2. Contents
• Mendelian Genetics (Monohybrid and dihybrid cross)
• Incomplete dominance and Co-dominance
• Epistasis and Gene Interaction
• Multiple Allelism
• Some relevant terminologies
• Mutation
• Classification of Genetic disorders
3. Mendelian Genetics
• Mendel chose garden peas (Pisum sativum) for his experiment
because it is an annual plant with well defined characteristics
• It can be grown and cross easily
• Garden peas have perfect flowers , flowers that contain both female
and male parts
Want to know more about Gregor Mendel ?
Visit https://www.nature.com/scitable/topicpage/gregor-mendel-a-private-
scientist-6618227/
6. • Principle of segregation
• The paired genes (allelic pairs) separate from one another and
distributed to each sex cells
For more visit:
https://www.nature.com/scitable/definition/principle-of-
segregation-301/
7. • Codominance- When both alleles of genes are able to express equally in a heterozygote;
It is called codominance
• Incomplete dominance- The heterozygous product is intermediate between dominant
and recessive alleles
9. • Principle of independent assortment
• The chance of two and more independent events occurring together
is the product of their separate occurrences
For more visit
https://www.nature.com/scitable/definition/principle-of-independent-
assortment-law-of-independent-302/
12. Epistasis and Gene Interaction
• Epistasis is an interaction at the phenotypic level of organization. The genes that are involved in a specific epistatic interaction may
still show independent assortment at the genotypic level.
• In such cases, however, the phenotypic ratios may appear to deviate from those expected with independent assortment.
• In the first decade of the twentieth century, British geneticists William Bateson and R. C. Punnett conducted research showing that
the shape of the comb in chickens was caused by the interaction between two different genes.
• Wyandottes have a "rose" comb, Brahmas have a "pea" comb, and Leghorns have a "single" comb.
• When Bateson and Punnett crossed a Wyandotte chicken with a Brahma chicken, all of the F1 progeny had a new type of comb,
which the duo termed a "walnut" comb. In this case, neither the rose comb of the Wyandotte nor the pea comb of the Brahma
appeared to be dominant, because the F1 offspring had their own unique phenotype.
• F1 progeny were crossed with each other, some of the members of the resulting F2generation had walnut combs, some had rose
combs, some had pea combs, and some had a single comb, like that seen in Leghorns .
• Because the four comb shapes appeared in a 9:3:3:1 ratio (i.e., nine walnut chickens per every three rose chickens per every three
pea chickens per every one single-comb chicken), it seemed that two different genes must play a role in comb shape
• Bateson and Punnett deduced that Wyandotte (rose-combed) chickens must have the genotype RRpp, while Brahma chickens must
have the genotype rrPP.
• A cross between a Wyandotte and a Brahma would yield offspring that all had the RrPp genotype, which manifested as the walnut-
comb phenotype.
• any chicken with at least one rose-comb allele (R) and one pea-comb allele (P) would have a walnut comb
• when two F1 walnut chickens were crossed, the resulting F2 generation would yield rose-comb chickens (R_pp), pea-comb
chickens (rrP_), and walnut-comb chickens (R_P_), as well as chickens with a new, fourth phenotype—the single-comb phenotype
• Based on the process of elimination, it could be assumed that these single-comb chickens had the rrpp genotype
13. Multiple Allelism
• A population of species contains different alleles of a single genes. It is
not the case a gene can have only two alleles. Rather the basis of
population genetics is to study alleles present of the same gene in a
population of the same species.
• One of the best example of multiple alleles are wings of a butterfly. The
different color and pattern of butterfly exemplifies multiple allelism.
14. • Single Gene Disorders refer to change in single nucleotide
It could be further divided into
Autosomal Dominant Disorder
Autosomal Receissive Disorder
X linked Receissive disorder
X linked Dominant disorder
15. Terminologies
• DNA , full form is deoxyribonucleic acid, carries the
genetic information
• Adenine, thymine, cytosine, guanine
Adenine
Thymine
Guanine Cytosine
16. • Gene: A part of DNA or RNA sequence is called genes.
Generally Genes code for proteins
• Humans have 3 billion base pairs and only 25000 genes
• Alleles : A gene can have multiple alleles , alleles are basically
different form of the same gene
17. • Phenotype: Concerned with external/observable
characteristics
• Genotype: Genetic sequence of any particular characteristic
19. • Chromosomes: Genes are packaged into a structure called
chromosome
• Humans have 23 pairs of chromosome
• Of those 1 pair sex chromosome and 22 pair autosome
20. • Ploidy-The number of sets of chromosomes in a cell or an
organism.
• Haploid means one set, diploid means two set ,triploid means 3 set
and so on
• Polyploidy refers to more than 2
21. • Chromatins made up of DNA, in prophase of mitosis the chromatin
fibers become coiled into chromosome
• Chromatids: One of the two identical parts of chromosome
• Centromere: The point where two chromatids attach
22. • Nucleotide: Made up of Phosphate , Base ( purine and pyrimidine ) ,
Pentose sugar
• Nucleoside: Nucleotide- Phosphate
23. • Codon: adjacent 3 nucleotides
• It codes for a specific amino acids
24. • Congenital disease
A disease present at the time of birth
• Hereditary disease
Diseases which are transmitted through generations , mainly through
gametes
25. • Mutations: A permanent heritable change in a gene or
chromosomal structure and important in several diseases
• Chemical Causes
Nitrous Acid
Alkylating Agents
5-bromouracil
Anti viral drug iododeoxy uridine
Benzpyrene in tobacco smoke
• Physical Causes
X-rays & ultraviolet light
Certain virus such as bacterial viruses
26. • Types of mutations
Point mutation
Insertion/Deletion
Frameshift mutations
Trinucleotide repeat mutations
27. • Point mutation: substitution of a single nucleotide base with another
base
https://evolution.berkeley.edu/dna-and-
mutations/a-case-study-of-the-effects-of-
mutation-sickle-cell-anemia/
28. • Deletion/Insertion: Addition or deletion of single or multiple
nucleotide base in DNA sequence changes the reading frame of it
• Example of deletion is cystic fibrosis and example insertion is
Hemophilia A
29. • Frameshift mutation: Frameshift mutation involves insertion/deletion of
one or two base pairs changing the reading in the coding sequence
• As the coding message is read in triplets , it results in a nonsense sequence
of amino acids
30. Trinucleotide Repeat mutations
• Trinucleotides are triplets of nucleotides that are repeated in tandem
many times over. The number of repeats varies in different individuals
• An example is CAGCAGCAGCAGCAGCAG.... Or (CAG)n
• A form of mutation characterized by stretch of three nucleotides
Example: Fragile X syndrome
31. • Types of Genetic disorders
Single gene defects/Mendelian Disorders
Disorders with multifactorial or Polygenic Inheritance
Cytogenetic Disorders
Disorders showing atypical patterns of inheritance
