1. Chromosomes
What do you notice about this karyotype from a human cell? Trisomy 21 (3 copies of chromosome 21) and the cell comes from a male (XY).

2. Trisomy Trisomy means having an extra copy of a chromosome.
Remember, chromosomes are extremely long stretches of DNA which code for 100’s and even 1000’s of genes.
Having an extra copy of so many genes can have profound phenotypic consequences.

3. Trisomy 21 Trisomy 21 is commonly called Down Syndrome
People with Down Syndrome have three copies of chromosome 21.
People with Down Syndrome have features that appear different than people who have two copies of chromosome 21.
Although rare, the odds have having a child with Down Syndrome increase in older mothers.

4. Down Syndrome Genotype Phenotype
Click the pictures to play the videos. Genotype: What sort of cell division is occurring in the video? (Meiosis) How do you know? Phenotype: how is someone with Down Syndrome different from you? How are they the same?
Genotype
Phenotype

5. Twins
Is it possible to have identical twins, where one twin is male and the other twin is female? Why or why not? How about fraternal (non-identical) twins?

6. Twins Identical Twins Fraternal Twins
Why are the twins in the video considered “1 in a million”? Reflecting on what you have learned, will you think differently about the way you respond to people who appear different from you? Why or why not?

7. Mosaics and X-inactivation
At what stage in embryonic development does X-inactivation occur? The Stem Cell stage (early in development)
Does X-inactivation occur in males? Do any male cells lack an X chromosome? Are all females mosaics? (Yes) What does that mean?