Pure tall plants are crossed with pure dwarf plants. In the $F_1$ generation,all plants were tall. These tall plants of the $F_1$ generation were selfed,and the ratio of tall to dwarf plants obtained was $3:1$. This is called:

Given below are two statements:
Statement $I$: Back cross is the crossing of $F_1$ hybrid with one of the two parents from which they were derived.
Statement $II$: Test cross involves the crossing of $F_1$ hybrid with its homozygous recessive parent.
In light of the above statements,choose the correct answer from the options given below.

$A$ human male is heterozygous for autosomal genes '$A$' and '$B$'. He is also hemizygous for the haemophilic gene '$h$'. What percentage of sperms will carry the '$abh$' genotype (in $\%$) (in $\%$)?

Mendel crossed true-breeding tall and dwarf plant varieties in his experiment. The tall character was dominant and the dwarf character was recessive. The recessive character appeared in:

In a monohybrid cross of plants with red and white flowered plants,Mendel got only red flowered plants. On self-pollinating these $F_1$ plants,he got both red and white flowered plants in a $3:1$ ratio. Explain the basis of using $RR$ and $rr$ symbols to represent the genotype of plants of the parental generation.

For geneticists,the year $1900 \, AD$ is of high significance due to: