1. Prophase I – Cell contains 2n chromosomes (n being the number of haploids — 2n because homologous pair from parent will make it x2); synapsis (homologous pair find each other); crossing over (exchange of genetic material)
2. Metaphase I – Spindle microtubules move homologous pairs to equator of cell; random orientation of chromosomes –> the paternal or maternal chromosome can be on either pole.
3. Anaphase I – One chromosome of each homologous pair is moved to each pole.
4. Telophase I – Chromosomes uncoil; reduction of chromosome from diploid to haploid; cytokinesis

There are 4 steps in Meiosis II:

1. Prophase II – The two chromatids of the remaining chromosomes condense and become visible.
2. Metaphase II – Spindle microtubules move homologous pairs to equator of cell
3. Anaphase II – Centromeres are separated and chromatids are moved to opposite poles
4. Telophase II – Nuclear envelope forms; cytokinesis

Non-disjunction refers to the failure of homologous chromosomes to separate during anaphase. The result is one gamete without any chromosomes (hence die), and a gamete that has an extra chromosome or is deficient in a chromosome. When this gamete is involved in human fertilization, the result will be an individual with either 45 or 47 chromosomes. This leads to a person with syndrome (collection of physical signs or symptoms) such as Down syndrome, or even just mental and growth retardation.

Down syndrome is due to non-disjuction that leaves an individual with three of chromosome #21. Symptoms include hearing loss, heart/vision disorders, etc.

Identifying Down Syndrome – standard clinical practice is administered for 'older' parents, known as the triple test or the quad test. These blood tests look for unusual levels of the chemicals alpha-fetoprotein (AFP) and human chorionic gonadotropin (HCG). For those who do not past these tests are then advised to take the following two procedures to obtain fetal chromosomes to produce a karyotype, an organized image of metaphase fetal chromosomes.

• Amniocentesis – Using the aid of ultrasound, a needle is passed through the mother's abdominal wall to withdraw a sample of amniotic fluid from the amniotic sac of a developing fetus. The cells of the amniotic fluid are cultured and used to prepare a karyotype.
• Chorionic Villus Sampling – Sampling tool is passed through the vagina, to retrieve cells from the placenta, specifically the chorion.

Chiasmata

• In prophase I, all the chromatids in the two homologous chromosomes become tightly associated in the process called synapsis. The group of homologous chromosomes are collectively called bivalent or tetrad.
• Recombination refers to the process by which offspring possess a combination of alleles distinct from either parents.
• Chiasmata is a physical manifestation of crossing over, where an X-shaped structure is formed between non-sister chromatids of bivalents. Chiasmata persists through metaphase I and plays a role in the prevention of non-disjunction.

DBQ: Risk of Chromosomal Abnormalities with Advancing Age of the Patent

1. The incidence of chromosomal abnormalities increases rapidly (exponentially) as a function of the increase in maternal age (beginning from the age 20).
2. * a.) Less than 2% b.) Less than 1 %
3. * Chromosomal abnormalities are a far occasion, and of those rare events trisomy 21 is the most common. One possible reason for this trend could be because chromosome # 21 is most susceptible to an error in segregation. Another reason is because other chromosomal abnormalities, specifically in larger chromosomes in the 'single-digits', could be fatal, and trisomy 21 happens to be harmful but not to the extent of killing a victim.
4. There are many risks involved in the postponing of having children. For one, the older the mother becomes, the higher chance of chromosomal abnormality incidences. Another risk is the fact that woman begin to experiencemenopause with age; menopause is a phase women face in their 40s – 50s, where female hormonal production decreases and signals the end of the fertile phase.

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