Mendelian genetics describes Gregor Mendel's experiments with pea plants that demonstrated inheritance of traits from parents to offspring. Through experiments crossing pea plants with different traits, such as seed shape and flower color, Mendel discovered the laws of inheritance including dominance, segregation and independent assortment. He found that traits are determined by discrete units (now known as genes) that are passed from parents to offspring. His work established the foundation of classical/Mendelian genetics and the principles of heredity.

1. Mendelian Genetics and its extension
By
N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia ( W.B) India
GENETICS

2. HYBRIDIZATION

3. GENETICS ?
 The proverb goes- like begets like’. The mother claims the
character of the offspring are identical to grandmother and the
father claims the characters bears by the offspring identical to
grandfather. Is it true? Questions relating to the nature and the
basis for the relationship have occupied the thoughts of man
since the dawn of the human civilization. Initially, the credit
goes to the almighty, God but systematic attempts to seek
answers to these questions began only in the post –renessiau of
the Europe and only in the eighteenth century, several scientists
began to explore the truth behind the truth. The studies lead the
foundation of the journey of the another domain of natural
science and the credit goes to Gregor Johann Mendel (1822-
1884). The branch of biology deals with the cause and
consequences along with the transmission of characters are
called genetics.

4. TERMINOLOGIES
 Factor : Mendel presumed that a character is determined by a
pair of factors or determiners (hereditary units) present in each
cells of the individual. These are known as genes in modern
genetics.
 Alleles or allelomorphs: allele is a Greek word which means “
belonging to one another”. It is used to refer to one member of
gene pair. According to Mendel, two genes representing two
alternative of a character are present on two separate
chromosomes but as the corresponding loci. For example, in a
gene pair Tt, T is present in one chromosome and t on the other
homologous chromosome but at the same locus., Each of them
is called an allele to the other .T is an allele to t and vice versa.
 Locus: The physical location of a particular gene in a
chromosome is called locus or loci.

5. TERMINOLOGIES
 Dominance: One member of an allelic pair of genes has the
ability to manifest to itself while the exclusion of the other
member. This is known as dominance. The trait bearing this
character is known as dominant traits. A capital letter (T) is
used to denote it.
 Recessive: One of the allelic pair of genes having no ability to
manifest itself during crossing is known as recessive. The trait
bearing such character is called recessive character as
expressed as small letter (t).
 Genotype & Phenotype: the genetic make up of an organism is
called genotype and the phenotype indicates its external
appearance. A pure round genotype is (RR) or heterozygous
(Rr).
 Homozygous & Heterozygous: Two genes identical(TT) for
particular character is called homozygous but two contrasting
gene pair for particular character (Tt) is called heterozygous.

6. GREGOR JOHAN MENDEL
 Born in 1822, Moravia near Brunn in Austria in a poor family,
Mendel initially joined as priest and then studied physics,
Mathematics, Philosophy In Vienna university. He returned to
Brunn, appointed as substitute Science teacher along with
priest in a local church. He began to collect pea seeds for his
experiment for seven years. He presented his findings before
the Natural History Society Of Brunn at two of its meetings
entitled a paper ‘Experiments in plant hybridization’ in 1866;
Mendel became much more involved in his inquisitiveness to
explore the beauty of the transmission of characters. He died on
1884 at the age of 62. He proposed three important principles-
Law of dominance, Law of segregation & law of Independent
assortment. After his death, three scientists ,Hugo de Vries,
Correns & Tschermak rediscovered his findings.

7. WHAT DID MENDEL DO?
 The main objects of Mendel experiments basically based on the
determination of pure line characters followed by the cross
pollination of the bisexual plants to establish the desired
outcomes in a repeatedly manner. He chose garden pea, Pisum
sativum with well defined characters for its different
advantages like availability several characters in two
contrasting forms, characters relatively large, plants bi-sexual,
self fertilized and homozygous character, duration of crop
comprises single season, easy to grow and handle etc.
 i. Crossed two parents having contrasting characters by
artificial cross pollination using emasculation of bisexual plants
followed by bagging of the designed female one.
 ii. The male and female gametes unite randomly to form zygote
 iii. If two varieties are crossed, only one trait is expressed
called dominant and others remain hidden called recessive.

