2/12/16
BIOLOGY 207 - Dr.Locke
Lecture#12 The Genetic Implications of Meiosis
Required readings and problems:
Reading: Open Genetics, Chapter 2
Problems: Chapter 2
Optional
Griffiths (2008) 9th Ed. Readings: pp 42-61
Problems: 9th Ed. Same as lecture #11
Campbell (2008) 8th Ed. Readings: Concept 14.1, 14.2
Concepts:
How do genes behave in meiosis?
1. Because genes exist on chromosomes, the behavior of chromosomes in meiosis
relates directly to gene behavior in heredity.
2. Observation of all four products of a single meiosis (a tetrad) allows us to track both
the chromosomes and their genes.
3. Analysis of tetrads confirms the predictions of gene behavior inferred from the
patterns of inheritance observed in organisms (Mendelian Genetics)
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 1
2/12/16
Genes are on chromosomes
Eukaryote chromosomes have”
telomeres(2), centromeres(1) & origins of replications(multiple): a linear, DNA duplex
telomere
centromere
telomere.
Each gene occupies a specific region of a particular chromosome - called a gene locus
The genes are in a linear order along the DNA
During chromosome replication each gene is replicated as the DNA is replicated.
An unreplicated chromosome has one chromatid
A replicated chromosome has two chromatids
-> the two chromatids have identical DNA sequences
-> identical copies of their genes -> sister chromatids
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 2
2/12/16
Drawing chromosomes:
DNA:
Chromosome:
Unreplicated:
Replicated chromosome:
NO butterfly chromosomes!
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 3
2/12/16
Tracking chromosomes and genes through Meiosis
In a diploid cell about to undergo meiosis
- each homolog in a chromosome pair has the same gene order as the other homolog
- but each may differ in the sequence -> different alleles
- one is paternal the other maternal in origin.
- genes go through process of meiosis as passengers on the chromosomes
- at the end --> Tetrad (all four products)
Yeast or molds
- all 4 products can be recovered and analyzed - called Tetrad Analysis
Humans or Drosophila
- the 4 products mix with the meiotic products of other meiocytes
- to analyse progeny -> ratios and frequencies
- gametes ratios and frequencies only inferred from their appearance in the progeny
- called random strand analysis
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 4
2/12/16
Patterns of Inheritance during Meiosis
Behavior of alleles at one gene locus
Segregation of Alleles – Mendel’s First Law
Genes that are different -> Alleles : - one from father - one from mother
Homologous pair
AM
AF
___AM______•_____
___AF______•_____
Follow the different chromosomes and their respective genes through the process of
meiosis
Conclusions:
-each of the 4 meiotic products
contains:
- one of the original 4 chromatids
- one of the original 4 alleles
in the meiocyte
-ratio of 2:2 or 1/2 gametes get one
chromosome 1/2 the other
-both chromosomes and alleles
segregate during meiosis
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 5
2/12/16
Behavior of alleles at 2 gene loci at different positions on the
same chromosome
Two possible situations:
1) If there is no crossover between the two gene loci:
- Alleles segregate together on the same chromosome
- A and B together and a and b together
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 6
2/12/16
2) If there is one crossover between the two gene loci
Two recombinant products:
- A and b now together in one meiotic product
- a and B now together in one meiotic product
- Alleles segregate from each other in Meiosis II
Two parental products
- the other two meiotic products are still AB and ab
Intra- chromosomal recombination – crossing over
Inter- chromosomal recombination – independent assortment
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 7
2/12/16
Extension of the 2 genes on the same chromosome
1) The frequency of crossing over is proportional to the distance between the
two loci.
The distance between two loci will determine how frequently a crossover will happen
Close: -If they are close, it will happen rarely
--> loci said to be linked (mostly parental combinations)
Distant: -If they are distant, it will happen frequently
-->partial linkage (some recombinant combinations)
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 8
2/12/16
2) More than two chromatids may be involved.
Simplest case:
- only 2 chromatids involved in one crossover
There can be 2 or more crossovers, which can include 2, 3, even all 4 chromatids.
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 9
2/12/16
Meiosis summary
1- Meiosis is a cellular process that segregates the chromosomes of a 2N meiocyte
into four products, each of 1N
2- segregates and shuffles genes (alleles)
3- promotes genetic diversity among the individuals in a population
(upon which natural selection can act. --> implications for evolution)
4- fundamental process in eukaryote organisms.
5- useful in understanding patterns of inheritance in ourselves, and the organisms we
wish to use.
Future lectures will examine and use the principles of
independent assortment and linkage.
Biol207 Dr. Locke section
Lecture#12
Fall'11
page 10