Q4. Assertion: the nucellar embryos in a mango seed are genetically identical to the mother plant. Reason: they develop from nucellar cells, with no reduction division and no fertilisation. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the nucellus is diploid tissue of the mother plant, and without meiosis or fertilisation the maternal genotype is retained, apart from possible mutation (reasoned from NCERT pages 22–23).
Q5. A plant has 2n = 24. How many chromosomes are there in a cell of a nucellar embryo, and in a cell of the endosperm of a normal seed?
24 in the nucellar embryo, which comes from diploid nucellus; 36 in the endosperm, which is triploid, 3 × 12 (NCERT pages 18–19 and 22–23).
Q6. A hybrid is heterozygous for three independent genes, AaBbCc. If its seeds form sexually by self-pollination, what fraction of the offspring is AaBbCc, like the hybrid?
For each gene, 1/2 of the offspring are heterozygous, so 1/2 × 1/2 × 1/2 = 1/8 are AaBbCc. (Mendel’s rules, NCERT Chapter 4.) If it produced only clonal apomictic embryos, all would retain AaBbCc, ignoring mutation (NCERT page 23).
Q7. A nucellar embryo develops without fertilisation, but a central cell is fertilised to support endosperm formation. Does this make the nucellar embryo sexual?
No. The embryo still arises asexually from maternal tissue. Fertilisation used for endosperm does not add a paternal genome to that nucellar embryo; embryo origin and nutritive-tissue origin must be distinguished.