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Genetics: Meiosis, Independent Assortment, Linkage, and Genetic Distance, Study notes of Cell Biology

The concepts of meiosis, independent assortment, linkage, and genetic distance. It covers topics such as parental and recombinant types, genetic distance calculation, and the effects of double crossing-over. The document also includes practice problems and examples for better understanding.

Typology: Study notes

2021/2022

Uploaded on 09/12/2022

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bg1
1
A
A
a
a
B
B
b
b
A a
B b
Meiosis
25% AB
25% Ab
25% aB
25% ab
Independent Assortment and Meiosis
A
Ba
ba
b
A
B
a
b
A
B
50% AB
50% ab
0% aB
0% Ab
Meiosis
Linkage and Meiosis
Crossingover and Recombinantion
Which are the Parental types and which the recombinant types?
A
Ba
ba
b
A
B
A
b
a
B
?% AB
?% ab
?% aB
?% Ab
Meiosis
Partial Linkage and Meiosis
Genetic Distance
The likelihood of recombination is roughly
proportional to the distance two genes are apart.
If two genes are close together on a
chromosome there will be few recombinants.
If two genes are farther apart on a chromosome
there will be more recombinants.
Genetic Distance
Genetic
Distance =Frequency of
Recombination (Rf)==
# of Recombinants
total # of progeny
In a Testcross
•Units for genetic distance centimorgans = map units
1 map unit = 1%Rf
An underestimate
of true Rf!!!
pf3

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A

A

a a

B

B

b

b

A aB^ b Meiosis

25% AB

25% Ab 25% aB 25% ab

Independent Assortment and Meiosis

AB^ ab^ ba

BA

ab

AB

50% AB

50% ab 0% aB 0% Ab

Meiosis

Linkage and Meiosis

Crossingover and Recombinantion

Which are the Parental types and which the recombinant types?

AB^ ab^ ba

BA

Ab

aB

?% AB

?% ab ?% aB ?% Ab

Meiosis

Partial Linkage and Meiosis

Genetic Distance

• The likelihood of recombination is roughlyproportional to the distance two genes are apart.

• If two genes are close together on achromosome there will be few recombinants.

• If two genes are farther apart on a chromosomethere will be more recombinants.

Genetic Distance

GeneticDistance = Frequency ofRecombination = (Rf )= # of Recombinantstotal # of progeny

In a Testcross

•Units for genetic distance – centimorgans = map units

1 map unit = 1%Rf

An underestimateof true Rf !!!

Outcome of Double Crossing-OverA b

Aa^ Bb a B

Double Crossing Over Mask Recombination

Therefore analysis of recombinant genotype willunderestimate the frequency of crossing over

Effect of Double Crossing-over

Actual frequency (%) of Recombinantbetween two loci

Observed frequency (%) ofRecombinant between loci

Practice Problem

In Drosophila, the droopy gene is 5 mapunits away from the curved gene. A

heterozygous female was generated by

mating a wildtype female with a doublerecessive male. If this heterozygote was

testcrossed and 1000 progeny wereexamined, how many would you expect to

be wildtype for both traits?

Detecting and Estimating Linkage

  • Why determine linkage? Help determine the chromosomal positionof a locus. Remember chromosomal position defines a gene.
  • To detect linkage generate a dihybrid and testcross the dihybrid.
  • First test for independent assortment (Chi square) in testcrossprogeny
    • If you fail to reject ind assort, then conclude the genes are ondifferent chromosome or are on the same chromosome but far

apart >45 mu.

  • If you reject independent assortment conclude the genes are onthe same chromosome and closer than 50 map units.

Generate a dihybrid and testcross^ Example

  1. Wildtype female X black curled male2. F1 Female X black curled male Testcross progeny784 wildtype762 black curled 234 curled210 black Are these genes independently assorting?Only test for linkage if reject independently assortment How far apart are the genes on the chromosome?Rf = # rec/total Progeny Why is this an underestimate of genetic distance?What results would you predict if you testcross a male F1 (remember in drosophila there is no crossing over in males)

Genetic Mapping

• Analyze the relative position on achromosome.

• Example

– Black and Curly are 22.3mu

– Black and Pink are 30.6mu

– Curly and Pink are 46.

• Rough map

• Effects of double crossovers