Heredity and Evolution
In this Chapter...
!
Accumulation of Variations During Reproduction
!
Heredity (Inheritance of Traits)
!
Mendel’s Contribution towards the Inheritance of Traits
!
Experiment Conducted by Mendel
! Sex-Determination
Through the process of reproduction individuals give rise to
Heredity (Inheritance of Traits)
new individuals that are similar (not same) to the parents.
Traits or characteristics, which are passed on from parents to
This similarity in progeny or offspring or child is due to
their offspring (generation to generation) are controlled by
transmission of characters or traits from parents to their
genes.
progeny.
A gene is a unit of DNA which governs the synthesis of one
The transfer of characters from parents to offspring is known
protein that constants a specific character of an organism.
as heredity and the process through which characters or traits
e.g. Inheritance of free or attached earlobes.
pass from one generation to another is called inheritance.
Accumulation of Variations During
Reproduction
The difference in the characters among the individuals of a
species is termed as variations. These variations
are accumulated by the process of sexual reproduction.
(a) Free earlobe
(b) Attached earlobe
Depending upon the nature of variations, different
individuals would have different advantages, the most
Rules for inheritance of traits Inheritance of a trait is related
important advantage of variation to a species is that it
to the fact that both father and mother contribute equally
increases the chances of its survival in a changing
towards the genetic makeup of their offspring, i.e. for each
environment.
trait two versions are available in the child.
Some Important Terms and Definitions Used in Heredity
Experiments Conducted By Mendel
Terms
Definitions
More than a century ago, Mendel worked out the main
Chromosome
A long thread-like structure in the nucleus. It appears
rules for inheritance. He performed following two
during cell division and carries genes.
experiments
Gene
A functional unit of heredity. It is present on chromosome.
It is a piece of DNA that codes for one protein that inturn
1. Monohybrid Cross : Inheritance of Traits for
determines a particular character (phenotype).
One Contrasting Character
Character
The feature or characteristic of an individual like height,
Mendel took pea plants with different characteristics
colour, shape, etc.
such as height (tall and short plants).
Trait
An inherited character, i.e. feature, which is normally
The progeny produced from them (F1 -generation
inherited and has its detectable variant too, e.g. tall and
plants) were all tall. Mendel then allowed F1 progeny
dwarf are traits of a character, i.e. height.
plants to undergo self-pollination.
Allele
One of the different forms of a particular gene, occupying
In the F2 -generation, he found that all plants were not
the same position on a chromosome.
tall, three quarter were tall and one quarter of them
Hybrid
An individual having two different alleles for the same trait.
were short. This observation indicated that both the
Dominant
An allele, whose phenotype will be expressed even in the
traits of shortness and tallness were inherited in
allele
presence of another allele of that gene. It is represented by
F1-generation. But, only the tallness trait was
a capital letter, e.g. T.
expressed in F1 -generation.
Recessive
An allele, which gets masked in the presence of a dominant
Two copies of the traits are inherited in each sexually
allele
allele and can only affect the phenotype in the absence of a
dominant gene. It is represented by a small letter, e.g. t.
reproducing organism.
Genotype
Genetic composition of an individual.
Phenotype
The expression of the genotype, which is an observable or
measurable characteristic.
Back cross
Crossing F1 hybrid with one of its parents,
e.g. Tt × tt or Tt × TT.
Monohybrid
A hybridisation cross in which inheritance of only one pair
Tall
Short
All tall offsprings
cross
of contrasting characters is studied.
(TT)
× (tt)
(Tt)
Dihybrid
A cross in which inheritance of two pairs of contrasting
P×P
F1
cross
characters is simultaneously studied.
Mendel’s experiment showing law of dominance
Homozygous
A condition in which an individual possesses a pair of
identical alleles controlling a given character and will
TT and Tt are phenotypically tall plants, whereas tt is
breed true for this character (e.g. occurrence of two
a short plant. For a plant to be tall, the single copy of
identical alleles for tallness in a P1 tall pea plant).
‘T’ is enough. Therefore, in traits Tt, ‘T’ is a dominant
Heterozygous A condition in which an individual has a pair of contrasting
trait, while ‘t’ is a recessive trait.
alleles for any one character and will not breed true for this
character (e.g. simultaneous existence of dominant and
recessive alleles in F1 - hybrid tall pea plant).
Gametes
Reproductive cells containing only one set (haploid) of
dissimilar chromosome.
