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MBG 2040 Chapter Notes - Chapter chapter 3 & 4 with study question answers: Epistasis, Genotype, Wild TypeExam

Molecular Biology and Genetics
Course Code
MBG 2040
James Uniacke
Study Guide

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We looked at patterns of inheritance for a variety of traits in cats in order to understand Mendel’s Law of
Independent Assortment, the principles of inheritance of dihybrid traits, and the ways in which genes interact
to produce a phenotype.
At the end of this Chapter, you will be able to:
solve problems dealing with multiple alleles
define, recognize, describe and apply Mendel’s second principle
solve genetic problems relating to dihybrid and trihybrid crosses
use the rules of probability to determine genotypic and phenotypic proportions
recognize modified dihybrid Mendelian ratios
understand examples of non-Mendelian inheritance
Chapter 3: Mendelism: The Basic Principles of Inheritance.
Pg 44-48, 53-56
Chapter 4: Extensions of Mendelism
Pg 62-77
Not all alleles display a simple dominant/recessive relationship. Not all phenotypes are determined by a
single gene. What happens to the essential Mendelian ratios when there are complex interactions between
alleles and/or genes?
More than two allelic forms of a gene can exist in a population, however, any given diploid individual
can possess only two different alleles of a given gene.
The phenotype must be determined from allelic relationship between the two alleles present in an
An allelic series supplies the information about the relationships between alleles.
The terms recessive, dominant, co-dominant, and incompletely dominant are terms that are reserved for
the relationship between alleles at a single gene.
The inheritance of two genes can be followed using Mendelian principles. Both Mendel’s first and second
principles are required to explain dihybrid patterns of inheritance.
Mendel’s first principle: Law of Segregation
Mendel’s second principle: Law of Independent Assortment
Multiple genes may contribute to a single phenotype. These genes interact in the determination of the
final phenotye. Modified Mendelian ratios represent different interactions between genes.
9:3:3:1 no interaction; modifier gene that modifies all phenotypes similarly
9:3:4 recessive epistasis
12:3:1 dominant epistasis
9:7 complementation
You could memorize these ratios, or you could note that all are based on a the core genotypic ratios
9/16 : 3/16 : 3/16 : 1/16. Then, simply figure out which classes are collapsing together. E.g. 12:3:1
= (9+3) : 3 : 1 because A/- B/- and a/a B/- look the same. Therefore the dominant B allele is masking
expression from the A locus.
Work through this logic with the other ratios.
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Lethal alleles produce a 2:1 ratio because one class is completely missing: 1:2:1
Complementation Test. A genetic cross that distinguishes between two or more mutations in a
single gene and two or more mutations in different genes.
Penetrance is the frequency with which a mutant phenotype (dominant or homozygous recessive
gene) is expressed in a population. e.g. the percentage of a/a individuals that exhibit the mutant
Expressivity is the extent of the severity of the phenotype from mild to strong.
Describe the effects of environment on phenotypic expression. This would include the effects of
age, sex, temperature and exposure to chemicals.
Chapter 3:
3.6, 3.24, 3.25
Chapter 4:
4.5, 4.6, 4.7, 4.11, 4.21, 4.27
3.6 In mice, the allele C for colored fur is dominant over the allele c for white fur, and the allele V for
normal behavior is dominant over the allele v for waltzing behavior, a form of discoordination. Give the
genotypes of the parents in each of the following crosses:
(a) Colored, normal mice mated with white, normal mice produced 29 colored, normal and 10 colored,
waltzing progeny;
(b) Colored, normal mice mated with colored, normal mice produced 38 colored, normal, 15 colored,
waltzing, 11 white, normal, and 4 white, waltzing progeny;
(c) Colored, normal mice mated with white, waltzing mice produced 8 colored, normal, 7 colored,
waltzing, 9 white, normal, and 6 white, waltzing progeny.
ANS: (a) colored, normal (CC Vv) white, normal (cc Vv) (b) colored, normal (Cc Vv) colored,
normal (Cc Vv); (c) colored, normal (Cc Vv) white, waltzing (cc vv).
3.24 In the following pedigrees, determine whether the trait is more likely to be due to a dominant or a
recessive allele. Assume the trait is rare in the population.
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