PinkMonkey Online Study Guide-Biology
7.9 Test Cross or Back Cross
Test cross is a simple method devised by Mendel to verify the genotype of the F1 hybrid.
When the F1 hybrid is crossed with the homozygous recessive parent, it is called a test cross.
Since, the F1 is crossed back with one of the parents, it is also called a back cross.
Test cross is also used for checking the correctness of Mendelís law of segregation (using a monohybrid test cross) and the law of independent assortment of characters (using a dihybrid test cross). For example,
(1) Monohybrid test cross : In a monohybrid cross of Tall x Dwarf, the F1 are all tall (Figure 1.1). Let us see what happens when this F1tall is test crossed with the homozygous recessive parent i.e. Dwarf with (tt) genotype.
We aim to check and verify two things (i) determine the genotype of F1 Tall and (ii) check the correctness of the law of segregation.
Thus, F1 tall is
back crossed (test crossed) with the homozygous recessive dwarf parent
and the progeny of the cross examined. We know that the recessive dwarf
with (tt) genotype will produce only one type of gametes (i.e., all with
(t) only). However, as regards the F1 tall,
there can be two possibilities:
If the F1 tall
is homozygous with (TT) genotype, it shall produce only one type of
gametes (i.e. all with (T) only). As a result, the progeny of the
cross should be all tall. (T) x (t) = (Tt) Tall.
If, on the other hand, the F1 tall is heterozygous with (Tt) genotype and, if Mendel's law of segregation of characters is correct, then the F1 should produce two types of gametes, (T) and (t) in equal proportion. The recessive dwarf parent produces only (T) type of gametes. The resulting test cross progeny should be 50% Tall and 50% Dwarf or in 1:1 ratio as follows:
The results obtained in the actual test cross experiments completely agree with the theoretical expectations. Thus it is proved that F1 Tall is a heterozygous dominant (monohybrid) with (Tt) genotype and that the alleles segregate during gamete formation.
(2) Dihybrid test cross : In the dihybrid cross of yellow
round X green wrinkled the F1, dihybrid
is a double heterozygous yellow round with (YyRr) genotype. It is
test crossed with the double homozygous recessive parent,( i.e.
green round with (yyrr) genotype). We aim to test the correctness
of Mendel's law of independent assortment of characters.
If the principle of independent assortment is correct, then as Mendel explained, the (YyRr) dihybrid should produce four types of gametes: YR, Yr, yR and yr in equal proportions. The recessive parent (yyrr) shall produce only one type of gametes i.e. yr type. It is expected that the maximum possible chance combinations between these gametes should produce four kinds of phenotypes in the ratio of 1:1:1:1 as follows.
The actual numbers of these four types of offspring obtained in one of Mendel's test crosses were: Yellow round = 55, Yellow Wrinkled = 49, Green Round = 51 and Green Wrinkled = 52. This gives an approximate 1:1:1:1 ratio as per the theoretical expectations and thus, confirms the law of independent assortment of characters.
Significance of back cross in plant breeding
It is a rapid method of improving a variety of crop.
It is an easier and quicker method of obtaining a desirable trait in a pure homozygous condition (true breeding trait).
Back cross is also use frequently in hybridization experiments because of its simple ratio. Moreover, this needs the analysis of fewer progeny.