1. MENDELIAN GENETICS
1. Mendel’s work
2. Monohybrid inheritance and principal of segregation
3. Dihybrid inheritance and the principal of independent
assortment
4. Test cross

2. INTRODUCTION
• Explaining the mechanism of inheritance
• The mechanism relates to the numbers of
characteristic of inheritance
• The simple characteristic leads to the simpler
crossing over mechanism and ration
• This is followed by excluding the mutation
effects that will be discussed later (chapter 4)

3. GREGOR MENDEL
• Study in University
of Vienna
• His parents has a
small farm in
Austria

4. GREGOR MENDEL (cont)
• Austrian monk
• Studied the
inheritance of traits in
pea plants
• Developed the laws of
inheritance
• Mendel's work was
not recognized until
the turn of the 20th
century

5. GREGOR MENDEL (cont)
• Between 1856 and
1863, Mendel
cultivated and tested
some 28,000 pea
plants
• He found that the
plants' offspring
retained traits of the
parents
• Called the “Father of
Genetics"

7. MENDEL’S PEA PLANT TRAITS

8. GREGOR MENDEL (cont)
• Mendel stated that
physical traits are
inherited as
“particles”
• Mendel did not know
that the “particles”
were actually
Chromosomes & DNA

9. GENETIC TERMINOLOGIES
• Character – heritable feature that varies among
individuals
• Trait – each variant for the character
• True-breeding – Plants homozygous for a
characteristic are true-breeding (Self-pollinate)
• Hybridization – mating or crossing over of two
true-breeding varieties
• P generation – parental generation/parent
• F1 generation – first filial generation (son)
• F2 generation – second filial generation

10. GENETIC TERMINOLOGIES (cont)
• Allele- alternate version of a gene
• Homozygote – pair of identical alleles for a character
• Heterozygote – two different alleles for a character
(Bb)
• Dominate allele – expressed in the heterozygote
• Recessive allele – not expressed in the heterozygote
• Homozygous dominant- BB
• Homozygous recessive - bb
• Genotype – genetic makeup
• Phenotype – appearance of an organism

11. TYPES OF GENETIC CROSS
1. Monohybrid cross - cross involving a single
trait
e.g. flower colour
2. Dihybrid cross - cross involving two traits
e.g. flower colour & plant height

12. PUNNET SQUARE
• Diagrammatic device for predicting the allele
composition of offspring from a cross between
individuals of known genetic makeup.
• 3 steps / generation = P gen, F1 gen, F2 gen
• Heterozygous allele - ?
• Homozygous allele - ?
• Phenotype - ?
• Genotype - ?

13. PUNNET SQUARE (cont)
• Can be used for monohybrid and also dihybrid
cross.

15. LAW OF INHERITANCE
Gregor Mendel introduce 2 laws
• Law of Segregation
• Law of Independent Assortment

16. LAW OF SEGREGATION
• Inherit only ONE characteristic @ Monohybrid
• Producing 3:1 of phenotypic inheritance
• Mendel use a large group of sample size to
explain this law
• Leads to a development of a model known as
Mendel’s Model

17. MENDEL MODEL
Four concepts in law of segregation
1. Alternative versions of genes account for variations in
inherited characteristics
2. For each character, an organism inherit two alleles,
one from each parent
3. If the two alleles at a locus differ, then one, the
dominant allele, determines the organism’s
appearance; the other, the recessive allele, has NO
noticeable effect in the organism’s appearance
4. The two alleles for a heritable character segregate
(separate) during gamete formation and end up in
different gametes

18. 1 . ALTERNATIVE VERSIONS OF GENES
ACCOUNT FOR VARIATIONS IN
INHERITED CHARACTERISTICS
• Have 2 choices of alleles
• Existing in two version
• Depending on the
phenotypic or characteristic
derive in the genetic make
up
• Eg. Purple flower and white
flower

19. 2. FOR EACH CHARACTER, AN
ORGANISM INHERIT TWO ALLELES,
ONE FROM EACH PARENT
• Each somatic cell in a
diploid organism has two
sets of chromosome
• Genetic locus represent
twice in diploid cell, once
in homolog of a specific
pair of chromosome

20. 3. IF THE TWO ALLELES AT A LOCUS
DIFFER, THEN ONE, THE DOMINANT
ALLELE, DETERMINES THE ORGANISM’S
APPEARANCE; THE OTHER, THE
RECESSIVE ALLELE, HAS NO NOTICEABLE
EFFECT IN THE ORGANISM’S
APPEARANCE
• The plant have more purple colour due to its
dominant allele, vice versa

21. 4. THE TWO ALLELES FOR A
HERITABLE CHARACTER SEGREGATE
(SEPARATE) DURING GAMETE
FORMATION AND END UP IN
DIFFERENT GAMETES
• An egg or sperm gets only one of the two
alleles that are present in the somatic cell of
the organism making the gamete
• The correspond depending on the types of
reproduction between meiosis and mitosis
• Further discussion after test cross

