1. MENDELIAN GENETICS

2. GARDEN PEAS Advantages
seeds easy
to obtain
characters easy
to score
crosses easily
controlled
short generation
time
large numbers
of progeny
use of statistics
in analysis

3. MENDEL'S POSTULATES Unit factors (genes) exist in pairs (alleles)
Dominance/Recessiveness
Alleles segregate into gametes in equal
frequencies
Alleles from different gene pairs assort independently into gametes

4. pure (true) breeding line
- a group of genetically identical individuals
that always produce offspring of the same
phenotype when mated to each other

5. F1 generation
- first filial generation; the progeny resulting
from the first cross in a series
F2 generation
- second filial generation; the progeny resulting
from a cross of the F1 generation
monohybrid cross
- a genetic cross between two individuals involving
only one character (e.g. AA x aa) in which the parents
possess different alleles of the character

6. MONOHYBRID CROSS: PARENTAL AND F1
Parents
R_ = Round
R is dominant
rr = wrinkled
r is recessive
F1 Gametes either R or r produced in equal frequencies
LAW OF EQUAL SEGREGATION
(pure breeding)
Gametes
F1 seeds
Gametes

7. F1 all Round (selfed or crossed to each other)
CONCEPT OF DOMINANT
AND RECESSIVE ALLELES
Round x wrinkled
F1 all Round (selfed or crossed to each other)
F2 3/4 Round; 1/4 wrinkled
Mendel concluded that one phenotype of the pea shape
character (wrinkled) is hidden in the F1 and reappears
in the F2.

8. F1 Seed Parent Rr Pollen Parent Rr X 1/2 R 1/2 r 1/2 R 1/2 r
1/4 RR /4 Rr /4 Rr /4 rr
1/2 Rr
3/4 Round
1/4 wrinkled
Genotypic Ratio = 1: 2: Phenotypic Ratio = 3: 1

9. Mendel’s First Law - Law of Equal Segregation
Mendel explained his data by invoking the concept of genes:
- each pea plant has a pair of alleles for one character;
one allele is dominant and the other is recessive
Mendel’s First Law - Law of Equal Segregation
- different alleles of one gene segregate into different
gametes in equal frequencies
(i.e. each gamete carries only one allele of a gene)
- the union of gametes to make a zygote is random
(it doesn’t matter which allele is in each gamete)
Proof: testcross - crossing a homozygous recessive individual to an
individual of unknown genotype to determine the unknown
genotype (e.g. R? x rr)

10. Parents RR x rr
F1 Rr (all round seeds)
F2 3:1 R_ : rr (phenotypic ratio)
Testcross (cross F1 individuals to homozygous recessive)
Rr x rr
Gametes R or r r
1:1 Rr : rr (phenotypic ratio)
If F1 were RR, all progeny from the test cross would be Rr
(phenotypically round)

11. dihybrid cross
a genetic cross involving two phenotypic
characters in which the parents possess
different alleles of each character
(e.g. round, green x wrinkled, yellow peas)

12. Mendel’s Second Law - Independent Assortment
DIHYBRID CROSS
Parents
(pure
breeding)
R_ Round
rr wrinkled
Y_ Yellow
yy green
Gametes
Mendel’s Second Law - Independent Assortment
- different gene pairs assort independently into gametes
F1 Progeny
F1 Gametes

13. PHENOTYPES OF F2 PROGENY IN DIHYBRID CROSS
Phenotypic Ratio Phenotypes Alleles Present
9 round, yellow R_Y_
3 round, green R_yy
3 wrinkled, yellow rrY_
1 wrinkled, green rryy

14. Punnett Square

15. The 9:3:3:1 phenotypic ratio observed in the
F2 is created by the random superimposition
of two independent 3:1 phenotypic ratios.
9/16 R_Y_
3/16 R_yy
3/16 rrY_
1/16 rryy
3/4 R_ 3/4 Y_
1/4 rr 1/4 yy
F1 cross: RrYy x RrYy

16. MENDEL IGNORED FROM
Mendel was an unknown researcher
Darwin’s Origin of Species (1859)
- continuous not discontinuous variation
Cytology of time could not explain hypothesis
- chromosomal theory of heredity developed by Theodore Boveri and Walter Sutton in early 1900's clearly showed link between chromosome segregation during meiosis and Mendel's unit factors (genes)
Unable to replicate results in all organisms
MENDEL REDISCOVERED IN 1900
by Carl Correns, Hugo DeVries, Eric Von Tschermak

17. Using branch (forked) diagrams to determine expected
phenotypic ratios from a cross
Parental cross RrYy x Rryy
(R and Y are dominant alleles)
1/2 Y_ 3/8 R_Y_
R_ 3/4
1/2 yy 3/8 R_yy
1/2 Y_ 1/8 rrY_
rr 1/4
1/2 yy 1/8 rryy
calculate frequencies for individual genes separately
* because events are independent multiply probabilities

18. In the cross Aa bb CC Dd Ee x Aa Bb Cc dd Ee,
what proportion of the progeny will have the genotype
AA bb CC dd EE?
calculate frequencies for individual genes
Aa x Aa 1/4 of the progeny will be AA
bb x Bb 1/2 of the progeny will be bb
CC x Cc 1/2 of the progeny will be CC
Dd x dd 1/2 of the progeny will be dd
Ee x Ee 1/4 of the progeny will be EE
* because events are independent multiply probabilities
Probability of AA bb CC dd EE individuals
= 1/4 x 1/2 x 1/2 x 1/2 x 1/4 = 1/128

19. In the cross AaBbCc x AaBbCC the expected number of
gametes = 2 x 2 x 2 = 8 and 2 x 2 x 1 = 4
genotypes = 3 x 3 x 2 = 18
phenotypes = 2 x 2 x 1 = 4
Assumes complete dominance
and recessiveness for all
gene pairs and independent assortment.
Don't blindly use 2n and 3n
rules described in text; they
can’t be used in situations
such as the one above