How do Mendel's experiments show that the
(I) traits may be dominant or recessive,
(ii) traits are inherited independently?
Answers
How do Mendel's experiments show that the (I) traits may be dominant or recessive,
$\longrightarrow$ When Mendel cross-pollinated pure tall pea plants with pure dwarf pea plants, only t plants were obtained in F, generation. On self pollinating the F, progeny, both tall and dwarf plants appeared in F, generation in the ratio 3: 1.
Appearance of tall character in both the F, and F, shows that it is a dominant character. The absence of dwarf character in F, generation and its reappearance in F, shows dwarfness the recessive character.
(ii) traits are inherited independently?
$\longrightarrow$ When Mendel first crossed pure-breed pea plants having round-yellow seeds with pure breed pea plants having wrinkled-green seeds, he found that only round-yellow seeds were produced in the first-generation. No wrinkled-green seeds were obtained in the generation From this, it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds.
When the F, generation pea plants having round-yellow seeds were cross-bred by sel pollination, then four types of seeds having different combinations of shape and colour were obtained in second generation (F₂). These were round-yellow, round-green, wrinkled yellow and wrinkled-green seeds.
Such a cross is known as dihybrid cross as two sets of corresponding characters are considered Mendel observed that along with round-yellow and wrinkled-green, two new combinations of characteristics, round-green and wrinkled-yellow, had appeared in the F, generation On the basis of this observation. Mendel concluded that though the two pairs of original characteristics (seed colour and shape) combine in the F, generation, they get separated and behave independently in the subsequent generation.
$\rm\underline\bold{Answer \purple{\huge{\checkmark}}}$
(a) Monohybrid cross between two pure breeding varieties always obtained hybrid progeny exhibiting one parental trait while the other trait was never expressed in F_1 generation. It suggested that two alleles of a gene can be either dominant or recessive and presence of the dominant allele in F_1 hybrids masks the expression of recessive one.
(b) A dihybrid cross between two pure breeding varieties obtains dihybrid progeny exhibiting the only dominant trait. However, selfing of F_1 hybrids obtains both parental and new combinations of traits in 9:3:3:1 ratio in F_2 generation suggesting an independent assortment of characters.