1. Chapter 11: Introduction to Genetics
Section 11-2: Applying Mendel’s Principles

2. Introduction
We cannot predict the future – If a parent carries 2 different alleles for a certain gene, there is no way to be sure which allele will be inherited by its offspring
The only thing we can do is predict the odds by applying Mendel’s principles

3. Probability
Mendel was very careful to categorize and count all of the offspring each time he preformed a cross
Whenever he crossed 2 plants that were hybrid for height (Tt) about ¾ of the offspring were tall and ¼ were short
Mendel realized that he could apply the rules of probability to explain his results
Probability is the likelihood that a particular event will occur
Think about a coin flip – the probability of flipping heads is the same as the probability of flipping tails – ½ or 50%
If you flip a coin three times in a row, the probability that it will land heads up every time is ½ x ½ x ½, or 1/8
Each flip is an independent event - past outcomes do not effect future outcomes

4. Using Segregation to Predict Outcomes
The way in which alleles segregate during meiosis is just as random as flipping a coin, therefore the rules of probability can be used to predict the outcomes of crosses
Let’s go through Mendel’s cross:
A true-breeding tall parent (TT) was crossed with a true-breeding short parent (tt)
Each F1 offspring received a T and a t – meaning they were all Tt, and tall
When the F1 plants make gametes, ½ of the gametes will receive the T allele and the other ½ will receive the t allele

5. Using Segregation to Predict Outcomes
The only way to get a short F2 offspring is for 2 t alleles to combine
The probability of 2 gametes coming together that both carry the t allele is ½ x ½, or ¼
You would predict ¼ offspring to be short, and the other ¾ to be tall
This 3:1 ratio of dominant to recessive plants appeared again and again, consistently for each of the seven traits

6. Using Segregation to Predict Outcomes
The reality is that not all organisms that look the same carry the same alleles – for example a plant that is tall could have two T alleles (TT) or is could have one T and one t (Tt)
All short plants have 2 t alleles (tt)
An organism that has two identical alleles for a gene – for example TT or tt – is said to be homozygous
Organisms with 2 dominant alleles are homozygous dominant
Organisms with 2 recessive alleles are homozygous recessive
Organisms that have two different alleles for a gene - for example Tt – are said to be heterozygous

7. Probabilities Predict Outcomes
Probabilities are used to predict the outcomes of large events
In genetics, the larger the number of offspring the closer the results will be to predicted outcomes

8. Genotype and Phenotype
An organism’s genotype is its genetic composition – the alleles it carries for each gene
It can be homozygous dominant, heterozygous, or homozygous recessive
An organism’s phenotype is its physical appearance, or how it looks
It can show the dominant phenotype or the recessive phenotype

9. Genotype and Phenotype
Genotype determines phenotype
Genotype: HD (TT)
Phenotype: Dominant (tall)
Genotype: Het (Tt)
Genotype: HR (tt)
Phentoype: recessive (short)
The genotype of an organism is inherited
Phenotype can also be influences by environment

10. Using Punnett Squares
Punnett squares are simple diagrams used to predict the results of genetic crosses
Used to predict genotype and phenotype possibilities of offspring using probability
Refer to sheet for how to make PS for one factor crosses

11. Independent Assortment
So far, we have only looked at one trait (1 gene) – called monohybrid crosses
Remember that Mendel also experimented to see what would happen if he followed two traits at the same time (2 genes)
These experiments are called dihybrid crosses

12. Dihybrid Cross: F1
Mendel crossed true-breeding plants with round, yellow peas with true-breeding plants with wrinkled green seeds
One plant HD for both traits – RRYY
One plant HR for both traits – rryy

13. Dihybrid Cross: F1 All of the F1 offspring produced round yellow peas
Punnett square shows that the genotype of each F1 offspring was RrYy, heterozygous for both seed shape and seed color

14. Dihybrid Cross: F2 Mendel used 2 F1 plants to create the F2 generation
Mendel observed that 315 of the F2 seeds were round and yellow, while another 32 seeds were wrinkled and green—the two parental phenotypes
But 209 seeds had combinations of phenotypes, and therefore genotypes, that were not found in either parent
The alleles for seed shape segregated independently of those for seed color
Genes that segregate independently do not influence each other’s inheritance – The Law of Independent Assortment
Refer to sheet for how to make PS for two-factor crosses

15. Summary of Mendel’s Principles
The inheritance of biological characteristics is determined by individual units called genes, which are passed from parents to offspring
Where two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive
In most sexually reproducing organisms, each adult has two copies of each gene that segregate from each other when gametes are formed
Alleles for different genes usually segregate independently of each other