The Human Karyotype & Nondisjunction

What’s a Karyotype?? Scientists use a microscope to take pictures of the condensed chromosomes during metaphase. They can match up the homologous chromosomes by size and shape. This arrangement of chromosomes is called a KARYOTYPE.

A closer look at a karyotype… The human karyotype has 23 pairs or 46 chromosomes Sex chromosomes

Remember, Meiosis results in the formation of FOUR daughter cells from ONE parent cell. Meiosis results in the formation of GAMETES (either eggs or sperm) which are then combined together to form a new individual.

The gametes come together… Egg (N), haploid Sperm (N), haploid Embryo (2N), diploid

Uh Oh! Problems with Meiosis… Usually Meiosis occurs with NO PROBLEMS. BUT, sometimes sex cells (gametes) are formed with irregular chromosomes numbers!! Nondisjunction: Homologous chromosomes DO NOT SEPARATE during meiosis. Gametes are produced with the WRONG NUMBER of chromosomes!!

Nondisjunction

Down Syndrome Results from having 47 chromosomes instead of 46 People with Downs Syndrome have 3 copies of chromosome 21 Have similar facial features, mental and physical disabilities

Turner Syndrome Female born with only 1 “X” chromosome (instead of 2) Do not develop into puberty (therefore, they can’t reproduce) Sometimes females have 3 “X’s”… these women CAN reproduce but are often taller and thinner than women with “normal” chromosome number

Klinefelter Syndrome Males born with 2 “X” chromosomes and 1 “Y” Produce less testosterone (Sex hormone) and can’t father children