1. Chapter 4 – Biology 11 Textbook
Mendelian Genetics
Chapter 4 – Biology 11 Textbook

2. Vocabulary Inheritance Heredity Hybrid Offspring Gene Allele Dominant
Recessive
Genotype
Phenotype
Segregation
Homozygous
Heterozygous
Monohybrid cross
Dihybrid cross
Punnett square
Pedigree chart
Law of independent assortment
Incomplete dominance
Codominance

3. Physical characteristics are inherited
Known to be true long before knowledge of genetics
Eg: offspring has same colour hair, same shape nose as parent
Not limited to humans – inheritance is present in plants and other animals that reproduce sexually
Passing of traits from parents to offspring is heredity

4. Gregor Mendel “father of genetics”
Performed experiments on inheritance with pea plants
Provided a basis for understanding heredity

5. Why pea plants? He used cultivated peas for his experiments
They grow rapidly
Their pollination is easily controlled
Pollen grains from the stamen come into contact with egg cells in the ovary
They can self-fertilize or be cross-fertilized
if self-fertilized: asexual reproduction, offspring are identical to parents
If cross-fertilized: sexual reproduction, offspring receives traits from 2 parents

6. Cross-fertilization

8. Why pea plants? Many characteristics with 2 forms
Seed colour
Seed shape
Height
Pod shape
Pod colour
Flower colour
Flower position
Only 2 options – easier to study

9. Previous theory: Crossing traits would “blend” the traits
Eg: smooth + wrinkled = slightly wrinkled
Short + tall = medium
Yellow + green = yellowish green
BUT: Mendel’s experiments proved – no blending
Either: smooth OR wrinkled
Short OR tall
Yellow OR green

10. Plus… Round seeds + wrinkled seeds = round seeds
Round seeds + round seeds = round seeds
Wrinkled seeds + wrinkled seeds = wrinkled seeds
1 trait out of the 2 always dominated
The other trait is recessive

11. Genes vs Alleles Genes control inheritance of particular traits
Alleles are the alternate forms of a gene

12. Mendel did not know these terms...
Mendel said factors (genes) controlled inheritance & there are different forms (alleles) of each factor
Gene: seed colour
Alleles: yellow & green
1 allele – dominant, the other – recessive
Yellow: dominant over green
Round: dominant over wrinkled

13. Genotype vs Phenotype The dominant allele controls the phenotype
Phenotype is the physical characteristic – what you see when the gene is expressed
Eg: seed colour, eye colour
Both alleles are represented in the genotype
Genotype tells you about the genes, specifically what alleles are present
Eg: smooth seed has one allele for smooth, one allele for wrinkled
OR smooth seed has two alleles for smooth

14. Genotype, cont’d… Convention:
Capital letter represents dominant allele
Lowercase letter (same) represents recessive allele
So: if yellow seed colour is dominant to green..
Yellow: Y
Green: y
Plants with both alleles (one from each parent) are hybrids

15. Mendel’s experiments Cross-fertilization of “pure-breeding” plants
Plants that always produced identical offspring
Example: he crossed a pure-breeding tall plant with a short plant
Characteristics did not blend
Only tall plants were produced
Tall height is dominant, short height is recessive

17. The principle of dominance
Whenever individuals with opposite characteristics are crossed, offspring express only the dominant characteristic
Organism has 2 alleles, but one is recessive and is hidden in the presence of the dominant allele
For the recessive allele to be expressed, the 2 alleles must be recessive

18. So… Phenotype: round seeds Possible genotypes: RR or Rr
Phenotype: wrinkled seeds
Genotype: rr
The genotype is either homozygous or heterozygous

19. Homozygous: both alleles are the same
RR, rr, YY, yy, TT, tt, etc…
Phenotype: shows either the dominant or recessive trait
Heterozygous: two alleles are different
Rr, Yy, Tt, etc…
Phenotype: only shows the dominant trait

20. After Mendel crossed the first generation, then he crossed the hybrids…
These are no longer “pure breeding” plants
The recessive phenotype showed up again!!!

21. When 2 hybrid plants were crossed, 75% showed the dominant trait, 25% showed the recessive trait.
So: F1 generation were hybrids, Yy, Rr, Tt
F2 generation could be pure or hybrids

22. Possibilities:
Tt – genotype, 1 dominant allele for tall, 1 recessive allele for short
Tall – phenotype – height of the offspring
TT – genotype, 2 dominant alleles for tall
Tall – phenotype
tt – genotype, 2 recessive alleles for short
Short - phenotype

23. So…
Recessive traits are never “lost” when crossed with dominant traits
The alleles remain in the population and can be expressed again when combined with other recessive alleles.
Let’s take a look at some different examples of monohybrid crosses (testing one allele at a time)

24. Monohybrid cross

25. 1.
Cross between plant that is heterozygous for purple flowers & a plant with white flowers
Use a Punnett Square to show the cross & to predict the possible genotypes & their proportions

26. 1. Punnett square: P p Pp pp Phenotypes: 50% of offspring: purple
50% of offspring: white
Genotypes:
50% heterozygous purple: Pp
50% homozygous white: pp
Punnett square: P p Pp pp

27. 2. Cross between 2 heterozygous tall plants Punnet Square: T t TT Tt
Phenotypes:
75% - tall
25% - short
Genotypes:
25% homozygous tall: TT
50% heterozygous tall: Tt
25% homozygous short: tt T t TT Tt tt

28. 3. Given: Offspring phenotypes:
Yellow seeds: 1578
Green seeds: 526
Work backwards to find the genotypes of the parents.

