1. Mendel, Punnet Squares Monohybrid Crosses,Test Crosses
3.3 Theoretical Genetics
Mendel, Punnet Squares
Monohybrid Crosses,Test Crosses

2. Early Beliefs of Mendel
The idea that biological traits are inherited has existed for over 6000 years (the time of the Babylonians) where pedigrees were depicted in cave paintings.

3. Gregor Mendel (1822 – 1884) Pioneer of genetics
Austrian monk who worked with garden peas
His experiments explained the mechanism of inheritance in plants
Also a basis for understanding heredity in general
This was not taken seriously in his lifetime (experiments involving inheritance were even frowned upon by the general public and religious groups especially)

4. 7 characteristics of peas were used in Mendel’s experiments:

5. Stem Length
Dominant
Recessive

6. Seed Shape
Dominant
Recessive

7. Seed Colour
Dominant
Recessive

8. Pod Shape
Dominant
Recessive

9. Pod Colour
Dominant
Recessive

10. Flower Colour
Dominant
Recessive

11. Flower Position
Dominant
Recessive

12. peas were used because they …..
are easy to grow
mature quickly
normally self pollinating and so he
found it easy to control crosses (Mendel cross-pollinated)

13. prior belief was that information in the blood of both parents was mixed when offspring conceived and so expect to see a combination of features from parents
So, if Mendel mixed round and wrinkled seeds it was expected that he should get a mixture of the two

14. Mendel proved this INCORRECT!
Round + Wrinkled = Round!!!
With the above traits  one trait always dominates another

15. Mendel’s Method
1. Chose plants that were purebred (spent years self-pollinating plants to get “pure” lines) (ex. tall plants self fertilized many times until tall plants produced seeds that would only produce tall plants)
EXAMPLE: round (RR) and wrinkled (rr)
(parent generation = P generation)

16. 2. Crossed two pure lines to get hybrids  these offspring are the first generation or filial generation (F1)
EXAMPLE: F1 = Rr (one from each parent)
F1 = Rr = all round!!

17. 3. Then cross F1 generation offspring to get the second generation (F2)

18. What will the offspring look like? Round? Wrinkled?
Rr x Rr F1
Gametes: R r R r
RR Rr Rr rr F2
F2 generation = 3 : 1 ratio
3 round : 1 wrinkled

19. He experimented with other characteristics and in every case found that F2 generation showed very close to a 3:1 ratio
Mendel concluded that sex cells (gametes) must contain only 1 allele.
Today we know this is true because of meiosis

20. Mendel said that there were units of inheritance that controlled the traits (he called them “factors”)
Today we know that “factors” = genes
He figured that there were different forms of a gene
Today we call these forms alleles

21. Once Mendel had formulated his laws he was able to describe the genotype of a plant.
He used letters to represent alleles
Capital letter = dominant allele
Small letter = recessive allele

22. Genotype: the alleles possessed by an individual for a given gene.
Ex: BB, Bb, bb
Phenotype: the physical manifestation of a gene.
Ex: brown eyes or blue eyes

23. Note: A “dominant allele” codes for a particular protein
Note: A “dominant allele” codes for a particular protein. A “recessive allele” usually means the absence of a protein.
Ex: Brown eyes are the result of the presence of a dominant allele. If the individual does not possess a dominant allele, they will have blue eyes.

24. Another rule to remember is:
the dominant allele is always expressed when the recessive allele is present (ex. Rr = round)

25. Genotypes
Homozygous Dominant: the possession of 2 dominant alleles for a given gene,
resulting in the dominant phenotype.
Ex: BB = brown eyes
Homozygous Recessive: the possession of 2 recessive alleles for a given gene, resulting in the recessive phenotype.
Ex: bb = blue eyes

26. Genotypes
Heterozygous (Dominant) = the possession of 1 dominant allele and 1 recessive allele for a given gene, usually resulting in the dominant phenotype (in genes that demonstrate complete dominance)
Ex: Bb = brown eyes

27. Mendel’s Laws
Law of Segregation - two alleles of a gene segregate during the formation of gametes (ex. a plant that is Tt forms gametes that are T or t)

28. Law of Segregation
This basically means, during meiosis when gametes are formed, only 1 chromosome from a homologous pair will be found in each gamete.

29. Mendel’s Laws
Law of Independent Assortment – Segregation for different pairs of alleles occurs independently (ex. a plant that is TtRr forms four types of gametes that are TR, Tr, tR, tr)

30. The homologous pairs line up randomly in metaphase 1 which means there are so many possible combinations of chromosomes in the gametes.

31. What’s your phenotype?

32. Tongue Rolling
Dominant
Recessive
Rollers
Non-rollers

33. Earlobes
Dominant
Recessive
Detached
Attached earlobes

34. Mid-digit Hair
Dominant = Hair
Recessive = no hair

35. Hairline
Dominant
Recessive
“Widow’s Peak”, v-shape
Straight Hair line

36. Thumb Crossing
Dominant
Recessive
Left over Right
Right over Left

37. Hitchhiker’s Thumb
Dominant
Recessive

38. Bent Pinky
Dominant
The pinky bends distinctly inward toward the fourth finger (ring finger)

39. Types of Ear Wax Dominant Recessive Wet (honey brown colour)
Dry (light coloured, flaky)

40. PTC (Phenylthiocarbamide) Tasting
The ability to taste PTC is a dominant trait
Tasters taste a bitter taste
Non-tasters do not notice a distinctive taste

41. Monohybrid Cross – follows the inheritance of 1 gene.
Dihybrid Cross – follows the inheritance of 2 genes.

42. Punnett square – a chart used by geneticists to show the possible combinations of alleles in offspring

43. Setting up a Punnet Square
Set up a 2 x 2 square

44. Setting up a Punnet Square
2. Write the alleles for parent 1 on the left side of the square, and the alleles for parent 2 on the right side.
Parent 1: Parent 2:
TT t t
t t T

45. Setting up a Punnet Square
t t T T t T t T T t T t