Modifications of Mendelian Ratios
Modifications of Mendelian Ratios BIO3010
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This 9 page Class Notes was uploaded by Samantha on Wednesday January 27, 2016. The Class Notes belongs to BIO3010 at University of Toledo taught by Dr Krishnamurthy in Winter 2016. Since its upload, it has received 17 views. For similar materials see Genetics in Biology at University of Toledo.
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Date Created: 01/27/16
Modification of Mendelian ratios Modifications of ratios o Alleles are passed on according to Mendelian principals o Phenotype does NOT always reflect a clear dominance or recessive relationship o Different factors that can affect phenotypic ratios More than one gene contributes to phenotype Sex chromosome linked Influence of genetics and environment Extranuclear inheritance Mitochondria and chloroplasts o Alleles alter phenotype Wild type Occurs most frequently in a population o Usually dominant o Gene product is functional o Wild type phenotype Mutant allele Modified genetic material May affect protein function Types: o Loss of function Reduces or eliminates function Null allele Reduces or eliminates function Recessive o Gain of function Enhances function of protein product Dominant allele only one copy is needed to see mutant phenotype o Ex) Oncogenes o Silent mutation Do NOT alter the phenotype, but he genotype is changed Mutation in genes produce new alleles – can lead to a change in phenotype Mutations alter the function of gene produce NonMendelian genetic ratios o Ratios occur when alleles of one gene DO NOT exhibit simples dominance and recessive o Incomplete (Partial) Dominance Where a cross between parents with contrasting traits results in an offspring that in a mix of the parents Ex) snap dragons; red x white = pink o ¼ red, ½ pink, ¼ red Cross red flower with white flower ALL F 1eneration with have pink flowers Heterozygous have intermediate (pink) color Some red pigment is produced but not enough to give the deep red color seen in wildtype homozygous Neither allele is dominant F 1cross Genotype 1:2:1 ratio (same as for monohybrid) Phenotype ¼ red, ½ pink, ¼ red Neither allele is dominant so phenotype ratio is identical to genotypic ratio TaySachs disease Example of disease in which heterozygous express only 50% of the enzyme activity found in homozygous normal individuals adequate for normal biochemical function common in many enzyme – linked disorders o NonMendelian Genetic ratios Codominance – where 2 alleles of a single gene produce two fully functional but distinct products Ex) MN blood type o Genotype Phenotype M M L L M L L N MN L L N N At the molecular level these are two distinct glycoprotein (protein with sugar side group) Different sugar side groups in a heterozygote is called codominance; situation in which two alleles produce two distinct, detectible gene products that are fully functional MN blood groups in humans M N L and L alleles encode for a glycoprotein found on the surface of RBCs, autosomal M and N are two alternative forms of the glycoprotein and an individual may express are on both of them N N M N M M Three expressions combos are possible, L L , L L , and L L Mating between L L and L L could produce of all three types (1:2:1 ratio) Codominance inheritance is characterized by distinct expression of the gene products of both the alleles, both alleles produce a similar but distinct product that are both functional NOT a blended phenotype like incomplete dominance – neither allele is dominant to the other – NO intermediate phenotype Multiple alleles Where there are more than two alleles of a single locus that produce fully functional but distinct products o Ex) ABO blood type Genotype Antigen Phenotype I IA A A I IO A A B B IB O B B I I B B I IB A&B AB I IO None O o At the molecular level these are sugars attached to lipids and proteins on the surface of RBC membranes One combo of alleles in the ABO system exhibits codominance o Similar to MN blood type Four phenotype exist; A, B, AB and O A o I Bproduces A antigen o I produces B antigen o I I produces A and B antigen Dominant to I but codominant to each other O o I produces no antigen o Bombay Phenotype A and B antigens are produced from H substance (precursor) Sugar molecules are added are added to H substance to produce either the A or B antigen If H substance is incompletely formed then the sugars to produce wither the A or B antigen cannot be added This results in a person having type O blood even though they have A B either I and or I alleles Caused by a recessive mutation in a different gene (designated h) First identified in a woman in Bombay in 1952 Woman had blood type O but one of her parents was blood type AB Can’t be O because the A or B allele are dominant to O A B Bombay phenotype – individual can be genotypically I and or I but phenotypically IO Occurs if they are deficient in and enzyme that allows the A or B sugar moleucles to be added Another (far more complex) example is eye color in fruit flies More than 1000 alleles can produce different eye colors that range from the wildtype brick red to the complete absence of pigment (white eyes) AO x AB – cannot produce children with type O blood o Proband – genotypically type A or B but phenotypically is tyrd O o 3 generation – If mother was type O then she could not give rise to offspring with type B blood o Mother must be geneotypically type B White locus is fruit flies 100 alleles at the white locus in fruit flies More than 100 alleles can produce different eye colors that range from the wildtype brick red to the complete absence of pigment NonMendelian genetic ratios Lethal o Recessive lethal – one wildtype copy is enough for the organism to survive but two copies of the mutant allele is lethal Ex) the agouti in the mice regulates hair color in mice o The normal allele gives agouti color Y o The A mutant allele is dominant to wiletype agouti allele (called yellow) results in a Mendelian deficiency Heterozygous have yellow coats mutant allele is doY nant Y A A A is dominant with respect to coat color phenotype Y Y A A homozygous die early in development (before birth – embryonic lethal) A is recessive with respect to lethality Y Y A A x A A – instead of 1:2:1 gives a 2:1 ratio since the A A are never born Example of a dominant lethal alleles in humans Presence of a single copy of the dominant allele causes death of the individual Huntington disease o Caused by autosomal dominant allele (H) o Heterozygotes – Hh – carry dominate lethal allele o Onset of disease symptoms delayed into adulthood Allows mutant allele to be passed on to next generation but affected individuals must reproduce before lethal allele is expressed Modification 9:3:3:1 dihybrid ratio Factors that affect the 3:1 monohybrid ratios o Incomplete dominance o Codominance o Multiple alleles o Lethal alleles Albinism o Chromosomal recessive disorder o Inherited in simple Mendelian fashion Ex) 3:1 ratio o ABO blood type – 3 alleles that determine the phenotype Does NOT adhere to normal Mendelian ratios o Dihybrid cross between two parents that are heterozygous for albinism gene and who both have ABO blood type Yields 6 phenotypes in a 3:6:3:1:2:1 ratio Gene Interaction o Many characteristics are under the control of several genes The products of different genes contribute to the development of a phenotype o Epistasis Occurs when the expression of one gene or gene pair masks or modifies the expression of another gene or gene pair Homozygous recessive allele at one locus (x) prevents expression of the alleles at a second locus (y) Ex) gene x could encode a transcription factor necessary for expression number of the y gene. The y gene is hypostatic to and x is epistatic to y Ex) Bombay phenotype (Hh) can affect AB blood type o The presence of hh causes an A or B individual to show an O phenotype o Bombay phenotype Homozygous recessive condition a one locus that masks expression at a second locus hh asks the expression of the IA and I alleles I B If a person’s genotype induces the I and I alleles, but he are hh, then they will express the type O phenotype o If you could examine enough offspring than a cross of I IA B A B Hh x I I Hh gives a modified ratio of 3:6:3:4 o IMPORTANT POINTS Following only one characteristic (blood type) – different from dihybrid cross – as opposed to blood type and pigmentation in previous cross o Because the ratios are in 16 it indicates that two genes are involved and are interacting during expression of the phenotype, even though only a single character was followed Example 1: Mouse coat color o Normal mouse coat color is agouti (A) Mice that are AA or Aa are agouti Mice that are aa are black o Albino mice: due to a recessive mutation at a second locus bb mice are albino Genotype at agouti locus does NOT matter when bb is present AAbb and Aabb and aabb mice are albino F1 cross AaBb x AaBb Recessive epistasis o bb genotype suppresses expression of the A gene Dominant epistasis o Occurs when a dominant allele at one locus masks expression of the alleles at a second locus Example 2: Pea flower color o Complementary gene interaction How does the crossing of two white pea plants give rise to a 9:7 ratio of purple to white plants? P1: AAbb x aabb – both are white F2 AaBb – purple Cross between whiteflowered pea plants All F1 plants are purple and F2 occurs in ratio of 9 purple to 7 white The plant needs to be homozygous recessive at least one of two alleles All cases of the modified dihybrid ratios have tow things in common Principles of segregation and independent assortment are still followed F2 ratios are expressed in 16ths – suggest two gene pairs are involved Genotype ratio = 9:3:3:1 Phenotypic ratio = 9:7 Biochemical pathway o Precursor intermediate product final product Colorless colorless purple Complementary gene interaction