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