Genetic recombination in Eukaryotes: crossing over, part 1

1
Genetic recombination in Eukaryotes crossing
over, part 1

1. Genes found on the same chromosome linked genes
2. Linkage and crossing over
3. Crossing over chromosome mapping

2
I. Genes found on the same chromosome linked
genes

• Conflicting cytological evidence, only a few
  dozen chromosomes/individual so must be several
  genes per chromosome
• cytological studies revealed only a few dozen
  chromosomes present, yet each species has
  thousands of genes
• highly likely that each chromosome would carry
  many hundreds/thousands of genes
• therefore not all genes could assort
  independently
• Testcross experiments revealed gene linkage
  observed deviations from the expected 1111
  ratio based upon independent assortment

3
If a testcross is done and the genes are on
separate chromosomes Aa/Bb x
aa/bb Aa/Bb aa/bb Aa/bb aa/Bb
1111 observed
2 genes, located on different chromosomes, will
segregate independently.
4
Chromosome is the unit of transmission, not the
gene

• Linkage two or more genes located on the same
  chromosome
• Linked genes are not free to undergo independent
  assortment
• Instead, the alleles at all loci of one
  chromosome, should in theory, be transmitted as a
  unit during gamete formation.

5
When two genes are compeletely linked, no
crossing over occurs therefore, each gamete
receives the alleles present on one chromatid or
the other AB or ab
6
II. Linkage and crossing over

• Crossing over breakage and rejoining process
  between 2 NONSISTER chromatids
• Crossing over produces recombinants
• The of recombinant gametes varies, dependent
  upon location of the loci. The closer the genes
  are, the less likely recombination will occur

NR parental R recombinant
7
Breakage and rejoining process between two
homologous non-sister chromatids -there can be
one or more cross-overs -the cross over can occur
anywhere
8
Parental gamete
RECOMBINANT Crossover gamete
Parental gamete
RECOMBINANT
Recombination Frequency the of
recombinants/total progeny
9
B. Recombination Frequency, unlinked genes v.
linked genes
1). In the case of unlinked genes, independent
assortment holds true Testcross
Heterozygous x homozygous mutant AaBb x
aabb Offspring the of recombinants the of
parental types Recombination Frequency (RF)
1/2 or 50
10
2). In the case of linked genes, no independent
assortment

• Offspring
• the of recombinants lt the of parental types
• Recombination Frequency the of
  recombinants/total progeny
• (RF) lt 1/2 or 50

11

• Crossing between adjacent non sister chromatids
  generates recombinants
• The two chromatids not involved in the exchange
  result in non-parental gametes
• The closer two loci are, the lower the RF (lt1/2)

12

• We can compare the RF to what one would expect
  with independent assortment
• RF Range 0 to 50
• RF significantly lt 50 - Linkage
• RF 50 - not linked

13
Recombination by Crossing Over points to keep
in mind

1. COs can occur between any two nonsister
   chromatids
2. If there is NO crossing over, only parental types
   will be observed
3. If there IS crossing over, RF will increase up to
   50
4. when the loci of two linked genes are very far
   apart, the RF approaches 50, 1111 ratio
   observed, thus transmission of the linked genes
   is indistinguishable from that of two unlinked
   genes

14
Morgan noted the proportion of recombinant
progeny varied depending on which linked genes
were being examined Testcross F1 results
As Morgan studied more linked genes, he saw that
the proportion of recombinant progeny varied
considerably.
pr pr vg vg x pr pr vg vg
pr vg 1339 pr vg 1195 pr vg
151 pr vg 154 y w 43 y w
2146 y w 2302 y w 22
RF 11
y y ww x yy ww
RF 1.4
15
III. Crossing over chromosome mapping

• Morgan thought the variations in RF might
  indicate the actual distances separating genes on
  the chromosomes.
• Sturtevant (Morgans student) compiled data on
  recombination between genes in Drosophila test
  crosses
• The closer the two linked genes, the lower the
  recombination frequency- thus RF may be
  correlated with the map distance between two loci
  on a chromosome

16
A. Linkage Maps

• Linkage of genes can be represented in the form
  of a genetic map, which shows the linear order of
  genes along a chromosome.
• Can also determine the distance between the
  genes. The recombinant offspring is correlated
  w/the distance between the two genes
• Variations in recombination frequency indicate
  actual distances separating the genes on
  chromosomes

17
B. Map Units

• Map Unit (m.u.) the distance between genes for
  which one product of meiosis out of 100 is
  recombinant
• RF of 1 1 m.u. or 1 cM
• e.g. if RF 12 between A B, and 28 between B
  C

A B
C
12 mu
28 mu
18
(No Transcript)
19
F1
F2
RF 1.3, therefore y is 1.3mu from w
males
females
20
F1
w 37.2 mu from m
F2
males
females
21
A plant of genotype A B a b Is test-crossed
to a b a b If the two loci are 10
m.u. apart, what proportion of progeny will be A
B / a b?
22
In the garden pea, orange pods (orp) are
recessive to normal pods (Orp), and sensitivity
to pea mosaic virus (mo) is recessive to
resistance to the virus (Mo). A plant with
orange pods and sensitivity is crossed to a
true-breeding plant with normal pods and
resistance. The F1 plants were then test-crossed
to plants with orange pods and sensitivity. The
following results were obtained 160 orange
pods/sensitive 165 normal pods/resistant 36
orange pods/resistant 39 normal
pods/sensitive calculate the map distance
between the two genes
36 39 .1875 x 100 18.8 m.u. 400
23
C. Mapping multiple genes Three-point mapping
Alfreds research

• Hypothesis when multiple genes are located on
  the same chromosome, the distance between the
  genes can be estimated from the proportion of
  recombinant offspring.
• Sturtevants First Genetic Map

24

• Sturtevants First Genetic Map
• The linear order of these genes can be determined
  using testcross data
• Examined 5 different genes y, w, v, m, r
• All alleles were found to be recessive and X
  linked.
• Crossed the double heterozygote female with
  hemizygous male recessive for the same alleles.
• Example yy ww x yw
• yw
• yw
• yw
• yw
• RF 214/21,736 0.0098

ww rr x wr wr wr wr wr RF
2,062/6116 0.337
1 mu between y w, 33.7 mu between w r
25
genes are arranged on the chromosome in a linear
order- which can be determined
26
The Complete Data

Alleles R./total RF
y and w 214/21,736 1
y and v 1,464/4,551 32.2
y and r 115/324 35.5
y and m 260/693 37.5
w and v 471/1,584 29.7
w and r 2,062/6116 33.7
w and m 406/898 45.2
v and r 17/573 3
w and m 109/405 26.9
27
y-w 1 m.u. v-r 3 m.u. y-m 37.5 m.u. w-r
33.7 m.u. w-v 29.7 m.u.
Suggesting that v is between r w, but closer
to r
Map distances more accurate between genes that
are closer together, as the RF approaches 50,
the value becomes more inaccurate as a measure of
map distance
y w v r
m
57.6
23.9
1
3
29.7
More accurate