1. Heredity
The Experiments of Gregor Mendel

2. Genetics is the scientific study of heredity
Study of patterns of inheritance & variations in organisms

3. History of Genetics Gregor Mendel Austrian monk
Notorious for his work with pea plants
He was a priest
He studied science & math at University of Vienna
He spent 14 years working in the monastery & teaching high school
He was in charge of the monastery garden

4. Gregor Mendel’s Experiment
Fertilization produces a new cell
Develops into tiny embryo encased in seed
Pea plants are self-pollinating
Pollen contains the male gametes and will fertilize the eggs of the same plant
Seeds produced by self-pollination inherit all characteristics from the single plant
Only have 1 parent

5. Gregor Mendel’s Experiment
Mendel’s garden had many pea plants:
Peas were true-breeding:
If allowed to self-pollinate, would produce offspring identical to themselves
Mendel wanted to produce seeds with 2 “parents”
By joining male & female reproductive cells from 2 different plants
Needed to prevent self-pollination

6. Gregor Mendel’s Experiment
To prevent plants from self-pollinating:
He cut off pollen-bearing male parts
Then dusted pollen from another plant onto the flower
This process is known as cross-pollination
It produces seeds that have 2 different “parents”

7. Mendel Cross Pollinated

8. Gregor Mendel’s Experiments
Mendel studied 7 different plant traits
Trait: specific characteristic that varies from 1 individual to another
Each trait had 2 contrasting characters (p.264 in text)
Mendel crossed plants with each of the 7 contrasting characters
studied their offspring

10. Gregor Mendel’s Experiments
P (parental) generation
original pair of plants
F1 (first filial) generation
offspring
FYI: Filius & filia: Latin words for “son” & “daughter”
Hybrids
offspring of crosses between parents with different traits

12. Gregor Mendel’s Experiments
After crossing:
all offspring had character of only 1 parent
Character from other parent “disappeared”
Mendel came up with 2 conclusions

13. Mendel’s Conclusions
Biological inheritance determined by factors passed from 1 generation to next
Chemical factors that determine traits =genes
Each trait is controlled by 1 gene
Each gene occurred in 2 contrasting forms
The different forms of a gene are alleles

14. Mendel’s Conclusions Law (Principle) of dominance:
Some alleles are dominant, others are recessive
Dominant: the primary or strong allele
Recessive: the secondary or weak allele
In Mendel’s experiments:
Plant height: tall=Dominant, short=Recessive
Seed color: yellow-Dominant, green-Recessive

15. Mendel’s 2nd experiment
Mendel’s question:
“Did the recessive alleles disappear or were they still present in F1 plants?”
Mendel’s experimental design:
allow the F1 plants to produce an F2 generation by self-pollination

16. Mendel’s 2nd experiment
Mendel’s results:
Traits controlled by the recessive alleles appeared!
Approx. ¼ of F2 plants showed traits controlled by recessive alleles

17. Mendel’s Conclusion Law (principle) of Segregation
Alleles for tallness & shortness segregated from each other during formation of gametes
When each F1 plant produces gametes:
2 alleles separate
Each gamete carries single copy of each gene
Each F1 plant produces 2 types of gametes
1 with allele for tallness
1 with allele for shortness
Law (principle) of Segregation

18. Homozygous vs. Heterozygous
Organism with 2 identical alleles for trait
Both chromosomes have same form of gene
Either both dominant or both recessive
Ex. MM or mm
Heterozygous
Organism with 2 different alleles for trait
Each chromosome has a different form of gene
1 dominant & 1 recessive
Ex. Mm

19. An example:

20. Genotype vs. Phenotype Genotype: Genetic makeup
Reveals type of alleles organism has inherited
Represented by a letter:
Capital-dominant
Lowercase-recessive
Examples:
TT = homozygous dominant genotype
tt = homozygous recessive genotype
Tt = heterozygous genotype

21. Genotype vs. Phenotype Phenotype: Physical characteristics
Description of way trait is expressed in organism
The way organism LOOKS
If genotype is:
TT or Tt  phenotype = tall
tt  phenotype = short

22. Phenotype vs. Genotype

23. Mendel’s 3rd experiment
Mendel’s question:
“Does the segregation of one pair of alleles affect the segregation of another pair of alleles?”
“Does the gene for seed shape have anything to do with the seed for seed color?”
Mendel’s experimental design:
He performed a two-factor (dihybrid) cross

24. Mendel’s 3rd experiment
Mendel’s results:
F1: RRYY x rryy (all offspring were round and yellow)
This wasn’t a surprise to Mendel
The real question was: in the gametes of these offspring would the dominant alleles stay together or would the segregate independently?
F2: RrYy x RrYy
315/556 were round & yellow
32/556 were wrinkled & green
209/556 were other combinations

25. Mendel’s Conclusion Law (principle) of Independent Assortment
Segregation of alleles of 1 trait does not
affect segregation of alleles of another trait
Genes on separate chromosomes separate independently during gamete formation (meiosis)
Law (principle) of
Independent Assortment

26. Summary of Mendel’s Principles
Inheritance of biological characteristics is determined by individual units called genes
Genes are passed from parents to offspring

27. Summary of Mendel’s Principles
When 2 or more forms (alleles) of the gene for a single trait exist:
Some forms of gene may be dominant
Others may be recessive
Law (Principle) of Dominance

28. Summary of Mendel’s Principles
In most sexually producing organisms:
Each adult has 2 copies of each gene
1 from each parent
These genes are separated from each other when gametes are formed
Law (Principle) of Segregation

29. Summary of Mendel’s Principles
Alleles for different genes usually segregate independently of one another
Law (Principle) of Independent Assortment