CBSE Class 10 Science 2019 · Paper 31/4/2 · Question 16

Heredity 5 marks Medium

(a) If we cross pure-bred tall (dominant) pea plants with pure-bred dwarf (recessive) pea plants we get pea plants of

F_{1}

generation. If we now self-cross the pea plants of

F_{1}

generation, then we obtain pea plants of

F_{2}

generation.

(i)What do the plants of $F_{1}$ generation look like ?
(ii)What is the ratio of tall plants to dwarf plants in $F_{2}$ generation ?
(iii)State the type of plants not found in $F_{1}$ generation but appeared in $F_{2}$ generation, mentioning the reason for the same. (b) What are homologous structures ? Give an example. Is it necessary that homologous structures always have common ancestors ?

Answer

$F_{1}$ plants are all tall and $F_{2}$ is $3 : 1$ tall to dwarf; homologous structures share a basic design and indicate a common ancestor.

(a) (i) All the plants of the

F_{1}

generation are tall (genotype Tt). (ii) Tall : dwarf =

3 : 1

in the

F_{2}

generation. (iii) Dwarf plants are not found in

F_{1}

but appear in

F_{2}

. Reason: tallness is dominant and dwarfness is recessive. The

F_{1}

plants (Tt) carry the factor for dwarfness, but it is masked by the dominant factor. During gamete formation the two factors (genes) segregate, so on self-pollination one-fourth of the

F_{2}

plants receive the dwarf factor through both gametes, the pollen and the egg (tt), and are dwarf. (b) Homologous structures are organs that have the same basic structure (structural plan) and origin but perform different functions. Example: the forelimbs of humans and the wings of birds. Yes: homologous structures are inherited from a common ancestor, and their common basic design is evidence of that common ancestry.

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CBSE Class 10 Science 2019 · Paper 31/4/2 · Question 16

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