Gene mapping and linkage

Question 1

Homologous recombination:

Answer 1

Crossing over during meiosis I (prophase I)
Reciprocal process
Results in recombinant chromosomes

Question 2

recombinant chromosomes

Answer 2

(non-parental chromosomes)

have different alleles than parental chromosome

Question 3

Parental chromosomes Nonrecombinant chromosomes

Answer 3

Chromosomes that do not cross-over retain all the same alleles as the parent chromosomes

Question 4

syntenic genes

Answer 4

Genes that are located on the same chromosome

Question 5

What does independent assortment predict?

Answer 5

50% parental gametes and 50% non parental gametes

Question 6

What happens when syntenic genes are located closely together?

Answer 6

Alleles may be unable to independently assort
 Results in genetic linkage
 Produces a distinctive pattern of gamete genotypes since they recombine less during meiosis
Denoted with a slash

Question 7

What happens if syntenic genes are very far apart?

Answer 7

hat are very far apart do assort following Mendel’s law of independent assortmen

Question 8

What is true for linked genes?

Answer 8

1. Linked genes are always syntenic and located near one another on the chromosome
    Syntenic genes that are very far apart will assort independently due to crossing over of homologous alleles
2. Genetic linkage leads to the production of different expected gamete frequencies
    Greater number of parental gametes (combinations of alleles) than recombinants expected compared to independent assortment
    Smaller number than expected for recombinant gametes
3. Crossing over is less likely to occur between linked genes that are closer
   together than those further apart
    Frequency of cross-over is roughly proportionate to the distance between a gene

Question 9

How can you detect genetic linkage?

Answer 9

compare observed frequencies of gamete genotypes/progeny phenotypes with the expected frequencies under the assumptions of independent assortment

If linked:
Parental gametes will be produced more than by chance (»50%)
Progeny will therefore show parental phenotypes more than expected
Recombinant gametes will be produced less than by chance («50%)

If unlinked:
Parental gametes with parental alleles will be produced the same amount as recombinant gametes
Produces 4 genetically different gamete combinations in a ratio of 1:1:1:1

Question 10

Compare independent assortment and genetic linkage

Answer 10

• 4 genetically different gametes are expected at 25% each when the genes assort independently
• when genes are linked parental gametes are much more frequent than expected

Question 11

Complete linkage

Answer 11

o recombination at all occurs between linked genes

A dihybrid produces two equally frequent gametes containing only parental allele combinations
Ie no recombinant alleles

Drosophila and Diptera males exhibit complete linkage on homologous chromosomes

Question 12

Incomplete linkage:

Answer 12

sulting recombination between the homologs produces a mixture of parental and recombinant gametes

more common

A dihybrid produces 4 genetically different gametes

Two are parental

Two are recombinants

The parental gametes have ~same frequency and equal >50% of gametes produced

The recombinant gametes have ~same frequency and equal <50% of gametes produced

Question 13

recombination frequency

Answer 13

Rate of recombination of syntenic genes

𝑟 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑟𝑒𝑐𝑜𝑚𝑏𝑖𝑛𝑎𝑛𝑡 𝑜𝑓𝑓𝑠𝑝𝑟𝑖𝑛𝑔/ 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑜𝑓𝑓𝑠𝑝𝑟𝑖𝑛𝑔

Question 14

How can recombination frequency be interpreted?

Answer 14

1. Crossing over occurs at a higher rate between genes that are further apart than those closer together
2. Linked genes with higher recombination frequencies are more distant from each other than other linked genes with lower recombination frequencies

Question 15

What did Bateson and Punnet discover?

Answer 15

Looked at flower colour and pollen grain shape
 When studied separately, the two traits generated
3:1 phenotypic ratio in the F2  When studied as a dihybrid:
 Crossed pure-breeding purple flowers and long pollen (PPLL) x pure–breeding red flowers and round pollen (ppll)
 F1: all offspring with purple flowers and long pollen
 F2: progeny showed more parental phenotypes
produced compared to recombinants
 ie did not produce 9:3:3:1 ratio
 Purple-long and red-round phenotypes&raquo_space; purple-round and red-long phenotypes
 Termed this “coupling” of the parental traits (i.e. PL and pl remained together)

Question 16

What did Morgan find?

