Linkage

Recombination

n The frequency of crossing over between 2 loci is proportional to the distance between them; depends on which loci are being studied

n Thus the percentage of recombinant gametes vary

n This correlation is the basis for constructing chromosome maps

MAP OF SELECTED CHROMOSOMES

Concepts on linkage

1. Departure from expected ratios should suggest linkage n E.g. consider the cross AaBb x aabb to

yield n 49 A-B- n 39 aabb n 10 A-bb n 9 aaB-

What info can be deduced from the cross? n The cross is a test cross and we expect a

1:1:1:1 ratio, this is for independent genes

n Single gene ratios: 52A-: 48aa and 51B-: 49bb

n Genes A & B do not seem to be behaving independently, thus they are LINKED

RF

n RF = recombinants/ total x 100 = 19/ 100 x 100 = 19% Therefore, 80% of the time the alleles stay

together. The genes are not completely linked

2. The most frequent classes will be the parental or non-recombinant classes n Consider the cross in number 1 n A & B are linked n Heterozygotes are either AB/ab or Ab/aB n The most frequent classes are A-B- and

aabb, thus they represent parental chrom.

nE.g. consider the cross nAaBb x aabb to yield

n49 A-B- n39 aabb n10 A-bb n 9 aaB-

3. Linkage does not change the expected numbers of phenotypes; only the frequencies are changed. n E.g. consider example in #1, we still saw 4

phenotypes. n The frequency with which the minority

phenotypes appear depends upon the distance between the 2 linked genes

4. In a test cross, 2 equally frequent majority classes and two equally frequent minority classes suggest linkage. n Refer again to the first example n A-bb and aaB- are also equal but in

minority

5. Reciprocal classes usually appear equally frequently. n Recombination results in the exchange of

parts of homologous chromosomes. n The even that produces one recombinant

class also produces the other class.

n If we draw the homologous chromosomes and indicate an exchange as follows:

A B a b The resultant chrom will be A b and a B One gamete will get AB, other ab

6. In crosses involving three genes that are each heterozygous, double crossover classes will usually be the least frequent. n Consider the cross

A b C x a b c a B c a b c

Draw the chromosomes and double exchange A___b___C a___B___c

n The resultant gametes from this double exchange will be A___B___C and a___b___c

Thus, these 2 phenotypic classes ABC and abc will be the least frequent

n In some cases with 3 linked genes, you will see only 6 or 7 phenotypic classes. If this happens, the 2 missing classes are reciprocal, the missing classes are then the DOUBLE CROSS OVER (DCO) classes

n E.g. AbC/aBc x abc/abc AbC 40 aBc 38 ABc 6

abC 5 aBC 5 Abc 3 ABC 3 What is the observed DCO?

n Group them into classes: n Parental or non-recombinant class (NRC) n Single cross over1 (SCO1) n Single cross over2 (SCO2) n Double cross over (DCO)