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Chapter 13

BIO342H5 Chapter Notes - Chapter 13: Dihybrid Cross, Gregor Mendel, Mendelian Inheritance

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Don Mc Kenzie

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The basic unit of heredity is the gene (composed of DNA located on chromosomes)
When genes exist in more than one form, they are called alleles
The genetic makeup of an individual is the individual’s genotype
o The physical manifestation of genetic makeup is the individual’s phenotype
Mendelian Genetics
Gregor Mendel took true-breeding individuals with different traits, mated them, and
statistically analyzed the inheritance of the traits in the progeny
A. Mendel`s First Law: Law of Segregation
o Genes exist in alternate forms (alleles)
o An organism has two alleles for each inherited trait (one from each parent)
o The two alleles segregate during meiosis
The gametes carry only one allele for any given trait
o If two alleles in an individual organism are different, only one is fully expressed
(dominant) while the other is silent (recessive)
Organisms that contain two copies of the same allele are homozygous
Organisms that carry two different alleles are heterozygous
1. Monohybrid
A cross between two true-breeding organisms with focus on a single trait
The individuals being crossed are the parental (P) generation
The progeny are the filial (F) generation
2. Punnett Square
One way of predicting the genotypes expected
3. Testcross
Only with a recessive phenotype can genotype be predicted with 100
percent accuracy
If a dominant phenotype is expressed, the genotype can be either
can be homozygous dominant or heterozygous
Only homozygous recessive organisms always breed true
Here an organism with a dominant phenotype of unknown genotype is
crossed with a homozygous recessive gene
The appearance of the recessive phenotype in the progeny
indicates that phenotypically dominant parent is heterozygous
B. Mendel’s Second Law: Law of Independent Assortment
1. Dihybrid Cross
Parents differing in two traits (which assort independently) is crossed
Known as independent assortment
The F1 generation will produce 4 different phenotypes in the ration of
2. Statistical Calculations
The probability of producing a genotype that requires the occurrence of
two independent events is equal to the product of the individual
probabilities that these events will occur (i.e. 1/16)
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The probability of producing a genotype that can be the result of more
than one event is equal to the sum of the individual probabilities that
these events will occur
3. Problem Solving (see p. 217)
The Chromosomal Theory of Inheritance
Diploids species have chromosomes pairs (homologues)
In diploids, alleles for a given trait are segregated
o One allele is located on one chromosome
o The other allele is found on its homologue
A. Segregation and Independent Assortment
o During the first meiotic division, the homologous pairs separate, and following
cytokinesis, the number of chromosomes per cell is reduced from 2N to N
This is the step in meiosis during which segregation and independent
assortment occur
B. Nonindependent Assortment: Genetic Linkage
o Not all traits assort independently in a dihybrid cross
These crosses (AaBb x AaBb) produce an F1 genotypic ration of 1:2:1
4 AABB: 8 AaBb: 4 aabb
The segregation pattern for a dihybrid cross is like that of a single trait
monohybrid cross
o Genes A and B are linked (i.e. located on the same chromosome) and so they
are inherited together
o Tightly linked genes recombine at a frequency close to 0 percent whereas weakly
linked genes recombine at frequencies approaching 50 percent
C. Recombination Frequencies: Genetic Mapping
o Recombinant chromosomes arise from the physical exchange of DNA between
homologous chromosomes paired during meiosis through crossing over (genetic
o The degree of genetic linkage is a measure of how far apart two genes are on
the same chromosome
The probability of a crossover b/w two points is generally directly
proportional to the distance b/w the points
Pairs of genes that are apart from each other on a chromosomes
have a higher probability of being separated during crossing over
o Recombination frequencies can be used to construct a genetic map
One map unit is defined as a 1 percent recombinant frequency
Recombination frequencies are roughly additive
If genes are found on a map in the order XYZ, the recombination
frequency b/w X and Y and between Y and Z will be roughly equal
the recombination frequency between X and Z
See Figure 13.6
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