linkage and mapping in eukaryotes Flashcards Preview

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Flashcards in linkage and mapping in eukaryotes Deck (40):
1

how was linkage discovered?

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

2

one genetic map unit (mu)=

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

3

genetic mapping

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.

4

gene linkage is all about

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

5

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

- 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

6

when genes are on diff chromosomes

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.

7

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

assort independnely at anaphase of meiosis.

8

when genes do not assort independently ..

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

9

linkage is present when..

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

10

when genes are not linked

50% will be recombinant

11

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

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

12

with crossing over..

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.

13

Genes on diff chromosomes -->

50% recombinant gametes after meiosis

14

Genes on the same chromosomes-->

less than or equal to 50% recombinant gametes after meiosis

15

genes with recombinant frequencies less than 50% are...

present on the same chromosome and therefore linked

16

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

1. on non-homologous chromosomes
2. located far apart on a single chromosomes

17

the further apart two genes are on a single chromsosme

the closer to 50% will the recombination frequency will be

18

when may crossing over not occur between linked gene

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

19

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

distance between the loci along the chromosome

20

at very small distances..

crossover is very rare and most gametes are parental

21

as the distance between the two genes increases

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

22

when genetic loci are very far apart on the same chromosome

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

23

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

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

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

24

frequency of recombination is the same as

the % of recombinant offspring

25

37.2% recombinant offspring means

37.2% frequency of recombination

26

linkage of genes can be represented on a

genetic map

27

a genetic map shows

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

28

unit of distance win linkage maps

map unit

29

1 map unit is equals to

1% recombination

30

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

1 map unit apart

31

multiple crossover events

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

32

when may multiple crossover not occur

when the genes are very close together

33

multiple crossovers can make mapping

inaccurate- if genes are far apart on the same chromosome

34

non parental genotypes arise due to

crossing over during meiosis 1 -recombinants

35

how to tell if a phenotype is recombinant or parental

usually recombinant is rare and parental si coomon

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

rare =
pr+vg-23
prvg+-21

therefore

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

therefore 11mu distance between pr and vg genes

37

if there are 3 genes, there are

3 possible gene orders (left, right or middle)

38

the gene in the middle must be the only one to

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

39

hints for three point crosses

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

40

who are responsible for the punnet crosses

bateson and saunders