Transmission genetics Flashcards
(16 cards)
When does crossing over occur?
During prophase 1:
Homologous chromosomes begin synapsis when Synaptonemal complex forms.
= Parental and maternal segments are exchanged.
Crossing over occurs during Synapsis of prophase 1 when bivalents form.
Held together by Synaptonemal complex.
With exchange sites (Chiasma) visible.
DNA after chiasma are swapped…
When are homologous chromosomes separated and when are sister chromatids separated?
How are sister chromatids separated?
During Reductional division, homologous chromosomes are seaprated (Meiosis 1)
During equational division, sister chromatids are separated. (Meiosis II)
During Metaphase, homologous chromosomes or sister chromatids will assemble on metaphase plate to undergo independent assortment.
Protein bridges between sister chromatids form due to Cohesin complex. When MT attaches to kinetochores of sister chromatids, Cohesin complex destroyed and leads to segregation.
What is independent assortment?
Within a pair of homologous chromosomes, maternal and paternal chromosomes can organise to face opposite poles, randomly and independently.
2^23 possible orientations.
How do X and Y chromosomes segregate?
They behave identically to the other autosomal chromosome pairs.
They contain PARs = Pseudo-autosomal regions where they contain homologous sequences which are enough to allow pairing during mitosis and meiosis.
What is Crossing Over?
Metaphse 1
After synaptonemal complex assembles in bivalents.
At the chiasma (afterwards).
The mutual exchange of genomic segments between non-sister chromatids of a pair of homologous chromosomes.
How can we estimate distance between 2 genes?
In Centimorgans:
% NPD + 1/2 % tetratype:
(NPD = 2 cross overs between, Tetra = 1 cross overs between).
In tetrad analysis of 2 genes, what is observed for 2 linked genes?
PD > Tetra > NPD.
For 2 linked genes, the closer together, the lower the chance of crossing over between the 2 genes which would result in non-parental combinations.
The most common ascus would be PD (where no cross over occurs).
The next would be tetratype = 1 cross over between.
Then finally NPD, which requires 2 cross overs between.
The higher % PD vs NPD suggests linkage, and the greater the difference, the closer the 2 genes are.
What do we expect between 2 genes which segregate independently?
PD = NPD.
In PD and NPD, there is NO cross over between genes and centromeres.
= There is an equal chance of independent assortment of 2 genes so PD and NPD are equal…
Tetratype is only possible when there is a crossing over between 1 of genes and centromere…
Because the genes are independent, the crossing over isnt between the genes but between the centromere and that respective gene.
How do we test single gene models in haploids and diploids?
When we have WT and a mutant for the same gene, crossing WT with mutant
Haploids = [A] x [B]
= giving 50% [A] and 50% [B]
Genotypically different for just 1 gene.
Diploids:
When genotypically different for 1 autosomal, nuclear gene =
F1 x F1 =
3/4 [A] and 1/4 [B} (When A is dominant to B).
Test Cross = F1 x a,b
= 50% [A] and 50% [B]
How can we test for Co-dominance for a single gene trait?
Both alleles are expressed, resulting in an intermediate phenotype between that of each homozygote.
F1 x F1 = Ab x Ab:
AA + Ab + bb:
1/4 x 1/2 x 1/4:
Intermediate is 50%, 25% is homozygote dominant and 25% homozygote recessive.
How do meiotic products appear for 2 independent genes when 2 homozygote parents are crossed. (Homozygotes but opposite (AB x ab)
Independent assortment of the 2 genes means that parental and recombined meiotic products are produced equally.
F(P) = F(R)
What to expect in a test cross between 2 independent genes?
F1 x homozygous recessive:
(A/a, B/b) x (a/a, b/b)
AB, ab, Ab, aB x ab
= (Aa, Bb)
= (aa, bb)
= (Aa, bb)
= (aa, Bb)
= all at 1/4 proportions.
25% each!
What is the advantage of test crosses?
Progeny directly indicates genotypes of F1 gametes and if their proportions are equal…
= Genotypes of gametes will be equal when independent.
= IF crossing over is occurring in the gametes than the proportions will be skewed.
In a F1xF1 of 2 independent genes, what do we expect to observe?
Due to independent assortment, all 4 gamete genotypes are equally probable.
= Observe a 9:3:3:1 ratio.
Each square is 1/16th chance.
What are clues of having linked genes?
When parental genotypes are much more frequent than recombined!!
= because on same gene, alleles are inherited together and dont segregate independently.
= Recombined genotypes require crossing overs.
= Cross over in interval between the 2 genes result in recombined!!
When do we cconclude 2 genes are genetically linked?
How about the distance?
2 genes are genetically linked and locaed on same chromosome when:
F(P)»_space;> F(R)….
When F(R’) and F(R’’) are equal.
and F(P’) and F(P’’) are also equal….
In non tetrad analysis:
1cM = 1% R
(Smaller % suggests closer)
