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Abnormal Meiosis: Genetic Disorders
Review: Human Chromosomes
There are 46 chromosomes (23 homologous pairs) in each somatic cell
22 pairs of autosomes
1 pair of sex chromosomes XX = Female, XY = Male
Karyotype – picture of chromosomes arranged according to shape and size
Normal Meiosis
46
23 23
23232323
Non-disjunction
“not coming apart”
failure of chromosome pairs to separate properly during cell division
produces gametes with 22 or 24 chromosomes
due to a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis
Results in a cell with an imbalance of chromosomes
Abnormal Meiosis I
46
24 22
22222424
ALL 4 Abnormal
Abnormal Meiosis II
46
23 23
23232224
Sister chromatids fail
to separate
Abnormal Meiosis
If the error occurs in meiosis I, ALL 4 of the gametes will be abnormal
If the error occurs in meiosis II, TWO of the gametes will be normal, and the other 2 gametes will be abnormal.
Trisomy Examples
Down syndrome (chromosome 21)
Edwards syndrome (chromosome 18)
Patau syndrome (chromosome 13)
Klinefelter syndrome (XXY, XXYY, XXXXY)
Down syndrome
Down Syndrome
1 in 800 live births Mild to moderate intellectual ability Round, full face Short height Large forehead
Down Syndrome
Edwards Syndrome
1 in 4,400 births
XX=80%XY=20%
Edwards Syndrome
• Small head• Developmentally
delayed• Internal organ
abnormalities• 90% die before
5 months of age
Patau Syndrome
Small headSmall or missing eyesHeart defectsExtra fingersAbnormal genitaliaDevelopmentally delayedMost die a few weeks after birth
Patau Syndrome
Klinefelter Syndrome
Klinefelter Syndrome
1 in 1000 live births Males are sterile Small testes; shortage of
testosterone Breast enlargement, reduced facial
and body hair Some boys are shy and/or have
learning disabilities
Prenatal testing
Structural Abnormalities
Mutation: any change to the DNA of a cell Deletions: A portion of the chromosome is
missing or deleted e.g. Wolf-Hirschhorn syndrome, and Jacobsen
syndrome
Structural Abnormalities
Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material
e.g. Charcot-Marie-Tooth disease type 1A
Structural Abnormalities
Translocations: When a portion of one chromosome is transferred to another chromosome
Structural Abnormalities continued…
Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted
Please Complete
Computer activity
Check and Reflect Sheet