linkage and mapping in eukaryotes Flashcards
how was linkage discovered?
in certain crosses, a larger number of offspring with ‘parental’ genotypes were generated than expected
one genetic map unit (mu)=
the distance between the genes for which 1% product of meiosis are recombinant
genetic mapping
how to determine if genes are linked on the same chromosome, and how to determine how far apart the genes are. This is the basis of genetic mapping.
gene linkage is all about
determining if two genes are linked and how far apart the genes are
example 1: two genes, each with two alleles A a and B b on separate chromosomes
- cross of AABB (from female) and aabb (from male)
- produces four gametes: AB- egg
ab- sperm
-F1: one pair of chromosomes is Aa and on the other chromosome Bb.
- possible gametes of F1 are:
1. A (one chromosome) B (one chromosome -parental
2. a and b-parental
3. A and b-recombinants
4. a and B -recombinants
each gamete has an expected proportion of 1/4
when genes are on diff chromosomes
50% of gametes produced by a doubly-heterozygous individual are recombinants , when compared to the gametes produced by its parents. The other 50% are parental.
if two genes occur on the same gene, they may not..
assort independnely at anaphase of meiosis.
when genes do not assort independently ..
genes are said to be linked and demonstrate linkage in genetic crosses
linkage is present when..
fewer than 50% of the gametes produced by a double heterozygote are recombinant
when genes are not linked
50% will be recombinant
example 2: the two genes are on the same chromosome (A and B are linked)
Female parent has a genotype of AB (on one chromosome) on both chromosomes and Male has genotype of ab (on one chromosome) on both chromosomes
- gametes: egg: AB and sperm:ab
- F1 : some AB/ab and some AB/ab
- when these F1 produce gametes some may crossover and some may not.
Gametes resulting from no crossover: AB- parental AB- parental ab-paretal ab-parental
Gametes resulting from crossover AB-parental ab-parental Ab-recombinant aB-recombinant
with crossing over..
we get recombination alleles on the same chromosomes
-since crossover occurs in the 4 strand stage of meiosis, and involves only two of the four chromatids, each crossover event results in 50% recombinant gametes and 50% parents gametes.
Genes on diff chromosomes –>
50% recombinant gametes after meiosis
Genes on the same chromosomes–>
less than or equal to 50% recombinant gametes after meiosis
genes with recombinant frequencies less than 50% are…
present on the same chromosome and therefore linked
Two genes that undergo independent assortment, indicated by a recombination frequency of 50% are either…
- on non-homologous chromosomes
2. located far apart on a single chromosomes
the further apart two genes are on a single chromsosme
the closer to 50% will the recombination frequency will be
when may crossing over not occur between linked gene
when the position of the genes on the chromosome are very near one another
the frequency with with which crossing over occurs between any two linked gene is proportional to
distance between the loci along the chromosome
at very small distances..
crossover is very rare and most gametes are parental
as the distance between the two genes increases
crossover frequency increases–> more recombinant gametes, fewer parental gametes
when genetic loci are very far apart on the same chromosome
crossing over nearly always occurs and the frequency of recombinant gametes approaches 50%
how test crosses can help us determined the crossing over frequency and therefore the distance between two loci
if there has been no crossover all of the gametes are parental
-however when crossover occurs some of them will be recombinants too
frequency of recombination is the same as
the % of recombinant offspring
37.2% recombinant offspring means
37.2% frequency of recombination
linkage of genes can be represented on a
genetic map
a genetic map shows
theinear order of the gens along a chrome, with distance proportional tot eh frequency of recombination
unit of distance win linkage maps
map unit
1 map unit is equals to
1% recombination
two genes that recombine with a frequency of 1% are said to be
1 map unit apart
multiple crossover events
unless genes are very close together, more than one crossover event can occur in a single meiotic division
when may multiple crossover not occur
when the genes are very close together
multiple crossovers can make mapping
inaccurate- if genes are far apart on the same chromosome
non parental genotypes arise due to
crossing over during meiosis 1 -recombinants
how to tell if a phenotype is recombinant or parental
usually recombinant is rare and parental si coomon
work out the distance between the genes when pr+pr+vg+vg+ x prprvgvg produces pr+pr vg+vg.
Then pr+pr vg+ vg is crossed with prprvgvg and this produces this ratio:
- prvg+ (parental)-165
- prvg(parental)-191
- pr+vg (recomb)-23
- prvg+(recomb)-21
rare =
pr+vg-23
prvg+-21
therefore
23+21=44 recomb/
400 total
=11%
therefore 11mu distance between pr and vg genes
if there are 3 genes, there are
3 possible gene orders (left, right or middle)
the gene in the middle must be the only one to
undergo double recombination- therefore in the double recombinants it is found alongside alleles which is not found along side the parents
hints for three point crosses
- look at the pattern- are there two or three classes of offspring (3 if 3 point)
- determine: common, uncommon and rare - determine parental and double recombinant types
- parent= common= ABC abc
- double recombinants-rare= Abc and abC - deduce gene order
- switches between parental and double recombinant-there C must be in the middle - calc distance between pairs of genes
who are responsible for the punnet crosses
bateson and saunders
