linkage and mapping in eukaryotes

Question 1

how was linkage discovered?

Answer 1

in certain crosses, a larger number of offspring with ‘parental’ genotypes were generated than expected

Question 2

one genetic map unit (mu)=

Answer 2

the distance between the genes for which 1% product of meiosis are recombinant

Question 3

genetic mapping

Answer 3

how to determine if genes are linked on the same chromosome, and how to determine how far apart the genes are. This is the basis of genetic mapping.

Question 4

gene linkage is all about

Answer 4

determining if two genes are linked and how far apart the genes are

Question 5

example 1: two genes, each with two alleles A a and B b on separate chromosomes

Answer 5

• cross of AABB (from female) and aabb (from male)
• produces four gametes: AB- egg
  ab- sperm

-F1: one pair of chromosomes is Aa and on the other chromosome Bb.

• possible gametes of F1 are:
  1. A (one chromosome) B (one chromosome -parental
  2. a and b-parental
  3. A and b-recombinants
  4. a and B -recombinants

each gamete has an expected proportion of 1/4

Question 6

when genes are on diff chromosomes

Answer 6

50% of gametes produced by a doubly-heterozygous individual are recombinants , when compared to the gametes produced by its parents. The other 50% are parental.

Question 7

if two genes occur on the same gene, they may not..

Answer 7

assort independnely at anaphase of meiosis.

Question 8

when genes do not assort independently ..

Answer 8

genes are said to be linked and demonstrate linkage in genetic crosses

Question 9

linkage is present when..

Answer 9

fewer than 50% of the gametes produced by a double heterozygote are recombinant

Question 10

when genes are not linked

Answer 10

50% will be recombinant

Question 11

example 2: the two genes are on the same chromosome (A and B are linked)

Answer 11

Female parent has a genotype of AB (on one chromosome) on both chromosomes and Male has genotype of ab (on one chromosome) on both chromosomes

• gametes: egg: AB and sperm:ab
• F1 : some AB/ab and some AB/ab
• when these F1 produce gametes some may crossover and some may not.

Gametes resulting from no crossover:
AB- parental
AB- parental
ab-paretal
ab-parental

Gametes resulting from crossover
AB-parental
ab-parental
Ab-recombinant
aB-recombinant

Question 12

with crossing over..

Answer 12

we get recombination alleles on the same chromosomes
-since crossover occurs in the 4 strand stage of meiosis, and involves only two of the four chromatids, each crossover event results in 50% recombinant gametes and 50% parents gametes.

Question 13

Genes on diff chromosomes –>

Answer 13

50% recombinant gametes after meiosis

Question 14

Genes on the same chromosomes–>

Answer 14

less than or equal to 50% recombinant gametes after meiosis

Question 15

genes with recombinant frequencies less than 50% are…

Answer 15

present on the same chromosome and therefore linked

Question 16

Two genes that undergo independent assortment, indicated by a recombination frequency of 50% are either…

Answer 16

1. on non-homologous chromosomes

2. located far apart on a single chromosomes

Question 17

the further apart two genes are on a single chromsosme

Answer 17

the closer to 50% will the recombination frequency will be

Question 18

when may crossing over not occur between linked gene

Answer 18

when the position of the genes on the chromosome are very near one another

Question 19

the frequency with with which crossing over occurs between any two linked gene is proportional to

Answer 19

distance between the loci along the chromosome

Question 20

at very small distances..

Answer 20

crossover is very rare and most gametes are parental

Question 21

as the distance between the two genes increases

Answer 21

crossover frequency increases–> more recombinant gametes, fewer parental gametes

Question 22

when genetic loci are very far apart on the same chromosome

Answer 22

crossing over nearly always occurs and the frequency of recombinant gametes approaches 50%

Question 23

how test crosses can help us determined the crossing over frequency and therefore the distance between two loci

Answer 23

if there has been no crossover all of the gametes are parental

-however when crossover occurs some of them will be recombinants too

Question 24

frequency of recombination is the same as

Answer 24

the % of recombinant offspring

Question 25

37.2% recombinant offspring means

Answer 25

37.2% frequency of recombination

Question 26

linkage of genes can be represented on a

Answer 26

genetic map

Question 27

a genetic map shows

Answer 27

theinear order of the gens along a chrome, with distance proportional tot eh frequency of recombination

Question 28

unit of distance win linkage maps

Answer 28

map unit

Question 29

1 map unit is equals to

Answer 29

1% recombination

Question 30

two genes that recombine with a frequency of 1% are said to be

Answer 30

1 map unit apart

Question 31

multiple crossover events

Answer 31

unless genes are very close together, more than one crossover event can occur in a single meiotic division

Question 32

when may multiple crossover not occur

Answer 32

when the genes are very close together

Question 33

multiple crossovers can make mapping

Answer 33

inaccurate- if genes are far apart on the same chromosome

Question 34

non parental genotypes arise due to

Answer 34

crossing over during meiosis 1 -recombinants

Question 35

how to tell if a phenotype is recombinant or parental

Answer 35

usually recombinant is rare and parental si coomon

Question 36

work out the distance between the genes when pr+pr+vg+vg+ x prprvgvg produces pr+pr vg+vg.

Then pr+pr vg+ vg is crossed with prprvgvg and this produces this ratio:

• prvg+ (parental)-165
• prvg(parental)-191
• pr+vg (recomb)-23
• prvg+(recomb)-21

Answer 36

rare =
pr+vg-23
prvg+-21

therefore

23+21=44 recomb/
400 total
=11%

therefore 11mu distance between pr and vg genes

Question 37

if there are 3 genes, there are

Answer 37

3 possible gene orders (left, right or middle)

Question 38

the gene in the middle must be the only one to

Answer 38

undergo double recombination- therefore in the double recombinants it is found alongside alleles which is not found along side the parents

Question 39

hints for three point crosses

Answer 39

1. look at the pattern- are there two or three classes of offspring (3 if 3 point)
   - determine: common, uncommon and rare
2. determine parental and double recombinant types
   - parent= common= ABC abc
   - double recombinants-rare= Abc and abC
3. deduce gene order
   - switches between parental and double recombinant-there C must be in the middle
4. calc distance between pairs of genes

Question 40

who are responsible for the punnet crosses

Answer 40

bateson and saunders