1. An Introduction to Mendel’s Laws
GENETICS:
An Introduction to Mendel’s Laws

2. Homologous Chromosome
We talked about the following terms:
Homologous Chromosome
From Dad
Chromosome #12
From Mom
Chromosome #12
A pair of chromosomes from mom & dad, that has the same genetic information in the exact same places on both chromosomes.

3. You have two copies of an allele… 1 from Mom, and 1 from Dad.
Allele (uh-lee-ul)
Characteristic of an organism, like skin color, which can be controlled by a single gene.
You have two copies of an allele… 1 from Mom, and 1 from Dad.
They are located on the same exact place on the homologous chromosomes.
Allele from Dad
Allele from Mom

4. The passing of traits, like eye color, from parents to offspring.
Heredity
The passing of traits, like eye color, from parents to offspring.
Gregor Mendel , an Austrian monk, was the first person to develop rules that accurately predicted the patterns of heredity.

5. Mendel used mathematics to explain how nature worked.
He decided to use pea plants to show how this worked.
In the beginning he tried an experiment just using the flower colors of the pea plant.

6. Mendel took purebred purple pea flowers.
This means their parents were also purple…
And decided to cross them (mate) with a purebred white pea flower.

7. He cross pollinated them by using a brush to move the pollen.
He called the purple & white flower he was to cross the Parent Generation or P1
He cross pollinated them by using a brush to move the pollen. X

8. All the offspring of the cross ended up being purple flowers
All the offspring of the cross ended up being purple flowers. He decided to call them the First Generation or F1 X P1 F1

9. So out of curiosity Mendel decided to cross a F1 generation flower with another F1 generation flower. X

10. X After multiple tries, he found out the ratio always came out to…
3 Purple : 1 White F1

11. He called this new generation the Second Generation or F2 generationX
F :1 ratio F1

12. Just to make sure it wasn’t a fluke or mistake, Mendel tried with other pea plant traits…X
All the F1 would be 1 type….
Smooth
Wrinkled X
Yellow Pea
Green Pea X
All the F2’s had a 3:1 ratio
Green Pod
Yellow Pod X
Tall Plant
Short Plant

13. Mendel’s Rules of Heredity “Law of segregation”
After years of work, Mendel came up with 4 rules that geneticists still use today
Mendel’s Rules of Heredity “Law of segregation”
1. For each inherited trait, an individual has two copies of the gene. One from each parent.
(allele)
Allele from Dad
Allele from Mom

14. 2. There are alternative versions of genes.
The flower can exist in either the white or purple form.
Allele from Dad
Allele from Mom
Same allele – black or white version

15. 3. When two different alleles occur, one may be completely expressed while the other may show no observable appearance.
When we crossed the P1 (purple) with the P1 (white) the white was not expressed in the F1 generation. X P1 F1

16. This leads us to 2 terms Mendel invented.
Dominant – the expressed form of the trait.
Recessive – the trait that was not expressed when a dominant trait was present X P1 F1
White could never be expressed as long as a Purple flower allele was present.

17. 4. When gametes form (sperm or egg or
4. When gametes form (sperm or egg or pollen) alleles separate independently “Law of Independent Assortment”
What Mendel was describing was meiosis before there ever was a term called “meiosis” XY
XY+XY X Y

18. - Modern Terms Used in Genetics -
Phenotype – the physical expression of an allele X P1 F1
Purple was always physically expressed

19. Genotype – the alleles that are present in the. individual
Genotype – the alleles that are present in the individual. (genes can’t be seen) X P1 F1
White was not expressed, but it’s allele is present in the F1 generation

20. PP = Pp = pp = Letter are often used in place of alleles…
Capital letters express dominant traits
Lower case letter express recessive traits.
P = purple flower p = white flower
PP =
Pp =
pp =
Purple
Purple
White

21. PP – homozygous dominant
Two more terms we must go over…
Homozygous – where two alleles are exactly the same.
PP – homozygous dominant
pp – homozygous recessive

22. Pp – heterozygous Heterozygous – where the two alleles are different.
- also called Hybrid
Pp – heterozygous

23. VOCABULARY ! Allele Heredity Purebred P1 generation F1 generation
Mendel’s rules of Heredity
Dominant
Recessive
Phenotype
Genotype
Homozygous
Heterozygous
Hybrid