Down syndrome in humans is due to?

Down syndrome in humans is due to?

A) Three copies of chromosome 21
B) Three X chromosomes
C) Three Y chromosomes
D) Two Y chromosomes
E) Monosomy

Ans: Three copies of chromosome 21

Down syndrome

This was the first autosomal aneuploidy, described by John Longdon Down (1866) under the name of mongoloid idiocy or mongolism.
• 21-Trisomy
• 47 chromosomes instead of 46 with three copies of chromosome 21 instead of  usual two.
 Down syndrome Karyotype

• Karyotype: XXY
(reason: Non-disjunction of chromosome of pair during meiosis)
 Reason for Down Syndrome: Non-disjunction of the chromosome of pair during meiosis
• The frequency of non-disjunction of chromosome pair 21 increases with maternal age.
• The frequency of abnormality is 1in 600 live births.
• Clinical symptoms: Male with slowly degenerating testes, enlarged breasts.
Problem 1

Down’s syndrome in humans is caused by non-disjunction. The chance of having a baby with Down’s syndrome increases as the age of the mother increases. At age 40, the probability of having a baby with Down’s syndrome is 0.018.
In 2016 the number of women aged 40 in the UK was estimated to be 500000. The pregnancy rate for women in the UK aged 40 is 14 pregnancies per 1000 women per year.
Calculate the number of babies with Down’s syndrome that was expected in 2016 in the UK
• number of pregnancies calculated - (500 000 ÷1000) x 14 = 7 000
• number of babies with Down’s syndrome calculated
(1.8 ÷ 100) x 7 000 = 126
or
14 x 500 = 7000 and then 7000 x 0.018 = 126