The document discusses Gregor Mendel's research on inheritance, highlighting his experiments with pea plants that established foundational principles of genetics, including the laws of segregation, dominance, and independent assortment. Despite his significant contributions, Mendel's work was largely ignored during his lifetime and only recognized posthumously. The document also explores the implications of Mendelian genetics, including its application to human traits and the ABO blood system.

2. Priya Jha
KV No3 Delhi
Cantt

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5. LEARNING OBJECTIVES
Discuss the methods Mendel
utilized in his research that led
to his success in understanding
the process of inheritance

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7. MENDEL’S FINDINGS WERE IGNORED
 In 1866, Mendel published the paper Experiments in plant
hybridisation (Versuche über plflanzenhybriden). In it, he
proposed that heredity is the result of each parent passing
along 1 factor for every trait. If the factor is dominant, it
will be expressed in the progeny. If the factor is recessive, it
will not show up but will continue to be passed along to the
next generation. Each factor works independently from the
others, and they do not blend.
 The science community ignored the paper, possibly because
it was ahead of the ideas of heredity and variation accepted
at the time. In the early 1900s, 3 plant biologists finally
acknowledged Mendel’s work. Unfortunately, Mendel was
not around to receive the recognition as he had died in
1884.

8.  Gregor Johann Mendel was a monk and teacher
with interests in astronomy and plant breeding.
He was born in 1822, and at 21, he joined a
monastery in Brünn (now in the Czech Republic).
The monastery had a botanical garden and
library and was a centre for science, religion
and culture. In 1856, Mendel began a series of
experiments at the monastery to find out
how traits are passed from generation to
generation. At the time, it was thought that
parents’ traits were blended together in
their progeny.

9. STUDYING TRAITS IN PEAS
 Mendel studied inheritance in peas
(Pisum sativum). He chose peas because they had
been used for similar studies, are easy to grow
and can be sown each year. Pea flowers contain
both male and female parts,
called stamen and stigma, and usually self-
pollinate. Self-pollination happens before the
flowers open, so progeny are produced from a
single plant.
 Peas can also be cross-pollinated by hand, simply
by opening the flower buds to remove their
pollen-producing stamen (and prevent self-
pollination) and dusting pollen from one plant
onto the stigma of another.

11. KEY TERMS
 Genetics : Branch of science that deals with
Heredity and variation.
 Heredity : It means the transmission of features
/ characters/ traits from one generation to the
next generation.
 Variation : The differences among the
individuals of a species/population are called
variations.

12.  Homozygous: of an organism in which both copies of
a given gene have the same allele
 Heterozygous: of an organism which has two
different alleles of a given gene
 Allele: one of a number of alternative forms of the
same gene occupying a given position on a chromosome

13.  Recessive traits are not expressed in
heterozygotes.
 For a recessive allele to be expressed,
there must be two copies of the allele.
 Dominant traits are governed by an
allele that can be expressed in the
presence of another, different allele.
 Dominant alleles prevent the
expression of recessive alleles in
heterozygotes.

14.  Law of segregation: a diploid individual possesses a
pair of alleles for any particular trait and each parent
passes one of these randomly to its offspring
 Independent assortment: separate genes for
separate traits are passed independently of one
another from parents to offspring
 Epistasis: the modification of the expression of a gene
by another unrelated one

15. TRAITS IN PEA PLANTS
 Mendel followed the inheritance of 7 traits in pea
plants, and each trait had 2 forms. He
identified pure-breeding pea plants that
consistently showed 1 form of a trait after
generations of self-pollination.

18. MENDEL’S LAWS
 Mendel discovered that by crossing true-breeding white
flower and true-breeding purple flower plants, the result
was a hybrid offspring. Rather than being a mix of the two
colors, the offspring was purple flowered. He then
conceived the idea of heredity units, which he called
“factors”, one of which is a recessive characteristic and the
other dominant. Mendel said that factors, later called
genes, normally occur in pairs in ordinary body cells, yet
segregate during the formation of sex cells. Each member
of the pair becomes part of the separate sex cell. The
dominant gene, such as the purple flower in Mendel’s
plants, will hide the recessive gene, the white flower. After
Mendel self-fertilized the F1 generation and obtained an F2
generation with a 3:1 ratio, he correctly theorized that
genes can be paired in three different ways for each trait:
AA, aa, and Aa. The capital A represents the dominant
factor while the lowercase a represents the recessive

19. MENDEL’S LAW OF DOMINANCE
 Mendel’s law of dominance states that in a
heterozygote, one trait will conceal the presence
of another trait for the same characteristic.
Rather than both alleles contributing to a
phenotype, the dominant allele will be expressed
exclusively. The recessive allele will remain
“latent,” but will be transmitted to offspring by
the same manner in which the dominant allele is
transmitted. The recessive trait will only be
expressed by offspring that have two copies of
this allele; these offspring will breed true when
self-crossed.

