another Gene's Alleles
0 This is where we see further modifications of the 9:3:3:1 ratio Type of gene interaction in which the effects of an allele at one gene hide the effects of alleles at another gene The allele doing the masking is "epistatic" to the gene that is being masked (the "hypostatic" gene) Let's look at 3 types of epistasis... 1. Recessive Epistasis Coat color in Labrador retrievers 3 coat colors Black Brown Yellow Modified ratio is... 9:3:4 from 0 9:3:4 The "ee" allele combination maskes the effects of the B or b alleles 2. Dominant Epistasis I 0 Summer squash color 3 colors White Green Yellow Modified ratio is... 12:3:1 from 0 12:3:1 Presence of "B" allele hides any action of the "A" or "a" allele 3. Dominant Epistasis II 0 Leg color in chickens 2 colors White Colored Modified ratio is... 13:3 from 0 13:3 At least one copy of "A" and the absence of "B" is needed to produce color GENE INTERACTION SUMMARY
0 For Some Traits, Homozygosity for a Mutant Allele at Any One of Two or More Genes Produces the Phenotype It takes many genes to generate a functional pathway Good example: Human hearing ~50 genes must have at least one dominant allele present in order for hearing to be normal Heterogeneous trait: mutation at any one of a number of genes can give rise to the same phenotype (i.e. impaired hearing) Complementation test: method of discovering whether a particular phenotype arises from mutations in the Impaired Impaired OK So, the recessive alleles are in different genes, while the other gene is homozygous dominant Impaired Impaired Impaired So, the recessive alleles are in the same gene Breeding Studies Help Decide How a Trait is Inherited
0 Specific breeding tests can help decide between two hypothesis Based on the results of additional crosses, you may be able to determine which of two hypotheses is correct Let's consider mouse coat colors... 0 HYPOTHESIS 1: 2 genes with recessive epistasis 0 HYPOTHESIS 2: 1 gene with incomplete dominance 0 The outcome of crossing a white F2 mouse with a PURE BREEDING brown mouse will help you decide which Hypothesis is correct (why could you not use a black mouse???) The Same Genotype Does Not Always Produce the Same Phenotype
0 Phenotype depends on penetrance and expressivity Modifier genes produce 2 effects on phenotype Penetrance: how many members of a population with a particular genotype show the expected phenotype High penetrance = high incidence of the phenotype Low penetrance = low incidence of the phenotype Expressivity: degree to which a phenotype is expressed Modifier gene: genes having a subtle, secondary effect Siamese car coloring & egg incubation temp. in alligators Environment can affect the phenotypic expression Continuous Variation: Still Mendelian Applications Discontinuous trait: clearcut traits, easy to score Many of the traits Mendel used in pea plants Some of ones we did in class (i.e. ear lobes) Continuous traits: trait that has a wide range of values Height Skin tone Weight Often referred to as "qualtitative traits" Usually polygenic: controlled by multiple genes Mouse Coat Color & Tail Length 0 Gene 1: agouti or other color patterns Dominance series epistasis epistasis Gene 2: black or brown Gene 3: albino or pigmented Gene 4: Short, long or somewhere in between tail polygenic ...
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This note was uploaded on 08/04/2009 for the course BIOL 2153 taught by Professor Larkin during the Fall '03 term at LSU.
- Fall '03