1. 10.2 Nondisjunction & Karyotypes

2. Disjunction
The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction.
When the separation is not normal, it is called nondisjunction.

3. nondisjunction
Nondisjunction ("not coming apart") is the failure of chromosome pairs to separate properly during meiosis stage 1 or stage 2.
This could arise from a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis.

4. Nondisjunction

5. nondisjunction
The result of this error is a cell with an imbalance of chromosomes. Such a cell is said to be aneuploid.

6. Aneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders (birth defects).
Aneuploidy occurs during cell division when the chromosomes do not separate properly between the two cells (nondisjunction). Chromosome abnormalities occur in 1 of 160 live births.
Most cases of aneuploidy result in termination of the developing fetus, but there can be cases of live birth; the most common extra chromosomes among live births are 21, 18 and 13.

7. Nondisjunction
Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.
This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for monosomy or trisomy .

8. Nondisjunction
Loss of a single chromosome (2n-1), in which the daughter cell(s) with the defect will have one chromosome missing from one of its pairs, is referred to as a monosomy.
Gaining a single chromosome, in which the daughter cell(s) with the defect will have one chromosome in addition to its pairs is referred to as a trisomy.

9. Nondisjunction
This is a cause of several medical conditions in humans, including but not limited to:
Patau Syndrome - trisomy of chromosome 13
Edward Syndrome - trisomy of chromosome 18
Down Syndrome - trisomy of chromosome 21
Klinefelter Syndrome - extra X chromosomes in males
- i.e. XXY, XXXY, XXXXY, etc.
Turner Syndrome - lacking of one X chromosome in
females - i.e. X0
Triple X syndrome - an extra X chromosome in
females
XYY Syndrome - an extra Y chromosome in males.

10. Prenatal Diagnosis

11. Amniocentesis
A small sample of the amniotic fluid surrounding the baby is removed using a syringe.
The fluid contains skin cells from the baby.
The skin cells are grown in the lab.

13. The chromosomes from the cells are magnified under a microscope and a picture is taken.
The chromosomes are cut out and arranged in homologous pairs in decreasing size order.
This is called a karyotype.

18. Down Syndrome trisomy of chromosome 21
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
MedlinePlus
PubMed Health

20. Down Syndrome (Trisomy 21)
Extra chromosome 21 in
every cell of the body
Karyotype = 47,XX+21 or 47,XY+21

22. As a woman gets older, her chances of having a baby with a chromosome abnormality increases
***remember, a woman is born with all of her egg cells, but meiosis is not yet complete
(egg development stops in prophase I until the follicle matures prior to ovulation)

23. Copy this address to visit an animation showing normal meiosis and nondisjunction in mothers of different ages.

24. Turner Syndrome lacking of one X chromosome in females - i.e. X0
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
MedlinePlus
PubMed Health

25. Turner Syndrome

26. Triple X syndrome an extra X chromosome in females
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference

27. Klinefelter Syndrome extra X chromosomes in males - i. e
Klinefelter Syndrome extra X chromosomes in males - i.e. XXY, XXXY, XXXXY, etc.
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
MedlinePlus
PubMed Health

28. Klinefelter Syndrome

29. XYY Syndrome an extra Y chromosome in males.
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference

30. Patau Syndrome trisomy of chromosome 13
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
PubMed Health

31. Trisomy 13

32. Edward Syndrome trisomy of chromosome 18
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
PubMed Health

33. Trisomy 18

34. Nondisjuction
Surprisingly, in plants, extra chromosomes can actually be helpful.
Sometimes it makes larger flowers and fruits!

35. Polyploidy

36. Gene linkage and maps
Genes on the same chromosome are usually linked and inherited together instead of independently.
It is the chromosomes that follow Mendel’s law of independent assortment, not the genes.
Linked genes can be separated as a result of crossing over.
Scientists have found that genes that are farther apart on a chromosome tend to cross over more often than genes that are close together.
Using this information, scientist can make chromosome maps that show the sequence of genes on a chromosome.