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BIOLOGY 2C03 (138)
Joe Kim (16)
Lecture 3

Lecture 3 – January 22, 2013.docx

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Joe Kim

BIO 2C03 2013 Lecture 3 – January 22, 2013  Cats are not peas  Multiple genes determine phenotypic traits  Dihybrid ratio – 9:3:3:1  Modified dihybrid ratio  Actors influencing phenotypic expression  Case study: Phenotypes in cats Multiple Alleles  Population – multiple alleles may exist  Individual (diploid) – only two alleles coexist in each cell Dominance/Allelic Series  Eg/ Multiple alleles contributing to fur coloration in cats o Five alleles: C = full color c = Burmese c = Siamese c = white, blue eyes c = albino, pink eyes o C > c = c > c > c o Where: > indicates the dominance = indicates incomplete dominance or codominance  The relationship between pairs of alleles is important  When determining phenotype, look at the dominance relationship between the two alleles in a single individual  The C gene codes for an enzyme called Tyrosinase  Comparison of wildtype and albino allele o A cytosine deletion in Tyrosinase at position 975 in exon 2, which causes a frameshift resulting in a premature stop codon nine residues downstream from the mutation  Frameshift Mutation – the loss of a single nucleotide causes the reading frame, the series of three nucleotide codons, to be read differently o This is a non-functional protein and the allele is described as a loss of function allele  The number of different genotypes is determined by the number of alleles in the population o C, c , c , c C (wildtype) o This gives, 15 genotypes of which there are five different homozygous genotypes and 10 different heterozygotes 1 BIO 2C03 2013  Loss of Function Allele – an enzyme or other protein is no longer being produced, is produced at lower levels or is nonfunctional  Wildtype Allele – a functional enzyme or other protein is produced o Used to refer to the most common phenotype (or genotype) found in a natural population  Haplosufficiency – wildtype allele is dominant over the loss of function allele because half as much protein is synthesized and this is sufficient to achieve the normal phenotype o For fur color, in the C/c genotype, half as much Tyrosinase is synthesized, but this is sufficient to achieve full coloration  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 o Eg/ Huntington’s disease  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 o Eg/ Tailless cats (Manx)  Law of Segregation (Gregor Mendel) – allele pairs separate or segregate during gamete formation and randomly unite at fertilization  Demonstrated by monohybrid crosses – one trait o Eg/ Aa x Aa  Examined dihybrid crosses – two traits o Eg/ Aa Bb x Aa Bb  Law of Independent Assortment (Gregor Mendel) – the inheritance pattern of one trait will notaffect the inheritance pattern of another trait o Each trait is inherited by the Mendelian ratio, 3:1, but together we see a phenotypic ratio of 9:3:3:1 o Eg/ cat color and tail length - the genotype of two independent traits show a 9:3:3:1 ratio2in the F generation; when parents are homozygous for each trait, their children in 1he F generation are heterozygous at both loci and only show the dominant phenotypes. If the children mate with each other, in t2e F generation all combination of coat color and tail length occur S gene: S = short tail s = long tail B gene: B = brown fur b = white fur 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  You can use a similar procedure for determining the proportion of genotypes and phenotypes if you are following more than two alleles o Eg/ Trihybrid crosses etc  Trihybrid: 64 combinations of gametes (8x8 Punnett Square) o 27 different 2 genotypes o 8 different 2 phenotypes o Ratio: 27:9:9:9:3:3:3:1 2 BIO 2C03 2013 o Eg/ In cats  White patches are caused by the dominant allele P, while p/p individuals are solid colored  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-colored and short-haired mates with a short-haired, solid- colored cat whose mother was long-haired and solid colored  What kinds of kittens can arise from this mating and in what proportions? Extensions of Mendelian Principles  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: interactions between genes o If there is complete dominance at two district traits: phenotypes = 9:3:3:1 o Dihybrid; phenotype = 9:3:3:1  Gene interactions may produce novel phenotypes that will modify the 9:3:3:12F phenotypic ratio (will still be based on it (total 16))  F 2henotypic Ration = 9:3:3:1 o Complete dominance at two gene pairs contributing to single trait; four distinct phenotypes o Effect: dilution of color due to a second gene  D/D = does not carry the dilute allele  D/d = cat is a carrier of the dilute allele 3 BIO 2C03 2013  d/d = carries two copies of the dilute allele; coat color is diluted  Molecular Explanation o The melanophilin gene affects the distribution of melanin granules within the cat’s hair o Dominant, dense allele (D) – produces dense pigmentation o Recessive, dilute allele (d) – results in pigment clumping, forming areas in the hair with no granules; causes the cat coat to dilute or lighten o The dilute effect is autosomal recessive, hence a cat requires two copies of the d allele for the coat to dilute  Epistasis – no new phenotypes are produced, instead the one gene masks or modifies the phenotypic expression of another gene  F 2henotypic Ratio – 9:3:4 – Recessive Epistasis o Homozygous recessives at one gene pair mask expression from the other gene o A/- b/b and a/a b/b have the same phenotypic o Similar for the pattern in cats  If the genotype is c/c, the mouse is white – regardless of the genotype at the other locus o Molecular Mechanism:  c/c = no pigment is synthesized; mice are white regardless of genotype at the A locus  recessive epistasis  The genotype at the A locus determines how the pigment is deposited if it is made:  Either black (a/a) or agouti (A/-)  F 2henotypic Ratio – 12:3:1 – Dominant Epistasis o One dominant allele at one gene masks expression from the other gene o A/- B/- and A/- b/b have the same phenotype  If the genotype is W/- the cat is white – regardless of the genotype at the other locus  Only in the w/w genotype, B/- is black and b/b is brown  F 2henotypic Ratio – 9:7 – Complementation o Mutations in two different genes produce the same mutant phenotype (see complementation in dihybrid) o Two different albinos 4 BIO 2C03 2013  We can observe complementation in human pedigrees  About 50 genes have recessive mutant alleles that can cause deafness in humans  Heterogeneous trait – a mutation in any one of a number of genes can give rise to the same phenotype  Phenotypic Ratio – 9:7 – Duplicate Recessive Epistasis o c/c masks expression of the A allele giving white fur o a/a masks expression of the C allele giving white fur Other modifications to our classic ratios  Essential Genes – loss of function alleles lead to a lethal phenotype o Eg/ Agouti gene; modifier that changed black to mottled grey o A/- = grey fu
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