#test cross

[enteringvulturedimension]

"no homo" i shouted as the test cross clearly showed the gene alleles to be heterozygous

[biol10005-blog]

Linkage and recombination

If loci are independantly assorting can cross a heterozygous with a homozygous recessive and we should produce 1AB: 1Ab: 1aB: 1ab ratio.

The homozygous parent does not contribute to the phenotype - you can see what the other parent has contributed.

Linkage

many more genes exist than chromosomes

several genes must exist on the same chromosome

consequently independent segregation of the alleles of these genes may not occur

AB/ab - cis arrangement

Ab/aB - trans arrangement

For the cis arrangement we expect only AB and ab gametes but Ab and aB are also produced! This is because during meiosis crossing over (chiasma) takes place and exchange (or recombination) of maternal and paternal chromatids occurs.

Non recombinant or parental offspring occur more frequently than recombinant offspring

Recombinant offspring only occur if there is a cross over between the loci being considered

The frequency of recombination is a measure of the relative distance between the gene loci (futher apart, more frequent recombination)

Relative distances between gene loci on the same chromosome are measured by recombination frequency

map units/ % recombination = total no. recombinants /  total no. offspring x 100

Loci are not linked if they are on the same chromosome but far enough apart for crossing over to occur often enough to produce equal numbers of recombinants and parental combinations. These loci are said to assort independently

Independent assortment occurs >50 mu/cM

3 point test cross

Everything to the right swaps so if ABC/abc and crossover occurs between A & B locus will get Abc/aBC (both B and C swap)

Highest frequencies (non recombinants)

Lowest frequencies (double recombinants)

Middle frequencies (single recombinants) 

NB: NO RECOMBINANTS IN MALE DROSOPHILA AND LUCILIA!!!

[biol10005-blog]

Test Cross, Back Cross and Reciprocal Cross

Test Cross A test cross is a cross to a homozygous recessive genotype.

It may be used to establish the genotype of an individual with the dominant phenotype.

        e.g: Genotype could be AA or Aa so crossing with aa will give us different results.

                AA x aa  = All Aa (All one phenotype)

                Aa x aa = 1 Aa: 1 aa (half one phenotype, half other)

The appearance of the recessive phenotype in the offspring of a test cross indicates that the unknown genotype was heterozygous.

A test cross can also occur with 2 or more genes involved.

Back Cross A cross of an F1 to one of its parents  OR A cross of an F1 to an individual with an identical genotype to the parent

Useful because you to know the genotype of the parent and can use that as a ‘constant’.

Reciprocal Cross

1st:  Male Phenotype A x Female Phenotype B 2nd: Male Phenotype B x Female Phenotype A 

Usual test for sex linkage as if it is sex linked these crosses will produce different results. If autosomal will produce same results for both crosses.