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. 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)

6. 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

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

8. 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)

9. 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!!

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

11. 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

12. 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

13. 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

14. 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
Punnett square – a chart used by geneticists to show the possible combinations of alleles in offspring

15. 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.

16. Mendel’s Laws
Law of Segregation - two alleles of a gene segregate during the formation of gametes (ex. a plant that is Rr forms gametes that are R or r)

17. 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, rr)

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

19. 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

20. 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

21. 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

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

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

24. 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

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