gene linkage and mapping

Question 1

independent assortment in meisosis 1

Answer 1

• double heterozygote equally likely to get any 4 haploid gametes
• which gives rise to 9:3:3:1
• this is where crossing happens

Question 2

genes are not always independent…..

Answer 2

• each chromosome contains multiple genes
• genes on the same chromosome are linked
• the alleles of genes already on a chromosome will tend to pass as a unit to gametes

Question 3

genetic recombinant chromsomes

Answer 3

• during meiosis the homologous chromosome pair up and physically exchange pieces of their dna
• new combinations of alleles can be genetic
• if recombination doesn’t occur than allele combinations will be the same as the parents
  (also called non parental gamates)

Question 4

crossing over produce recombinant chromosomes

Answer 4

• completley random
• there is a tendancy for parental alleles to stay together

Question 5

how can we determine if genes are linked?

Answer 5

you can test cross to a homozygous individual and it can tell you if a gene is assorting independently or is linked
- Aa;Bb x aa;bb
- equal numbers if the loci assort independently
(as the homozygous recessive parent does not contribute to offspring phenotype you can see the other parent contributes)

Question 6

chi test to determine if loci are linked

Answer 6

null hypothesis: there is no significant difference beteween the expected and observed frequencies (that they are independently linked 1:1:1:1 ratio)
- alternative hypothesis: there is significant difference between the expected and observed frequencies, implkies the genes may be linked

Question 7

cis arrangment

Answer 7

the 2 dominant alleles A,B are on the same chromosome A,B/ab

Question 8

trans arrangment

Answer 8

1 dominant allele and one recessive allele are on the same chromosome Aa/ab

Question 9

recombination frequency

Answer 9

• The proportion of gametes that are recombinant varies on how often crossovers occur between the two loci
• if a cross never happens RF=0%
• If one cross happens RF=%50

Question 10

map distance

Answer 10

• If two genes are very close together, small chance of crossover between them, very few recombinants offspring
  
  • If two genes are further apart, high chance of crossover= lots of recombinants
  • Therefore the % of recombinant offspring is correlated with the distance between 2 genes
  • % defined as map distance
    1 map unit (mu) is defined as the distance between two loci that will generate 1 % recombination between them