$A$ back-cross is a cross between ........

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

Sickle-cell anemia has not been eliminated from the African population because,........

The inheritance of $ABO$ blood group is an example of which of the following conditions?
$I -$ Complete dominance,$\quad II -$ Co-dominance
$III -$ Multiple allelism,$\quad IV -$ Incomplete dominance

Match the terms in Column-$I$ with their descriptions in Column-$II$ and select the correct option.

Column-$I$	Column-$II$
$(A)$ Dominance	$(1)$ Many genes govern a single character.
$(B)$ Co-dominance	$(2)$ In a heterozygous organism, only one allele expresses itself.
$(C)$ Pleiotropy	$(3)$ In a heterozygous organism, both alleles express themselves fully.
$(D)$ Polygenic inheritance	$(4)$ $A$ single gene influences many characters.

Which of the following is the significance of dominance?