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 (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.

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.

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 Referencehttp://ghr.nlm.nih.gov/condition/down-syndrome

MedlinePlushttp://www.nlm.nih.gov/medlineplus/downsyndrome.html

PubMed Health http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001992/

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/ch10expl.htm

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 Referencehttp://ghr.nlm.nih.gov/condition/turner-syndrome

MedlinePlushttp://www.nlm.nih.gov/medlineplus/turnersyndrome.html

PubMed Healthhttp://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001417/

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

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 Referencehttp://ghr.nlm.nih.gov/condition/klinefelter-syndrome

MedlinePlushttp://www.nlm.nih.gov/medlineplus/klinefelterssyndrome.html

PubMed Healthhttp://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001420/

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

Patau Syndrome trisomy of chromosome 13

Places to find out more information from the U.S. National Library of Medicine :

Genetics Home Referencehttp://ghr.nlm.nih.gov/condition/trisomy-13

PubMed Healthhttp://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002625/

Trisomy 13

Edward Syndrome trisomy of chromosome 18

Places to find out more information from the U.S. National Library of Medicine :

Genetics Home Referencehttp://ghr.nlm.nih.gov/condition/trisomy-18

PubMed Healthhttp://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002626/

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.