10.2 Nondisjunction & Karyotypes

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 Syndrometrisomy of chromosome 21 Places to find out more information from the U.S. National Library of Medicine : Genetics Home Reference http://ghr.nlm.nih.gov/condition/down-syndrome MedlinePlus http://www.nlm.nih.gov/medlineplus/downsyndrome.html PubMed Health http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001992/

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. http://www.mhhe.com/biosci/genbio/biolink/j_explorations/ch10expl.htm

24. Turner Syndromelacking 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 http://ghr.nlm.nih.gov/condition/turner-syndrome MedlinePlus http://www.nlm.nih.gov/medlineplus/turnersyndrome.html PubMed Health http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001417/

25. Turner Syndrome

26. Triple X syndromean extra X chromosome in females Places to find out more information from the U.S. National Library of Medicine : Genetics Home Reference http://ghr.nlm.nih.gov/condition/triple-x-syndrome

27. Klinefelter Syndromeextra 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 http://ghr.nlm.nih.gov/condition/klinefelter-syndrome MedlinePlus http://www.nlm.nih.gov/medlineplus/klinefelterssyndrome.html PubMed Health http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001420/

28. Klinefelter Syndrome

29. XYY Syndromean extra Y chromosome in males. Places to find out more information from the U.S. National Library of Medicine : Genetics Home Reference http://ghr.nlm.nih.gov/condition/47xyy-syndrome

30. Patau Syndrometrisomy of chromosome 13 Places to find out more information from the U.S. National Library of Medicine : Genetics Home Reference http://ghr.nlm.nih.gov/condition/trisomy-13 PubMed Health http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002625/

31. Trisomy 13

32. Edward Syndrometrisomy of chromosome 18 Places to find out more information from the U.S. National Library of Medicine : Genetics Home Reference http://ghr.nlm.nih.gov/condition/trisomy-18 PubMed Health http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002626/

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.