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Extensions of Mendelian Principles- Genetic Interactions.docx

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Bhagwati Gupta

September 23 , 2013 Biology 2C03: Genetics Extensions of Mendelian Principles: Genetic Interactions Module 3 Outline: Cats are not Peas - At the end of this module, you will be able to: solve probles dealing with multiple alleles 1. Define, recognize, describe and apply Mendel’s second principle 2. Solve genetic problems relating to dihybrid and trihybrid crosses 3. Use the rules of probability to determine genotypic and phenotypic proportions 4. Know how and when to use a testcross 5. Recognize modified dihybrid Mendelian ratios 6. Test your theory that a Mendelian ratio is consistent with your data using a Chi-squared test Multiple Alleles - Mutations anywhere along the gene can still give a loss of function, but are considered different alleles - Population: multiple alleles may exist - Individual (diploid): only two alleles coexist in each cell Example of the Multiple Alleles Contributing to Fur Coloration in Cats - Five alleles:  C: full colour  c : Burmese (mutation at a particular locus)  c : Siamese (mutation at a different allele)  c: white, blue eyes  c : albino, pink eyes - C>c =c >c>c (dominant/allelic series)  If each individual has two alleles, the dominant/allelic series tells you which one will be dominant - Where:  > indicates  = indicates incomplete dominance or codominance  c /c = mink s a - Ex, for a cat to be Siamese they must have the c allele with either a c or c allele - This represents a dominance series or allelic series. Important is the relationship between pairs of alleles The C Gene Codes for an Enzyme Called Tyrosinase - The albino allele (the most recessive allele in this series) contains a cytosine deletion in Tyrosinase at position 975 in exon 2 - Causes a frame shift resulting in a premature stop codon residues downstream from the mutation – loss of a single nucleotide that alters the reading frame - This is a non-functional protein and the allele is described as a loss of function allele - Definition of frame-shift mutation: the loss of a single nucleotide causes the reading frame, the series of three neucleotide codons, to be read differently The Number of Different Genotypes is Determined by the Number of Alleles in the Population - Number of genotypes is determined by the number of different possible allele combinations in the population - c, c . c , c , C (wildtype) - This gives, 15 genotypes of which there are five different homozygous genotypes and 10 different heterozygotes Biochemical Definition of Wildtype - Wildtype allele: a functional enzyme or other protein is produced in the cell - Loss of function allele: an enzyme or other protein is no longer being produced, is produced at lower levels, or is nonfuncitonal, or at least subfunctional - Sometimes the term wildtype is used to refer to the most common phenotype (or genotype) found in a natural population - Often the wildtype allele is dominant over the loss of function allele - Half as much protein is synthesized yet this is often sufficient to achieve the wildtype phenotype (heterozygotes) - This is called haplosufficiency - For fur colour, in the C/c genotype, half as much Tyrosinase is synthesized, but this is sufficient to achieve full colouration Because there are Always Exceptions to the Rule - It is not always the case that the dominant allele is ‘normal’ and the recessive allele is a mutation - Dominant alleles can be gain of function mutations, in which the mutant allele produces a protein that has increased (detrimental) function - E.g. Huntington’s disease is an autosomal dominant disease which manifests the phenotype later in life; 50% chance of passing on the allele to your offspring - Dominant allele can be a loss of function alleles, but in the heterozygote, half as much protein is synthesized and this is not sufficient for a normal phenotype; haploinsufficient – inheriting both copies of the mutation is a lethal genotype - E.g. tailless cats (Manx) Gregor Mendel Defined the Principle of Segragation - Mendel’s principle of segregation: 1) Each individual organism possesses two alleles encoding a trait 2) Alleles separate when gametes are formed 3) Alleles separate in equal proportions - He did this using monohybrid crosses, e.g. Aa x Aa - What would happen if he followed two traits at the same time? Mendel’s Law of Independent Assortment - The inheritance pattern of one trait will not affect the inheritance pattern of another trait - Examined dihybrid crosses e.g. Aa Bb x Aa Bb  Taking true breeding round yellow seeds with true breeding wrinkled green seeds producing heterozygotes  Four possible phenotypes  9:3:3:1 ratio in the F p2enotype - Each trait is inherited by the Mendelian ratio, 3:1, but together we see a phenotypic ratio of 9:3:3:1 Another Example: Cat Colour and Tail Length - 9 brown fur, short tail - 3 brown fur, long tail - 3 white fur, short tail - 1 white fur, long tail - 9:3:3:1 - 9 = two dominant traits - 1 = two recessive traits More Complex Problems: Tribybrid Cross - You can use a similar