Question

A homozygous sweet pea plant with blue flowers ($RR$) and long pollen (${R_O}{R_O}$) is crossed with a homozygous plant having red flowers ($rr$) and round pollen (${r_o}{r_o}$). The resultant ${F_1}$ hybrid is test crossed. Which of the following genotypes does not appear in its progeny.
A. $Rrr{r_o}$
B. $RrR{r_o}$
C. $rr{r_o}{r_o}$
D. $rr{R_o}{r_o}$

Answer 1

Experiments are performed by Mendel on genetics. The description of mechanisms of hereditary processes and formulation of principles and the experiments performed are included under Mendelism. A cross in which there is a study of the inheritance of two pairs of contrasting traits, this type of cross is called a dihybrid cross. When ${F_1}$ individuals (individuals of first-generation) are crossed with any of the parents, it is called the back cross. A test cross is one of the types of backcross.

Complete answer:
According to Mendel's conclusion, different types of characters present in plants assorted independently during inheritance. This is known as the law of independent assortment and it is based on the ${F_2}$ generation of a dihybrid cross.
When first generation progeny is crossed with recessive parents then it is called a test cross. The genotype of the homozygous plant with blue flowers and long pollen will be $RR{R_O}{R_O}$ and the genotype of the other homozygous parent with red flowers and round pollen will be $rr{r_o}{r_o}$.

| $r{r_o}$
---|---
$R{R_o}$| $Rr{R_o}{r_o}$
$R{r_o}$| $Rr{r_o}{r_o}$
${R_o}r$| ${R_o}rr{r_o}_{}$
${r_o}r$| ${r_o}rr{r_o}$ $$

The gametes produced will be $R{R_o}$ and $r{r_o}$. Now the ${F_1}$ progeny produced will be $Rr{R_o}{r_o}$. According to the test cross, this ${F_1}$progeny will be crossed to recessive individuals with genotype $rr{r_o}{r_o}$. Now the gametes from the ${F_1}$progeny will be $R{R_o}$ , $R{r_o}$, ${R_o}r$ and ${r_o}r$. The gamete produced from the recessive individual will only be of one type $r{r_o}$.
When the gamete $R{R_o}$fuse with the gamete $r{r_o}$the resultant individual will be $Rr{R_o}{r_o}$.Similarly, $RrR{r_o}$, $R{r_o}r{r_o}$, $rr{R_o}{r_o}$ and many more progenies will be produced but the progeny $Rrr{r_o}$ will not be formed.
Hence, the correct answer is option (A).

Note: In a test cross, phenotypes and genotypes ratio are the same. Test cross helps to find out the genotype of a dominant individual. Mendel postulated mainly three conclusions named as the law of dominance, the law of segregation and law of independent assortment. There is no exception to the law of segregation in a sexually reproducing organism.