Need at least one wild type allele of each gene pair to produce the final product Product of the two genes interact to influence the development of a common phenotype o Complementation analysis If tow independently isolated mutations cause the same phenotype how can you determine if the mutations occur in the same or in different genes? Ex) fruit fly with wingless mutation isolated in two labs in Canada and the US Both mutations are recessive Complementation Analysis cross the two mutant strains Two possible outcomes depending on whether the mutations are in the same gene or different genes Two possible outcomes In case one: the mutations are not alleles of the same gene so complementation occurs. Because the mutations are in separate genes the offspring will be heterozygous at both loci and will develop wings In case two: the mutations are alleles of the same gene so no complementation occurs. Because the mutation are in the same gene offspring are all homozygous recessive and NO wing development Complementation group Mutations that are present in a single gene belong to the same complementation group Useful for studying mutation that affect the same trait Possible to predict the number of genes involved in determining that trait o Xlinkage In males the Y chromosomes acts as a homologue of the X chromosome during meiosis Y chromosome only contains a few are also found on the X chromosome Unique patterns of inheritance of Xlinked gene was first described by Thomas Hunt Morgan while studying the whiteeye mutation in fruit flies Sex chromosome pair of unlike chromosomes that are involved in sex determination Fruit flies and mammals males are XY and females are XX Y acts as homolog to X during meiosis but contains genes that are not found on X and genes on X are not on Y – hemizygous Xlinkage describes the transmission and expression of the genes located on the X chromosome Genes on X have a unique pattern of inheritance Inheritance of gene present on X but not on Y results in modification of Mendelian ratios F1 crosses W F Redeyed female x whiteeye male R R Whiteeyed female x redeyed male W R Reciprocal crosses did not give the same results Thomas Morgan Studied eye color inheritance in fruit flies Established that inheritance pattern of whiteeyed color is directly related to the sex of the parent carrying the mutant allele Reciprocal crosses between white and red eyed parents did not yield identical results as the case in normal monohybrid cross White locus is on the X chromosome – the trait is Xlinked X linkage of the white locus is fruit flies Mutation on the X chromosome is revealed in the presence of a Y chromosome or when two mutant X chromosomes are present o F1 and F2 results of Morgan’s reciprocal cross Differences in phenotypic ratios in F1 and F2 generations in dependent on which P1 parent is whiteeyed Explanation of Xlinked crosses Xlinked crosses o Malescannot be heterozygous for Xliked genes Hemizygous for genes found on the X chromosome Xlinked trait is homozygous recessive will be passed on from mother to all sons o Crisscross pattern of inheritance Phenotypic expression Genotype of an organism is not always directly expressed in its phenotype o Individual genes and gene product do not exist in a closed system o Influenced by diverse factors both within and between cells and the environment Gene expression and the resultant phonotype are often modified are often modified through the interaction between an individuals genotype and external environment Penetrance percentage of individuals that show at least some degree of expression of a mutant genotype o Ex) If 20% of mutant individuals show a wildtype like appearance then the mutation gene had an 80% penetration Expressivity range of expression of a gene Variable expressivity Degree of expressivity can be affected by genetic as well as environmental factors Disease onset affects inheritance of genetic disorders Late onset genetic diseases o Different sets of genes are expressed at different times during growth and development of an organism o Huntington disease: caused by an autosomal dominant mutation, average age of onset is 38. If a person does not know they have the mutation than they can reproduce and have a 50% chance of passing on mutation Genetic anticipation Have a progressively earlier age of onset with each subsequent generation Also displaying increased severity of the disorder in each successive generation Myotonic dystrophy: trinuclear DNA sequences in the DM gene number of repeat segments increased in successive generation Extra nuclear Inheritance Yeastpetite colonies due to defects in electrons transport in mitochondria DNA can grow anaerobically Maternal transmission o Only offspring of affected mothers get the disease o Offspring of affected fathers do OT inherit the disease o Caused by aberrant mitochondria which are inherited from the mother
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