This Grade 12 IEB Life Sciences document provides a comprehensive summary of abnormal meiosis and its resulting genetic disorders, including non-disjunction conditions like Down syndrome and Klinefelter's syndrome. With clear explanations of chromosomal mutations and their effects on physical and m...
Abnormal Meiosis
Chromosomal non-disjunction occurs when chromosomes fail to separate correctly during meiosis.
‣ This results in a condition called aneuploidy where zygote has an extra or a missing
chromosome.
‣ A!ected individuals have physical and mental characteristics called syndromes.
Note:
Diagnosis - identifying the nature of an event. (e.g illness)
Prognosis - predicting how the event will develop.
Klinefelters syndrome
‣ occurs in males when a boy is born with an extra copy of the X chromosome (XXY)
‣ This occurs when sex chromosomes in the egg split unevenly
Symptoms:
- Sparse body hair
- Enlarged breasts
- Small testicles and penis
- Not very deep voice
- May interfere with sexual development an fertility
Down syndrome
‣ occurs in children who are born with an extra (third) copy of chromosome number 21 in their cells
this is known as trisomy
How does this come about?
- during oogenesis the two number 21 chromosomes do not separate properly during anaphase
- Result is an egg with two number 21 chromosomes instead of one.
- If egg is fertilised, zygote will have three number 21 chromosomes, and thus each cell has 47
chromosomes instead of 46.
Characteristics:
- varying degrees of mental retardation
- Distinctive "attened facial features and slightly slanted eyes ( due to skin folds at corners of eyes )
- Short stubby #ngers, largish head and ears may develop di!erently
- These can be accompanied by heart defects and other health problems.
- Tend to have a happy and loving nature
- NO CURE!
How common is Down syndrome?
- Relatively common a!ecting 1 in 900 births
- The chances of having a baby with down syndrome increases with the age of the mother - This is
know as the maternal age e!ect, as chromosome have more di$culty separating with age
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