1. Mistakes in Meiosis
Meiosis Lecture 4

2. Nondisjunction –
Failure of homologous chromosomes OR chromatids to separate at anaphase

3. Nondisjunction
Results in ANEUPLOIDY = one or more chromosomes have extra or missing copies
AFTER FERTILIZATION: Cell with only 1 copy of a chromosome instead of 2
= MONOSOMY
Cell with 3 copies of a
chromosome instead of 2
= TRISOMY

4. Errors in mitosis or meiosis can result in changes in phenotype
Changes in chromosome number often result in:
New phenotypes
Sterility caused by triploidy and increased vigor of other polyploids
human disorders with developmental limitations
Trisomy 21 (Down syndrome)
XO (Turner Syndrome)

5. Karyotype can show: Sex of baby Missing or extra chromosomes
Major deletions or translocations
Can’t see individual gene changes

6. Down syndrome (Trisomy 21)
Most common chromosomal abnormality (1 in 800 births)
Similar facial features Slanted eyes / Protruding tongue Mild to severe mental retardation
50% have heart defects that need surgery to repair
Both older (35+ years) and younger (under 16 years) mothers are more at risk.

7. Some individuals with Down syndrome have the normal number of chromosomes but have all or part of a third chromosome 21 attached to another chromosome by translocation.

8. Turner syndrome (X0)
1 in 5000 births
Females have only one X chromosome
Small size
Broad chest
Slightly decreased intelligence
35% have heart abnormalities
Hearing loss common
Reproductive organs don’t develop at puberty

9. http://www. akdeniz. edu
Klinefelter syndrome
1 in 2000 births live births
Males have extra X chromosomes
(Can be XXy, XXXy, or XXXXy)
Taller than average
Normal intelligence have male sex organs, but are sterile. may be feminine characteristics,
Often not discovered until puberty when don’t mature like peers
Presence of BARR BODIES