WEEK 3: LINKAGE, RECOMB, AND GENETIC MAPPING Flashcards
if two genes are linked on the same chromosome will they assort independently?
- NO!!
What do genetic maps show?
- The position of genes (loci) in relation to each other along the chromosomes
What are 4 reasons why generating genetic maps are good?
- Can determine whether mutations affect different genes or not (mapping provided the first indication that there were two forms of Haemophilia)
- Can CLONE genes using their map position (e..g CFTR gene)
- Enhances our ability to predict inheritence patterns (counselling)
- Very useful for genome sequencing projects
What is one form of Haemophilia linked to (gene) ?
- The colour blindness gene
What are genes on the same chromosome said to be?
- Linked
What is the F2 ratio for two genes on two different homologous pairs of chromosomes?
- 9:3:3:1
Does exchange occur in linked genes?
- NO - no crossing over
Which structure on the chromosome is the physical evidence of crossing over during meiosis?
- Chiasmata - evidence of crossing over in Meiosis I
Can linked genes be separated by recombination in meiosis?
- YES
If the genes assort independently, what is the predicted phenotypic ratio in the test cross progeny?
- 1:1:1:1 (unlinked)
What is the coupling phase known as?
- When the dominant alleles of the different genes stayed together as did the recessive alleles
What is the repulsion phase known as?
- Where one dominant and one recessive allele stay together e.g. A and b stay together and a and B stay together
Which test can be used to distinguish between independent assortment and linkage?
- Chi squared test
What is the recombination frequency?
- The number of recombinants OVER the total progeny *100
If crossovers never occur between two gene loci what is the recombination frequency (RF)?
- 0
What is the maximum RF?
- 50%
If one crossover always occurred, then what would the RF be and what is this the same for?
- 50%
- The same for independent assortment
Why is the RF never >50?
- because there are always (nearly) two non-crossover gametes from a single meiosis
What can a testcross be used to measure and explain it?
- RF
- Heterozygote for two or more genes and homozygous recessive for all genes –> can determine whether recombination has occurred during gamete formation in heterozygous parent
With test crosses to measure RF, are we looking at the gametes for one or both parents?
- Only 1 parent
In a test cross, what do the phenotypes of the zygotes equal and what does this tell us?
- The genotypes of the heterozygous parent gametes
- Tells us that the phenotypes of the offspring tells us the number of RECOMBINANT and parental gametes.
If two genes are very close together is there a small or large chance of crossover?
- A SMALL chance of crossover
if two genes are FAR APART is there a small or large chance of a crossover occurring?
- A LARGE chance with lots of recombinant offspring
What is the percentage of recbominant offspring correlated with and what is this called?
- The distance between two genes
- Called the MAP distance
