(b)  Tt nad Tt let's use Mendel's cross of tall and dwarf pea plants as an example.

The ${\mathrm{F}}_1$plants of genotype Tt are self-pollinated. (both tall (T) but with dwarf (t) alleles).

Phenotypic ratio :  Tall  :  Dwarf

                               3     :   1

Genotypic ratio :  Pure tall :  Hybrid  :  Pure dwarf

                                   1     :   2         :   1

The letters T and t are used to represtn the alleles of the gene that determine plant height by convention. The upper case letter (T) represents the dominant allele and the recessive allele (t) is represented by the same letter in lower case.

Thus, the tall parent's plants having heterozygous alleles results in offsprings which comprise of both tall and dwarf plants.

For the parental cross, both the parents are true-breeding plants, the tall plant is homozygous for the allele 'T', while the dwarf plant is homozygous for hte dwarf allele 't'. Mendel tracked each trait through two generations.

When true bleeding plants were crossed to each other, this is called a parental cross and offspring comprise the first filial or ${\mathrm{F}}_1-\mathrm{generation}$. When the members of the ${\mathrm{F}}_1-\mathrm{generation}$ were crossed, this produced the ${\mathrm{F}}_2-\mathrm{generation}$ or second filial generation.

A cross between true-breeding tall and dwarf plants of the parent generation yield phenotypically tall plants.

The cross between TT and Tt is called back cross, which results into two homozygous and two heterozygous dominant gametes. The cross between Tt and tt is called test cross which results into 1:1 ratio of gametes.