Notes: When Mistakes Occur Packet 7- Cell Reproduction Pg. 10-11

The diagram to the right, called a karyotype, shows what the chromosomes look like when we stain them (which causes bands) and match them up according to size.

From an adult, blood is often taken to get cells for a karyotype. For an unborn baby, two methods are used to get fetal cells: Chorionic Villi Sampling (CVS)- take a piece of placenta Amniocentesis (take amniotic fluid sample)

The pair of chromosome that have the same length and banding pattern are called homologous pairs.

The first 22 pairs of chromosomes are the non-sex chromosomes and are called autosomes. Pair #1 is the shortest Part #22 is the longest

The last or 23rd pair of chromosomes differ in males and females and are called sex chromosomes. Females have two of the same (large) chromosomes which are referred to by the symbols XX. Males have two different chromosomes, one large called X and one small called the Y chromosome. Sex chromosomes

When mistakes occur…. Sometimes during meiosis pairs of chromosomes don't separate from each other. This non-separation is called nondisjunction. The end result is the production of an egg or sperm that has too many or too few chromosomes.

Normal Division Nondisjunction

Nondisjunction can occur with autosomes or sex chromosomes. Trisomy is the formation of a zygote (offspring) with one extra chromosome. In a human this would form a gamete with 24 chromosomes total. If it unites with a normal gamete (which would have 23 chromosomes) then there would be 47 total chromosomes. The issues that arise will depend upon which chromosome has three copies. In Down Syndrome, there are three 21st chromosomes…a total of 47 chromosomes in every cell. Monosomy is the formation of a zygote that is missing one chromosome.

Usually zygotes with an abnormal number of chromosomes are miscarried (or aborted spontaneously by the body). However, if combinations are not lethal (deadly), the individual may survive to birth, but still be seriously affected.

Normal Karyotype

46XY 46 XX

Autosomes Sex Chromosomes

Patau’s Syndrome

Edward’s Syndrome

Down’s Syndrome

Turner’s Syndrome

Triple X Syndrome

Klinefelter’s Syndrome

Jacob’s Symdrome

Chromosomal makeup is written with: Total number of chromosomes per cell Followed by the symbols for the sex chromosomes in each cell If there is a defective chromosome, it is identified by its number (in parenthesis)

If you make a karyotype and the genetic diagnosis is: 46XY Does the child have the correct number of chromosomes? Yes Is it male of female? Male If there is a problem, what is it? No problem

46 XY

If you make a karyotype and the genetic diagnosis is: 47 XX (18) Does the child have the correct number of chromosomes? No, one extra. Is it male or female? Female If there is a problem, what is it? Edwards Syndrome.

47 XX (18)