Gamete Formation and Genetic Variation
GAMETE FORMATION
• The production of gametes is called __________________ and results in the formation of __________ and _________.
• The process of male gametogenesis is called _____________________ and the process of female gametogenesis is called __________________.
SPERMATOGENESIS
• Takes place in the ___________.
• It all starts with a diploid germ cell called a ____________________.
• This cell undergoes meiosis I and meiosis II and eventually becomes 4 haploid __________ cells
• Sperm can either be made throughout the year as in ___________ or only during breeding season as seen in ___________________.
OOGENISIS
• Takes place in the ___________.
• Starts with a diploid germ cell called an _____________.
• Undergoes meiosis I and meiosis II
• The cytoplasm does not equal divide and the larger of the 2 cells after meiosis I is called a _________________, the smaller cell is called a _______________ and is not a viable sex cell
• As the primary oocyte undergoes meiosis II, the cytoplasm unequally divides again and only ONE cell becomes an _______ (ovum) and the other cell is again a _____________ and is not a viable sex cell
• The unequal cytoplasmic division is to give the ovum enough nutrients to support the zygote in the first few days of fertilization
• Therefore after meiosis I and meiosis II, there is only ____ haploid ovum
Spermatogenesis & Oogenesis
Meiosis & Genetic Variation
• Through crossing over and random segregation of the chromosomes during meiosis results in _______________________.
• When there are errors in meiosis these results in genetic errors throughout the entire organism. Why?
NONDISJUNCTION (Abnormal Meiosis)
• Occurs when homologous chromosome pairs move to the same pole during meiosis.
• The result is that one daughter cell is ____________ one chromosome and the other will have an _________ chromosome.
• When cells have too much information or too little information, the cells will not function properly.
• Since this most commonly occurs in the gametes, once the zygote is formed the entire organism will have this abnormal chromosomal count.
• In humans nondisjunction will produce gametes with _____ and _____ chromosomes.
• When a zygote is created from a cell that contains 23 and 24 chromosomes the individual will have a diploid count of ______. This condition is referred to as _____________ because there are 3 chromosomes instead of just 2 on one pair of the homologues.
• If the zygote created from cells that contain 22 and 23 chromosomes, the individual will have a diploid number of _____. This condition is called _______________ because there is just one chromosome instead of the usual homologous pair.
• Once the cells of these zygotes begin to divide, each cell of the body will contain greater than or less than 46 chromosomes.
NONDISJUNCTION DISORDERS
• The most common type of trisomy is __________________.
• There was an unequal division of the 21st homologous pair and that is why this disorder is sometimes called _____________.
• Another nondisjunction order that occurs in the sex chromosomes is called _____________________.
• This occurs when both ____ chromosomes migrate to one pole and the other cell has ____ X chromosomes.
• When the egg with no X chromosomes is fertilized by a normal sperm cell with an X chromosome a female will be produced with ___________________. They never sexually mature and are unable to pass on this defect. Most are usually miscarried before the first 20 weeks of pregnancy.
• _______________________ is caused by nondisjunction in either sperm or egg. The child inherits two X chromosomes and a single Y chromosome. The child appears to be a male at birth, but as he enters puberty, he produces high levels of female sex hormones. These males are sterile.
KARYOTYPE CHARTS
• During genetic screening, a __________________ can be done which arranges all of the chromosomes in pairs. From here any chromosomal disorders and even the gender can be predicted from this chart.
• Technicians take a small tissue sample and treat it with a solution that causes mitosis.
• They then treat the sample with another solution to stop mitotic division at metaphase seeing that the chromosomes are the most condensed and the centromeres is the most discernable.
• The cells are then placed on a slide and stained. A photograph is then taken and enlarged.
• The chromosomes are then cut out and matched up with their homologous pair.
• Finally the pairs are aligned at their centromeres in decreasing size. The sex chromosomes are always placed last.