1. Biology 250 Mendelian Genetics
It’s all about Gregor Mendel

2. The History of Genetics
Genetics - The field of biology devoted to understanding how characteristics are transmitted from parents to offspring.
Believe it or Not! Genetics began with the work of an Austrian monk named Gregor Mendel (now known as the “father” of genetics!)

3. The History of Genetics (continued)
Some important facts about Gregor Mendel:
His task at the monastery was to tend the gardens. (This is where he was first introduced to his favorite test subject: peas)
He had taken courses in both science and mathematics including statistics. (This enabled him to see patterns in his experimental results which would otherwise have gone unnoticed.)
Genetics gave rise to heredity – the study of the transmission of characteristics from parents to offspring

4. Mendel’s Experiment
Mendel Experimented with pea plants. Qualities of the pea plant which made it useful for this purpose:
Has 7 characteristics which occur in 2 contrasting traits. (purple OR white flowers for example)
Pea plants can either self-pollinate (fertilize themselves) or cross-pollinate (fertilize each other)

5. Mendel’s Experiment (continued)
Plant Terminology:
Pure – Plants which are pure for a specific trait will always produce offspring with that trait. (Example: A pure purple flowering plant will always produce seeds which produce purple flowering plants.)
Strain – A group of plants which are pure for a specific trait

6. Mendel’s Experiment (continued)
Terms dealing with generations:
P1 (Parental Generation) – the starting generation of the experiment.
F1 (First Filial Generation) – the generation produced by crossing the parental generation. (Example: If your parents are the P1 generation, you are the F1 generation)
F2 (Second Filial Generation ) – the generation produced by the F1 generation (Example: if your parents are the P1 and you are the F1 then your children will be the F2)

7. How are these two terms similar/related?
Vocabulary Review!
Genetics
How are these two terms similar/related?
Heredity
Pure
Strain
F1 Generation
F2 Generation
P1 Generation

8. Mendel’s Experiment (continued)
Mendel allowed pea plants to self-pollinated until they were pure for a specific trait. These plants were his P1 Generation.
He crossed the members of his P1 Generation (white flower plant with purple flower plant) to produce the F1 generation
He allowed the F1 generation to self-pollinated to produce the F2 generation.

9. Mendel’s Results & Conclusions
Mendel concluded that each trait was controlled by a pair of alleles – alternative form of a gene
Dominant Allele – Must be expressed when present. Will mask the appearance of other alleles.
Recessive Allele – Will only show if no dominant allele is present.

10. Alleles
Dominant alleles are abbreviated with capital letters (A, B, D)
Recessive alleles are abbreviated with lower case letters (a, b, d)
Each trait will have TWO alleles. (AA, Aa, aa)
If an individual has two dominant alleles for a gene (AA) they are said to be HOMOZYGOUS DOMINANT
If an individual has two recessive alleles for a gene (aa) they are said to be HOMOZYGOUS RECESSIVE
If an individual has one dominant and one recessive allele for a gene they are said to be HETEROZYGOUS

11. Practice Practice Practice
RR = ?
rr = ?
Rr = ?
Genotype - The genes an organism has. (AA, Aa, aa)
Phenotype – The physical appearance (purple or white flowers)
In peas, purple is dominant to white so:
R = purple
r = white

12. Probability The likelihood that a specific event will occur.
Can be expressed as a decimal, fraction, or percentage.
0.75
75%

13. Predicting the results of crosses
Punnett Square – Diagram used to predict the probability of a specific genotype or phenotype from a cross between two individuals.
Let’s try some Punnett Squares!
Does NOT show you the actual offspring. It shows the likelihood of having a specific type of offspring.

14. Reporting results of Punnett Squares
If you are asked for the genotypic ratio you need to give the ratio of the genotypes.
1AA: 2Aa: 1aa
If you are asked for the phenotypic ratio you need to give the ratio of the phenotypes
3 purple : 1 white

15. Performing a Testcross
We will always know the genotype of an individual with the recessive phenotype.
Why?
To determine the genotype of an organism with the dominant phenotype, we perform a testcross.
In a testcross you cross the organism of unknown genotype with a homozygous recessive individual.
If A= purple and a = white
AA = purple
Aa = purple
aa = white
The only way to get the recessive phenotype is to have the aa genotype!

16. Let’s practice a testcross!
Testcross continued…
How will this help us determine the genotype?
If the unknown individual is homozygous dominant then….
If the unknown individual is heterozygous then….
Let’s practice a testcross!
All offspring show the dominant phenotype!
Half the offspring will have the dominant phenotype and half will have the recessive phenotype!

17. Let’s see how it’s done! Dihybrid Crosses
Sometimes you want to cross two individuals while looking at TWO different traits (flower color AND seed texture).
Flower color: A = purple a = white
Seed texture: R = smooth r = wrinkled
Write the genotype of an individual who is heterozygous for flower color and homozygous dominant for seed texture.
Write the genotype of an individual who is homozygous recessive for flower color and heterozygous for seed texture.
Let’s see how it’s done!
AaRR
aaRr

18. Special Types of Inheritance
We’ve been working with complete dominance – when one allele is completely dominant over the other.
Sometimes alleles show incomplete dominance – when both alleles influence the phenotype of a heterozygote and the phenotype is a blend.

19. Example of Incomplete Dominance
Four o’clock flowers show incomplete dominance.
R = red
r = white
Rr = pink

20. Special Types of Inheritance cont.
Codominance – When neither allele is dominant or recessive to the other and BOTH are expressed in a heterozygote.
R = red
R’ = white
RR’ = red and white
Example: The roan horse.