Mendel's law of inheritance, test cross, punnett square.

2. History

3. Term gene was coined
by:
W. Johannsen
3 February 1857 – 11 November 1927

4. Father of genetics:
Gregor Johann
Mendel
20 July 1822 – 6 January 1884

5. Father of modern
genetics:
W. Bateson
8 August 1861 – 8 February 1926

6. Father of experimental
genetics:
T.H. Morgan
September 25, 1866 – December 4, 1945

7. Father of Human and
Biochemical genetics:
A. Garrod
25 November 1857 – 28 March 1936

8. Mendel’s Laws of Inheritance
● Genetics is the branch of biology, which
deals with the inheritance, as well as the
variation of characters from parents to
their offsprings.
● Inheritance is the process by which
characters or traits are passed from parent
to their progeny i.e. from one generation to
the next. It is the basis of heredity.
● Variation is the degree by which progeny
differs from their parents. One of the
causes of variation in living things was
hidden in sexual reproduction.

9. ● Gregor Johann Mendel, also known as ‘Father of
Genetics’, for the first time conducted experiment to
understand the pattern of inheritance of variations in
living things.
● He conducted hybridization experiments on garden
pea (Pisum sativum) for seven years and proposed the
laws of inheritance in living organisms.
● His observation became the foundation of modern
genetics and the study of heredity.
Mendel’s Experiment
(Inheritance of one gene)

10. Steps in
making a
cross in pea

11. He selected garden pea plant as a sample due to the following reasons:
● Pea plant are readily available on large scale and has bisexual flowers.
● Peas are self- pollinated and be cross- pollinated also.
● These are annual plants with short life cycle. So, several generations
can be studied within a short period.
● Pea plants could be easily raised, maintained and handled.
● Pea plants differ in distinct/ contrasting characteristics, which
provide many easily detectable contrasting characteristics.
Mendel conducted artificial/ cross pollination experiments using several
true- breeding pea lines.
Mendel selected 14 true- breeding pea plant varieties, as pairs, which were
similar except for one character with contrasting traits.
Mendel’s Experimental Material

13. ● Mendel observed one trait or character at a time, e.g. he
crossed tall and dwarf pea plants to study the inheritance
of one gene.
● He hybridised plants with alternate forms of a single trait
(monohybrid cross). The seeds produced by these crosses
were grown to develop into plants of Filial1 progeny or F1-
generation (F1 plants).
● He then self pollinated the tall F1- plants to produce plants
of Filial2 progeny or F2- generation (F2 plants).
● In later experiments, Mendel also crossed pea plants with
two contrasting characters known as Dihybrid Cross.
Mendel’s Experimental Procedure

15. ● In F1- generation, Mendel found that all pea plants were tall and none was
dwarf.
● He also observed other pair of traits and found that F1- plant always resembled
either one of its parent and the trait of other parent was not found.
● In F2- generation, he found that some of the offsprings were ‘dwarf’, i.e. the
character which were not seen in F1- generation were expressed in F2-
generation.
● These contrasting traits (tall/ dwarf) did not show any mixing either in F1 or in
F2- generation.
● Similar results were obtained with the other traits that he studied. Only one of
the parental traits that he studied. Only one of the parental traits was
expressed in F1- generation, while at F2- generation stage, both the traits were
expressed in the ratio of 3:1.
● Mendel also found identical results in dihybrid cross as in monohybrid cross.
● The trait that appeared in the F1- generation is called dominant trait, while the
other trait is recessive trait.
● In tall/ dwarf trait, tallness is dominant over dwarfness that is recessive.
Mendel’s Observation on Experiment

16. Following inferences were made by Mendel based on his observations:
● He proposed that some ‘factors’ passed down from parent to offsprings through the gametes
over successive generations. Now-a-days, these factors are known as Genes.
● Genes are hence, the units of inheritance.
● Genes which code for a pair of contrasting traits are known as alleles or allelomorph,
i.e. they are slightly different forms of the same gene.
● Genes occur in pairs in which, one dominates the other called dominant factor expresses
itself, while the other remains hidden and its factor called recessive.
● Allele can be similar in case of homologous (TT or tt) and dissimilar in case of heterozygous
(Tt).
● In a true-breeding tall or dwarf pea variety, the allelic pair of genes for height are identical
or homologous.
● TT and tt are called genotype of the plant, while the term tall and dwarf are the phenotype.
● When the tall and dwarf plant produce gametes, by the process of meiosis, the alleles of the
parental pair segregation and only one of the allele gets transmitted to a gamete.
● During fertilisation, the two alleles, T from one parent and t from another parent are united
to produce a zygote, that has one T and one t allele or the hybrids have Tt.
● Since, these hybrids contain alleles which express contrasting traits, the plants are
heterozygous.
Mendel’s Inferences

17. The production of gametes by the
parents, the formation of zygotes, the
F1 and F2- generations can be
explained by a diagram called punnett
square. It was developed by British
geneticist Reginald C Punnett.
The 1/4 : 1/2 : 1/4 ratio of TT: Tt: tt is
mathematically condensable to the
form of the binomial expression (ax
+by)2 , that has the gametes bearing
genes T or t in equal frequency of ½.
The expression is expanded as given
below :
(1/2T + 1/2 t) 2 = (1/2T + 1/2t) X
(1/2T + 1/2t)
= 1/4 TT + 1/2Tt + 1/4 tt

18. This is a method devised by Mendel to
determine the genotype of an organism.
In this cross, the organism with an
unknown dominant genotype is crossed
with the recessive parent, instead of
self- crossing.
E.g. in a monohybrid cross, between
violet colour flower (W), and white
colour flower (w), the F1- hybrid was a
violet colour flower. If all the F1-
progeny are of violet colour, then the
dominant flower is homozygous and if
the progenies are in 1:1 ratio, then the
dominant flower is heterozygous.

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