Download Genes are on chromosomes

Lecture 14; Feb 2007
Biology 207; Section B2; Good
The Genetic Implications of Meiosis
Readings:
Griffiths et al (2004)
8th Edition: pp. 94-103; 7th Edition: Ch. 3 pp 76-82; 179-186
Assigned Problems:
8th Ch. 3: 20, 39; 7th Ch. 3: 21, 22
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)
Genes are on chromosomes
Figure of Eukaryote chromosome
Chromosomes have telomeres (2), centromeres (1) & origins of replications (multiple)
Each chromosome is a long, linear, DNA duplex telomere centromere telomere.
Genes are regions of DNA along the chromosome.
Each gene occupies a specific region of a particular chromosome - called a gene locus
During chromosome replication (S-phase of the cell cycle) each gene is replicated as
the DNA is replicated.
A replicated chromosome has two chromatids
-> the two chromatids have identical DNA sequences
-> identical copies of their genes - sister chromatids
The genes are in a linear order along the DNA
It is the same order on each chromatid.
Drawing chromosomes:
DNA:
Chromosome:
Replicated chromosome:
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Lecture 14; Feb 2007
Biology 207; Section B2; Good
Tracking chromosomes and genes through Meiosis
In a diploid cell about to undergo meiosis
- have a pair of homologous chromosomes
- each homologous chromosome has the same gene order as the other homolog
- but each may differ slightly 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 (eg. Sordaria in your labs)
- all 4 products can be recovered and analyzed (can look at MI and MII)
- called Tetrad Analysis
Humans or Drosophila
the 4 products mix with the meiotic products of other meiocytes
can still can do analysis of progeny
ratios and frequencies of gametes only inferred from their appearance in the progeny
called random strand analysis
Patterns of Inheritance
Behavior of One Gene Locus – Segregation of Alleles
Genes can be slightly different. - Alleles : - one from father - one from mother
We can label the alleles on each chromosome.
Each gene on each chromosome can be labeled as to which parent it came from Homologous pair
AM ___AM______•_____
AP ___AP______•_____
Fig 3-35(8th) 3-26(7th) : to 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 chromatids in the meiocyte
-each of the 4 meiotic products contains one of the original 4 genes in the meiocyte
-ratio of 2:2 or 1/2 gametes get one chromosome 1/2 the other
-chromosomes said to segregate
-genes on those chromosomes also said to segregate during meiosis
Behavior of 2 different genes at different positions on the same chromosome
-chromosomes go through meiosis
Two possible situations:
1. If no cross-over between the two gene loci:
- Alleles segregate together on the same
chromosome - A and B together and a and b together
2. If there is a cross-over between the two gene loci
- Alleles segregate from each other in Meiosis II
Two recombinant products:
- A and b now together in one meiotic product
- a and B now together in one meiotic product
Two parental products
- the other two meiotic products are still AB and ab
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Lecture 14; Feb 2007
Biology 207; Section B2; Good
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Extension of the 2 genes on the same chromosome
1) Frequency of crossing over is proportional to the distance between the two loci.
The distance between two loci will determine how frequently this cross-over effect will
happen
Close:
-If they are close, it will happen rarely
--> loci said to be linked
Distant:
-If they are distant, it will happen frequently
-->partial linkage
2) More than two chromatids may be involved.
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Lecture 14; Feb 2007
Biology 207; Section B2; Good
Simplest case:
- only 2 chromatids involved in one cross-over
There can be 2 or more cross overs, which can include 2, 3, even all 4 chromatids.
Meiosis
Meiosis is a process that segregates and shuffles genes (alleles) and thereby promotes
genetic diversity among the individuals in a population and upon which natural selection
can act. --> implications for evolution
It is a fundamental process in eukaryote organisms.
The process of meiosis is also very useful in understanding patterns of inheritance in
ourselves and the organisms we wish to use.
We will return to the process of meiosis in future lectures to examine and use the
principles of independent assortment and linkage.
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