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.
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