1. Mendelian Genetics

2. Heredity – the passing of traits from
parents to offspring
Genetics - The study of heredity

3. Gregor Mendel “Father of Genetics” Austrian monk who studied
patterns of inheritance in
Pisum sativum, the garden pea.
Used math to
discover patterns of inheritance
and developed genetic rules to
explain them.

4. Only 2 traits for each characteristic (Table 1)
Why Study Peas?
Only 2 traits for each characteristic (Table 1)
Characteristic
Contrasting Traits
Alleles
Dom vs. Rec
Dom vs. Rec
P p
A a
Y y
R r
I i
G g
T t

5. Genotype PP (homozygous dominant) Pp (heterozygous)
The genetic make-up of an organism
Examples:
PP (homozygous dominant)
Pp (heterozygous)
pp (homozygous recessive)
We’re the SAME
We’re the SAME
Homo = “Same”
Hetero = “Different”

6. Phenotype The external appearance of an organism Genotype Phenotype
PP (homozygous dominant) = Purple
Pp (heterozygous) = Purple
pp (homozygous recessive) = White
Curly
Straight

7. A cross involving 1 characteristic or gene.
Mendel’s Experiments
Monohybrid cross
A cross involving 1 characteristic or gene.

8. Predicting the Outcomes of Monohybrid Crosses:
Punnett Square
A chart devised by Reginald Punnett in 1905
Used to simulate a cross &
predict the likelihood of all possible outcomes.
Sperm
Eggs

9. Steps For Using a Punnett Square:
Write the genotypes of the individuals to be crossed.
Ex #1: Perform a P1 cross for plant height:
TT X tt

10. Steps For Using a Punnett Square:
Draw the Punnett square below them.
TT X tt

11. Steps For Using a Punnett Square:
Simulate meiosis by writing the types of sperm the male can produce above the square and the types of eggs the female can produce to the left. T
Eggs t
Sperm
Law of Segregation X T T t t

12. Steps For Using a Punnett Square:
Simulate fertilization by filling in the boxes of the square
TT X tt T
Eggs t
Sperm
Genotypic ratio of F1:
TT : Tt : tt
0 : 4 : 0
100% Tt
Phenotypic ratio of F1:
Tall : short
4 : 0
100% Dominant Tt Tt F1 Tt Tt

13. Mendel’s Conclusions:
Organisms inherit 2 copies of a gene (one from each parent)
2. Genes exist in alternative forms (P vs. p).
* Today the different versions of a gene are called alleles.
When an organism inherits alleles that are different hybrid, one may totally mask or hide the other.
* Mendel described the trait that was expressed in the F1 the dominant trait - capable of masking its contrasting trait.
* He called the trait that was lost in the F1 only to reappear in the F2 the recessive trait - the trait that is masked.
4. When gametes are formed, the alleles for each gene separate independently (Law of Segregation)

14. Mendel’s findings were published before chromosomes were discovered, so scientists couldn’t explain them.
Walter Sutton (1900)
Proposed the Chromosome theory
States that genes are located on chromosomes. p P
Chromosomes occur in pairs, therefore, genes
also occur in pairs – one allele per homologue

15. Simulate an F1 cross: Ex #2: Given: B = Black fur b = brown fur
(heterozygous X heterozygous)
Bb X Bb
Genotypic ratio of F2:
BB : Bb : bb
: : 1
Phenotypic ratio of F2:
Black : Brown
3 : B b BB Bb B Bb bb b

16. How can we find out which one?
Dwarfism in dogs is caused by an undesirable recessive allele:
D = Normal height d= dwarfism
Normal and dwarf German Shepherd litter mates.
What is the genotype of a dwarf dog? dd
What is the genotype of a normal dog?
DD or Dd
How can we find out which one?

17. Perform a test cross
Procedure used to determine the genotype of an individual with a dominant phenotype.
Mate the unknown with a homozygous recessive individual.
Enables us to determine the genotype of unknown individual.

18. Test Cross
If we have a normal dog, we can breed it with a dwarf to determine if it carries a hidden dwarf allele or
DD X dd
Dd X dd d d d d D D Dd Dd Dd Dd D Dd Dd d dd dd
If unknown is heterozygous, offspring will be
50% normal, 50% dwarfs.
If unknown is homozygous dominant, offspring will be 100% normal.