The purpose of this presentation is to help you understand the movements of chromosomes in meiosis and how Mendelian genes segregate in meiosis. This is the key to understanding the fundamentals of genetic principles.
A series of short movies will demonstrate the segregation of alleles during meiosis. The hypothetical organism is 2N = 4 and thus has two pairs of homologous chromosomes - remember one chromosome of each pair was donated by the gametes that formed this individual. This is indicated by color - blue chromosomes are from the paternal parent and red are from the maternal parent.
This presentation is a two dimensional artistic representation of a process that occurs in three dimensions. For clarity, we have arranged the chromosomes in a flat plane.
Meiosis is a series of two nuclear divisions called meiosis I and meiosis II. These two divisions are each divided into further phases:
 | Prophase |
| Metaphase |
| Anaphase |
| Telophase |
| In this initial example, we are considering two genes, A/a and B/b that are located on different chromosomes. The A/a gene is located at the tip of one arm of a large metacentric chromosome and the B/b gene is near the centromere of a smaller chromosome. The cell (meiocyte) undergoing meiosis is heterozygous for both genes and thus its genotype is AaBb. Note that one of the gametes that formed this organism must have had the genotype AB (blue parent) and the other gamete must have been ab (red parent). The arrangement of the genes on the chromosomes prior to meiosis is diagrammed below: |
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Prior to the beginning of meiosis, all the DNA replicates during the premeiotic S-phase to form identical pairs of sister chromatids for each chromosome.
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