Chromosome Mapping In. Eukaryotes Flashcards
How did scientists study chromosomes before the whole genomes were known?
Scientists could isolate chromosomes in metaphase and identify the individuals chromosomes by staining them with dye to get a banding pattern
Scientists worked on identifying regions of the chromosome that they called “markers” and recorded the locus
Now that we have sequenced the genome, we can assign the genes to the appropriate regions of the chromosome to complete the map
How did the original scientists figure out which genes were on which chromosomes?
Chromosome mapping: genes responsible for phenotype were studied
What can be shown by test crosses?
If we perform test crosses, genes which are found close together the same chromosome, they are said to be linked
Original genetic studies showed that if multiple genes were on the same chromosome, they would be transmitted as a single unit
What does it mean to be”segregating together”?
If genes on the same chromo are close together, they are usually transmitted together, and this is called SEGREGATING TOGETHER
What does it mean to be “segregating away”?
If genes on the same chromosome and far from each other, they have a good chance of a recombination event happening between them and they DO NOT transmit together, and this is called segregating away
What is the frequency of crossing Over between any two loci? How did this aid the scientific community?
The frequency of crossing over between any two loci on a single chromosome is proportional to the distance between them 1% =1 mu(map unit)
This correlation allowed early scientists to construct chromosome maps which indicated the relative locations of the genes on the chromosomes
How did crossing over in Meiosis 1 aid scientists?
This crossing over in meiosis 1 results in the reshuffling/recombination of the alleles between homologs
Scientists used this to map out a chromosome and decipher how alleles are organized on a chromosome based on the probability of a crossover occurring between any two given alleles
What is linkage?
Two genes on a single pair of homologs; exchange occurs between two non sister chromatids
What does independent assortment result when two heterozygous genes are on two different pairs of chromosomes? (Scenario A)
Independent assortment: two genes on two different homologous pairs of chromosomes
Results of gamete formation when two heterozygous genes are on two different pairs of chromosomes
4 distinct gametes AB Ab aB aB
What does independent assortment result in when two heterozygous genes are on the same pair of homologs, but with exchange between nonsister chromatids?
result in when two heterozygous genes are on the same pair of homologs, but with exchange between nonsister chromatids
4 distinct gametes: AB Ab aB ab
If two linked genes (on same chromosome) are far apart, there is almost always be recombination between non sister chromatids and up to 50% of the gametes will have recombination
- 1:1:1:1 ratio of the 4 gametes
- Problem: similar results to scenario A with two unlinked and independently assisting genes
What happens if two linked genes are far apart?
If two linked genes (on same chromosome) are far apart, there is almost always be recombination between non sister chromatids and up to 50% of the gametes will have recombination
- 1:1:1:1 ratio of the 4 gametes
- Problem: similar results to scenario A with two unlinked and independently assisting genes
What are the results of gamete formation when two heterozygous genes are on the same pair of homologs with no exchange between them?
results of gamete formation when two heterozygous genes are on the same pair of homologs with no exchange between them
-2 distinct gametes, only parental/nonsister cross over gametes
AB
ab
How much crossing over and recombination occurs in each tetrad?
- During mitosis, a limited number of crossing over and recombination occurs in each tetrad
- Recombination occurs randomly along the length of the tetrad
If crossing over occurs 100% of the time, how much of the gametes are rearranged?
Only 50% of the gametes are rearranged
The closer two loci are to each other…
The less likely that a single crossover will occur between them
What are map units?
Percentage of rearranged gametes
What is the max number of % recombination and map units between two genes on the same chromosome?
50%
This suggests that a chiasma is formed between two genes 100% of the time
Two nonsister chromatids have rearranged. Two parental non-sister chromatids are unchanged….
This would appear as independent assortment
Single crossover occurs between two nonsister chromatids. What is the first possible scenario of the resulting gametes?
The exchange does not alter the linkage between the alleles of the two genes
Only parental gametes are formed
AB
ab
The exchange is undetected
Single crossing over occurs. What is the second possible outcome ?
The exchange separates the alleles of the two genes
Recombinant gametes are detectable: AB Ab aB ab
The percentage of recombinant alleles gives an idea of how close/far alleles are from each other
How were flies to find out two genes were linked?
Each parent is homozygous for the alleles eye color and thickness of wing veins
Normal wild type is dominant hv+ and bw+ (thin veins with red eye) and this is what F1 inherits
F1 is interbred
F2 generation produces 1:2:1 phenotypic and genotypic ratio
This is characteristic of complete linkage when the genes are very close together
In linkage ratio testcross, what is the zygosity of the parent?
Testcross parent Homozygous for both alleles