29. 3. Offspring phenotype: Possible genotypes: Yellow seeds: 1578
Green seeds: 526
Find the ratio: 1578/526 = 3/1
Possible genotypes:
Yellow seeds: Yy, YY
Green seeds: yy

30. 3. Punnett Square: Yellow
Fill in the phenotypes, we know there is a 3:1 ratio of yellow to green
Yellow
Green

31. 3.
Next…
We know that the only possible genotype for green is yy, so each parent must have contributed a recessive allele. y
Yellow
Green yy

32. 3.
We also know that each of the yellow offspring must have at least one dominant yellow allele y
Yellow Y
Green yy

33. 3. Now fill in the rest of the square!
We can now see that both parents had heterozygous yellow seeds (Yy) Y y
Yellow YY Yy
Green yy

34. 4.
Homozygous, dominant green pods crossed with a plant with recessive yellow pods
Punnett square:
Phenotypes:
100% green pods
0 yellow pods
Genotypes:
100% heterozygous green: Gg G g Gg

35. Test Cross
Cross that allows the biologist to determine the genotype of an individual for a specific trait
Must be crossed with a known (homozygous) recessive individual
If any offspring show the recessive trait, the unknown genotype was heterozygous

37. Pedigree Chart Visual representation of phenotypes within a family
Can be used to trace the passing of an allele from parents to offspring

38. Dihybrid Crosses
Laws that apply to a monohybrid cross (one trait) apply to a dihybrid cross (2 traits)
Dominant alleles remain dominant, recessive alleles remain recessive
Can test for more than one allele at the same time
Due to law of independent assortment

39. Law of Independent Assortment
Genes that are located on separate chromosomes are inherited independently of each other
So, the expression of one gene does not affect the expression of another
Remember independent assortment during meiosis?

41. Dihybrid cross: example 1
Trait 1: dominant allele: A
recessive allele: a
Trait 2: dominant allele: B
recessive allele: b
2 heterozygous individuals crossed:
AaBb x AaBb

42. Dihybrid cross AB Ab aB ab AABB AABb AaBB AaBb AAbb Aabb aaBB aaBb

43. Dihybrid cross AB Ab aB ab AABB AABb AaBB AaBb AAbb Aabb aaBB aaBb
Phenotypes:
9/16 = 2 dominant traits shown
3/16 = dominant trait A shown, recessive trait B shown
3/16 = recessive trait A shown, dominant trait B shown
1/16 = 2 recessive traits shown

44. Multiple Alleles
It is possible to have more than one allele for a particular gene
Increases the complexity of gene expression
Therefore, there are more than 2 ways for the gene to be expressed
Results in a dominance hierarchy
Common examples: eye colour in fruit flies, blood types (A,B,O)

45. Dominance hierarchy Example: gene with 4 different colour options
Colour 1  C1C1, C1C2, C1C3, C1C4 (whenever C is present, it is expressed)
Colour 2  C2C2, C2C3, C2C4, (C2 is expressed when C1 is not present)
Colour 3  C3C3, C3C4 (C3 is only expressed when gene is homozygous, or heterozygous with C4)
Colour 4  C4C4 (C4 is only expressed when homozygous)
Dominant over
Dominant over
Dominant over

46. Examples
Cross individual who is homozygous for C2 with individual who has genotype C1C4
Punnett Square:
50% expressing Colour 1, genotype C1C2
50% expressing Colour 2, genotype C2C4 C2 C1
C1C2 C4
C2C4

47. C3C4 x C1C2
C1C4 x C2C3
Try these on your own.

48. Incomplete Dominance
“Blending” of traits can occur in nature, even though Mendel did not see it in pea plants
2 alleles are equally dominant
Intermediate phenotype produced by heterozygous genotype

49. Incomplete dominance Example: Colour 1 = Colour 2
C1C1 = colour 1 phenotype
C2C2 = colour 2 phenotype
C1C2 = colour 3 phenotype
Eg: snapdragon flower colour
Red + white = pink

51. Codominance
Both alleles are expressed at the same time in the offspring
Not blended
C1C1 = colour 1 phenotype
C2C2 = colour 2 phenotype
C1C2 = both colours seen in the phenotype
Eg: shorthorn cattle

52. Codominance

53. Human Blood Types Controlled by multiple codominant alleles
Inherit 1 allele from mother, 1 allele from father
6 different genotypes possible:
Blood types
For simplicity IA A IB B i O

54. Possible blood genotypes:
Allele from Parent 1
Allele from Parent 2
Genotype of offspring
Blood types of offspring A AA B
AB* AB O AO BB BO OO
*A is codominant with B,
A & B are dominant over O

55. Rhesus Factor
Rh factor is inherited independently from A, B or O alleles
Rh+ or Rh-
Finding blood
type is a dihybrid
cross involving A, B &
O alleles & the Rh factor
Rh factor
Possible genotypes
Rh+
Rh+/Rh+ Rh+/Rh-
Rh-
Rh-/Rh-

56. To do: p. 147: Case Study: A Mystery
Read the evidence, examine the pedigree chart & examine the Blood types of each family member
Note that you are told that the murderer is the child of Lord Hooke who is not biologically related to him.
Work together in small groups (2 or 3) to complete the analysis questions (a,b,c).

57. Sex-linked traits Remember: ♀  2 X chromosomes (XX)
♂  1 X, 1 Y chromosome (XY)
During meiosis, the XY pair acts as homologous
Actually cannot carry the same genes because the Y chromosome is smaller than the X chromosome

59. Sex-linked traits
If a gene is located on the part of the X chromosome that is missing from the Y chromosome… then the ♂ cannot be homozygous for that trait.
Phenotype for that gene is determined by the X chromosome
e.g. eye colour in fruit flies, red-green colour blindness in humans