8. WHAT DID MENDEL DO?
 He gave the formulae for determining the numbers of i0
different types of gametes produced by F1, ii) different
genotypes in F2, iii) homozygous genotypes and iv)
heterozygous genotypes in F2 for the segregation of factors
anticipated for the expression of the phenotypic traits.
 He made a clear distinction between the external appearance
(Phenotype) and the genetic make up (genotype). He classified
the F2 individuals with dominant phenotype into pure and
hybrid forms on the basis of types in the succeeding progenies.
 On the basis of the observation of the single characters
crossing( Monohybrid), he explored three important outcomes-
 Law of unit characters- Each character controlled by one factor
 Law of dominance- Phenotypically expressed treated as
dominance.
 Law of segregation- Segregations of the two factors having the
chance during gamete formation.

9. MONOHYBRID CROSS
 A hybrid that differs with respect of one pair of genes is called
a monohybrid. In the Pea plant, a smooth (SS) seed flower
plant crossed with a wrinkled seed (ss) fives F1 hybrid (Ss) that
are all smooth in seed shape. Such a cross is known as
monohybrid cross. It has in fact a gene for smooth and a gene
(allele) for wrinkle seed shape. Smooth seed shape are
dominant over wrinkle seed shape. The SS peas are pure for
smooth shape and would produce all smooth seed offspring.
The same held true for the ss peas in respect of wrinklelness.
The ss pea seeds are phenotypically smooth but carried the
factor for wrinkleness as a recessive. Two of these crossed
would again give SS. Ss, Ss and ss .The 3:1 ratio is a
phenotype and the genotype ratio (SS: Ss:ss) is 1:2:1 as shown
in the next slide.

10. CHECKER BOARD
Male/Female S s
S SS (Homozygous,
dominant)
Ss (Heterozy7gous,
dominant)
s Ss (Heterozygous,
dominant)
ss (homozygous,
Recessive)

11. MENDEL FINDINGS
 From the above experiment, Mendel concluded three important
principles-
 Law of unit characters,
 Law of dominance,
 Law of Segregation.
 Law of unit character- This theory advocates the presence of
factor responsible for the expression of a particular character as
experimentally proved by Mendel. The pod shape either
smooth or rough is determined by the presence of unit factors
viz. S &s. If SS or Ss, the phenotypic expression will be
smooth but if it becomes ss, then it will be rough in pure
recessive homozygous form. Thus , the law of unit characters
states the factor responsible for the expression of any unit
character.

12. DETERMINATION OF
HOMOZYGOUS OR HETEROZYGOUS
 As told earlier, the character having physical manifestation is
known as phenotypic character and the hidden factor(gene)
responsible for the external manifestation is called genotypic
character. But how it becomes confirms the homozygous or
heterozygous nature of the expression because this is very
important clues before conducting any type of hybridization
programme irrespective or plants or animal hybridization
programme. The test which is consulted in this regard known
as the test cross and it is also a type of back cross as the
crossing is done in between the traits of which the testing
individual derived from the parent one. So, it is usually called
that all test crosses are back cross in other hand. Now question
comes the definition of the test cross and how it is conducted to
have the result in this regard.

13. TEST CROSS
 A cross between the F1 hybrid and a strain having the recessive
form of character is known as test cross. The purpose of the test
cross is to obtain evidence that segregation for the alleles as a
single gene in the F1 hybrid produces two types of gametes in
equal frequencies. According to the law of segregation, the F1
(Tt) from a cross between TT & tt seeded pea varieties would
produce gametes of T & t in equal frequencies. The strain with
recessive trait( tt, dwarf) used in test cross, would produce only
one type of gametes i.e t. Union of the gametes from F1 with
those from the test cross parent, therefore, is expected to
produce two type of seeds. Tt (tall) and tt (dwarf). The
frequencies of these two types of seeds are expected to be
equal. Mendel devised and made suitable test cross in this
study. He found that the ratio in test cross progeny was always
1 dominant : 1 recessive that indicate the appearance of
character in equal frequencies.

14. TEST CROSS

15. ALL TEST COROSS ARE BACK CROSS BUT
NOT VICE VERSA
 A method of verification of an unknown genotype is the
behavior of its alleles in test crosses. It is a crossing the
genotype back to the homozygous recessive parent. Generally,
when F1 is crossed with any of the parent irrespective to
dominant or recessive is called back cross. So, in this logic, all
test crosses are back cross but all back crosses are not test cross
unless the F1 is crossed with homozygous recessive parent. To
test the validity of the theory of inheritance, Mendel used a test
cross in which the F1 plants were mated to recessive
homozygous. The F1 plant heterozygous for a single pair of
alleles should produce two type of germ cells in equal numbers
following meiosis and so yields half of the dominant progeny
and half of the recessive progeny as found in the aforesaid
cross but if it is done with homozygous dominant, all will have
dominant characters.

