How many sentence are correct ?

$(i)$ Every gene contains the information to express a particular trait.

$(ii)$ Gene responsible for $\beta$ - thalassemia is present on $21^{st}$ pair of autosomal chromosome.

$(iii)$ Mendel's approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.

What is the correct sequence of the following events

$1.$Formation of the chromosome theory of heredity

$2.$Experiments which proved that $DNA$ is the hereditary material

$3.$Mendel's laws of inheritance-discovery

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Choose correct answer :

$(i)$ Queen Elizabeth shows a number of haemophilic descendents.

$(ii)$ Variation in blood group prevalences due to incomplete dominance.

$(iii)$ Drosophila has three pairs of autosomal chromosomes.

$(iv)$ In birds, female is heterozygous.

Marriages between close relatives and cousins is not advisable because

Pick out the correct statements.

$(1)$ Haemophilia is a sex­linked recessive disease.

$(2)$ Down’s syndrome is due to aneuploidy.

$(3)$ Phenylketonuria is an autosomal recessive gene disorder.

$(4)$ Sickle cell anaemia is an $X$-­linked recessive gene disorder.

A man can inherit his $X-$chromosome from his