Lec7_linkage and Crossing Over

LINKAGE AND

CROSSING OVER

Application of Mendel’s Rules assumes:

1. One allele completely dominates the other

2. All genes have 2 allelic forms

3. All traits are monogenic (affected by only one locus)

4. All chromosomes occur in homologous pairs

5. All genes assort independently

6. An allele is completely expressed when either dominant or homozygous

7. Each trait is controlled by a different set of factors

BATESON AND PUNNETT’S EXPERIMENT

Cross PP LL x pp ll (purple and long pollen) (red and round

pollen) F1 is Pp Ll selfed F1: Pp Ll F2 results in table Very significant deviation from expected Mendelian

ratio 9:3:3:1 The null hypothesis for chi square test with 2 genes

is that the genes assort independently. These genes do not assort independently.

phenotype observed exp ratio exp num

P_ L_ 284 9/16 215

P_ ll 21 3/16 71

pp L_ 21 3/16 71

pp ll 55 1/16 24

B+P GENES IN A TEST CROSS

Purpose of a test cross: the offspring phenotypes appear in the same ratio as the gametes in the parent being tested.

Here, we want to see how many gametes are in the original parental configuration (PL or pl) and how many are in the recombinant configuration (Pl or pL). The parental types have the same combination of alleles that were in the original parents, and the recombinant types have a combination of the mother’s and father’s alleles.

Original parents: PP LL x pp ll F1 test cross: Pp Ll x pp ll

Pheno-type

obs

purple long

392

purple round

116

red long 127

red round 365

total 1000

MORE TEST CROSS Parentals: 392 PL + 365 pl = 757. 757/1000 total offspring = 75.7% parental Recombinant: 116 Pl + 127 pL = 243. 243 /1000 = 24.3% recombinant.

If the genes were unlinked, 50% would be recombinant. These genes are linked, with 24.3% recombination between the P gene and the L gene.

If the genes were right on top of each other, that is, the two phenotypes were both caused by the same gene (pleiotropy), then there would be 0% recombination between them.

The percentage of recombinants is always between 0% and 50%, and the percentage of parentals is always between 50% and 100%.

COUPLING VS. REPULSION

The original test cross we did was PL/pl x ppll. Among the offspring, PL and pl were parental types, and pL and Pl were the recombinant types. There was 24.3% recombination between the genes.

The condition of having the dominant alleles for both genes on the same parental chromosome, with both recessives on the other parental chromosome, is called “coupling”: the P and L genes are “in coupling phase”.

The opposite condition, having one dominant and one recessive on each parental chromosome, is called “repulsion”. Thus, if the original parents were P l x p L, their offspring would have the genes in repulsion phase: Pl / pL.

TEST CROSS IN REPULSION Now do the test cross in repulsion: Pl / pL x p l

Here, the parental types are P l and p L, and the recombinant types are P L and p l.

The numbers of offspring in each type are quite different from the originals.

However, the percentage of recombinants is the same: 24.3%.

123 P L + 120 p l = 243 recombinant offspring.

243/ 1000 total offspring = 24.3 %

The percentage of recombination depends on the distance between the genes on the chromosome, and NOT on which alleles are on which chromosome.

phenotype

obs

P L 123

P l 372

p L 385

p l 120

total 1000

WHAT IS LINKAGE? Linkage is defined genetically: the failure of two genes to

assort independently.

Linkage occurs when two genes are close to each other on the same chromosome.

However, two genes on the same chromosome are called syntenic.

Linked genes are syntenic, but syntenic genes are not always linked. Genes far apart on the same chromosome assort independently: they are not linked.

Linkage is based on the frequency of crossing over between the two genes. Crossing over occurs in prophase of meiosis I, where homologous chromosomes break at identical locations and rejoin with each other.

DISCOVERY OF LINKAGE

In 1900, Mendel’s work was re-discovered, and scientists were testing his theories with as many different genes and organisms as possible.

William Bateson and R.C. Punnett were working with several traits in sweet peas, notably a gene for purple (P) vs. red (p) flowers, and a gene for long pollen grains (L) vs. round pollen grains (l).

LINKAGE The term linkage has two related

meanings Two or more genes on the same

chromosome are physically linked Two ore more genes on the same

chromosome are genetically linked Tend to be transmitted as a unit

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LINKAGE ……

The human karyotype has 46 chromosome All genes on a single chromosome are physically

linked to each other Chromosomes can be called

linkage groups - 22 autosomal linkage group - X linkage group - Y linkage group - Genes located far apart from

each other on the same chromosome may assort independently

- Crossing over can occur between these genes

Complete linkage – Genes that are linked but never crossover

Incomplete linkage – Genes that are linkedand can crossover.

FACTORS AFFECTING LINKAGE

1. Age2. Temperature3. X-Rays

Significance of Linkage

• Keeps parental / racial specific traits together.

• Valuable traits of new variety maintained.

• Disallows desirable mixing of traits.

• No. of linkage groups = No. of chromosome pairs. This shows genes are located on chromosomes.

Crossing Over

• Morgan discovered crossing over when studying two genes on X chromosome in Drosophila.

• Morgan proposed that the chiasmata visible on chromosomes were regions of crossing over.

• Occurs between nonsister chromatids.

Steps of Crossing- over

• Synapsis of homologous chromosomes – Zygotene

• Tetrad formation- Pachytene

• Crossing over- Pachytene

• Disjunction

FACTORS AFFECTING CROSS OVER

1.Temperature

2. X-Rays

3. Chemicals

4. Age

5. Interference

6. Sex

Significance of Crossing-over

• Produces new combinations of traits.

• Forms raw material for evolution.

• Establishes concept of linear arrangement of genes.

• Helps to determine loci of genes in the chromosomes.

Crosses of Two X-linked genes

Expect only parental types if no crossing over occurs - Confirm this for yourself with a Punnett square.

Morgan’s Interpretation

• Recombination was caused by linear arrangement of genes and crossing over.

• Frequency of recombination was determined by distance between genes:

– y and w recombination rate = 1.3%

– w an m recombination rate = 37.2%

– Therefore y and w were closer together on the chromosome, while w an m are farther apart.

Sturtevant and Mapping

• Sturtevant, Morgan’s undergraduate student, discovered frequency of crossing over between each pair of the 3 genes:

– yellow, white 0.5%

– white, miniature 34.9%

– yellow, miniature 35.4%

Do you see a pattern?

Sturtevant’s Interpretation

• Sturtevant reasoned that recombination frequencies were probably additive, so order of genes on chromosome was yellow-white-miniature.

34.9

Map Units

• One map unit (centimorgan, cM) = 1% recombination between two genes

– yellow and white are 0.5 cM apart

– yellow and miniature are 35.4 cM apart

– white and miniature are (35.4-0.5) = 34.9 cM apart

• In Drosophila, crossing over occurs only in females, never in males.

You only see recombination when it occurs between the genes you are watching!

Single Crossovers: Non-crossover (Parental) and Crossover (Recombinant) Gametes

Double Crossovers

Types of Double Exchanges:Not All are Detectable