Lecture 9/23 - E2 Flashcards
(20 cards)
A dihybrid cross is performed between AaBb x AaBb. Given this pathway: Colorless precursor —–(A blocks)—> Yellow Pigment ——– (B)—-> Green Pigment
Which of the phenotypic ratios do you expect in the offspring?
9: A-B- = Colorless
3: A-bb = Colorless
3: aaB- = green pigment
1: aabb = yellow pigment
Answer: 12 Colorless: 3 Green: 1 Yellow
Genes on _______ chromosomes always assort independently and produce _______ numbers of all possible gamete combinations
1) Different
2) Equal
_____ _____ do not sort independently.
Linked genes
What are linked genes?
Syntenic genes (nearby genes on the same chromosome) which are bassed down together in gametes more frequently than ex[ected from independent assortment
If there is a greater than 50% frequency of parental genotypes/phenotype gametes produced then…
The genes are closer together.
- If less than 50%, genes are farther apart.
If there is less than 50% frequency of non-parental genotypes/phenotype gametes produced then….
The genes are closer together
- If greater than 50%, genes are farther apart
If 100% of the gametes are parental then…
There is complete linkage where no recombination occured between the two genes in meiosis.
Parental frequency + nonparental always =
100% because both parental will be produced at the same frequency while both nonparentla will be produced at the same frequency
In Drosophila melanogaster, Genes A and B are separated by 1/1000th the length of a chromosome. Which cross do you expect to detect in recombinant progeny in the next generation?
1) AB/ab (female) x ab/bb (male)
2) AB/ab (male) x ab/ab (female)
3) AB/AB (female) x ab/ab (male)
Only females undergo recombination
Answer: AB/ab (female) x ab/bb (male)
Because females cannot be homozygous for both because of the recombination
How do you calculate the recombination frequency (r)?
r = (number of recombinants) / (total number of progeny)
What is recombination frequency a function of?
The physical distance between two genes
- the greater the distance, the more recombination observed
What is 10.7% recombination in centimorgans?
10.7 centimorgans
If a cross produces 30% recombinant offspring, each parental class type (P1 and P2) must represent approximately _____% of the progeny.
Total parental percentage = 70%
70/2 for each parent = 35%
Why is gene mapping used?
To determine the order and distance between genes on a chromosome.
True breeding parents (P= Ab/Ab and aB/aB) are crossed. A test cross of the resultant F1 produces the progeny ratios: 11% AB, 13% ab, 38% Ab, and 38% aB. How far apart are the A and B linked genes?
Add the AB and ab
11% + 13% = 24%
Single stranded crossover event
Four dsDNA molecules (chromatids) in all combinations, half the gametes are recombinants
Double Crossover between two genes
Where there are two crossover events, occurs much less frequently
When is a chromosome considered recombinant?
If the alleles don’t look like the parents.
Example: Parents have AB and ab, if the gametes look like aB or Ab, then those are recombinant
How do you tell which phenotypes are the parental, single recombination, or double recombination based on the number observed for a 3 point cross?
Parental - largest number
Single - intermediate number
Double - smallest number
How can you tell which gene is in the middle using the number observed in a 3 point cross?
Look at the double crossover events.
See which gene differs from the parentals
Example: Parents are vga and VGA, double cross over shows vGa and VgA
Therefore, G is the gene in the middle