16. LAW OF INDEPENDENT ASSORTMENT
 According to this law, the segregation of two or more
characters in the same hybrid is independent to each other.
Thus any allele of one gene is equally likely to combine with
any allele of the other gene and pass into the same gamete.
Independent segregation of two genes produces four different
type of gametes in equal proportion. A random union among
these gametes give rise to 16 possible zygotes. The zygotes
yield a phenotypic ration of 9:3:3:1 which is known as typical
dihybrid ratio. The genotypic ratio is 1:2:2:4:1:2:1:2:1.As the
two different traits were considered in this context, it is called
dihybrid test .The independent segregation of two genes means
that one of the two alleles of one gene can combine any one of
the two alleles of the other gene and pass into the same gamete.
The frequencies of all such combinations are equal. The result
was confirmed by the hybridization of the consideration of two
characters as follows.

17. DIHYBRID EXPERIMENT
 A cross between two parents differing in two traits or in which
only two traits are considered called dihybrid cross. Mendel
raised separately two pure varieties of garden peas, one with
yellow cotyledon, round seed and another with green
cotyledon, wrinkled seed. From the cross between these two
parental (P) generation plants, the offspring’s in the F1
generation were all with yellow cotyledon and round seed.
 When these F1 plants were self-fertilized, the offspring’s of F2
generation were of four types in the ratio 9:3:3:1 –
 (a) Yellow cotyledon, round seed (b) Yellow cotyledon,
wrinkled seed (c) Green cotyledon, round seed and (d) Green
cotyledon, wrinkled seed. The offspring’s showed that two
pairs of contrasting characters combined in every possible way.
Mendel carried out dihybrid experiments with all the chosen
characters in different combinations and got the similar results.

18. DIHYBRID CROSS

19. EXPLANATION
 1. As the parental plants were pure, so their genotypes will be
homozygous – YYRR and yyrr producing YR and yr gametes
respectively.
 2. The F1 dihybrid will be heterozygous for both the traits
(YyRr).
 3. As all the F1 plants were with yellow cotyledon and round
seed, so allele Y for yellow cotyledon is dominant over allele y
for green cotyledon and allele R for round seed is dominant
over allele r for wrinkled seed.
 4. The appearance of all the four possible phenotypic
combinations in F1 in the ratio 9:3: 3 :1 is possible if the two
pairs of characters are believed to behave independent of each
other. Each pair of contrasting characters bear no permanent
association with particular other character.

20. EXPLANATION
 5. If the F1 plant (YyRr) produces only parental gametes (YR,
yr), then in F2 only two types of phenotypes (parental) are
expected. But the appearance of four types of phenotypes in F2
(two parental and two new types) confirms the production of
four types of gametes (YR, Yr, yR, yr) in equal frequency.
 The appearance of two new types of phenotypic combinations
– yellow cotyledon, wrinkled seed and green cotyledon, round
seed in addition to parental phenotypic combinations requires
the production of Yr and yR gametes in addition to YR, yr
gametes by F2 plants.
 6. Thus the allele Y may be associated with the allele R as well
as r in equal frequency, giving rise to YR and Yr gametes
respectively. Similarly, the allele y may be associated with the
allele R as well as r in equal frequency giving rise to yR and

21. EXPLANATION
 yr gametes respectively. Thus four types of gametes viz.’, YR,
Yr, yR and yr will be produced in the ratio 1 : 1 : 1 : 1.
 7. These four types of gametes (both male and female) will
unite in sixteen possible combinations to produce nine types of
genotypes in the ratio 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 and four
types of phenotypes in the ratio 9:3: 3 : 1.
 8. The similar ratios will result even if the characters are
present in different parental combinations: yellow cotyledon,
wrinkled seed X green cotyledon, round seed. This further
proves that the inheritance of individual character is
independent of the other characteristics.
 Mendel was fortunate in selecting his experimental material. It
is self-fertilizing species but fertile hybrids can be produced
and all the seven characters chosen by him showed independent
assortment without any linkage.

22. GENETIC EXPLANATION
 The round yellow & wrinkled green varieties used as parents ,
may be assigned as RRYY & rryy
 The round yellow will produce all RY gametes & green
wrinkled will produce ry gametes,
 The union between RY & ry gametes from two parents will
produce RrYy progeny in F1,
 In the F1, both Rr & Yy will segregate at the same time and
the segregation will take place independently to each other.
 In the F2 generation, the selfing in between the two F1 will
produce four different kind of gametes- RY, Ry, rY & ry and
the crossing in between the 4 gametes will produce 16 different
type of combination having 4 altogether phenotypic traits as
told earlier. It indicates, that the two pairs of characters may
stay together but can be assorted independently if they have the
pleasure to do so.

