What is the 2n value of chromosomes for us?
2n = 46
therefore we have 23 chromosomes from each parent
What genetic variation is gained at the metaphase plate?
We can get new combinations of alleles based on how the chromosomes align on the metaphase plate
What will our haploid set of chromosomes be?
The haploid set (n=23) will be a random combination of
chromosomes from maternal & paternal sets
With genes 2 different chromosomes?
We have 2 possibilities and equal chance of getting an allelic variation the same from a parent or getting a non-parental allelic combination
What occurs when there are 2 genes on the same chromosome?
We will get the same allelic gamete combinations as the parents had
What is a linkage group?
Linkage group: a collection of genes that do not assort independently into gametes because they are
located on the same chromosome.
Are genes in a linkage group all the same?
Not all genes in a linkage group are the same:
* strongly linked genes (physically close loci)
* weakly linked genes (physically distant loci)
* genes very far apart assort independently
If 2 genes are linked what would you expect?
If two genes are “linked” would you expect…
* 9:3:3:1 ratio from the cross AaBb × AaBb?
* 1:1:1:1 ratio from a test cross (AaBb × aabb)?
NO!
WHY?
… because Mendelian ratios assume the principle of independent
assortment.
What is a meiocyte?
A meiocyte is a diploid cell in which meiosis can take place
Difference between before and after meiosis?
sister chromatids are all genetically identical.
Crossing over occurs….
we get 50% non-recombinant and 50% recombinant therefore the sister chromatids are not identical
After crossing over…?
linear DNA has been exchanged between non-sister chromatids.
What occurs if 2 genes are very close on the same chromosome?
only crossovers here will produce Ab and aB but this is very unlikely and we will have no recombination
What occurs if 2 genes are close on the same chromosome?
more opportunities for crossover to produce Ab and aB but not in every meiosis event.
we might observe 40/40/10/10
What occurs if the 2 genes are very far apart on the same chromosome?
If loci are far apart we will; see crossovers in every meiosis event. ALWAYS separated by recombinant events.
What is recombination?
Recombination: assortment of alleles into novel (non-parental) combinations
What is inter-chromosomal recombination?
Inter-chromosomal recombination: assortment of alleles for genes on different chromosomes
into novel combinations.
Mechanism of inter-chromosomal recombination?
mechanism: random alignment of chromosome pairs (metaphase I) and segregation (anaphase I) in meiosis
What is intra-chromosomal recombination?
Intra-chromosomal recombination: assortment of alleles for genes located on the same
chromosome into novel combinations.
Mechanism of intra-chromosomal recombination?
mechanism: exchange of DNA by crossing over (prophase I) in meiosis
When are genes independent?
Important conclusion: Genes are independent (un-linked) if they are
(i) located on different chromosomes, or
(ii) located very far apart on the same chromosome.
In a population of meiocytes what is the observed proportion?
Consider a population of meiocytes:
The observed proportion of the different gamete types for two genes on a chromosome
(parental and recombinant) depends on the distance between them.
complete linkage?
Complete linkage: only parental gamete types are produced. (no recombinant gametes)
* crossing over never occurs.
* genes must be very very close.
R-type gametes = 0%
independence?
Independence: parental and recombinant gametes are produced with equal frequencies (50:50)
* crossing over occurs in every meiosis (in every meiocyte)
* genes must be vary far apart.
R-type gametes = 50%
Incomplete linkage?
Incomplete linkage: parental gametes are produced with greater frequency than recombinant gametes. (a.k.a.
partial linkage)
* crossing over occurs in some meioses.
* frequency of crossing over depends on distance
0%< R-type < 50%
