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Linkage. chromosome theory of inheritance. Mendelian genes have specific loci (positions) on chromosomes Chromosomes undergo segregation and independent assortment during meiosis. Figure 12.2. /. /. /. /. 1. 1. 1. 1. 4. 4. 4. 4. P Generation. Yellow-round seeds ( YYRR ).

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Linkage

Linkage


Chromosome theory of inheritance

chromosome theory of inheritance

  • Mendelian genes have specific loci (positions) on chromosomes

  • Chromosomes undergo segregation and independent assortment during meiosis


Linkage

Figure 12.2

/

/

/

/

1

1

1

1

4

4

4

4

P Generation

Yellow-round

seeds (YYRR)

Green-wrinkled

seeds (yyrr)

y

Y

r

R

r

R

Y

y

Meiosis

Fertilization

r

y

R

Y

Gametes

All F1 plants produce

yellow-round seeds (YyRr).

F1 Generation

R

R

y

y

r

r

Y

Y

Meiosis

LAW OF SEGREGATION

The two alleles for each

gene separate.

LAW OF INDEPENDENT

ASSORTMENT Alleles of

genes on nonhomologous

chromosomes assort

independently.

r

r

R

R

Metaphase

I

y

Y

Y

y

1

1

r

r

R

R

Anaphase I

Y

Y

y

y

Metaphase

II

r

R

R

r

2

2

y

Y

y

Y

y

Y

Y

y

Y

y

y

Y

r

R

R

r

r

r

R

R

yr

yR

Yr

YR

YR

F2 Generation

An F1 F1 cross-fertilization

3

3

Fertilization results in the

9:3:3:1 phenotypic ratio

in the F2 generation.

Fertilization

recombines the R and

r alleles at random.

9

: 3

: 3

: 1


Linked genes

linked genes

  • Genes located on the same chromosome that tend to be inherited together

  • Do not independently assort


What would be the expected phenotypic ratio assuming independent assortment

What would be the expected phenotypic ratio assuming independent assortment?

GgLl

ggll


Linkage

What statistic would you use to verify that your observed values differed from your expected values?What would be the expected values?Hmm…not 1:1:1:1;more like 1:1


Crossing over explains the presence of the recombinants

Crossing over explains the presence of the recombinants


Most look like parents but some are recombinants due to crossing over

Most look like parents, but some are recombinants due to crossing over


Linkage

% Recombinant

Frequency or

= # map units apart two genes are


You try

You try…

  • What is the frequency of recombinants?How many map units apart are the two genes?

  • 500 offspring

  • 7 grey short recombinants

  • 8 black long recombinants


Linkage map

linkage map

  • a genetic map of a chromosome based on recombination frequencies

  • the farther apart two genes are, the higher the probability that a crossover will occur between them

  • therefore the higher the recombination frequency


You try1

You try…

  • Flies can have black bodies (b), vestigial wings (vg), and cinnabar eyes (cn).

  • The cross-over frequencies are listed below:

  • b-cn 7.5%

  • b-vg 17%

  • vg-cn9.5%

  • Draw the chromosome map. Label the distance btw the genes.


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