1. Gamete formation and errors in meiosis

2. Gamete Formation Gametogenesis – process of creating sperm and eggs
Spermatogenesis – male gamete formation
Occurs in the testes
Immature spermatogonia (germ cells) may continually duplicate through mitosis or undergo meiosis to eventually develop into sperm cells
In some species this can occur year round while in other species this is limited to certain times in the year

3. Spermatogenesis
A diploid germ cell (spermatogonium) undergoes meiosis to create 4 haploid cells
Following meiosis II, most cytoplasm is lost and a tail develops to allow locomotion  sperm
The entire process, from spermatogonium to functional sperm, takes approximately 76 days
At any one time, different cells may be in a different stage of the development process  this allows sperm cell production to stay steady at about 200 million sperm per day

4. Gamete Formation Oogenesis – female gamete formation
Occurs in the ovaries
Diploid germ cell (oogonium) undergoes meiosis to create 1 haploid cell
After meiosis I, cytoplasm does not split equally and the majority goes to the secondary oocyte. The other cell is called a polar body and it is not a viable sex cell.
After meiosis II, the cytoplasm is again unequally divided and only one cell is viable as a sex cell (egg or ovum)

5. oogenesis
When a female is born, primary oocytes have already entered meiosis I but remain suspended in prophase I until puberty
Starting at the first menstrual cycle, meiosis will resume one oocyte at a time, once a month
Production of ova continues from puberty until menopause (between 40 and 55)
Oocytes left in ovaries nonfunction/unresponsive and no longer released

6. Abnormal Meiosis
Nondisjunction – failure of chromosomes to separate properly
2 homologous chromosomes move to the same pole during meiosis
One of the daughter cells will be missing 1 chromosome, while the other retains an extra chromosome

7. Nondisjunction
In humans, nondisjunction produces gametes with 22 and 24 chromosomes
The gamete with 24 chromosomes has both chromosomes from one of the homologous pairs
If joins with normal gamete, = 47 chromosomes in zygote rather than 46
Once zygote begins dividing, each cell of the body will contain 47 chromosomes
Trisomy: there are 3 homologous chromosomes instead of a homologous pair
Monosomy: there is a single chromosome instead of a homologous pair

8. Down syndrome
A trisomic disorder in which a zygote receives 3 homologous chromosomes for chromosome pair number 21
Common traits: round, full face, enlarged and creased tongue, short height, large forehead, mental retardation
Woman in 40s has 1 in 40 chance of having a child with Down syndrome
1/600 babies

9. Turner syndrome: monosomic disorder - a female has a single X chromosome
Sex chromosomes undergo nondisjunction
Appear female but do not usually develop sexually, short and have thick, widened necks  1/3000 female babies
Klinefelter syndrome: trisomic disorder - male carries an XXY condition
Nondisjunction in sperm or egg
Appears to be male at birth, but begins producing high levels of female sex hormones once entering sexual maturity  Males are sterile
1/500 male babies

10. Mitosis vs Meiosis Rap Battle Video: https://www. youtube. com/watch
(3:01)