Atypical Meiosis

BC Science Probe 9Section 4.5

Pages 127-129

Nondisjunction

• Sometimes errors happen during meiosis and the homologous chromosomes do not separate.

• This is called nondisjunction.

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.

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.

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.

Down Syndrome

• Down Syndrome affects about 1 in 800 babies born.

• Heart conditions, hearing loss and hypothyroidism are common among people with Down Syndrome.

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.

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.

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.

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

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.

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

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).

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.