GENETICS 2581 FINAL
EXAM REVIEW
5:30pm April 14th, 2023
, Sex and Meiosis
Recombination and Negative Interference
Meiosis
• DNA is replicated during Interphase
• Meiosis consists of 2 division events:
o Meiosis I: segregation of homologous chromosomes
▪ Produces 2 cells, each containing 1 homologous chromosome from each
homologous pair
▪ Metaphase I: cell undergoes reduction from 2n = 46 → n = 23
▪ Major difference from mitosis is alignment of homologous chromosomes
▪ Often thought of as modified mitosis
o Meiosis II: segregation of sister chromatids
▪ Produces 4 gametes, each containing 1 sister chromatid from each
homologous pair
Problem Solving:
• Counting chromosomes = counting centromeres
• Chances that a given version of a given number of homologous chromosomes end up in
the same gamete:
o 0.5n, where n = number of chromosomes
1
,Formation of the Tetrad and Occurrence of Recombination
• During Prophase I, synapsis—the alignment of homologous chromosomes—occurs
• Two homologous chromosomes from a homologous pair form a synaptonemal complex
between two homologs from the same homologous pair
o The synaptonemal complex formed during meiosis I is called the tetrad because
it consists of 4 sister chromatids
• During meiosis I when homologous chromosomes form a synaptonemal complex / tetrad,
can undergo recombination
o Recombination: the physical exchange of information resulting in a portion of an
arm of one homologous chromosome being exchanged with a portion of an arm
from the other homologous chromosome
▪ Exchange between sister chromatids from different homologs belonging to
the same homologous pair
o Generates genetic diversity
Mendel’s Laws
Law of Random Segregation
• Every individual possesses 2 alleles for a given gene
• Only one allele is passed onto their offspring (each gamete contains one allele)
• Two gametes fuse to form a zygote such that offspring inherit one maternal allele and one
paternal allele for the same gene
• Heterozygotes will undergo meiosis obeying Mendel’s Law of Segregation to produce a
2:2 ratio of segregation
o 2 gametes will receive one allele
o 2 gametes will receive the other allele
Law of Independent Assortment
For unlinked genes (ie., genes on different chromosomes):
• Inheritance of alleles for one gene is independent of inheritance of alleles for a different
gene
• Alleles for different genes are segregated into gametes independently of one another
• Each gamete has an equal probability of inheriting either allele because pairs of
homologous chromosomes line up randomly at the metaphase plate during Metaphase I
of meiosis I
2
, Gene Mapping and Recombination
• Generally, recombination is equally likely to occur at each location along a chromosome
• There are more possible sites for recombination to occur between genes further apart on a
chromosome than between genes which are closer together
• The frequency of recombination between two genes is proportional to the distance
between the genes
o Higher likelihood of recombination between genes further apart
o Smaller likelihood of recombination between genes closer together
• It is possible to use recombination between homologous chromosomes during meiosis I
to map genes on chromosomes
• Gene mapping can be done via 2 strategies:
o 2 point test cross
o 3 point test cross
Two-Point Test Cross
i) Initial parental cross using parental genotypes that will produce a heterozygote in the F1
generation
• F1 heterozygote must have four distinct alleles for the two genes
o AaBb
• Parental genotypes must be homozygous for each gene
o Homozygous = dominant alleles are in cis and recessive alleles are in cis
• 2 options for parental genotypes:
o AABB x aabb
o AAbb x aaBB
ii) Cross the F1 heterozygote whose meiosis we want to study to an individual homozygous
for the recessive alleles in both genes (aabb)
iii) Look at the phenotypes of the progeny from this cross in the F2 generation to analyze the
meiosis of the F1 heterozygote
• The recessive homozygote in F1 will always contribute the recessive allele for both genes
to the F2 offspring
o In the F2 offspring, identify the recessive alleles contributed from the
homozygous recessive parent
• The other alleles in each offspring were contributed by the F1 heterozygote (whose
meiosis we want to study)
o 2 offspring genotypes will be of parental type: alleles contributed by the F1
heterozygote will be in the same combinations as they were in the initial parental
generation
▪ Ie., AB and ab
o 2 offspring genotypes will be of recombinant type: alleles contributed by the F1
heterozygote are in different combinations than they were in the initial parental
