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Gregor Mendel
Book:
“Experiments in Plant
Hybridization”
Why Mendel was successful…
He studied single traits He used LARGE sample sizes Introduced Ratios
Terms to KNOW Allele: 1 of 2 or more different forms of a gene.
Represented as a letter. 2 alleles make a genotype. Genotype: Genetic Make-up represented as 2
alleles, one from each parent. Ex: “AA, Aa, aa” Phenotype: Physical Characteristic. Ex: Blue eyes. Dominant: Allele that will be expressed.
Represented as a capital letter. Ex: “A” Recessive: Allele that is “hidden.” Represented as
a lower case letter. Ex: “a” Homozygous: 2 of the same alleles
Example: AA or aa
Heterozygous: 2 different alleles Example: Aa
MORE Terms to Know
Monohybrid: Individual that is heterozygous for one trait. Ex: Aa
Monohybrid Cross: Genetic Cross between 2 heterozygotes. Ex: Aa x Aa
First and Second Parental Generations P1 and P2
First and Second Filial Generations (offspring- filial means child in Latin) F1 and F2
Law of Dominance: If a dominant allele is present, the dominant phenotype will be expressed.Model Genotype
symbolGenotype description
Phenotype Type of organism that exhibits genotype
RR Homozygous Dominant
Round Pure bred
rr Homozygous recessive
Wrinkled Pure bred
Rr Heterozygous Round Hybrid
Law of Segregation
During the formation of gametes (in meiosis), alleles of gametes separate (segregate).
Each G allele has separated from the other.
Punnett SquareDiagram showing the possible
combinations of offspring from a genetic cross
r r
R Rr Rr
R Rr Rr
Parents
Offspring
R r
R RR Rr
r Rr rr
r r
R Rr Rr
R Rr Rr
Pisum sativum Pea PlantsP1= round (RR) x wrinkled (rr)
F1 = round (Rr)P2= round (Rr) x round (Rr)
F2 = 5,474 round, 1,750 wrinkled
Types of Ratios1. Genotypic
Homozygous Dominant : Heterozygous: Homozygous Recessive
2. Phenotypic Dominant : Recessive
Example: genotypic ratio ________ ; phenotypic ratio _______
RR RR
Rr Rr
R
R
r
R
2:2:0 or 1:1:0
4:0 or 1:0
Monohybrid Cross Ratios
RR Rr
Rr rr
R
R
r
r
1:2:1Genotypic Ratio:
Phenotypic Ratio:
3:1