20. MENDEL’S LAW OF SEGREGATION
 Mendel’s Law of Segregation states that a diploid
organism passes a randomly selected allele for a
trait to its offspring, such that the offspring
receives one allele from each parent.
 Apply the law of segregation to determine the
chances of a particular genotype arising from a
genetic cross
 When gametes are formed, each allele of one
parent segregates randomly into the gametes,
such that half of the parent’s gametes carry each
allele.

22. PUNNETT SQUARE
 The Punnett square is a square diagram that is
used to predict the genotypes of a particular cross
or breeding experiment. It is named after
Reginald C. Punnett, who devised the approach.
The diagram is used by biologists to determine
the probability of an offspring having a particular
genotype.

24. OBSERVATIONS OF MONOHYBRID CROSS
 1. All F1 progeny were tall (no medium height
plant (half way characteristic)
 2. F2 progeny ¼ were short, ¾ were tall
 3. Phenotypic ratio F2 – 3:1 (3 tall : 1 short)
 Genotypic ratio F2 – 1 : 2 :1 = TT : Tt : tt

25. CONCLUSIONS
 1. TT and Tt both are tall plants while tt is a
short plant.
 2. A single copy of T is enough to make the plant
tall, while both copies have to be ‘t’ for the plant
to be short.
 3. Characters/Traits like 'T' are called dominant
trait (because it express itself) and ‘t’ are
recessive trait (because it remains suppressed)

27. MENDEL’S LAW OF INDEPENDENT
ASSORTMENT
 Independent assortment allows the calculation of
genotypic and phenotypic ratios based on the
probability of individual gene combinations.
 Mendel’s law of independent assortment states
that genes do not influence each other with
regard to the sorting of alleles into gametes:
every possible combination of alleles for every
gene is equally likely to occur. The independent
assortment of genes can be illustrated by the
dihybrid cross: a cross between two true-breeding
parents that express different traits for two
characteristics.

28. The distribution of one pair of
alleles into gametes does not
influence the distribution of
another pair.
The genes controlling different
traits are inherited
independently of one another.

29. PARENT GENERATION ---> ROUND GREEN SEEDS
x WRINKLED YELLOW SEEDS

31. OBSERVATIONS
 1. When RRyy was crossed with rrYY in F1
generation all were RrYy round and yellow seeds.
 2. Self pollination of F1 plants gave parental
phenotype and two mixtures (recombinants
round yellow & wrinkled green) seeds plants in
the ratio of 9:3:3:1

32. CONCLUSIONS
 1. Round and yellow seeds are DOMINANT characters
 2. Occurrence of new phenotypic combinations show that
genes for round and yellow seeds are inherited
independently of each other

33. PRINCIPLE OF SEGREGATION
 Genes occur in pairs (like chromosomes).
 During gamete production, members of
each gene pair separate.
 During fertilization, the full number of
chromosomes is restored (allele pairs are
reunited).
 Homozygous- same allele at same locus
on both members of a chromosome pair.
(i.e.TT, tt)
 Heterozygous- two different alleles at
the same locus on a chromosome pair.

34. SEX DETERMINATION :
PHENOMENON OF DECISION OR
DETERMINATION OF SEX OF AN OFFSPRING

36. THIS SHOWS THAT HALF THE CHILDREN WILL
BE BOYS AND HALF WILL BE GIRLS. ALL
CHILDREN WILL INHERIT AN X CHROMOSOME
FROM THEIR MOTHER REGARDLESS WHETHER
THEY ARE BOYS OR GIRLS. THUS SEX OF
CHILDREN WILL BE DETERMINED BY WHAT
THEY INHERIT FROM THEIR FATHER, AND NOT
FROM THEIR MOTHER.

37. MENDELIAN INHERITANCE IN HUMANS
 Mendelian principles apply to over 4,500
human traits.
 The human ABO blood system is an example
of a simple Mendelian inheritance.
 The A and B alleles are dominant to the O
allele.
 Neither the A or B allele are dominant to
one another (codominant and both traits are
expressed).

38. CODOMINANCE
 Codominance occurs when two versions, or
“alleles,” of the same gene are present in a living
thing, and both are expressed. Instead of one
trait being dominant over the other, both traits
appear.
 Codominance occurs when both alleles show
dominance, as in the case of the AB blood
type (IA IB) in humans. Furthermore, the human
ABO blood groups represent another deviation
from Mendelian simplicity since there are more
than two alleles (A, B, and O) for this particular
trait.

40. ABO GENOTYPES AND ASSOCIATED
PHENOTYPES
Genotype
Antigens on
Red Blood
Cells
ABO Blood
Type
(Phenotype)
AA, AO A A
BB, BO B B
AB A and B AB
OO None O

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