What process leads to the inheritance of novel alleles on DIFFERENT chromosomes?
Independent assortment of chromosomes at meiosis I
What process leads to the inheritance of new combinations of alleles on the SAME chromosomes?
A chiasma at meiosis
What happenes in meiosis I?
Chromosomes replicate, form 2 chromatid pairs and then the two chromosomes form bivalents
Define bivalent
a pair of homologous chromosomes.
What happens in meiosis II?
microtubules attach to cetromeres, leaving 4 products and supposedly 2 products
If parental genotypes AA BB and aa bb produce progeny of AB, ab, Ab and aB, which are the recombinants and which are parental? How come?
AB and ab are parentals
Ab and aB are recombinants
- independent assortment at bivalent level
Parents AA bb x aa BB produce?
Which are recombinants?
Ab ab (parental)
AB ab (recombinants)
The frequency of recombination between genes on different chromosomes is what? Always?
50%
YES
Chiasma
Where paired chromosomes remain in contact during the first metaphase of meiosis at which corssing over and exchange of genetic material can occur between strands.
Genes that show < 50% recombination are said to be….
Linked (on the same chromosome)
Recombinants come in ____
Pairs
How is recombination frequency calculated?
No. recombinants/ expected progeny
1cM = 1% recombination = ______
1 map unit
Max frequency of recombination between any 2 genes is what?
50%
Parental phenotypes are the most ____
common
Double recombinant frequencies are the most ____
uncommon
Crossing over gives __________ products
reciprocal
Why does genetic map never add up to distance between furthest away genes?
correct for double recombinants
Top tip for working with 3 point test cross tables
always write out the full genotypes of parents
How can you tell which gene is in the middle of a genetic map for linked genes?
look at double recombinants and see which remain parental
What does it mean for the coefficient of coincidence to be below 1 ?
How is it calculated?
less double recombinants than expected
OBSERVED freq of doubles/ EXPECTED freq of doubles
What is interference?
Calculated?
Crossing over one time in a chromosome makes it less likely for a second cross over on the same chromosome
1 - coefficient of coincidence
Is recombination uniform among individuals/ species?
NO
- varies greatly between species
- varies between sexes
BUT distribution is not random
What is a physical map?
The actual length of the gene distances in base pairs
Why is the female genetic map longer ?
There is more recombination points, more chiasma perhaps?
What stabilises bivalents at meiosis I?
Chiasma
What holds sister chromosomes together?
Cohesin
Is it possible for mitotic recombination to occur? Why?
Can lead to…
YES
When the chromosomes line up in mitosis to be separated, they can also line up differently/flipped
As a result of this, there can be a loss of heterozygosity
Describe the use of a complementation test
if two mutant homozygotes are crossed and produce wild type offspring, then the mutations are in different genes and complementation has occurred
What is intragenic complementation?
result of a gene coding for an enzyme that is an homomultimer (has more than one protein subunit)
- This means that alleles of the same gene can complement
Structure of a T4 bacteriophage
- head with packaged DNA
- collar
- tube
- sheath covers a tail
- base plate
How are T4 phages used for complementation tests?
plaques on a lawn of bacteria
- due to lysis
- it is key in this experiment that recombination is given time to occur, hence why it is isn’t recombination as then they would be able to grow immediately
Distinguish between a genetic map and a physical map
Genetic and physical maps illustrate the arrangement of genes and DNA markers on a chromosome. The relative distances between positions on a genetic map are calculated using recombination frequencies, whereas a physical map is based on the actual number of nucleotide pairs between loci.