6. Chromosomal Variation

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

How many chromosomes do you expect to see in karyotypes of the parents?

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

Explain how this is possible.

A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the second and third chromosomes. Her chromosomes, compared to those in a normal karyotype, are diagrammed to the right. This woman is phenotypically normal. Does this surprise you? Why or why not? Under what circumstances might you expect a phenotypic effect of such a rearrangement?

A normal chromosome and its homolog carrying a paracentric inversion are shown here. The dot (·) represents the centromere.

Normal    ABC • DEFGHIJK

Inversion  abc • djihgfe

Diagram the alignment of chromosomes during prophase I.

From the following list, identify the types of chromosome changes you expect to show phenotypic consequences.

Paracentric inversion