BIOL239 Study Guide - Final Guide: Dna Polymerase Iii Holoenzyme, Ribosome-Binding Site, Allele Frequency

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Published on 12 Apr 2013
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Mendelian Genetics
Why garden peas?
Easy to cross fertilize
Large numbers of offspring
Short growing season
Terms
Phenotype: observable characteristic (largely determined by genotype). Commonly
referred to as a trait
Genotype: genetic make-up
Monohybrid Cross: matings between individuals that only differ in one trait
Gene: unit of inheritance
Allele: alternative form of a single gene
Polymorphic: more than one wildtype allele; allele frequency greater than 1%
Monomorphic: only one wildtype allele; only one allele above 1% frequency
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 fertilization.
At the DNA level, alleles vary in nucleotide sequence. This can result in:
o New amino acid sequence
o Change in the amount of protein
o E.g. Garden peas R” allele gives the pea a round shape because this version of the
starch branching enzyme is normal. The r allele has a different amino acid
sequence and the enzyme is non-functional (i.e. no branched starch).
Genetic Crosses
Test Cross: Breeding an unknown genotype with a homozygous recessive will reveal its
genotype. If it was homozygous dominant, all progeny are 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.
The Law of Sum: The probability of either of two mutually exclusive events occurring is
the sum of their individual probabilities
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Dihybrid Cross: matings of individuals that differ in 2 traits. More phenotypic classes are
seen because there are more allele combinations for independent assortment. Mating
between dihybrids produces a 9:3:3:1 phenotypic ratio.
Multihybrid Cross: matings that differ in 3 or more traits
o Huge punnet square. Use a simpler method. E.g., Aa Bb Cc Dd x Aa Bb Cc Dd
o Probability of an AA bb Cc Dd offspring: ¼ x ¼ x ½ x ½ =1/64
o Modifications of Mendelian Ratios
Crossing: when a gene has more than 2 alleles, reciprocal crosses help determine
dominance relationships by crossing pure breeding lines. A dominance series shows
most dominant to least dominant.
Modifications of Mendelian Ratios
Types of Dominance
Incomplete Dominance: the F1 Hybrid doesn’t resemble either parent (intermediate
phenotype).
o E.g. Flower pigments. A red flower and a white flower produce all pink F1’s. The
F2’s will have a 1:2:1 phenotypic ratio.
Codominance: Both traits are expressed in the F1 Hybrid.
o E.g. A dotted parent and a spotted parent mate to make an F1 that is all dotted
and spotted. The F2 generation also has a 1:2:1 ratio.
Blood type is also an example of codominance.
o IA = makes enzyme to add A sugar
o IB = makes enzyme to add B sugar
o IO or i = no enzymes - no sugars added
o IAIB = both enzymes to add both A + B sugars
Alleles and Inheritance
Allele Frequency: frequency of a given allele within a population
o Mutant Allele Less than 1%
o Wild-type Allele Greater than 1%
Pleiotropy: Multiple phenotypic effects from one allele (E.g., sickle cell anemia).
Recessive Lethal: Homozygotes for this allele die. F2 is 2:1 (when heterozygotes are
crossed).
o E.g. The yellow allele in mice is the dominant for colour, but recessive for
lethality.
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Multifactorial Inheritance
Arising from the action of two or more genes (polygenic), or from interactions between
genes and the environment. F2 is 9:3:3:1 (same as dihybrid cross).
Complementary Gene Action: two or more genes can work in tandem, in the same
biochemical.
Pathway to produce a particular trait. F2 is 9:7.
Heterogeneous Trait: a mutation at any one of a number of genes can give rise to the
same phenotype
Complementation Testing: determines if 1 gene or 2 or more are involved in producing a
trait.
o If the mutation is on a different gene, complementation can happen.
o No complementation on the same gene.
Epistasis: a gene interaction in which the effects of an allele at one gene hide the effects
of alleles at another gene.
Recessive: when both alleles are recessive, it hides expression.
Dominant: dominant allele hides expression.
Hypothesis Testing: can determine mode of inheritance from F2 ratios (but not in
humans).
o Must use pedigrees in humans.
Penetrance vs. Expressivity
Penetrance: percentage of the population with a particular genotype that demonstrate
the expected trait.
Expressivity: the degree or intensity with which a particular genotype is expressed in a
phenotype within a population.
o E.g. Retinoblastoma
75% penetrance (25% do not develop the disease)
of those who do get the disease, some get in it only one eye (30%
expressivity)
Sex-linked traits
Due to genes ON the X or Y chromosome
Sex-limited Traits: affect a structure or process that is found in one sex but not the other
Sex-influenced Traits: show up in both sexes but their expression may differ between
the two sexes
Note: The environment can also influence the phenotype of a given genotype.
(Himalayan coat)
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Document Summary

Phenotype: observable characteristic (largely determined by genotype). Monohybrid cross: matings between individuals that only differ in one trait. Allele: alternative form of a single gene. Polymorphic: more than one wildtype allele; allele frequency greater than 1% Monomorphic: only one wildtype allele; only one allele above 1% frequency. The two alleles for each trait separate (segregate) during gamete formation then unite at random, one from each parent, at fertilization. At the dna level, alleles vary in nucleotide sequence. This can result in: new amino acid sequence, change in the amount of protein, e. g. Garden peas r allele gives the pea a round shape because this version of the starch branching enzyme is normal. The r allele has a different amino acid sequence and the enzyme is non-functional (i. e. no branched starch). Test cross: breeding an unknown genotype with a homozygous recessive will reveal its genotype. If it was homozygous dominant, all progeny are heterozygous and have the same phenotype.

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