23. DIHYBRID TEST CROSS
 Mendel tested his theory by crossing the F1 (double
heterozygous) to a completely recessive i.e wrinkle green. If
Mendel’s hypothesis was correct, the progeny would be of four
kinds: round yellow, round green, wrinkle yellow and wrinkle
green in a ration of 1:1:1:1 as expected from dihybrid
backcross to the double recessive parent. Mendel obtained , in
the test cross progeny, 55 round yellow ( RRYY), 51 round
green (RrYy), 49 wrinkle yellow ( rrYY) and 53 wrinkle green
(rryy). This is approximately the predicted ratio of 1:1:1:1.The
frequencies of four classes was ¼, ¼, ¼ & ¼ yielding the
above ratio. Dihybrid test cross (involving genes segregate
independently) do actually yield four types of progeny in the
ratio of 1:1:1:1. This test also used to know the genotype of the
dominant pair of traits-either homozygous or heterozygous at
the onset of breeding programme in designing And
development.

24. DIHYBRID TEST CROSS

25. CHROSOMAL BASIS OF INHERITENCE
 Mendel findings was in the middle of the 19th century when the
experimental genetics was not in the right track due to lack of
modern appliances and knowhows.In the early of the 20th century,
Mendel findings was rediscovered by a number of his followers and
the darkness about the inheritance of the acquired character and its
transmission became crystal clear. Nowadays, Mendel discovery has
been explained on the basis of the chromosomal basis of inheritance
pattern.
 The phenomenon of segregation has been explained on the behavior
of chromosomes during meiosis. According to Mendelism, the two
alleles separate and go to different gametes. In the F1 hybrid, one
allele of one gene is located in one chromosome and the other allele
is present in the other members of homologous chromosomes.
During normal cell division. Two homologous chromosomes pair
during zygotene and each split longitudinally to form two chromatids
being joined with each other in different points.

26. PHYSICAL BASIS OF SEGREGATION
 The two bivalents homologous chromosomes each carrying
one of the two alleles , start moving to opposite poles at
anaphase I , as a result, each pole has haploid (N) number of
chromosomes. The two sister chromatids of each chromosome
separate and migrate to opposite poles in anaphase II. The cell
divides into two halves but do not separate after the first
meiotic division. By the end of the telophase II, the one parent
cell produces four daughter cells (tetrad) .Each having one
chromatid from each homologous pair of chromosomes. Two
of the four cells receive the sister chromatids from one of the
two homologues while the other two receive the sister
chromatids from other homologues. So, the two of the four
cells receive the same allele of the gene, while the remaining
two cells contain other allele of the allele pair. Thus, each of
the two nuclei formed after the meiotic division has only one of
the two alleles of every gene.

27. CROSSING OVER FOR SEGREGATION
 At prophase stage, each homologous chromosomes of the pair
has two sister chromatids. Occasionally, non-sister chromatids
of a homologous pair of chromosomes exchange similar
portions of their chromosome, a phenomenon called crossing
over. When such crossing over takes place between the genes
and centromere, the sister chromatids of the chromosome as a
result now carry different alleles of the gene in question.
Therefore, a separation of the two chromosomes of a
homologous pair at anaphase I does not lead to segregate of the
two alleles of the gene. The two alleles of the gene, however,
segregate at anaphase II when the sister chromatids of each
chromosome moves to the opposite poles . In such situation,
the two alleles of a gene segregate during second meiotic
division. Thus, the segregation of the homologous
chromosomes during meiosis is the reason behind the
segregation of the two alleles of a gene located in identical
position in the homologous chromosomes.

28. CROSSING OVER

29. PHYSICAL BASIS OF INDEPENDENT
ASSORTMENT
 Independent assortment for two genes can be explained by
assuming that the two genes are located in two different
chromosomes. The two alleles of a gene will be located in the
two homologues of the concerned chromosome. Independent
separation of these two pairs of chromosomes at anaphase I of
meiosis will lead to the independent assortment of the genes
located in them. Two alleles of a gene occupy the same position
in two homologues of a chromosome. The fixed location of a
gene in a specific location is called locus(loci). During
prophase –I , the two chromosomes pairs containing r locus ( R
& r) and y (Y & y) locus form two bivalents. R and r will be
present in one bivalent while Y and y will be present in another.
If the orientations of the two homologues pairs are independent
at metaphase-I, there will be4 four possible combinations of the
two chromosome pairs-RY, Ry, rY, ry in 1:1:1:1.

