When two genetic loci produce identical phenotypes in $cis$ as well as in $trans$ position,they are considered to be

Pick out the correct statements.
$I.$ Haemophilia is a sex-linked recessive disease
$II.$ Down's syndrome is due to aneuploidy
$III.$ Phenylketonuria is an autosomal dominant gene disorder
$IV.$ Phenylketonuria is an autosomal recessive gene disorder
$V.$ Sickle cell anaemia is an $X$-linked recessive gene disorder

Tjio and Levan's contribution is very significant because they

Which disease is caused by an $X$-linked recessive gene but is not lethal?

Which one of the following pairs is an example of intergenic interaction?

The science which links heredity with environments is known as: