Heredity
(inheritance) is the transmission of characters from parents to offspring.
Evolution is the change in the characteristics of a species over several generations.
ACCUMULATION
OF VARIATION DURING REPRODUCTION
Inheritance
from previous generation provides common basic body design & minute changes
for next generation. Second generation gets inherited differences and newly
created differences.
In asexual
reproduction, the resultant individuals get only very minor differences due to
small inaccuracies in DNA copying. However, in sexual reproduction, greater
diversity is generated.
The variations
in a species have no equal chances of surviving in the environment. Based on
the nature of variations, different individuals would have different kinds of
advantages. E.g. Bacteria that can withstand heat will survive better in a heat
wave.
Selection
of variants by environmental factors is the basis of evolution.
HEREDITY
Inherited Traits
A child
bears all the basic features of a human being. However, it does not look
exactly like its parents.
Human populations
show a great variation. E.g. Variation in ear lobes. Most of the individuals
have free ear lobes (dominant trait) and some have attached ear lobe (recessive
trait).
Rules for the Inheritance
of Traits – Mendel’s Contributions
Father and
mother contribute equal amounts of genetic material to the child. i.e., each
trait is influenced by paternal and maternal DNA. Thus, for each trait there
will be two versions in each child.
Gregor
Johann Mendel (1822–1884) worked out
the main rules of such inheritance.
Mendel
used several contrasting visible characters of garden peas. E.g. round/wrinkled
seeds, tall/short plants, white/violet flowers etc.
He crossed
a tall plant & a short plant. In first generation (F1 progeny), all plants
were tall. There were no halfway characteristics (no ‘medium-height’
plants). This means that only one parental trait (tall) was expressed. The
expressed trait is called Dominant. The suppressed trait is called Recessive.
Mendel
allowed F1 tall plants to reproduce by self-pollination. The second-generation
(F2) progeny was 75% tall and one quarter (25%) short. This
indicates that both traits (tall and short) were inherited in the F1 plants,
but only the tallness trait was expressed. Thus he proposed that two copies of factor
(now called genes) control traits in sexually reproducing organism. They
may be identical or different, based on the parentage.
Genotypic ratio (TT: Tt:
tt) = 1:2:1
To confirm
genotypic ratio, each F2 plants are crossed with a pure recessive (tt) variety.
It is called Test cross.
TT & Tt are tall plants, while tt is a short
plant. It means a single copy of ‘T’ can make the plant tall (dominant trait), but
two copies of ‘t’ (tt) is needed to get short plant (recessive trait).
Cross of
coloured flowered & white flowered plants:
Here,
F1 produced all coloured flower. So it is dominant trait and white flower is recessive.
F2 produced coloured flowered plants and white flowered plants in 3:1 ratio.
When a tall
plant with round seeds and a short plant with wrinkled seeds
is crossed, the F1 progenies will be tall and round seeded. i.e., tallness and
round seeds are dominant traits.
If the
F1 progeny undergo self-pollination, the following types of F2 progeny are
produced:
|
Parental type combination |
New combinations |
|
o Tall, round seeds. o Short, wrinkled seeds. |
o Tall, wrinkled seeds. o Short, round seeds. |
New
combinations are formed due to independent inheritance of tall/short trait and
round seed/wrinkled seed trait.
Similarly,
formation of new combinations of traits in F2 when factors controlling for seed
shape & seed colour recombine to form zygote leading to form F2
offspring.
How do these Traits get
Expressed?
DNA is the information source to make proteins in a cell.
A
section of DNA that provides information for one protein is called the gene
for that protein.
Genes
control traits producing proteins. E.g.
Plant height
depends on a growth hormone. It is synthesised due to an enzyme
(protein). This enzyme is synthesised due to a gene.
o Efficient enzyme → more hormone → tall plant.
o Alteration of the gene → less efficient enzyme → less hormone →
short plant.
According
to Mendelian experiments, both parents contribute DNA equally (copies of same
genes) to the progeny. Thus each pea plant inherits 2 sets of all genes. For
this, each germ cell must have only one gene set.
In
Mendel’s experiment, the characteristics ‘R’ and
‘y’ independently inherited because they are not linked. This indicates that
each gene set is not in a single DNA thread (i.e., not in a whole gene set),
but in separate independent pieces, each called a chromosome. Thus, each
cell has two copies of each chromosome, one each from male (paternal) &
female (maternal) parents. Every germ cell takes one chromosome (maternal or
paternal).
When
two germ cells combine, they restore the normal chromosome number in the
progeny. It ensures the stability of DNA of the species. Such a mechanism of
inheritance is used by all sexually reproducing organisms. Asexually reproducing
organisms also follow similar rules of inheritance.
Sex Determination
There
are different strategies for sex determination.
Some species
rely entirely on environmental cues. E.g. In a few reptiles, the temperature at
which fertilised eggs are kept determines whether
they become male or female.
In animals
such as snails, individuals can change sex. It indicates that their sex is not
genetically determined.
In human
beings, the sex is genetically determined. The genes inherited from parents
decide whether an individual be boy or girl.
All
human chromosomes are not paired. Most human chromosomes have a maternal and a
paternal copy, and have 22 such pairs. But one pair (sex chromosomes) is
not always a perfect pair. Women have a perfect pair called XX. But men have
a mismatched pair in which one is a normal-sized X while the other is short
called Y (XY).
Half
the children will be boys and half will be girls.
All
children inherit an X chromosome from their mother.
Sex of
the children is determined by father. A child who inherits X chromosome from
father will be a girl, and one who inherits Y chromosome will be a boy.
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