GENETICS Trisomy 21 or Down syndrome occurs when there is a

GENETICS: Trisomy 21 or Down syndrome occurs when there is a normal diploid chromosomal complement of 46 chromosomes plus one (extra) chromosome 21. Such individuals therefore have 47 chromosomes. Assume that a mating occurs between a male with Down syndrome and a normal 46-chromosome female. What proportion of the offspring would be expected to have Down syndrome?

Solution

In normal cases every human being inherits a set of 23 chromosomes from mother and a set of 23 chromosomes from father. But in some rare occasions there will be an extra copy of a chromosome, usually in chromosome 21, which leads to miscarriage of the baby and if the baby survives it will be Down\'s syndrome.

Usually a male with Down syndrome cannot reproduce successfully, but if he could,

The child will also have Down\'s syndrome

Down’s syndrome Male                                                 CROSS                          Normal Female

( Chromosome 14,21,21(two copies))                                                      (Chromosomes 14,21)

                                                                                  RESULTS IN -

F1 Progeny                    1(Normal):2(Miscarriage):1(Down’s syndrome)