1. Aim: How were traits discovered?
Mendelian Genetics
Aim: How were traits discovered?

2. What is Genetics?
The branch of biology that studies the ways which hereditary information is transmitted from parents to offspring.
DNA: chemical which carries the biological traits.
Genes: contain large units of DNA & are specifically arranged on chromosomes.
2 genes (alleles) for each trait in an organism.

3. Mendelian Genetics
Gregor Mendel is considered the “Father of Genetics”.
Studied one trait at a time.
After crossing (breeding) pea plants with contrasting traits, he analyzed the (data) resulting new generations & came up with 3 laws:
1. Law of Dominance
2. Law of Segregation
3. Law of Independent Assortment

5. Mendel’s Law of Dominance
In a cross between 2 “pure” contrasting traits, only 1 of these traits is visible in the next generation.
1. The trait that shows up = DOMINANT
2. The trait that does not appear = RECESSIVE
Examples: Tall is dominant over short.
Yellow seeds are dominant over green seeds.

6. Mendel’s Law of Dominance
A letter is used to represent alleles (genes).
1. A capital letter = DOMINANT.
2. A lower case letter = RECESSIVE.
Example: T = tall pea plant (dominant)
t = short pea plant (recessive)

7. Mendel’s Law of Dominance
Genotype refers to the genetic make up of an individual. This is represented by 2 letters for each trait.
1. Pure or homozygous: both alleles for the trait are the same.
Example: TT = pure tall
tt = pure short
2. Hybrid or heterozygous: the 2 alleles for the trait are different.
Example: Tt = hybrid tall

8. Mendel’s Law of Dominance
Phenotype refers to the physical characteristics or traits of an individual.
Example: Tall or short
Genotype
Phenotype TT
Looks tall Tt tt
Looks short

9. Law of Dominance Experiments
Key: T = tall allele Each box represents t = short allele a 25% chance occurrence.
Cross 1 pure tall & 1 pure short
Parent = TT x tt
Offspring = Tt, Tt, Tt, Tt
Genotype: 4 hybrid Tt
Phenotype: 4 hybrid tall plants T t Tt

10. Law of Dominance Experiments
Key: T = tall allele
t = short allele
Cross 2 hybrid tall
Parent = Tt x Tt
Offspring = TT, Tt, Tt, tt
Genotype: 1 TT, 2 Tt, 1 tt
Phenotype: 3 tall, 1 short T t TT Tt tt

11. Mendel’s Law of Segregation
During gamete formation
(meiosis), the 2 genes
(alleles) responsible for
each trait separate from
each other so that each
gamete contains only
1 gene (allele) for each trait.

12. Mendel’s Law of Independent Assortment
Different traits are inherited independently of one another.
1. Dihybrid cross: crossing 2 traits at one time.
2. During meiosis, alleles for different traits are separated & distributed to gametes independently from one another.