generation
3
EXAM REVIEW
5:30pm April 14th, 2023
, Sex and Meiosis
Recombination and Negative Interference
Meiosis
• DNA is replicated during Interphase
• Meiosis consists of 2 division events:
o Meiosis I: segregation of homologous chromosomes
▪ Produces 2 cells, each containing 1 homologous chromosome from each
homologous pair
▪ Metaphase I: cell undergoes reduction from 2n = 46 → n = 23
▪ Major difference from mitosis is alignment of homologous chromosomes
▪ Often thought of as modified mitosis
o Meiosis II: segregation of sister chromatids
▪ Produces 4 gametes, each containing 1 sister chromatid from each
homologous pair
Problem Solving:
• Counting chromosomes = counting centromeres
• Chances that a given version of a given number of homologous chromosomes end up in
the same gamete:
o 0.5n, where n = number of chromosomes
1
,Formation of the Tetrad and Occurrence of Recombination
• During Prophase I, synapsis—the alignment of homologous chromosomes—occurs
• Two homologous chromosomes from a homologous pair form a synaptonemal complex
between two homologs from the same homologous pair
o The synaptonemal complex formed during meiosis I is called the tetrad because
it consists of 4 sister chromatids
• During meiosis I when homologous chromosomes form a synaptonemal complex / tetrad,
can undergo recombination
o Recombination: the physical exchange of information resulting in a portion of an
arm of one homologous chromosome being exchanged with a portion of an arm
from the other homologous chromosome
▪ Exchange between sister chromatids from different homologs belonging to
the same homologous pair
o Generates genetic diversity
Mendel’s Laws
Law of Random Segregation
• Every individual possesses 2 alleles for a given gene
• Only one allele is passed onto their offspring (each gamete contains one allele)
• Two gametes fuse to form a zygote such that offspring inherit one maternal allele and one
paternal allele for the same gene
• Heterozygotes will undergo meiosis obeying Mendel’s Law of Segregation to produce a
2:2 ratio of segregation
o 2 gametes will receive one allele
o 2 gametes will receive the other allele
Law of Independent Assortment
For unlinked genes (ie., genes on different chromosomes):
• Inheritance of alleles for one gene is independent of inheritance of alleles for a different
gene
• Alleles for different genes are segregated into gametes independently of one another
• Each gamete has an equal probability of inheriting either allele because pairs of
homologous chromosomes line up randomly at the metaphase plate during Metaphase I
of meiosis I
2
, Gene Mapping and Recombination
• Generally, recombination is equally likely to occur at each location along a chromosome
• There are more possible sites for recombination to occur between genes further apart on a
chromosome than between genes which are closer together
• The frequency of recombination between two genes is proportional to the distance
between the genes
o Higher likelihood of recombination between genes further apart
o Smaller likelihood of recombination between genes closer together
• It is possible to use recombination between homologous chromosomes during meiosis I
to map genes on chromosomes
• Gene mapping can be done via 2 strategies:
o 2 point test cross
o 3 point test cross
Two-Point Test Cross
i) Initial parental cross using parental genotypes that will produce a heterozygote in the F1
generation
• F1 heterozygote must have four distinct alleles for the two genes
o AaBb
• Parental genotypes must be homozygous for each gene
o Homozygous = dominant alleles are in cis and recessive alleles are in cis
• 2 options for parental genotypes:
o AABB x aabb
o AAbb x aaBB
ii) Cross the F1 heterozygote whose meiosis we want to study to an individual homozygous
for the recessive alleles in both genes (aabb)
iii) Look at the phenotypes of the progeny from this cross in the F2 generation to analyze the
meiosis of the F1 heterozygote
• The recessive homozygote in F1 will always contribute the recessive allele for both genes
to the F2 offspring
o In the F2 offspring, identify the recessive alleles contributed from the
homozygous recessive parent
• The other alleles in each offspring were contributed by the F1 heterozygote (whose
meiosis we want to study)
o 2 offspring genotypes will be of parental type: alleles contributed by the F1
heterozygote will be in the same combinations as they were in the initial parental
generation
▪ Ie., AB and ab
o 2 offspring genotypes will be of recombinant type: alleles contributed by the F1
heterozygote are in different combinations than they were in the initial parental
generation
3
