Select the correct statement:

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.

Which of the following is true about linkage

$I.$ It is phenomenon in which more recombinants are produced in $F_{2}$-generation

$II.$ More parental combination are produced in $F_{2}$-generation

$III.$ Genotype which are present in $F_{1}$ hybrid. Reappear in high frequency in $F_{2}$-generation

$IV.$ It is a phenomenon in which two chromosome are linked

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

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.