30. PHYSICAL BASIS OF INDEPENDENT
ASSORTMENT
 Thus, an independent movement of the two pairs of
chromosomes , each carrying alleles of one gene , leads to an
independent segregation and assortment of the alleles of the
two genes in question. The independent orientation and
movement of chromosomes can be evidenced under
microscope by heteromorphic homologues pairs of
chromosomes in which the two members differ from each other
in their morphology and form heteromorphic bivalent. Such
heteromorphic pair of unequal size was discovered by
Carothers in 1913 in an insect Brachystola. Thus, an
independent separation of two pairs of chromosomes, each pair
carrying alleles of one of the two genes, will lead to an
independent segregation of the alleles of the two genes.

31. LIMITATIONS OF MENDEL’S LAWS
 1. Mendel assumed that characters controlled by a single pair
of genes. But gene interactions result a new trait or modify
trait. Multiple characters control quantitative traits.
 2. In each of the seven characters as studied by Mendel,
dominance of one allele over the other was rule. However, in
some cases, the F1 heterozygous was intermediate in between
the two homozygote. This shows incomplete dominance. The
ideal example is the flower color of four o’ clock plant-
Mirabilis jalapa; red flower crossed with white flower appears
pink color that does not follow the Mendelian rule.
 3. Mendel predicted of only one alternative forms of each
character he studied. He assumed only two allomorphs of each
locus.-one dominant and other recessive. But multiple alleles
existence reject this concept.
 4.Mendel seven characters wren fortunately distributed in
seven different pairs of chromosomes but in other cases,

32. LIMITATIONS OF MENDEL’S LAWS
 this does not happen. He did not face the consequences of
linkage and crossing over-the two magic of reality. Had he
studied more characters he would have come up with more
complicated phenomenon of linkage and crossing over. So, he
was quite fortunate enough to have these inferences from his
experiment and this was one of the cause and consequences of
his outstanding performances.
 Mendel assumed that factor was particulate but he had no idea
about its biochemical nature and this was quite obvious in his
contemporary. Neither DNA nor the gene and its existence was
explored.
 Contrasting characters, mathematical knowledge, experiment
with care and elaborateness are some of the features for his
success in this endeavor.
 The limitations of Mendel was an opportunity by his
successors to unfold of new chapter of in this domain.

33. SOME PROBLENS CAN BE ADDRESSED
 1. All test crosses are back crosses but not all back crosses are
test crosses-justify.
 2.Determine the different gametes produced by the following:
a) Aabb, ii) AABb iii) AaBbDd iv) Tt rR
 3.In man, brown eyes (B) are dominant over blue(b) and dark
hair ® to red (r). A man has brown eyes and red hair. He
marries a woman with blue eyes and dark hair. Give the
genotypes and phenotypes of the parent and children.
 4.A dwarf pea plant with yellow seeds is crossed with tall plant
with green seeds. Give the genotypes and phenotypes of F1, the
gametes produced by F1 and the genotypes and phenotypes
with checker board.
 5.An individual having three pairs of chromosomes has
received the centromeres. A, B, and D from his father and a, b,
and d from mother. List the various combinations of
centromeres obtaining its gametes.( Consider the centromeres
as genes)

34. CONCLUSION
 Mendel was considered as “father of genetics’ due to his urge
to explore the cause and consequences about the cause of the
biological characters and its mode of transmission following
the rule of mathematics in inheritance pattern. William Batson (
1909) stated “ The study of heredity thus becomes an organized
branch of physiological science, already abundant in results,
and in promise unsurpassed”. Although the principles of
Mendel laid the foundations of genetics, still, it developed a lot
of questions after the post- Mendelian research outcomes.
Irrespective of monohybrid, dihybrid or trihybrid experiments,
different other issues like incomplete dominance, lethal factor,
multiple alleles, pleiotropy, co-dominance, complementary
gene interaction, supplementary gene action, epistasis,
hypostasis etc are now become crystal clear as far as the cause
and consequences that to be explored in the next PPT.

35. THANKS FOR YOUR JOURNEY
 References:
 1. Google for images,
 2. Principles of Genetics- Basu & Hossain,
 3. A textbook of Botany (Vol III) Ghosh, Bhattacharya, Hait
 4. Fundamentals of Genetics- B.D. Singh,
 5.A Textbook of genetics- Ajoy Paul
 This presentation has been made without any financial interest,
to enrich open source of information. The presenter
acknowledges the followings to develop this PPT.