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Genetics Review- Mendelian Genetics to Sex-Linked Traits

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University of Waterloo
BIOL 239
Christine Dupont

GENETICS REVIEW Mendelian Genetics Belyaev – artificially bred foxes ... happy foxes vs. Aggressive foxes Gregor Mendel  Laws of inheritance  Experimented with garden peas Why Garden peas?  Easy to cross fertalize  Large numbers of offspring  Short growing season Terms:  Phenotype – observable characteristic, can also be behaviour, referred to as a trait  Genotype- genetic make-up, version of the gene  Monohybrid cross – matings between individuals that only differ one trait. Example tall vs short, yellow vs green.  Gene - unit of inheritance. Example: one gene in humans for eye colour.  Allele – alternative form of a single gene. Example: ^ multiple versions of eye colour.  Polymorphic – more than one wiltype allele, allele frequency greater than 1%, several alleses that occurs normally in a population.  Monomorphic – only one wild type alle, only one allele above 1%, one alle that is normally present in a population. Example: sickle cell anemia. Mendel’s Experiment  Used selective breeding to produce true-breeding lines of peas  Found a consistent pattern of inheritance Mendel’s Law of Segregation:  The two alleles for each trait separate (segregate) during gamete formation, then unite at random, one from each parent at fertalization.  At the DNA level, alleles vary in nucleotide sequence. This can result in: o New amino acid sequence o Change in amount of protein o Example: garden peas. R gives the pea a round shape because the version of starch branching enzyme is normal. The r allele as a different amino acid sequence and the enzyme is non-functional (ie. No branched starch) Mendel’s Law of Independent Assortment  During gamete formation, different pairs of alleles segregate independently of each other.  Dominant allele G does not always travel and pair with dominant W.  Law of Segregation describes how different alleles of a gene behave. Law of independant assortment describes how different genes behave. Monohybrids  Phenotypic ratio: 3:1  Genotypic ratio 1:2:1 Genetic Crosses  To determine genotype, use a test cross.  Test crosses – breeding an unknown genotype with a homozygous recessive indivudal will reveal it’s genotype. If it was homozygous dominant, all offspring would be heterozygous and have the same phenotype. If it was heterozygous, we would see a 1:1 phenotypic ratio.  The law of product: The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself. AND IS KEY WORD.  Example: coin toss – chance of heads twice in a row. Probability of a head and a head. ½ x ½ = ¼  The law of sum: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities. OR IS KEY WORD.  Example: dice throw – chance of an even number = probability of a 2, 4, or a 6. 1/6 + 1/6 + 1/6 = ½.  Dihybrid Crosses – Matings between individuals that differ in two traits. There are more phenotypic classes seen because there are more allele combo’s for independant assortment. 9:3:3:1 phenotypic ratio.  Multihybrid Cross – Matings between individuals that differ in three or more traits. Eg. Aa Bb Cc Dd x Aa Bb Cc Dd ... branching diagram, or... probability of an AA bb Cc Dd offspring : ¼ x ¼ x ½ x ½ = 1/64 -Modifications of Mendels Ratios  Crossing – When a gene has more than 2 alleles, reciprocal crosses help determine dominance relationships by crossing pure breeding lines. Modifications of Mendelian Ratios Types of Dominance:  Incomplete Dominance – The F1 hybrid doesn’t resemble either purebred parent. There is often an intermediate phenotype. It doesn’t look like dominant or recessive phenotype.  Example: flower pigments. A red flower and a white flower produce all Pink F1s. The F2s will have a genotypic and phenotypic ratio of 1:2:1 – one AA (red), two Aa (pink) and one aa (white).  Codiminance – alternative traits are both visible in F1 hybrid. F2 generation also genotypic and phenotypic ratio of 1:2:1.  Blood type is also an example of codominance. A IB= makes enzyme to add sugar A I = makes enzyme to add sugar B I or I = no ezymes = no sugars added I I = makes both enzymes to add sugars A and B A B I and I are codiminant to each other, but they are both dominant to the I allele. Note: A gene may have more than two alleles  Reciprocal crosses cna be done between pure breeding lines (homozygous) representing all phenotypes to establish the dominance series.  Ex. Bunnies: cc = albino, c c = Himalayan, c c = chinchilla, and c c = wildtype.  Wild is dominant over Himalayan, wild is dominant over albino. Himalayan is dominant over albino.  C (wildtype) has a frequency of greater than 99% therefore is a monomorphic gene Alleles and Inheritance  Allele frequency – percentage of total number of copies of a gene in a population represented by a particular allele.  Wild type allele: greater than 1%  Mutant allele: less than 1%  Monomorphic: gene with only one wild type allele, all other mutant.  Polymorphic: gene with more than one wild type allele.  Pleiotropy: m
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