1. Atypical Meiosis BC Science Probe 9 Section 4.5 Pages 127-129

2. Nondisjunction Sometimes errors happen during meiosis and the homologous chromosomes do not separate. This is called nondisjunction.

3. Nondisjunction If one of the gametes that fuses during fertilization has the wrong number of chromosomes, the zygote will have either too much or too little genetic information.

4. Nondisjunction For example: – If a gamete with 22 chromosomes fuses with a gamete with 23, the zygote will have 45 instead of 46. Most of the time this causes the zygote to not divide and it dies.

5. Down Syndrome Down syndrome (DS), also called Trisomy 21, is a condition in which extra genetic material causes delays in the way a child develops, both mentally and physically.

6. Down Syndrome Down Syndrome affects about 1 in 800 babies born. Heart conditions, hearing loss and hypothyroidism are common among people with Down Syndrome.

7. Patau Syndrome Trisomy 13, also called Patau syndrome, is a chromosomal condition associated with severe intellectual disability and physical abnormalities in many parts of the body.

8. Patau Syndrome Individuals with trisomy 13 often have heart defects, brain or spinal cord abnormalities, very small or poorly developed eyes, extra fingers and/or toes, a cleft lip or a cleft palate, and weak muscle tone.

9. Turner Syndrome (X0) Most girls are born with two X chromosomes, but girls with Turner syndrome are born with only one X chromosome or they are missing part of one X chromosome.

10. Turner Syndrome (X0) Short stature Lack of sexual development a low hairline at the back of the neck drooping of the eyelids differently shaped ears that are set lower abnormal bone development a larger than usual number of moles on the skin extra fluid in the hands and feet

11. Trisomic X (XXX) XXX syndrome (also called Trisomy X or Triple X) is caused by the presence of an extra ‘X’ chromosome in every cell.

12. Trisomic X (XXX) Tall stature (height) Possible mild facial characteristics Normal IQ, but may be 10-15 points below siblings Speech and language delays (50%) poor coordination introverted, difficulty with interpersonal relationships

13. Klinefelter Syndrome (XXY) Affected individuals typically have small testes that do not produce as much testosterone as usual. A shortage of testosterone can lead to delayed or incomplete puberty, breast enlargement, reduced facial and body hair, and an inability to have biological children (infertility).

14. Klinefelter Syndrome (XXY) Children with Klinefelter syndrome may have learning disabilities and delayed speech and language development. They tend to be quiet, sensitive, and unassertive, but personality characteristics vary among affected individuals.