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Nondisjunction is a rare event that occurs in when paired homologous chromosomes or sister chromatids do not separate and move to opposite poles during anaphase. It results in loss of gain of a chromosome.

Depending on when it occurs (meiosis I or II), nondisjunction can affect progeny phenotypic and genotypic ratios differently. When it occurs in meiosis I, both homologues end up in one daughter cell. When it occurs in meiosis II, one of the four daughter cells ends up with both sister chromatids from one chromosome, and the other meiotic cell involved does not have any copies of that chromosome. Some examples of genetic abnormality caused by nondisjunction are Down syndrome (trisomy 21), Turner female (XO), and Klinefelter syndrome (XXY).

Bridge's experiment on Drosophila

In Drosophila, the gene for eye color is on the X chromosome and red eye color is dominant over white eye color.

Thus, a homozygous white female mating with a red (single allele) male would in theory have all red-eyed daughters and white-eyed sons. However, nondisjunction means that some female progeny lose their paternal red allele and wind up with white eyes.

Parentalwhite-white female x red male
ExpectedAll daughters are red-eyed;
All sons are white-eyed
ObservedApproximately 0.1% of total female progeny is white-eyed instead of red-eyed due to nondisjunction.