1. Gregor Mendel
The Father of Genetics

3. Mendel
Modern genetics had its beginnings in an abbey garden, where a monk named Gregor Mendel documented a particular mechanism of inheritance.
He discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.

4. Why peas? Peas are normally self-pollinating. Peas are easily grown,
Peas mature quickly
Peas produce many seeds
Peas show several pairs of obvious contrasting traits
The use of plants also allowed strict control over the mating.

5. Pea’s contrasting traits
Tall plant vs. dwarf plant
Yellow seeds vs. green seeds
Smooth seeds vs. wrinkled seeds
Inflated pod vs. constricted pod
Purple flower vs. white flower
Green pod vs. yellow pod
Axial flower vs. terminal flower

7. Mendel experiments Mendel spent several years self-pollinating
the pea plants in order
to establish purebred
plants.

8. Mendel’s experiments
These pure-breeding plants were called the P generation.
Mendel had strains of peas that were pure breeding for height: the plants were either tall or dwarf
He had strains that had either yellow pods or green pods, inflated pods or constricted pods.

9. Mendel’s experiments
When a pure breeding tall plant is crossed with a pure breeding dwarf plant, all the resulting plants are called hybrids.
All the hybrids are called the F1 generation ( first filial generation)
All of the F1 generation were tall

11. Mendel’s experiment
The same pattern was observed in each of the 7 contrasting characteristics.
In the F1 generation, all of the plants showed one characteristic.
Mendel called this characteristic the dominant trait
The trait that was not expressed in F1 was called the recessive trait

12. Mendel’s experiment Next, Mendel crossed the F1 generation plants.
He wanted to know if the F1 plants were identical to the P generation
So he crossed two of the F1 plants

13. Mendel’s experiment
The resulting plants (the F2 generation) yielded 3 tall plants and 1 dwarf plant.
A ratio of 3:1

14. Mendel’s experiment
This meant that the tall F1 plants ( the hybrids) were different than the tall P generation plants (the purebred plants) even though they were all tall.

15. Mendel’s experiment
Mendel repeated this experiment with all of the traits and the results were similar.
The F2 generation displayed a phenotypic ratio of 3:1

16. Mendel’s explanation
Units of inheritance, called factors (we now call them genes) were involved
For any given characteristic there were several different forms (now called alleles)
Each plants phenotype (what they look like) was determined by a pair of alleles that can be identical or different

17. Mendel’s explanation
In a hybrid plant one allele of a pair has the ability to express itself while the other one is not expressed.
Recessive characteristics are expressed only when there is no dominant allele present

18. Mendel’s explanation
When gametes are produced, the members of each pair of alleles are separated into different reproductive cells.
A gamete can contain only one allele of a particular characteristic.
When fertilization occurs these alleles unite to give the zygote the necessary pair of alleles.

19. Mendel’s first law The Law of Segregation
Members of a pair of alleles for a given trait are segregated when gametes are formed

20. Genotype The genotype represents the genes that exist in the organism
Mendel used letters of the alphabet to represent genes.
Capital letters represent the dominant allele
Lower case letters represent the recessive allele

21. Genotype A purebred tall plant would have a genotype of TT
A purebred dwarf plant would have a genotype of tt
Genotypes with identical alleles are called homozygous
Genotypes with different alleles are called heterozygous

22. The sex cells of a TT plant would contain only the dominant allele T
The sex cells of a tt plant would contain only the recessive allele t
The F1 generation would all have the genotype Tt
The phenotype of all the F1 generation would be tall

24. Mendel’s impact
Mendel’s theories of inheritance, first discovered in garden peas, are equally valid for figs, flies, fish, birds and human beings.
Mendel’s impact endures, not only on genetics, but on all of science, as a case study of the power of hypothesis/deductive thinking.

25. A Punnett Square
Punnett squares illustrate the possible outcomes of a particular cross (the genotype of the offspring)
Mendel’s first experiment could be illustrated with the following Punnett square: TT X tt
tt gametes: t
TT gametes: T Tt

26. A 2 x 2 Punnett Square
The F1 generation cross is a monohybrid cross: Tt x Tt
Tt gametes: T t TT Tt tt
tall
dwarf
Phenotypic ratio =
3:1

27. Single Trait Analysis
In humans the ability to taste PTC, T is dominant to non-tasting t. Determine the expected genotypic and phenotypic ratios resulting from a cross between a heterozygous taster and a non-taster.
Tt (heterozygous taster) gametes: t T Tt tt
taster
non-taster
tt (non-taster) gametes:
Genotype ratio: 1:1 (Tt:tt)
Phenotype: 1:1

28. Practice Problem In humans, the allele A, for pigment formation
is dominant to the allele for a, the inability to
form pigment.
aa individuals are albino
Determine the expected genotypic and
phenotypic ratios expected from a cross
between two individuals heterozygous for
this trait.

29. Solving Punnett Square Problems GRASP method
Given:
pigment formation (A) is dominant to the inability to form pigment (a)
aa individuals are albino
The cross is Aa x Aa
Required:
The expected genotypic and phenotypic ratio of the cross
Analysis:
Solution:
Paraphrase:

30. Solving Punnett Square Problems GRASP method
Analysis:
Solution:
Paraphrase:
The phenotypic ratio is 3:1 normal : albino
The genotypic ratio is 1 : 2: 1 AA:Aa:aa
A a AA Aa
A a Aa aa

31. Recognizing Hybrids
A geneticist crosses two parent plants that have the dominant trait of
purple flowers.
When the resulting seeds are planted the geneticist observes that 145 of
the F1 plants have the recessive trait of yellow flowers and 430 of the F1
plants have purple flowers.
How can you explain these results? What are the genotypes of the parent
plants and the F1 plants?
Given: F (purple flowers) is dominant to f (yellow flowers). Both of the P1
plants possess at least one F gene: F__ × F__
Required: The genotypes of the parents and F1 plants
Analysis:
The key to this question is the appearance of the ff (yellow plants) in the F1.
We know that yellow flowers can only appear if the plants are homozygous recessive for yellow flowers (ff)
Produce a Punnett square of the offspring.

32. Let F be the dominant allele for flower colour: purple and f be the recessive allele for colour: yellow
Phenotypic ratio is 430:145
gametes:
gametes:

33. Solution:
The appearance of the recessive trait in the
phenotype of the F1 plants can only occur if
they are homozygous recessive (ff). This can
only happen if both of the purple parent plants
are heterozygous and each parent contributed
the recessive allele to these yellow plants. In
addition, recognize that the ratio of purple
plants to yellow plants is approximately 3:1.
This ratio indicates a monohybrid cross.

34. Dihybrid crosses
Dihybrid crosses show the genotypes of offspring using 2 different genes on two different chromosomes
TtRr x TtRr hybrid tall plants with round seeds
Gametes produced by independent assortment: TR, tr, Tr, tR
These gametes would be present in equal numbers (eg. ¼ of the total number)

35. Dihybrid cross
TtRr x TtRr TR Tr tR tr

36. Dihybrid cross TtRr x TtRr The expected phenotypic ratio:
9/16 tall round plants, 3/16 tall wrinkled plants
3/16 dwarf round plants, 1/16 dwarf wrinkled plants TR Tr tR tr
TTRR
TTRr
TtRR
TtRr
Ttrr
ttRr
ttrr
TTrr
ttRR

37. Test cross
A test cross is a cross with an individual whose genotype is being tested to a recessive individual
We know that the recessive individual must be homozygous so it can only produce one type of gamete
This way we can determine the genotype of the test individual.