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BIOL239 Study Guide - Final Guide: Dihybrid Cross, Allele Frequency, Genotype Frequency

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Christine Dupont
Study Guide

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Biology 139
Mendelian Genetics
Why garden peas?
Easy to cross fertilize
Large numbers of offspring
Short growing season
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 then 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 gives the pea a round shape because this version of the Starch
Branching Enzyme is normal. The r allele as 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 the 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.

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The law of sum: The probability of either of two mutually exclusive events occurring is the sum
of their individual probabilities
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 Mendellian 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
Types of Dominance
Incomplete Dominance The F1 Hybrid doesn’t resemble either parent (intermediate
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.
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

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Heterogeneous Trait - A mutation at any one of a number of genes can give rise to the same
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
Note: the environment can also influence the phenotype of a given genotype. (Himalayan coat)
Conditional lethality occurs with a particular type of allele which is lethal under only certain
o Conditions are said to be permissive or restrictive
Pedigree Analysis
Many traits do not show the Mendelian pattern of inheritance because most traits in humans are
influenced by more than one gene.
Most single gene traits in humans are life-threatening or disabling
Examples of single-gene traits: Thalassemia, Sickle-cell disease and PKU.
Thalassemia: there is a mutation in front of the coding region for the B-globin chain of
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