Back-cross

A back-cross is the cross between an individual and its descendent. The purpose of making a back-cross is to cultivate a homozygous mutation gene in order to observe its expression.

Back-crosses are always conducted, but not in human, because it is immoral.

The need and process of a back-cross is as follow.

In a genetic experiment, a male Drosophila was treated with a mutagen, and mutation was suspected to have occurred.

After the treatment, examine the Drosophila to see whether there is any change. If there is any chance, the mutation gene must be a dominant gene. If not, carry out the back-cross to check whether there is any recessive gene produced.

Procedure of the back-cross

(1)	Make a cross between the experiment male and a virgin pure bred female to get the offspring. If the mutation gene is not sex-linked, a half of the offspring should carry the mutation gene.
(2)	Make a cross between the experiment male and some offspring virgin females. This is the back-cross. Examine their offspring to see whether new traits can be found. If new trait is found, the individuals presenting the trait must be homozygous. If no new trait is found, the experiment may be failing, or the mutation gene may have the lethal effect.

Note : All female Drosophila can intercourse once only. In the first intercourse, the mating apparatus of the female will be damaged. So, they cannot have the second intercourse. Also, the femal will store the sperms in the first intercourse and fertilize the eggs thereafter. So, no second intercourse is effective.

(23.05.2007)