Gene Linkage and Genetic Mapping

Question 1

gene linkage

Answer 1

some genes may be located on the same chromosome therefore are inherited together (linked genes)
altered progeny ratio from the expected 9:3:3:1 segregation ratio (if 2 genes are tightly linked, the progeny will show only the parental combinations)
anaphase 1 of meiosis

Question 2

independent assortment

Answer 2

two genes on two different homologous pairs of chromosomes
assort independently because they’re on different chromosomes
4 different gametes

Without linkage: AaBb (genes inherited independently/on different chromosomes)

Question 3

linkage

Answer 3

two genes on a single pair of homologs (no exchange occurs)
2 gametes inherit dominant form for both genes
2 gametes inherit recessive form for both genes

With linkage: AB/ab or Ab/aB

Question 4

Coupling (cis)

Answer 4

recessive alleles of both genes are on one chromosome and the dominant alleles are on the corresponding chromosome

produce more wild-type gametes (?)

Question 5

Repulsion (trans)

Answer 5

recessive allele of a gene and a dominant allele of another gene are on one chromosome and the corresponding dominant and recessive alleles are on the homolog

produce less wild-type gametes (?)

Question 6

Complete linkage

Answer 6

exists between 2 genes only if located very close together on the same chromosome

linkage ratio illustration
1:2:1 genotypic and phenotypic ratios
(trans 2:1 and cis 3:1)

Question 7

linkage group

Answer 7

the set of genes present together in a chromosome
1 linkage group = pair of homologous chromosomes
ex: humans = 23 pairs of chromosomes = 23 linkage groups

Question 8

recombination

Answer 8

the process by which offspring derive a combination of genes different from that of either parent

• results from crossing over (prophase 1 of meiosis effects crossing over)
• 4 gamets: 2 parental and 2 recombinants
• % of recombination individuals is related to the distance between the 2 genes along a chromosome

Question 9

recombination frequency

Answer 9

the % of total progeny that are recombinants
RF%= (recombinants/total progeny) * 100

linkage phase (coupling or repulsion) does not affect the amount of crossing over

Question 10

T.H. Morgan

Answer 10

Drosophila (fruit fly) experiment
basis for X chromosome being associate with sex determination
Sex-linked traits
genes controlling eye color on X chromosome
linked genes are inherited together (linkage)

Question 11

linkage map

Answer 11

shows linear order and relative distance of genes along a chromosome

• uses the idea of recombination frequency to map the distance between genes
• low recombination= close genes
• high recombination= far genes

Question 12

map distances

Answer 12

map unit: the distance between 2 genes that result in 1% RF

1 map unit = 1 centiMorgan = 1 % recombination frequency

Question 13

genetic map

Answer 13

indicates the recombination frequency between genes

Question 14

genetic distance

Answer 14

generally correlated with physical distance on the chromosome

-the greater the physical separation between genes along a chromosome, the greater the genetic distance between them

Question 15

physical distance

Answer 15

number of nucleotides

Question 16

What is the maximum amount of recombination that can occur between any two linked genes?

Answer 16

50%

Question 17

RF near 50% suggests:

Answer 17

1. The two genes are on different chromosomes

2. The two genes lie far apart on the same chromosome

Question 18

physical map

Answer 18

physical map shows distances in terms of a physical measurement such as base pairs of DNA sequence

Question 19

Multiple crossovers

Answer 19

when 2 genes are located relatively far apart on a chromosome it’s possible that more than one crossover can occur between them

even #s= no recombination
odd #s= recombination

Question 20

double crossing over

Answer 20

if 2 crossovers involve the same chromatid
-no recombination can be detected between the 2 genes and the genetic distance between them would be underestimated (less observed recombination suggesting that genes are closer together than they actually are)

Question 21

types of multiple crossovers

Answer 21

2-strand double exchange= no detectable recombinants
3-strand double exchange= 50% detectable recombinants
4-strand double exchange= 100% detectable recombinants

Question 22

Three Point Mapping

Answer 22

analysis of 3 linked genes; determination of gene order

• the parental genotypes are represented by the two classes with the highest number of progeny (non-recombinant individuals)
• double recombinant genotypes are represented by the classes with the fewest # of progeny
• if recombinant of a gene represents the least abundant genotype, it must lie in the middle

-illustrated map units are additive
calculated distance