PRINCIPLES OF INHERITANCE Class XII Biology Part I

1. PRINCIPLES OF INHERITANCE
AND
VARIATION
DR. RAMA KANT UPADHYAY
PART- 1

2. GREGOR JOHANN MENDEL
❖ Conducted hybridization experiments on garden peas (Pisum
sativum) for seven years
❖ Proposed laws of inheritance
❖ 7 contrasting features of pea plant
❖ Use pure-line plants
❖ Around 29,000 pea plants were used in Gregor's Mendel
hybridization experiments.
Why pea plant:
(i) Bisexual flower
(ii) Self-pollinated
(iii) Contrast features
(iv) Pure line

3. SOME BASIC TERMS
➢ Chromosome
➢ Chromatid
➢ Genes / Factors
➢ Alleles
➢ Traits
➢ F1 generation
➢ Offspring/ progeny
➢ Cross
➢ Homozygous
➢ Heterozygous
➢ Bivalent
➢ Dominant
➢ Recessive
➢ Codominance
➢ Pure line
➢ Self cross
➢ Test cross
➢ Back cross Genotype
➢ Phenotype
➢ Monohybrid
➢ Dihybrid
➢ Punnet square

4. LAW OF DOMINANCE
• when parents with pure, contrasting traits are crossed together, only one form of the trait appears in the next
generation.
• The trait that appears in the next generation is known as a dominant trait, while the trait that does not express
is called a recessive trait.
(i) Characters are controlled by discrete units called factors.
(ii) Factors occur in pairs.
(iii) In a dissimilar pair of factors one member of the pair dominates
(dominant) the other (recessive).
• The law of dominance is used to explain the expression of only one
of the parental characters in a monohybrid cross in the F1 and the
expression of both in the F2
• It also explains the proportion of 3:1 obtained at the F2
P
F1
Cross
Purple- Dominant
White- Recessive

5. LAW OF SEGREGATION
only one of the two gene copies present in an
organism is distributed to each gamete (egg or sperm
cell) that it makes, and the allocation of the gene
copies is random.
This law is based on the fact that the alleles do not
show any blending and that both the characters are
recovered as such in the F2 generation though one of
these is not seen at the F1 stage.
Though the parents contain two alleles during
gamete formation, the factors or alleles of a pair
segregate from each other such that a gamete
receives only one of the two factors.

6. LAW OF INDEPENDENT ASSORTMENT
‘when two pairs of traits are combined in a hybrid, segregation
of one pair of characters is independent of the other pair of
characters’.
In other words, during gamete formation, one pair of trait
segregates from another pair of traits independently.
❑ It contributes to genetic diversity.
❑ Describes how different genes independently separate from
one another when reproductive cells develop.

7. CODOMINANCE
Codominance is a genetic inheritance
pattern where both alleles of a gene are
expressed equally, resulting in different
traits in an individual.
Example- Blood Group A and B
Dominance
Dominance
Dominance
Dominance
Codominance
Codominance
Recessive
Multiple alleles control a trait.
MULTIPLE ALLELISM
Example- The human blood group is controlled
by three alleles IA, IB, and IO

8. CHROMOSOMAL THEORY OF INHERITANCE
Sutton: studied chromosomes and meiosis in grasshoppers.
Boveri: studied chromosomes and meiosis in sea urchins.
In 1902 and 1903 Sutton and Boveri published independent papers proposing
the chromosome theory of inheritance
• The genes are present on the chromosomes in a linear fashion
• Each organism has a fixed number of chromosomes which occur in two sets referred to as diploid.
• Chromosomes from each homologous pair are sorted randomly into pre-gametes.
• Parents form gametes that contain only half of their chromosomes
• Gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as
their parents
• Factors described by Mendel are the genes that are the actual physical units of heredity
• One set is received from the male parent and the other from the female parent.
• Pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried.
• Sutton united the knowledge of chromosomal segregation with Mendelian principles
Boveri–Sutton
• The behavior of chromosomes was parallel to the behavior of genes and used chromosome movement to
explain Mendel’s laws.
• Chromosomes as well as genes occur in pairs.
• The two alleles of a gene pair are located on homologous sites on homologous chromosomes.

9. INCOMPLETE DOMINANCE
Incomplete dominance is a genetic phenomenon that
occurs when a cross between two genetically
different parents produces offspring with an
intermediate phenotype.
Reason: neither allele is completely dominant over
the other, resulting in a phenotype that shows a
blended feature
Example- Dog flower (snapdragon or Antirrhinum)

10. REDISCOVERY OF MENDEL’S LAW
❖ de Vries, Correns, and von Tschermak independently rediscovered Mendel’s results on the inheritance.

11. LINKAGE AND CROSSING OVER
➢ Thomas Hunt Morgan worked on tiny fruit flies (Drosophila melanogaster).
➢ He discovered the basis for the variation due to sexual reproduction.
Why Drosophila melanogaster ??
• They could be grown on a simple synthetic medium.
• Short life cycle (about two weeks).
• Single mating could produce a large number of progeny flies.
• Clear differentiation of the sexes
• It has many types of hereditary variations that can be seen with low power microscopes.

12. • When the two genes in a dihybrid cross were situated on the same chromosome, the proportion of
parental gene combinations was much higher than in the non-parental type.
• This is due to linkage (parental features)and recombination (nonparental features).
Observation
Linkage:
• physical association of genes on a chromosome
• Closely linked genes tend to be inherited together in progenies.
• Linkage may be complete or incomplete
• Linkage is directly proportional to the distance between adjacent genes.
Crossing over-
• Exchange of chromatids takes place.
• Formation of a new combination (recombination) and causes variation in offspring.
• Inversely proportional to the distance between adjacent genes.
•If the two genes are located on the same chromosome, the percentage of parental combination should be more as
compared to the non-parental combination.
•Sturtevant Morgan’s student found the location of linkage in the chromosome. He found the site by frequency of the
recombination through the gene mapping process.

15. POLYGENIC INHERITANCE
The traits are controlled by three or more genes.
Example: Human skin colour
In a polygenic trait, the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive.
Three genes A, B, and C control skin colour/Skin pigmentation.
A, B, C- Dominant- Dark skin colour
a, b, c- Recessive- light skin colour
https://byjus.com/neet/polygenic-inheritance/

16. PLEIOTROPY
• Single gene exhibit multiple phenotypic expressions.
• Example: Phenylketonuria disease
• The disease is caused by a mutation in the gene that codes for the enzyme phenylalanine hydroxylase (single gene
mutation).
• Character- mental retardation, reduction in hair and skin pigmentation.
https://www.geeksforgeeks.org/pleiotropy/
https://byjus.com/biology/pleiotropy/

17. SEX DETERMINATION

18. Sex determination in honey bee
• An offspring formed from the fusion of a sperm and an egg develops as a female (queen or worker)- 2n (32)
• An unfertilized egg develops as a male(drone)- n (16) by parthenogenesis.
• This is called haplodiploid sex determination.
• Males produce sperm by mitosis, they do not have a father and thus cannot have sons, but have a grandfather
and can have grandsons.

19. MUTATION
Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the
genotype and the phenotype of an organism.
Loss (deletions) or gain (insertion/duplication) of a segment of DNA, result in alteration in chromosomes.
Since genes are known to be located on chromosomes, alteration in chromosomes results in abnormalities or
aberrations.
Point Mutation: Mutation also arises due to changes in a single base pair of DNA. Example- Sickle cell anemia.
Deletions and insertions of base pairs of DNA, cause frame-shift mutations.
Factors that induce mutations (mutagens)- Chemical and physical factors, Radiation.

22. THANK YOU!
Image source: Google image