(ii) Double fertilization is found only in angiosperms. It is the
fusion of two male gametes, brought by a pollen tube,
with two different cells of the same female gametophyte,
in order to produce two different structures. One male
gamete fuses with the egg, resulting in a diploid zygote.
This is called syngamy or generative fertilization. The
second male gamete fuses with two haploid polar nuclei
(or the diploid secondary nucleus), resulting in a triploid
primary endosperm nucleus. This is called triple fusion or
vegetative fertilization.
- (i) The given figure shows the sectional view of the
human ovary.
(ii) The structure labelled a i.e., primary follicle is a pre-birth
structure of a female individual. Oogenesis starts during
embryo development where millions of oogonia are
formed whose development arrests at primary follicle
stage. No more oogonia, thus no more primary follicles,
are formed after birth.
(iii) The structure labelled C is called Graafian follicle. It
responds to LH surge by rupturing and releasing the
secondary oocyte, resulting in ovulation.
- (i) both of the parents in generation I do not express
the trait, yet it appears in the progeny. It indicates that
the trait in question is a recessive trait which was present
in heterozygous parents and is expressed in the progeny
which is homozygous for the trait.
(ii) It is an autosomal trait. If it has been a sex linked trait
(either X linked or y linked) then the father in generation I,
must express the trait as it has only one chromosome of
each type (X and y) and thus recessive genes on both
the chromosomes must be expressed.
(iii) Generation I → aa and aa
Generation II → Second child - aa
Third child - aa
- (i) ‘a’ represents start codon auG.
‘b’ may represent any of the stop codons i.e., uaa/uaG/
uGa.
(ii) auG codes for methionine and it is the initiation codon.
uaa, uaG and uGa do not code for any amino acid, but
bring about termination of polypeptide synthesis.
(iii) During elongation of the polypeptide chain, peptide bond
is formed between the amino acid carried by trNa at
P-site, and the newly arrived amino acid carried by trNa
attached at a site, both on the smaller subunit of the
ribosome. The reaction is catalysed by enzyme peptidyl
transferase.- (i) Hardy-Weinberg’s principle states that allele
frequencies in a population are stable and constant from
generation to generation. The gene pool, (the total genes
and their alleles in a population) remains a constant. This
is called genetic equilibrium.
according to this principle, sum total of all the allelic
frequencies is 1. In a diploid organism, the frequency
of allele a and allele a is represented as p and q
respectively.
The frequency of aa individuals in a population i.e. the
probability that an allele a with frequency of p appears on
both the chromosomes of a diploid individual is simply the
product of the probabilities, i.e., p^2. Similarly, frequency
of aa is q^2 , of aa is 2pq. Hence, p^2 + 2pq + q^2 = 1. This
is the binomial expression of (p + q)^2.
When frequency measured, differs from expected values,
the difference indicates the extent and direction of
evolutionary change. Disturbance in genetic equilibrium,
or Hardy-Weinberg equilibrium, i.e., change of frequency
of alleles in a population would then be interpreted as
resulting in evolution.
(ii) The major factors known to affect Hardy-Weinberg
equilibrium are as follows :
(a) Gene migration or gene flow and genetic drift
(b) Mutation and genetic recombination
OR
Natural selection is the process by which those organisms
that are best suited to their environment survive and
reproduce. One of the most striking example, which
demonstrates the action of natural selection is the case
of peppered moth, Biston betularia, commonly found
in parts of england. Two types of moths, white winged
moths and dark winged or melanised moths occur in
nature. Due to industrial smoke and soot, the pale tree
trunks became more and more blackened. as a result,
the light coloured moth variety stood out in contrast
to its background, increasing the possibility of being
easily detected and eaten by their predators, such as
birds, in much greater number than the dark melanic
variety. In course of time, there was a gradual decrease
in the number of light coloured moths and increase
in the number of dark coloured, melanic variety. also,
evolution favoured the melanic moths to reproduce more