In a cross between true-breeding red-flowered (RR) and
true breeding white-flowered plants (rr), the F 1 (Rr) was
pink. When the F 1 was self-pollinated the F 2 resulted in
the following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White.
Genotypic ratio was similar to Mendelian monohybrid cross
and phenotypic ratio changed from 3 : 1 to 1 : 2 : 1. R was
not completely dominant over r and this made it possible to
distinguish Rr (pink) from RR (red) and rr (white).
- Reasons for selecting pea plant are: (i) Pure varieties of pea
were available. (ii) Pea plants showed a number of easily
detectable contrasting characters. (iii) The plant is self
pollinated, but it can be cross bred manually. (iv) F 1 hybrids
are fertile.
OR
Coelacanth is considered to be the missing link between
fishes and the first four-limbed animals (amphibians). These
were the ancestors of modern day frogs and salamanders.
- RNA was regarded as the first genetic material as the
essential life processes, such as metabolism, translation,
splicing, etc. evolved around RNA. RNA acted as catalyst, but
being a catalyst, it was reactive and highly unstable. DNA
evolved from RNA with chemical modifications and is more
stable. DNA being double stranded with complementary
strands is not only more stable but also resists changes as it
has evolved a process of repair. RNA was replaced by protein
enzymes for bicatalysis which was more stable and efficient. - There are three types of natural selection which play an
important role in evolution. These are:
(i) Stabilising selection : It is balancing selection, which
favours average sized individuals and eliminates small
sized individuals.
(ii) Directional selection : It is a progressive selection in
which population changes towards one particular
direction.
(iii) Disruptive selection : It is diversifying selection that
favours both small-sized and large-sized individuals. It
eliminates member with mean expression.- Oswald Avery, Colin MacLeod and Maclyn McCarty worked
to determine the biochemical nature of ‘transforming
principle’ in Griffith's experiment. They purified biochemicals
(proteins, DNA, RNA, etc.) from the heat-killed S cells to see
which ones could transform live R cells into S cells. They
discovered that DNA alone from S bacteria caused R bacteria
to become transformed. They also discovered that protein-
digesting enzymes (proteases) and RNA-digesting enzymes
(RNases) did not affect transformation, so the transforming
substance was not a protein or RNA. Digestion with DNase
did inhibit transformation, suggesting that the DNA caused
the transformation and thus, they concluded that DNA is
the hereditary material. - Thalassemia is an autosomal recessive blood disorder,
transmitted to the offspring, when both the parents are
heterozygous, i.e., carrier of the disease. The defect arises
either due to mutation or deletion of genes controlling
synthesis of one of the globin chains of haemoglobin.
Imbalanced synthesis of globin chains of haemoglobin
causes anaemia.
Depending on the globin chain affected, thalassemia is of
3 types: alpha (α) thalassemia, beta (β) thalassemia and
delta (δ) thalassemia. - Semi-conservative replication of DNA was explained by
experiment of Messelson and Stahl. They grew E.coli in a
medium containing^15 NH 4 Cl as the only nitrogen source for
many generations. The result was that^15 N was incorporated
into newly synthesised DNA. Then they transferred the cells
into a medium with normal^14 NH 4 Cl and took samples at
various defined time intervals as the cells multiplied, and
extracted the DNA that remained as double-stranded
helices. The various samples were separated independently
on CsCl gradient to measure the densities of DNA. The
DNA that was extracted from the culture one generation
after the transfer from^15 N to^14 N medium had a hybrid or
intermediate density. DNA extracted from the culture after
another generation was composed of equal amounts of this
hybrid DNA and of ‘light’ DNA. The experiment proved that
DNA replicates semi-conservatively.