10 2 Nondisjunction Karyotypes Disjunction The normal separation

Disjunction • The normal separation of chromosomes in meiosis I or sister chromatids in

nondisjunction • Nondisjunction ("not coming apart") is the failure of chromosome pairs to separate

nondisjunction • The result of this error is a cell with an imbalance of

Aneuploidy • Aneuploidy is an abnormal number of chromosomes, and is a type of

Nondisjunction • Nondisjunction can occur in the meiosis I or meiosis II, phases of

Nondisjunction • Loss of a single chromosome (2 n-1), in which the daughter cell(s)

Nondisjunction This is a cause of several medical conditions in humans, including but not

Amniocentesis • A small sample of the amniotic fluid surrounding the baby is removed

• The chromosomes from the cells are magnified under a microscope and a

Down Syndrome trisomy of chromosome 21 Places to find out more information from the

Down Syndrome (Trisomy 21) • Extra chromosome 21 in every cell of the body

• As a woman gets older, her chances of having a baby with

Copy this address to visit an animation showing normal meiosis and nondisjunction in mothers

Turner Syndrome lacking of one X chromosome in females - i. e. X 0

Triple X syndrome an extra X chromosome in females Places to find out more

Klinefelter Syndrome extra X chromosomes in males - i. e. XXY, XXXXY, etc. Places

XYY Syndrome an extra Y chromosome in males. Places to find out more information

Patau Syndrome trisomy of chromosome 13 Places to find out more information from the

Edward Syndrome trisomy of chromosome 18 Places to find out more information from the

Nondisjuction • Surprisingly, in plants, extra chromosomes can actually be helpful. • Sometimes it

Gene linkage and maps • Genes on the same chromosome are usually linked and

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10. 2 Nondisjunction & Karyotypes

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.

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.

Nondisjunction

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

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.

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.

Nondisjunction • Loss of a single chromosome (2 n-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.

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, XXXXY, etc. • Turner Syndrome - lacking of one X chromosome in females - i. e. X 0 • Triple X syndrome - an extra X chromosome in females • XYY Syndrome - an extra Y chromosome in males.

Prenatal Diagnosis

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.

• 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.

Down Syndrome trisomy 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 Medline. Plus http: //www. nlm. nih. gov/medlineplus/downsyndrome. html Pub. Med Health http: //www. ncbi. nlm. nih. gov/pubmedhealth/PMH 0001992/

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

• 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)

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/ch 10 expl. htm

Turner Syndrome lacking of one X chromosome in females - i. e. X 0 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 Medline. Plus http: //www. nlm. nih. gov/medlineplus/turnersyndrome. html Pub. Med Health http: //www. ncbi. nlm. nih. gov/pubmedhealth/PMH 0001417/

Turner Syndrome

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 http: //ghr. nlm. nih. gov/condition/triple-x-syndrome

Klinefelter Syndrome extra X chromosomes in males - i. e. XXY, 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 Medline. Plus http: //www. nlm. nih. gov/medlineplus/klinefelterssyndrome. html Pub. Med Health http: //www. ncbi. nlm. nih. gov/pubmedhealth/PMH 0001420/

Klinefelter Syndrome

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 http: //ghr. nlm. nih. gov/condition/47 xyy-syndrome

Patau Syndrome trisomy 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 Pub. Med Health http: //www. ncbi. nlm. nih. gov/pubmedhealth/PMH 0002625/

Trisomy 13

Edward Syndrome trisomy 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 Pub. Med Health http: //www. ncbi. nlm. nih. gov/pubmedhealth/PMH 0002626/

Trisomy 18

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

Polyploidy

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.