linkage.pdf, Exams of Genetics

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Linkage

-Genes on the same chromosome are

called linked

Human

-23 pairs of chromosomes, ~35,

different genes expressed.

• average of 1,500 genes/chromosome

Following Meiosis

Parental chromosomal types- complete linkage and no crossing over.

Seldom occurs. Nonparental chromosomal types

• result from crossing-over, recombination

of alleles.

Crossing-Over

-Frequency of crossing-over between linked

genes is proportional to the distance between them. -Possible if two genes are close that

crossing-over will not be detected.

• crossing-over has been used to construct

chromosome maps.

Three possibilities for two genes:

Fig. 6.1 in text.

a. On separate chromosomes so segregate

independently.

b. Both genes on same pair of

chromosomes, no crossing-over

so always stay together.

c. Both genes on same pair, crossing over,

produces recombinant gametes.

Linkage ratio

If complete linkage, a unique F

2

phenotypic ratio results. 1:2:1 phenotypic and genotypic ratio Test cross produces 1:1 ratio

Genetic Symbols P

1

: bw

hv

bw

hv

X

bw

hv

bw

hv

Genes located on same chromosome will show evidence of linkage. Linkage groups - number equals haploid number

of chromosomes

.

Complete linkage is rare, usually

some crossing-over.

Thomas Hunt Morgan