22. Example of
MONOHYBRID CROSS

23. P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR x rr
r r Genotype: Rr
Phenotype: Round
R Rr Rr
Genotypic
Ratio: All alike
R Rr Rr Phenotypic
Ratio: All alike

24. P1 Monohybrid Cross Review
• Homozygous dominant x Homozygous
recessive
• Offspring all Heterozygous (hybrids)
• Offspring called F1 generation
• Genotypic & Phenotypic ratio is ALL ALIKE

25. F1 Monohybrid Cross
• Trait: Seed Shape
• Alleles: R – Round r – Wrinkled
• Cross: Round seeds x Round seeds
• Rr x Rr
R r Genotype: RR, Rr, rr
Phenotype: Round &
R RR Rr wrinkled
G.Ratio: 1:2:1
r Rr rr
P.Ratio: 3:1

26. F1 Monohybrid Cross Review
• Heterozygous x heterozygous
• Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
• Offspring called F2 generation
• Genotypic ratio is 1:2:1
• Phenotypic Ratio is 3:1

27. HOW DOES THE PEAS LOOK LIKE?

29. •Genotypic Ratio
&
•Phenotypic Ratio

30. TEST YOURSELF!
1. Between blue flower, BB and yellow, yy
2. Between small leaf, ff and big leaf, Ff

31. LAW OF INDEPENDENT ASSORTMENT
• TWO characteristics at the same time @
Dihybrid cross
• Eg. Leaf colour and leaf size
• Using both dominant and recessive alleles in
each of the characteristics.

32. INDEPENDENT ASSORTMENT in
CHROMOSOME

33. • Mendel performed dihybrid crosses in plants that were
true-breeding for TWO traits.
• E.g a plant with green pod colour and yellow seed,
cross-pollinated with a plant that had yellow pod
colour and green seeds.
• Green pod colour = GG
• Yellow seed colour = YY
• Yellow pod colour = gg
• Green seed colour = yy
• The resulting F1 generation were all heterozygous for
green pod colour and yellow seeds (GgYy)

34. DIHYBRID CROSS
• Involves two pairs of contrasting traits

36. DIHYBRID CROSS
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
Phenotypic ratio 9:3:3:1
copyright cmassengale 36

37. DIHYBRID CROSS
• Traits: Seed shape & Seed colour
• Alleles: R round
r wrinkled
Y yellow
y green
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinations

38. DIHYBRID CROSS
RY Ry rY ry
RY
Ry
rY
ry

39. DIHYBRID CROSS
RY Ry rY ry
Round/Yellow: 9
RY RRYY RRYy RrYY RrYy
Round/green: 3
Ry RRYy RRyy RrYy Rryy
wrinkled/Yellow: 3
rY RrYY RrYy rrYY rrYy wrinkled/green: 1
9:3:3:1 phenotypic
ry RrYy Rryy rrYy rryy ratio

40. HYPOTHESIS/CONCLUSION
• The alleles of seed colour and seed shape sort
into gametes independently of each other.
• Phenotypic ratio for IA = 9:3:3:1

41. TEST CROSS
• To determine if an individual exhibiting a
dominant trait is homozygous or heterozygous
for that trait.
• If all offspring display the dominant
phenotype, the individual in question is
homozygous dominant; if the offspring
display both dominant and recessive
phenotypes, then the individual is
heterozygous

42. TEST CROSS (cont)
• In some sources, the ‘test cross’ is defined as
being a type of backcross between the
recessive homozygote and F1 generation.
• F1 progeny are mated back to one of their
parents (or to individual with a genotype
identical to the parent)
• Backcross is often used synonymously with
testcross.

43. TEST CROSS
A mating between an individual of unknown genotype
and a homozygous recessive individual.
• Example: bbC__ x bbcc
BB = brown eyes
Bb = brown eyes
bb = blue eyes bC b___
CC = curly hair bc
Cc = curly hair
cc = straight hair

44. TEST CROSS
Possible results:
bC b___
C bC b___
c
bc bbCc bbCc or bc bbCc bbcc
copyright cmassengale 44

46. If the plant being tested is If the plant being tested is
homozygous heterozygous

47. • G?W? X ggww
– (G=yellow; g=green; W=round; w=wrinkled)
– What will the expected phenotypic ratios be for
the above testcross?

49. SUMMARY of MENDEL’S LAW
LAW PARENT CROSS OFFSPRING
DOMINANCE / True-
TT x tt 100% Tt
breeding
tall x short tall
Tt x Tt 75% tall
SEGREGATION
tall x tall 25% short
INDEPENDENT RrGg x RrGg 9/16 round seeds & green pods
3/16 round seeds & yellow pods
ASSORTMENT round & green x 3/16 wrinkled seeds & green pods
round & green 1/16 wrinkled seeds & yellow pods