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Nondisjunction in Humans

In the diagrams below, models of chromosomes have been constructed using beads. The ovals represent cells. Each cell shows either one chromosome, one homologous pair of chromosomes, or no chromosomes.

Normal Meiosis

Normal human cells have 23 pairs of chromosomes. One of these pairs is sex chromosomes and the other 22 pairs are autosomes. The two chromosomes shown in the diagram represent a homologous pair. They could represent any of the pairs of autosomes (either sex) or two X chromosomes (females).

The diagram shows the metaphase alignment in the primary spermatocyte or primary oocyte (top) and secondary spermatocyte or secondary oocyte (middle). The chromosomes found in the sperm or the egg and polar bodies are seen in the bottom.

Each of the gametes (bottom row) have one chromosome shown. For example, if the chromosomes in the diagram represent chromosome 21, then each gamete has one chromosomes 21.

There is a total of 23 chromosomes in each of the gametes in the bottom row.

Nondisjunction, Meiosis I

The two chromosomes shown in the diagram represent a homologous pair. They could represent any of the pairs of autosomes (either sex) or two X chromosomes (females).

The diagram shows the metaphase alignment in the primary spermatocyte or primary oocyte (top) and secondary spermatocyte or secondary oocyte (middle). The chromosomes found in the sperm or the egg and polar bodies are seen in the bottom.

Two of the gametes (bottom row) have a chromosome missing and two have an extra chromosome. For example, if the chromosomes in the diagram represent chromosome 21, then the two gametes shown in the bottom left do not have a chromosome 21 and the two gametes shown in the bottom right have two chromosomes 21.

There is a total of 22 chromosomes in the two of the gametes on the left 24 in the other two gametes.

Nondisjunction, Meiosis II

The two chromosomes shown represent a homologous pair. They could represent any of the pairs of autosomes or two X chromosomes.

This slide shows the metaphase alignment in the primary spermatocyte or primary oocyte (top) and secondary spermatocyte or secondary oocyte (middle). The chromosomes found in the sperm or the egg and polar bodies are seen in the bottom.

The two gametes shown on the bottom left are normal. The third gamete from the left has one chromosome missing and the fourth gamete has one extra chromosome. For example, if the chromosomes in the diagram represent chromosome 21, then the two gametes shown on the bottom left each have one chromosome 21, the third gamete does not have a chromosome 21 and the fourth gamete has two chromosomes 21.

There is a total of 23 chromosomes in the two of the gametes on the left, 22 chromosomes in the third gamete from the left and  24 chromosomes in the fourth gamete from the left.

Normal Spermatogenesis showing sex chromosomes

The short chromosome represents a Y chromosome and the long chromosome represents the X chromosome. The 22 pairs of autosomes are not shown.

The diagram shows the metaphase alignment in the primary spermatocyte (top) and secondary spermatocyte (middle). The chromosomes found in the spermatids and sperm are shown in the bottom row. 

There is one X chromosome in each of the two gametes shown in the bottom left. The other two gametes each have one Y chromosome.

There is a total of 23 chromosomes in each of the gametes (bottom row).

Nondisjunction, Meiosis I, Males

The short chromosome represents a Y chromosome and the long chromosome represents the X chromosome.

This slide shows the metaphase alignment in the primary spermatocyte (top) and secondary spermatocyte (middle). The chromosomes found in the spermatids and sperm are seen in the bottom.

There are no sex chromosomes in the two gametes shown on the bottom left. The other two gametes each have one X and one Y chromosome.

There is a total of 22 chromosomes in the two of the gametes on the left 24 in the other two gametes.

Nondisjunction of the X Chromosome, Meiosis II, Males

The short chromosome represents a Y chromosome and the long chromosome represents the X chromosome.

This slide shows the metaphase alignment in the primary spermatocyte (top) and secondary spermatocyte (middle). The chromosomes found in the spermatids and sperm are seen in the bottom.

There is one Y chromosome in each of the two gametes shown on the bottom left. The third gamete from the left does not have a sex chromosome and the fourth from the left has two X chromosomes.

There is a total of 23 chromosomes in each of the two of the gametes on the left, 22 chromosomes in the third gamete from the left and 24 chromosomes in the fourth gamete from the left.

Nondisjunction of the Y Chromosome, Meiosis II, Males

The short chromosome represents a Y chromosome and the long chromosome represents the X chromosome.

This slide shows the metaphase alignment in the primary spermatocyte (top) and secondary spermatocyte (middle). The chromosomes found in the spermatids and sperm are seen in the bottom.

There are two Y chromosomes in the gamete shown on the bottom left. The second gamete from the left does not have a sex chromosome. The other two gametes each have one X chromosome..

There is a total of 24 chromosomes in the gamete shown in the bottom left, 22 chromosomes in the second gamete from the left, and 23 chromosomes in the other two gametes.