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Making Parental Lines for a Dominant Autosomal Allele |
Fixing a new dominant allele:
Assumptions:
It is possible to obtain a homozygous line for the dominant phenotype through a series of genetic crosses.
The exact breeding procedure will depend on the fecundity and the mating systems of your organism. The more progeny the organism produces, the better. Females who can produce several sequential broods or litters by different males are more desirable than females who produce only a single brood or who mate only once and then store sperm.
Defined Phenotypes- the heterozygote.
Getting the appropriate homozygous line for a dominant trait requires that the experimenter be very flexible. In general, the dominant trait is going to be in the population in the heterozygous condition.
Step 1: intercross the known heterozygotes.
Step 2 : Isolate the AA from the Aa individuals (or How can tell them apart?) The answer to this question depends on the mating systems of you organisms. There are several genetic crosses which are possible. For a few organisms there are some sophisticate methods for getting homozygous organisms.
| 1. | Test Cross | If the organism produces fairly large litters (8 of more) and the males
and females live long enough and also can remate (without storing sperm) you
can test cross individuals with the dominant phenotype (AA or Aa) to
homozygous recessive individuals (aa). |
| If any of the resultant progeny show the recessive phenotype, you can
discard that individual as a founder of your homozygous line because it was
heterozygous (Aa). |
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| If your cross produces more than eight progeny and all of the progeny have the dominant phenotype, there is a high probability that the
tested individual was homozygous (aa). If the organism is long-lived, you
might do several test crosses with the same individual (easier for males
than for females). |
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| Test crosses work well with zebra fish and mice. |
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| This is fairly labor intensive. Since on average only one in three organisms will be homozygous (AA), you might have to test cross 60 individuals to get 10 pairs of homozygous parents (10 males, 10 females). |
| 2. | Pairwise cross | Drosophila females only mate once so it is not possible to use test cross information. With animals such as Drosophila, where test crossing does not work, you have to do many different pairwise crosses. | ||||||||||||||||||||||||||||||||||||||||||
| A male and female each with the dominant phenotype are placed in a single vial (a pairwise mating). | ||||||||||||||||||||||||||||||||||||||||||||
| These crosses will be of three types with the most desirable cross (AA x AA being the rarest - about 1 in 9 crosses). The crosses and their probabilities are seen below. | ||||||||||||||||||||||||||||||||||||||||||||
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| For every 9 pairwise crosses that you set up, you will immediately discard 4/9 of them since the recessive phenotype showed up. | ||||||||||||||||||||||||||||||||||||||||||||
| Of the remaining 5 crosses, only one of them, on average is what you want (AA x AA) and the other 4 are (AA x Aa). In none of these 5 crosses will a recessive phenotype show up among the progeny. | ||||||||||||||||||||||||||||||||||||||||||||
| Progeny from all crosses with only dominant phenotypes are mass-mated. With Drosophila where rearing costs are minimal, you would use 10 or more pairs per bottle and multiple bottles per cross. | ||||||||||||||||||||||||||||||||||||||||||||
| A single bottle may produce more than 1000 progeny. If a single recessive phenotype shows up among the progeny, the entire bottle should be discarded. In any vial where there are only dominant phenotypes, you have probably reached homozygosity. | ||||||||||||||||||||||||||||||||||||||||||||
| If only dominant phenotype show up in a bottle, you have probably reached homozygosity. | ||||||||||||||||||||||||||||||||||||||||||||
| Your suspected homozygous lines will be observed over generations for the occurrence of recessive phenotypes. In addition, a random sample might be test crossed to confirm that the flies are homozygous. | ||||||||||||||||||||||||||||||||||||||||||||
| 3. | A Sad Truth | You may never get a homozygous line: If an organism has low fecundity (one or two progeny at a time) and doesn't remate, it may take many generations before you are confident that it is homozygous (AA). | ||||||||||||||||||||||||||||||||||||||||||
| 4. | "GeneticTricks" | For a few organisms there are a few tricks for getting homozygous lines for a dominant traits. To review these:inactive links | ||||||||||||||||||||||||||||||||||||||||||