1. Mendelism: The Basic Principles of Heredity
Md. Habibur Rahaman (HbR)
Lecturer
Dept. of Biology & Chemistry
North South University

2. Heredity
What genetic principles account for the transmission of traits from parents to offspring?
One possible explanation of heredity is a “blending” hypothesis - The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green
An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea - Parents pass on discrete heritable units (or genes)

3. Gregor Mendel
Gregor Mendel (1822 – 1884) – Austrian monk who first discovered the basic rules of inheritance – his work was rediscovered in 1900 and he came to be known as the Father of Genetics
Documented a “particulate” mechanism of inheritance through his experiments with garden peas

4. Genetic Terminology
Locus – specific location of a gene on a chromosome
Homologous chromosomes carry the same type of gene, located at the same place (same type, but may not be identical)
Alleles – alternative forms of the same gene. e.g., – human blood types produced by three different alleles A, B, and O
Homozygous – an organism that has the same allele on both homologous chromosomes at a given gene locus
Heterozygous – when the two homologous chromosomes have different alleles at a given gene locus (e.g., hybrid)

5. Genetic Terminology
Pure-breeding (true-breeding) – It involves creating offspring by mating two parents that are genetically similar, homozygous (opposite of a hybrid)
Phenotype – the physical appearance of an organism
Genotype – the actual combination of alleles carried by an organism
Genome – the whole set of the genetic information of an organism
Dominant traits- traits that are expressed
Recessive traits- traits that are covered up
Monohybrid crosses—Crosses between parents that differed in a single characteristic

6. Mendel’s Success Mendel worked with the pea plant, Pisum sativum
Easy to cultivate, rapid growth, produce many offspring
He focused on traits that were easy to contrast
He selected pure-breeding plants (self-fertilizing)
Traits were located on separate chromosomes (he was lucky!)
No linkage among the traits

7. Antagonistic traits
Dominant
Recessive

8. True Breeding Plants
Mendel also made sure that he started his experiments with varieties that were “true-breeding” X

9. Monohybrid Crosses
When Mendel crossed contrasting, true-breeding white and purple flowered pea plants all of the offspring were purple
When Mendel crossed the F1 plants, many of the plants had purple flowers, but some had white flowers
A ratio of about three to one, purple to white flowers, in the F2 generation
P Generation
(true-breeding
parents)
Purple
flowers
White 
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation

10. Mendel’s Rationale
In the F1 plants, only the purple trait was affecting flower color in these hybrids
Purple flower color was dominant, and white flower color was recessive
Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F2 offspring
There are four related concepts that are integral to this hypothesis

11. Heredity Concepts
Alternative versions of genes account for variations in inherited characters, which are now called alleles
For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice
If the two alleles at a locus differ, the dominant allele determines the organism’s appearance
The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers

12. Mendel’s Monohybrid Cross
White
(pp)
Purple
(Pp)
Gametes
Gametes p p P p
Purple
(PP)
Purple
(Pp) P P
Gametes Pp Pp PP Pp
Gametes p P Pp Pp Pp pp
F1 generation
All purple
F2 generation
¾ purple, ¼ white
The Punnett Square

13. Test Cross
To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype: PP
If all offspring are purple;
unknown plant is
homozygous. P pp p Pp
Recessive
phenotype
Dominant
Phenotype
Gametes
Alternative 1 – Plant with dominant phenotype is homozygous ? Pp
If half of offspring are
white; unknown plant
is heterozygous. P p pp
Recessive
phenotype ?
Alternative 2 – Plant with dominant phenotype is heterozygous
Dominant
Phenotype
Gametes

14. Dihybrid Cross
Monohybrid cross: determined that the 2 alleles at a single gene locus segregate, when the gametes are formed
Dihybrid cross: determined that alleles at 2 different gene loci segregate independently of one another
Also known as law of independent assortment
Basically, an extension of the law of segregation

15. Dihybrid Cross Crossed pea varieties that differed in two traits
Mendel crossed 2 pure-breeding plants: round yellow seeds X green wrinkled seeds
Seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r)
Seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y)

16. Dihybrid Cross RrYy What are the expected F1 gametes?
True-breed parents
Parental Gametes RY ry
RrYy
F1 Offspring
What are the expected F1 gametes?
What should be the phenotypic ratio for the F2?

17. F2 with independent assortment Phenotypic ratio is 9 : 3 : 3 : 1RY Ry rY ry RR YY Yy Rr yy rr RY Ry rY ry
Phenotypic ratio is 9 : 3 : 3 : 1

18. Summary of Mendel’s Principles
Mendel’s Principle of Uniformity in F1:
F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents
Why? Some traits are completely dominant over other traits
Mendel’s Law of Segregation:
Recessive characters masked in the F1 progeny of two true-breeding strains, re-appear in a specific proportion of the F2 progeny
Two members of a gene pair segregate (separate) from each other during the formation of gametes
Mendel’s Law of Independent Assortment:
Alleles for different traits assort independently of one another
Genes on different chromosomes behave independently in gamete production

19. Exceptions To Mendel’s Original Principles
Incomplete dominance
Co-dominance
Polygenic traits
Epistasis
Pleiotropy

20. Incomplete dominance
Neither allele is dominant and heterozygous individuals have an intermediate phenotype
For example, in Japanese “Four o’clock”, plants with one red allele and one white allele have pink flowers:
P Generation
F1 Generation
F2 Generation
Red
CRCR
Gametes CR CW 
White
CWCW
Pink
CRCW
Sperm Cw
1⁄2
Eggs
CR CR
CR CW
CW CW

21. Does it support blending inheritance?
Incomplete Dominance
Gametes CR CW
CRCR CR
CRCR
CRCW
Gametes CW
F1 generation
All CRCW
CRCW
CWCW
F2 generation
CWCW
1 : 2 : 1
Does it support blending inheritance?

22. Co-dominance
Neither allele is dominant and both alleles are expressed in heterozygous individuals
Example ABO blood types

23. Misconception of Dominant Phenotype
A genetic factor/trait may be dominant in one species, but can be recessive or incompletely dominant in other species
Even though the two genetic factors appear to produce identical phenotypes (red flower trait in both the pea and the Four O’clock)
A factor for black fur may be dominant in one species (guinea pigs) but recessive in another (sheep)

24. Polygenic Traits
Most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits.
Polygenic traits often show continuous variation, rather then a few discrete forms:

25. Epistasis
When two or more genes influence a trait, an allele of one of them may have an overriding effect on the phenotype
Labrador retrievers: a dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat
However, a second gene locus controls the color. Dogs that are homozygous recessive at this locus (ee) will have golden fur no matter which alleles are at the first locus
Gene 1: Represented by B
: Controls color
Gene 2: Represented by E
: Controls expression of B

26. Epistasis BBEE or BbEe --> Black retrievers
bbEE or bbEe --> Brown retrievers
BBee, Bbee, or bbee --> Golden retrievers
Golden
Brown
Black

27. Pleiotropy
This is when a single gene locus affects more than one trait
For example, in Labrador retrievers the gene locus that controls the pigment in the hair also affects the color of the nose, lips, and eye rims

28. Slides are provided just to guide your study
Slides are provided just to guide your study. Please follow books/reading notes for exam. Thank You!!!