Down’s syndrome
• Chromosome analysis has shown that, while the additional chromosome may be derived from either the mother
or father, the ovum is the source, in about 95 per cent of 47, + 21 trisomy or Down’s syndrome cases.
• Before the development of techniques involving polymorphic markers, that clearly distinguish paternal from
maternal homologues, this conclusion was supported by the more indirect evidence, derived from studies of the
age of mothers giving birth to infants, afflicted with Down’s syndrome.
• The relationship between the incidence of Down’s syndrome births and maternal age, illustrates the dramatic
increase, as the age of the mother increases.
• While the frequency is about 1 in 1000 at maternal age 30, a tenfold increase to a frequency of 1 in 100 is
noted at age 40.
• The frequency increases still further to about 1 in 50 at age 45.
(^202530354045)
Maternal age (years)
20
18
16
14
12
10
8
6
4
2
Incidence of Down’
s syndrome births
(per 1000)
Fig.:Incidence of Down syndrome births contrasted with maternal age.
• While the nondisjunctional event that produces Down’s syndrome seems more likely to occur during oogenesis in
women between the age of 35 and 45, we do not know with certainty why this is so. However, one observation
may be relevant.
• In human females, meiosis in all eggs is initiated during fetal development. Synapsis of homologues occurs and
recombination is initiated. Then, oocyte development is arrested in meiosis I.
• Thus, all primary oocytes have been formed by birth.
• Then, once ovulation begins at puberty, meiosis is re-initiated in one egg during each ovulatory cycle and
continues into meiosis II.
• The process is once again arrested after ovulation and is not completed unless fertilization occurs.
• In this suspended state, the chromosomes may become unpaired.
• The longer, the time in arrested state, the greater, the chance for unpairing and nondisjunction.
• These statistics obviously pose a serious problem, for the woman, who becomes pregnant late in her reproductive
years.
• Genetic counselling early in such pregnancies is highly recommended.
• Such counseling informs prospective parents about the probability that their child will be affected, and educates
them about it.
Intext Practice Questions
- Define euploidy. Distinguish it from aneuploidy.
- Explain any one intra-chromosomal and one inter-chromosomal aberration.
- How can you distinguish between a child suffering from Edward’s syndrome from another
one suffering from Patau’s syndrome?