Three copies of chromosome 21 in a child with Downs syndrome have been analysed using molecular biology technology to detect any possible DNA polymorphism with reference to different alleles located on chromosome - 21. Results showed that out of three copies two of the chromosomes of the child contain the same alleles as one of the mother's alleles. Based on this when did the non-disjunction event most likely occur?

Q. Out of the three copies of chromosome $21$ in a child, two have come from the mother. Based on this when did the nondisjunction event most likely occurred.

Q. "Paternal chromosome contain one allele and maternal chromosome contain other allele both these alleles constitute one gene, one gene controls one character" Is this statement true.

Q. Fill in the blanks.
Exchange of chromosome segments between maternal and paternal chromatids during meiosis I is called as ............

Q. The exchange of chromosome segments between maternal and paternal chromatids during meiosis is called as

Q. How many of the following statements are correct?
I. Three scientists independently rediscovered Mendel’s work.
II. Coloured bodies called lysosomes can be visualised inside the nucleus by staining cells.
III. By 1890, the chromosome movement during meiosis had been studied.
IV. Mitosis is a kind of equational division and meiosis is reductional division.
V. Chromosomes and genes occur in pairs.
VI. Two alleles of a gene are located on a homologous site on homologous chromosomes.