Mendelian Genetics
Gregor Mendel (1822-1884)
Responsible for the laws
governing Inheritance of
Traits
Gregor Johann Mendel
• Austrian monk• Studied the inheritance
of traits in pea plants• Developed the laws of
inheritance• Mendel’s work was not
recognized until the turn of the 20th century
Gregor Johann Mendel• Between 1856 and
1863 Mendel cultivated and tested some 28,000 pea plants
• He found that the plants’ offspring retained traits of the parents
• Mendel was called the “Father of Genetics”
Site of Gregor Mendel’s experimental garden in the Czech Republic
Particulate Inheritance
- Mendel stated that physical traits are inherited as “particles”
- Mendel did not know that these “particles” were actually genes on chromosomes.
- Remember genes are segments of DNA that code for a particular protein!
Genetic Terminology
Trait- any characteristic that can be passed from parent to offspring
Heredity- passing of traits from parent to offspring
Genetics- study of heredity
Types of Genetic Crosses
Monohybrid cross- cross involving a single trait
Example: Flower color
Dihybrid cross- cross involving two traits
Example: flower color and plant height
Punnett Squares
Used to help solve genetics problems
Designer “Genes”
Alleles- two forms of a gene (dominant and recessive)
Dominant- stronger of the two genes and expressed in a hybrid (represented by a capital letter)
Recessive- shows up less often in a cross and is hidden when a dominant gene is present (represented by a lower case letter)
More Terminology
Genotype- gene combination for a trait
(Example: RR, Rr, or rr)
Phenotype- the physical feature resulting from a genotype
(Example: Red or white)
Genotype and Phenotype in FlowersGenotypes of alleles:
R = red flowerr = yellow flower
All genes occur in pairs, so 2 alleles affect a characteristic
Possible combinations are:Genotypes: RR Rr rrPhenotypes: RED RED
YELLOW
GenotypesHomozygous genotype:
gene combination involving 2 dominant or 2 recessive alleles (RR or rr); also called purebred
Heterozygous genotype: gene combination of one dominant and one recessive allele (Rr); also called hybrid
Mendel’s Pea Plant Experiments
Why peas, Pisum sativum?
• Peas can be grown in a small area
• Produce lots of offspring
• Produce pure plants when allowed to self-pollinate
• Can be artificially cross-pollinated
Reproduction in Flowering Plants
Pollen contains sperm produced by the stamen
Ovary contains eggs found inside the flower
Mendel’s Experimental Methods
- Mendel hand-pollinated flowers using a paintbrush
- He snipped the stamens (male parts) to prevent self-pollination
- Covered each flower with a cloth bag
Generation “Gap”P1 Generation- the parental
generation in a breeding experiment
F1 Generation- the first generation of offspring in a breeding experiment
F2 Generation- the second generation of offspring in a breeding experiment (from breeding individuals from the F1
generation)
Following the Generations
Cross 2 pure plants TT and ss
Results in all hybrids (Ts)
Cross two hybrids and get 3 tall and one short (TT, Ts, and ss)
Mendel’s LawsLaw of Segregation-
during the formation of gametes (sex cells; aka eggs or sperm), the two alleles responsible for a trait separate from each other
Alleles for a trait are then “recombined” at fertilization to produce the genotype of the offspring
bb
b b
Law of Segregation
PRACTICE
1. A hybrid male yellow canary (Yy)
Yy
2. A pure female white flower (rr)
rr
If a male red flower (Rr) and a pure white flower (rr) from above got their gametes
together (fertilization)
what kind of combinations could be formed?
X
• Male red flower (Rr)• Female white flower
Rr
Sperm 1 Sperm 2
rr
Egg 1 Egg 2
+
+
+
+
=
=
=
=
1 1
1 2
2 1
2 2
Genotype Phenotype
Law of Dominance
In a cross of parents that are pure for different traits, only one form of the trait will appear in the next generation
All the offspring will be heterozygous and express only the dominant trait.
GG x gg = all Gg
Law of Independent Assortment
Alleles for different traits are distributed to sex cells independently of one another.
This law can be illustrated using dihybrid crosses.
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