procedure for determining the proportion of genotypes and phenotypes if you are following more than two alleles:  E.g. trihybrid crosses (and more…) or why I learned to stop using Punnett squares  Tryhibrid:  64 combinations of gametes (an 8x8 Punnett square)  27 different F 2enotypes  8 different F2phenotypes  ratio: 27:9:9:9:3:3:3:1 Take Home Problem - In cats, white patches are caused by the dominant allele P, while p/p individuals are solid coloured - Short hair is caused by a dominant allele, S, while s/s cats have long hair - A long-haired cat with patches whose mother was solid-coloured and short- haired mates with a short-haired, solid coloured cat whose mother was long- haired and solid-coloured - What kinds of kittens can arise from this mating and in what proportions Extensions of Mendelian Principles or Cats are Not Peas - How do multiple genes interact to determine a single phenotype? What Happens When Multiple Genes often Contribute to a Single Characteristic - What happens when many genes contribute to a single phenotype: interaction between genes - F1 cross: A/a B/b x A/a B/b, this is still a dihybrid cross so we still have to consider Mendelian principles - If there is complete dominance at two distinct traits: phenotypes = 9:3:3:1 - Four potential outcomes - Gene interactions may produce novel phenotypes that will modify the 9:3:3:1 F2 phenotypic ratio - But will still be based upon it – total 16 Complete Dominance - D/D = this cat does not carry the dilute gene - D/d = this cat is a carrier of the dilute gene - d/d = this cat carries two copies of the dilute gene. The coat colour is diluted. - Black dilutes to blue, chocolate dilutes to lilac, cinnamon dilutes to fawn, and red dilutes to cream Molecular Explanation for Complete Dominance of Cat Colouration - The dilute gene (or melanophilin) affects the distribution of melanin granules within the cats’ hair - Dominant or dense allele (D): produces dense pigmentation - Recessive dilute (d) allele: results in pigment clumping, forming areas in the hair with no granules. This causes the cat coat to dilute or lighten - The dilute effect is autosomal recessive, hence a cat requires two copies of the d allele for the coat to dilute Complementation - White x white = white - White x white = white - White offspring mate = black - F2 phenotypic ratio = 9:7 Complementation – Two Different Albinos - White (c/c) x white (c/c) = white (c/c) - White (a/a) x white (a/a) = white (a/a) - White (c/c) x white (a/a) = black - Mutations in two different genes produce the same mutant phenotype - F2 phenotypic ratio = 9:7 - P: white (c/c) x white (c/c) and white (a/a) x white (a/a) - F1: white (c/c, A/A) x white (C/C, a/a) - F2 phenotypic ratio= 9:7 black (C/c, A/a) We Can Observe Complementation in Human Pedigrees - About 50 genes have recessive mutant alleles that can cause deafness in humans - A heterogenous trait: a mutation in any one of a number of genes can give rise to the same phenotype - Complementation: 9:7 - Non complementation: September 24 , 2013 Types of Natural Selection What Happens when Multiple Genes Contribute to a Single Characteristic? - Examples from last class  Complete dominance  Complementation - P: A/A b/b x a/a B/B; F1 A/a B/b - What gametes are produced? - Complete dominance (9:3:3:1) Complementation - Complementation occurs when two strains of an organism with different homozygous recessive mutations that produce the same phenotype, produce offspring of the wild-type phenotype when mated or crossed - Complementation will only occur if the mutations are in different genes - The other genome supplies the wild-type allele to “complement” the mutated allele - Complementation will not occur if the mutations are in the same gene What Happens When Multiple Genes Contribute to a Single Characteristic? - What gametes are produced? - 9:7 Take Home Problem - In cats, white patches are caused by the dominant allele P, while p/p individuals are solid coloured - Short hair is caused by a dominant allele, S, while s/s cats have long hair - A long-haired cat with patches whose mother was solid-coloured and short- haired mates with a short-haired, solid coloured cat whose mother was long- haired and solid-coloured - What kinds of kittens can arise from this mating and in what proportion - Ppss x ppSs Answer to Take Home Problem - ¼ PpSs: patches, short hair - ¼ ppSs: solid, short hair - ¼ Ppss: patches, long hair - ¼ ppss: solid, long hair - Does this agree with your answer Genetic Epistasis - 9:7 – observing complementation, assume that two alleles are acting on the genotype - Epistasis: the masking of the expression of one gene by another. No new phenotypes are produced - The epistatic gene does the masking - The hypostatic gene is masked - F2 phenotypic ratio = 9:3:4 – recessive epistasis  Homozygous recessives at one gene pair mask expression from the other gene  A/-b/b and a/ab/b have the same phenotype  bb gives recessive phenotype - F2 phenotypic ratio = 12:3:1 dominant epistasis  One dominant allele at one gene masks expression from the other gene  B locus does not matter, as long as there is
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