Frequency of Down’s syndrome increases when the maternal age is
Above $35$ years
Below $35$ years
During $1^{st}$ pregnancy
In mothers of at least $3$ children
Polytene chromosomes in salivary glands of are formed as a result of
$MN$ factors are due to
Albinism is a congenital disorder resulting from the lack of the enzyme
Persons who are colourblind can not distinguish
When normal and mutant alleles are present on opposite chromosomes of homologous pair, the heterozygotes are called as