Linkage, Recombination & Mapping 1 Flashcards
How is a dihybrid cross shown?
If genes are on different chromosomes there is a ‘;’ so Aa ; Bb
If genes are on the same chromosome there is a ‘/’ and the alleles are put together on each side So Ab/ab
If gene location is unknown there is a ‘•’ between the genes so A/a • B/b
What are linked genes?
Genes located close together on same chromosome and in turn are inherited together on the same chromosome
How do genes on the same chromosome that are linked break up sometimes?
Recombination during meiosis 2
Important note to remember:
2 Alleles that are at the same locus (i.e A a) are never on the same chromosome they are on homologous chromosomes.
How can a test cross show gene linkage?
Crossing a heteozygote with a homozygous recessive organisms at both loci.
If genes are linked then heterozygous genes will be inherited together.
If unlinked then they assort independently and result in more offspring.
Example:
Aa • Bb X aa • bb
If linked:
ABab = 50%
abab = 50%
If unlinked:
AaBb
aabb
Aabb
aaBb
What is the type of recombination that takes place between genes on different chromosomes - random segregation of chromosomes?
Interchromosomal recombination (Mendel)
Number of recombinant types in progeny = 50%
Number of parental types in progeny = 50%
What is intrachromosomal recombination?
Linked genes are prevented from assorting independantly during meiosis so the number of recombinants is less than 50%. (genes are being swapped around between homologous chromosomes)
When does crossing over take place? What happens in this process?
Prophase 1 of meiosis. It is an exchange of genetic material between non-sister chromatids.
How can recombination frequency be calculated?
Number of recombinant progeny/Total number of progeny x 100 %
What is the configuration of linked genes referring to?
The arrangement of alleles on homologous chromosomes.
Coupling = wild type alleles are found on one chromosome and the mutant alleles are found on the other chromosome.
Repulsion: Wild type and recessive allele (Dominant and mutant) are found on the same chromosome.
The non parental offspring will be arranged based on which configuration is present.
How can results of a cross that resemble the ratios of independent assortment be evaluated for gene linkage?
Using a chi-square test
How is chi-squared calculated?
(Observed - Expected)^2/Expected
This number is n in df = n-1 gives degrees of freedom then a table is used to estimate P.
if P > 0.05 then chance is responsible for deviation between expected and observed values
13600
13600
