Obtaining Parental Lines - Sex-Linked XX-XY

Mutant Allele is Completely Dominant


How do Geneticists Get a New Parental Line?

In the following discussion,  genotypes in black (A2A2 or A2Y ) are associated with the wildtype phenotype. Genotypes in blue (A1A1 & A1A2 or A1Y) are associated with the dominant mutant phenotype.

 If the mutation is an X-linked dominant it is actually easier to fix the allele than if the allele was autosomal.  This is because the genotype of the male is always known.  Furthermore, the phenotype of the males for an X-linked trait depends entirely on the genotype of the mother.   That is:

As previously discussed it is possible to obtain a generation of organisms for a dominant trait where the females have the single dominant phenotype representing the two genotypes A1A1 and A1A2 and the males have the genotypes A1Y and A2Y.

The A2Y wildtype males are discarded and mutant males A1Y are mated to individual females who may be either  A1A1 or A1A2All of the daughters from each of these matings will be wildtype.  The sons, however, will reveal their mother's genotype.

  1. If a wildtype son occurs, the mother was A1A2 and all of the progeny (males and females) can be discarded.

  2. If only mutant males occur  (A1Y) there is a good possibility that their mother was A1A1.  Thus,  and all of the sisters are homozygous (A1A1). 

In the second case the mutant males  (A1Y) can then be mated to their sisters to (A1A1) establish a pure-breeding line. 

Table 1:
 
The mutant males are individually mated to their sisters. 

These mating are of two different types:

Cross 1a: This cross produces both mutant males and mutant females.  These can be intermated to produce a homozygous parental line. 

Cross 1b:   If a wildtype son occurs, the mother was A1A2 and all of the progeny can be discarded.  The mutant  progeny might be kept for additional breeding (always preserve the allele) but the wildtype A2Y males would be discarded.

If only mutant sons occur, there is a good possibility that the mother was A1A1 and all of the daughters are homozygous.  The mutant males can then be mated to their mutant sisters to establish a pure-breeding line.
 

Cross 1a Mutant x Mutant
  &   %
  A1A1 x A1Y
       
Progeny A1A1   A1Y
       

 

Cross 1b Mutant x Mutant
  &   %
  A1A2 x A1Y
       
Progeny A1A1   A1Y
  A1A2   A2Y

 

This is fairly reliable method if sufficient sons are produced as in Drosophila.  In less fecund species, such as the mouse, one would continue to monitor the offspring from brother-sister matings and discard any animals from crosses in which a wildtype pup occurred. 

 In species with very low fecundity (e.g., sheep) it might be impossible to get a homozygous line.

Mendelian Genetics Index

 

Related Tools

Parental Lines 1
How to Obtain a Parental Line
  a) Autosomal Trait  
  b) Sex-linked XX-XY
  c) sex-linked ZZ-ZW
2
How to Obtain a Parental Line for a Dominant Allele
a) Autosomal Trait 
b) Sex-linked XX-XY 
c) sex-linked ZZ-ZW
3
How to Breed a Homozygous Line for more than one gene 4
F1 cross 5