Test cross
Crossing F1 heterozygote with homozygous recessive
parent, e.g. F1 hybrid tall plant (Tt) with pure dwarf
plant (tt).
Tall
Tall
Tall
Tall
Tall
Short
F1
F2
(Tt)
×
(Tt)
(TT)
(Tt)
(Tt)
(tt)
Mendel’s experiment showing law of segregation
Mendel’s Contribution towards the
In F2 -generation, both the characters are recovered,
Inheritance of Traits
though one of these is not seen in F1 stage. During
The Austrian monk, Gregor Johann Mendel is known as Father of
gamete formation, the factor or allele of a pair segregate
Genetics. He performed many experiments on pea (Pisum sativum)
from each other.
plant related to hybridisation.
Thus, the phenotypic ratio is 3 : 1 and the genotypic ratio
He studied seven pairs of contrasting characters in pea plants and
is 1 : 2 : 1 for the inheritance of traits for one contrasting
only one character at a time.
character, i.e. monohybrid cross.
2. Dihybrid Cross : Inheritance of Traits for Two
sufficient protein will be produced for normal body functions.
Visible Contrasting Characters
If the gene for a specific protein is altered, the protein will be
less efficient or will not be functional at all.
Mendel took pea plants with two contrasting characters, i.e.
one with a green round seed and the other one with a yellow
Mechanism of Inheritance
wrinkled seed.
When the F1 progeny was obtained, they had round and
Both the parents contribute a copy of the same gene to their
yellow seeds, thus establishing that round and yellow are
progeny. Each germ cell thus, has one set of gene, present as
dominant traits.
chromosome. Each cell of the body will have two copies of
Mendel then allowed the F1 progeny to be self-crossed
each chromosome, one inherited from each parent.
(self-pollination) to obtain F2 progeny. He found that seeds
When two germ cells combine, they restore the normal
were round yellow, round green, wrinkled yellow and some
number of chromosomes in the progeny. This ensures the
were wrinkled green.
stability of the DNA of species. Such mechanism of
The ratio of plants with above characteristics was 9 : 3 : 3 :1,
inheritance explains the result of Mendel’s experiments. It
respectively (Mendel observed that two new combinations
is used by all sexually and asexually reproducing organisms.
had appeared in F2 ).
Sex-Determination
In F2 -generation, all the four characters were assorted out
independent of the others. Therefore, he said that a pair of
It is the process by which sex of a newborn individual is
alternating or contrasting characters behaves independently
determined. Different strategies can determine sex in
of the other pair. For example, seed colour is independent of
different species. For example, in reptiles environment
seed coat. The independent inheritance of two separate traits
factors such as temperature at which fertilised eggs are kept
shape and colour of seeds is schematically shown below
determine sex of the offspring. The determination of sex
occurs largely by genetic control in human beings. In human
Parents
RRyy
×
rrYY
beings, there are 23 pairs of chromosomes, out of which
(Round green)
(Wrinkled yellow)
22 pairs are autosomes and one pair is sex-chromosomes.
Gametes
Ry
rY
Females have a perfect pair of sex chromosome
(homogametic), but males have a mismatched pair
F2
-generation
(heterogametic) in which one is X (normal sized) and the
RrYy
(Round yellow)
other is Y-chromosome (short in size).
×
Hence, an egg fertilised by X-chromosome carrying sperm
F1
F1
results in a zygote with XX, which becomes a female and if an
egg is fertilised by Y-chromosome carrying sperm, it results in a
F2-generation
RY
Ry
rY
ry
XY zygote that becomes male.
RY
Thus, the sex of the children will be determined by what they
inherit from their father. A child who inherits an
RRYY
RRYy
RrYY
RrYy
X-chromosome will be a girl and one who inherits a
Ry
Y-chromosome will be a boy. The inheritance of sex in
RRYy
RRyy
RryY
Rryy
humans is diagrammatically shown below
rY
RrYY
RrYy
rrYY
rrYy
ry
XY
XX
RrYy
Rryy
rrYy
rryy
Ratio
F2
-generation
315 round yellow
9
Male
Female
108 round green
3
101 wrinkled yellow
3
Gametes
32 wrinkled green
1
X
Y
X
556 seeds
16
Expression of Traits
Zygote
XX
XY
Cellular DNA is the source of information for making
proteins in the cell. A section of DNA that provides
information for one particular protein is called a gene for that
Offsprings
protein. Expression of trait in body depends on the
Female
Male
functioning of a gene. If the gene is working normally,
Sex-determination in human beings