1. Mendelian Genetics

2. Background
We know that traits are passed on from one generation to the next (parents to offspring).
How they are passed on was a mystery until around 1900.
Gregor Mendel, the father of modern genetics, used pea plants to study how traits were passed on and described his discoveries in his laws of inheritance.

3. Mendel used pea plants to study seven traits, each of which had two distinct forms.
For example, seed pod color was either green or yellow and plant height was either tall or short (dwarf).

4. Mendel set up crosses between specific plants to see what traits the next generation would have.
For example, he crossed a tall plant with a short plant.

6. What Mendel saw was that all of the plants in the next generation (F1 ) were tall.
There were no short plants.
But one of the parents was short. How could all of the offspring be tall?

7. He then crossed 2 tall plants from the F1 generation.
The results were surprising: about 75% of the next generation (F2 ) were tall and 25% were short.
The short trait had reappeared!

8. Mendel’s Discoveries
Mendel studied the inheritance of traits in experiments with thousands of pea plants.
He found that each trait was controlled by a gene on homologous chromosomes.
The two alternate forms of that gene (one from each parent) are called alleles.

10. Dominant and Recessive Alleles
If there are two different traits and one is always expressed if at least 1 allele is present, that allele is considered dominant.
A capital letter represents a dominant allele.
Ex: T represents the allele for tall plant height.

11. Dominant and Recessive Alleles
If a trait is not expressed unless both alleles for that trait are present, the trait is considered recessive.
The lowercase letter of the dominant allele represents a recessive allele.
Ex: t represents the allele for short plant height.

12. Dominant and Recessive Alleles
Example:
Round seeds are dominant to wrinkled seeds.
Dominant allele: R (round)
Recessive allele: r (wrinkled)
Remember to use the same letter! The dominant allele is capitalized, the recessive is lower case. R r

13. Phenotype and Genotype
The phenotype is the physical appearance of the trait.
Tall and short plant heights are phenotypes.

14. Phenotype and Genotype
The genotype is the genetic makeup for the trait and includes both alleles in a homologous pair.
TT and Tt are the genotypes for a tall plant.
tt is the genotype for a short plant.

16. Homozygous and Heterozygous
Homozygous is the term we use when an organism has two identical alleles for a trait.
Ex: TT (homozygous dominant)
tt (homozygous recessive)
Heterozygous is the term we use when an organism has two different alleles for a trait; also called carriers.
Ex: Tt

18. Mendel’s Laws
The rules of inheritance are referred to as Mendel’s Laws.
They apply to all organisms that sexually reproduce.
There are exceptions to these laws, and they will be discussed later.
Many traits do follow Mendel’s laws and are called Mendelian traits.

19. Mendel’s Laws
The LAW OF SEGREGATION states that each pair of genes segregates, or separates, during meiosis.

20. Law of Segregation
sperm
egg

21. Mendel’s Laws
The LAW OF INDEPENDENT ASSORTMENT states that for 2 characteristics, gene pairs segregate into gametes randomly and independently of each other.
It produces all possible combinations of alleles in the gametes.
Dihybrid crosses are used to show this.

22. Law of Independent Assortment
All possible combinations of alleles from each parent are shown.

23. Mendel’s Laws
According to the LAW OF DOMINANCE, the dominant allele is always expressed and the recessive allele can be hidden.
Red (R) is dominant over white (r).
So, the recessive allele is hidden in the F1 generation.

24. Punnett Squares and Probablility
A Punnett square is a simple graphical way of predicting all of the potential combinations of genotypes that can occur in the offspring, given the genotypes of the parents. 
It is used to determine the probability of genotypes, which determines the phenotypes.
Probability can be expressed as a ratio, fraction or percent.
Example: 3:1 or 3/ or 75%

25. To set up a Punnett square, you first draw a 4-square grid:

26. Next, you put the alleles of one parent across the top and those of the other parent down the left side (law of segregation). 
For example, if parent genotypes were RR (round peas) and rr (wrinkled peas), the setup would be: r r R R

27. Next, fill in the boxes by copying the row and column-head letters across or down into the empty squares. 

28. So the probability that the offspring will have round peas (Rr) is 100% (4/4).
The probability that the offspring will have wrinkled peas (rr) is 0% (0/4). r r Rr Rr R
round
round Rr Rr R
round
round

29. Try this one. Tall plant height (T) is dominant to short (t).
Parents: Tt X Tt T t TT Tt T tt t Tt

30. What % will be tall? _________ What % will be short? ________
What percent are heterozygous? _______
3/4 = 75%
1/4 = 25%
2/4 = 50% T t TT Tt T
tall
tall tt t Tt
tall
short

31. Punnett Squares and Probablility
What are the genotypes of the parents?
_________
What are the phenotypes of the parents?
________________
What percent of the offspring will be heterozygous? ______
Yy X yy
yellow and green
50%

32. Next…. Tape this on page 60 of your notebook.
Practice what you learned today on the assignment you’re about to get.

33. Homozygous or Heterozygous
Aa _____________
RR _____________
Tt ______________
Pp ______________
YY ______________
Homozygous
Heterozygous
Heterozygous
Homozygous

34. Phenotypes Purple is dominant to white
Purple flowers
PP _____________
Pp _____________
pp ______________
Purple flowers
White flowers

35. Genotypes Round is dominant to wrinkled
RR, Rr rr

36. What % will be round? _________ What percent are heterozygous? _______
What % will be homozygous recessive? ________
3/4 = 75%
2/4 = 50%
1/4 = 25% R r RR Rr R
round
round rr r Rr
wrinkled
round

37. DiHybrid