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Genetic Linkage and Mapping. A. a. A. __•_______. __•_______. __•_______. a. __•_______. Notation. A. Two homologous chromosomes, four chromatids total. Aa Diploid Adult. — • —————. — • —————. a. Haploid gametes (single chromatid). 1/2 A : 1/2 a. Dihybrid Cross.

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notation

A

a

A

__•_______

__•_______

__•_______

a

__•_______

Notation

A

Two homologous

chromosomes,

four chromatids total

Aa

Diploid

Adult

—•—————

—•—————

a

Haploid gametes

(single chromatid)

1/2 A : 1/2 a

dihybrid cross
Dihybrid Cross

AA BB Female aa bb Male

A B a b

—•———————•——————•———————•—————

P:—•———————•——————•———————•—————

A B a b

Gametes: Egg (A B) Sperm (a b)

A B a b

—•————— ——•———— —•————— ——•————

slide5

F1 adult

after the S phase

A B

—•———————•—————

F1:—•———————•—————

a b

4 gamete types equally frequent

A B

—•———————•—————

F1:—•———————•—————

a b

4 gamete types, equally frequent

A B

—•————— ——•————

A b

—•————— ——•————

a B

—•————— ——•————

a b

—•————— ——•————

__________

__________

__________

__________

f2 of dihybrid cross when genes not linked
F2 of dihybrid cross when genes not linked
  • What proportion of the gametes of the double heterozygote are recombinant?
but what if the genes are on the same chromosome
But what if the genes are on the same chromosome?

(What if the genes are linked?)

They do not assort independently at meiosis

two linked genes
Two linked genes

Female Male

A B a b

—•——————————•—————————

P:—•——————————•—————————

A B a b

Gametes: Egg (A B) Sperm (a b)

A B a b

—•————————— —•————————

no crossing over between markers

A B

—•—————————

F1:—•—————————

a b

No crossing over between markers

A B

—•—————————

a b

—•—————————

A B

—•—————————

a b

—•—————————

crossing over in 4 strand stage

A B

—•—————————

F1:—•—————————

a b

Crossing overin 4-strand stage

A B

—•—————————

a b

—•—————————

A b

—•—————————

aB

—•—————————

no crossover crossover
No crossover Crossover

A B

—•—————————

F1:—•—————————

a b

A B

—•—————————

F1:—•—————————

a b

A B

—•—————————

a b

—•—————————

A B

—•—————————

a b

—•—————————

A b

—•—————————

aB

—•—————————

A B

—•—————————

a b

—•—————————

when genes are on the same chromosome but far apart
When genes are on the same chromosome, but far apart:
  • Crossing over between them will be common, approaching 100% of the time

In this case, what proportion of all F1 gametes will be recombinant (have a combination of alleles that was not present in the original true-breeding parents)?

crossing over in 4 strand stage16

A B

—•—————————

F1:—•—————————

a b

Crossing overin 4-strand stage

A B

—•—————————

a b

—•—————————

_________

_________

_________

_________

A b

—•—————————

aB

—•—————————

slide17

Genes that are so far apart on the same chromosome that crossing over occurs between them in 100% of meioses behave as if they are unlinked:_____ parental combinations_____ recombinants

what if genes are close together
What if genes are close together?
  • Say that crossing over occurs between the genes in 20% of meioses
  • What proportion of F1 gametes will be recombinant?
slide19

80%

20%

A B

—•—————————

F1:—•—————————

a b

A B

—•—————————

F1:—•—————————

a b

A B

—•—————————

a b

—•—————————

A B

—•—————————

a b

—•—————————

A B

—•—————————

a b

—•—————————

A b

—•—————————

aB

—•—————————

genes with 50 recombination are on the same chromosome

Genes with < 50% recombination are on the same chromosome

Genes that have 50% recombination (independent assortment) are either on non-homologous chromosomes or __________________________

slide22

The frequency of crossing over between two genes on the same chromosome is proportional to the distance between the genes

This is the basis of genetic mapping

slide23
At small distances crossing over is rare and few gametes are recombinant (r is near 0%)
  • As distance between genes increases, crossover frequency increases, and more gametes are recombinant (__ < r < ___)
  • At greater distances, crossover nearly always occurs and r --> ____
how would you tell if a gamete produced by a double heterozygote is recombinant or parental

How would you tell if a gamete produced by a double heterozygote is recombinant or parental?

How would you determine the genotypes of gametes produced by a doubly-heterozygous individual?

two linked genes26
Two linked genes

Female Male

bw hv+ bw+ hv

—•——————————•—————————

P:—•——————————•—————————

bw hv+ bw+ hv

Gametes: Egg (A B) Sperm (a b)

bw hv+ bw+ hv

—•———————— —•————————

slide27

bw hv+

—•—————————

F1:—•—————————

bw+ hv

bw hv

—•—————————

—•—————————

bw hv

X

bw hv+

—•—————————

bw+ hv

—•—————————

bw hv

—•—————————

Par

Par

Rec

Rec

bw hv

—•—————————

bw+ hv+

—•—————————

slide28

If 15% of the offspring of the testcross are recombinant, what proportion of meioses had crossover between the brown and the heavy-veined genes?

if 15 of the gametes are recombinant the genes are said to be map units apart on a genetic map

If 15% of the gametes are recombinant, the genes are said to be ___ map units apart on a genetic map

bw hv

___ map units

unit of distance in linkage map is a map unit 1 map unit is equal to 1 percent recombination

Unit of distance in linkage map is a map unit: 1 map unit is equal to 1 percent recombination

A map unit is also equivalent to the physical distance along a chromosome which will experience 1 crossover event in every ____ meiotic divisions

two x linked genes
Two X-linked genes

Female Male

w m w+ m+

—•——————————•—————————

P:—•——————————•————/

w m

Gametes: Egg (A B) Sperm (a b)

w m w+ m+

—•———————— —•————————

—•———/

slide33

w+ m+

—•—————————

F1:—•—————————

w m

w m

—•—————————

—•————/

X

w+ m+

—•—————————

w m

—•—————————

w m

—•—————————

Par

Par

—•————/

Rec

Rec

w+ m

—•—————————

w m+

—•—————————

62 8 non recombinant offspring 37 2 recombinant offspring

62.8%non-recombinant offspring,37.2% recombinant offspring

How far apart (in map units) are the genes?

___ map units

if two genes are 50 map units apart what proportion of recombinant offspring would you observe

If two genes are 50 map units apart, what proportion of recombinant offspring would you observe?

What would you conclude about these two genes?