Answer 16

the parental phenotypes predominated
and
Parental phenotypes are produced when the gametes in the F1 female predominantly contain X-chromosomes with one of the original parental sets of alleles- sex linked trait so its based on crossing over in females
i.e. w+m+always went together and wm

Sperm are wm/Y

Recombination occurs on homologous X chromosomes- occurs when we don’t get the parental,

i.e. w+m or wm+

Question 17

Morgans crossing over hypothesis

Answer 17

each homolog initially contains identical sister chromatids
a single cross over produces 2 recombinant chromatids
completion of meiosis produces 2 parental gametes and 2 recombinant gametes

Question 18

Two-point test-crosses

Answer 18

Dihybrid F1 is crossed to a pure-breeding mate with recessive phenotypes
 Morgan used eye colour (red: pr+, purple: pr) and wing size (full wing size: vg+, vestigial wing: vg
 P: pr+ vg+/pr+ vg+ x pr vg/pr vg
 F1: Red eyed and full wing (pr+ vg+/pr vg)
 F2 (test-cross progeny): F1 females x pr vg/pr vg males
 Males only contributed recessive alleles  Females contributed 1 of 4 gamete
combinations- females controlled phenotype

Question 19

i

Answer 19

Males have no cross-over on the X- chromosome, so only produces parental gametes- bc they have only one x chromosome wd don’t get crossing over between an x and y, if it has dominant allele it will produce dominant gamete, hemizygous

Females have a single cross-over event, producing 2 parental gametes and 2 recombinant gametes

Morgan expected a 1:1:1:1 phenotypic ratio in the progeny

Morgan actually observed 1:1 ratio in the parentals, and a 1:1 ratio in the recombinants BUT- similar to sex linked traits where if we compared between the sexes we didn’t see the 3:1 ratio but if we compared just females we saw it

Parentals&raquo_space; recombinants

Parentals&raquo_space; 50%

Recombinants &laquo_space;50%

Question 20

Cytological evidence of recombination

Answer 20

First demonstrated simultaneously by Harriet Creighton/Barbara McClintock in 1931 and Curt Stern (Droosphila)
 Harriet Creighton and Barbara McClintock did it in corn (Zea mays)
Chromosome 9 homologs, using kernel colour (c1) and starch type (wx)
Microscopically, one homolog appeared normal and carried c1 and Wx
Other was altered with a knob (C1 end) and a translocated bit of chromosome 8 (wx end)
Observed a physical exchange of genetic information on chromosomes

Question 21

Genetic Linkage Mapping

Answer 21

Smaller recombination frequencies indicated genes were closer together
 Larger recombination frequencies indicated genes were further apart

Question 22

Why should we be concerned about linked genes?

Answer 22

In humans, hereditary elliptocytosis

Elliptical instead of biconcave red blood cells

Usually autosomal dominant (E_)

Has variable penetrance and expressivity

Confers some resistance to malaria

The gene for HE is 4 map units from the gene that determines your Rh factor (R=+, r=-)

In humans, other examples

Hair colour and eye colour (usually)

Blond hair, blue eyes

Brown hair, brown eyes

Human height and shoe size

White fur in cats and deaf

Red hair and freckles

Question 23

What is the max frequency you can get?

Answer 23

Max frequency you can get is 50% for complete independent assortment

Question 24

chiasma interference (I)

Answer 24

Reduction in the observed number of DCOs relative to the number expected if the 2
SCO events happened independently

Indicates the influence of some process(es) that limit the number of crossovers in
short length chromosomes
Compares the frequency of observed DCO events with the number of expected DCOs
Represents the proportion of DCO that are expected but not produced in the
experiment

Question 25

Negative interference

Answer 25

Sometimes, we can get more DCOs produced than expected

Interference will be 0 if observed DCOs = expected DCOs

Question 26

What factors affect recombination in genomes ?

Answer 26

Differences in recombination due to sex

Differences in recombination due to age, temperature-dependent variation

Genetic hotspots and coldspots

Question 27

Discuss why sex cause recombination to vary

Answer 27

In Drosophila, recombination is exclusive to females only and does not occur in males- bc females have 2 x

Heterogametic organisms have lower recombination in general

Male mammals (XY)

Female birds (ZW)

But not limited to just sex chromosomes!

In humans, recombination more frequent in females

Larger recombination map

Question 28

Hotspots

Answer 28

Recombination occurs more often than average based on the number of nucleotides present

Question 29

Coefficient of coincidence

Answer 29

Observed DCO / Expected DCO