1. And Probability of Inheritance
History of Genetics
And Probability of Inheritance

2. Gregor Mendel Considered the father of genetics
He was a monk who enjoyed caring for
a garden of pea plants
These pea plants can self-pollinate
(fertilize one of its own eggs with its own pollen) or
cross-pollinate (allow pollen from another pea plant
to fertilize its eggs)
Mendel created stocks of true-breeding pea plants
True-breeding means that if these plants self- pollinate they will always create offspring with the same traits as the parent plant

3. Dominance Mendel looked at several traits in his pea plants:
Tall or short
Yellow or green seeds
Green or yellow pods
Round or wrinkled seeds
Gray or white seed coat
Smooth or constricted pod shape
He noticed when he crossed true-breeds of each there was always the same outcome.

4. Dominance
So, every time he fertilized a true-breeding tall plant with pollen from a true-breeding short plant, the offspring were tall (no matter which way the fertilization occurred)

5. Law of Dominance
Mendel realized that something was being passed on from generation to generation, today we recognized that as genes
Each gene has different alleles, or different genetic make-up that leads to a different expression of that gene
We each get two alleles, one from each parent
This lead Mendel to create the law of dominance:
Some alleles are dominant and some are recessive
Tall = dominant pea plants
Short = recessive pea plants

6. F1 Generation
Segregration
Mendel was still curious about whether or not the recessive alleles had disappeared in the offspring
So, Mendel took the offspring (F1 generation) of the true tall and true short and bred them
After breeding several F1 plants with other F1 plants Mendel noticed that the recessive allele reappeared in about ¼ of the F1’s offspring (F2 generation)
F2 Generation

7. Segregation
Mendel concluded that the dominant allele in the F1 generation masked the recessive allele, but these alleles could be separated again in the next generation
Mendel presumed that the alleles separated from one another during formation of the gametes (sex cells)
Mendel came up with the Law of Segregation:
When each organism produces sex cells, the two alleles separate so that each gamete only carries only one single copy of each gene

8. Hybrids = Heterozygous
Therefore, the F1 generation were all heterozygous
Heterozygotes carry two different alleles for the same gene
Homozygotes carry the two of the same allele
A capital letter always represents the dominant allele, and a lower-case of the same letter is the recessive
T T t t  T t
True breeding tall plants
True breeding short plants
Hybrid tall plants

9. Probability
The fact that ¼ of the pea plants in the F2 generation were short was important, because probability plays a role in genetics
Probability is the likelihood that a certain event will occur (like getting a child with blue eyes)
The easiest way to figure out genetic probability is using Punnett squares

10. Vocabulary to Know
Heterozygous – having two different alleles for a gene
Ex:
Homozygous – having the same allele for a gene
Genotype – the genetic make-up or two alleles that are present
Phenotype – the physical expression of a set of alleles

11. Punnett Squares
A punnett square is simply a way to visualize Mendel’s Law of Segregation
One parent goes on top and one on the right,
then just fill in
the two alleles
they have in the
boxes and
check your
outcome. T t

12. Punnett Square Now try it with the F1 generation plants (hybrids)
Now you can see why ¼ of the F2 generation of pea plants ended up short

13. Extreme Punnett Squares
Now Mendel was curious to see what would happen if he crossed a plant with two true-breeding traits and a plant with the two recessive true-breeding alleles of that gene.
So, he took a true-breeding round seed, yellow seed and crossed it with a true-breeding wrinkled green seed pea plant.
We’ll need a bigger punnett square!

14. Di-Hybrid Cross

15. F1 Generation Cross

16. Independent Assortment
Mendel found that when he crossed the F1 generation F2 plants developed with traits that were not there in either parent (example: wrinkled yellow seeds)
This lead Mendel to come up with the Law of Independent Assortment:
Genes for different traits can segregate independently during the formation of gametes.
What this means is that the inheritance of one gene does not necessarily affect other genes because they may be located on separate chromosomes.

17. Mendel’s Laws
Law of Dominance – Some alleles are dominant and some are recessive
Law of Segregation - When each organism produces sex cells, the two alleles separate so that each gamete only carries only one single copy of each gene
Law of Independent Assortment - Genes for different traits can segregate independently during the formation of gametes