1. Mendelian Inheritance
Classical Transmission Genetics

2. Mendel & The Laws Of Inheritance
Gregor Johann Mendel ( )
Austrian monk
Conducted studies from ,
Quantitative analysis

3. Mendel Used Pea Plants as Experimental Organism
Hybridization
mating (cross) btwn 2 individuals w/ different characteristics of a trait
eg Purple-flowered X white-flowered
Hybrids
Offspring resulting from a mating
Mendel observed no blending
For each trait, offspring had characteristic of one or the other of the parents
eg either purple or white flowers, not both, not light purple

4. Mendel Studied Seven Traits That Bred True
A variety (phenotype) that always produces the same phenotype is termed true-breeding or pure-breeding
Strain; Breed; Agrivar; Variety;
homozygosity

5. Mendel’s Experiments Crossed two variants differing in only one trait
a monohybrid cross

6. X x x

7. MENDEL’S MONOHYBRID CROSS DATA
P Cross
F1 generation
F2 generation
Ratio
Tall X dwarf stem
All tall
787 tall, 277 dwarf
2.84:1
Round X wrinkled seeds
All round
5,474 round, ,850 wrinkled
2.96:1
Yellow X Green seeds
All yellow
6,022 yellow, 2,001 green
3.01:1
Gray X white seed coat
All purple
705 gray, white
3.15:1
Axial X terminal flowers
All axial
651 axial, 207 terminal
3.14:1
Smooth X constricted pods
All smooth
882 smooth, 229 constricted
2.95:1
Green X yellow pods
All green
428 green, 152 yellow
2.82:1
F1 showed only one of the parental traits
F2 showed ~ 3:1 ratio of both parental traits

8. Interpreting the Data
Data suggested a particulate theory of inheritance
Mendel postulated :
There are 2 discrete hereditary factors for a trait, one from each parent
ALLELES
2. Factors in parent may be identical or different
HOMOZYGOUS or HETEROZYGOUS
3. When the two factors of a single trait are different
One is dominant - effect can be seen
The other is recessive - is masked

9. T t
gametes
Dwarf allele masked
Tt x Tt

10. Monohybrid Crosses
P (parental): (tall) x (short) - phenotypes T/T x t/t - genotypes F1 (1st filial): T/t (tall) F1 x F1: T/t x T/t (self or sib cross) F2 (2nd filial): 1 : 2 : 1 - genotypic ratio 3 : 1 - phenotypic ratio
T/T
T/t T/t
t/t

11. Law of Segregation
There are two alleles of each gene in an organism.
Any given offspring receives only 1 allele of each gene
Ergo - the two alleles of a locus are arranged into separate gametes during meiosis and each gamete receives only one or the other allele of any given gene.
Each allele of a locus is placed individually into separate gametes during meiosis

12. Monohybrid test cross test cross
cross to test whether dominant phenotype corresponds to homozygous dominant genotype or heterozygous genotype
T/T vs T/t

13. Dominant phenotype F1 x recessive homozygote
Monohybrid test cross
Dominant phenotype F1 x recessive homozygote
T/T x t/t or T/t x t/t or 1
T/t
T/t & t/t
1 : 1

14. Law of Segregation
Each allele of a locus will be placed into separate gametes during meiosis.
This is true for diploid organisms only

15. Mendel’s Experiments Dihybrid crosses i.e.
Crossing individuals that differ in two traits
i.e.
Trait 1 = Seed texture (round vs. wrinkled)
Trait 2 = Seed color (yellow vs. green)
2 possible inheritance patterns

16. YR/YR Y/Y; R/R yr/yr y/y; r/r YR/yr Y/y; R/r YR/yr yr/yr YR/YR YRyr
F2 generation
yr/yr
YR/yr
YR/YR

18. DATA FROM ONE OF MENDEL’S DIHYBRID CROSSES
P Cross
F1 generation
F2 generation
Round, Yellow peas X
wrinkled, green peas
All
round, yellow
315 round, yellow seeds
101 wrinkled, yellow seeds
108 round, green seeds
32 green, wrinkled seeds

19. Interpreting the Data
F2 phenotype combinations not present in the parentals
Round & Green
Wrinkled & Yellow
These phenotypes = nonparentals
Results
contradict linked assortment hypothesis
support independent assortment hypothesis

20. Law of Independent Assortment
If the genes, assort independently the predicted phenotypic ratio in the F2 generation would be 9:3:3:1
P Cross
F1 generation
F2 generation
Ratio
Round,
Yellow X wrinkled, green
All round, yellow
315 round, yellow
101 wrinkled, yellow
108 round, green
32 green, wrinkled
9.1
2.9
3.1
0.9
Mendel’s data very close to expectations
Law of Independent assortment
Either allele of one locus may go together with either allele of a second, unlinked locus

21. Dihybrid Testcross T/t; Y/y x t/t; y/y T/t; Y/y T/t; y/y t/t; Y/y
If genes assort independently, the expected phenotypic ratio among the offspring is 1:1:1:1

22. Forked-line Method (fork diagram)
Calculate predicted ratios of offspring by multiplying probabilities of independent events
P yellow, round x green, wrinkled
Figure: 03-06
Caption:
Computation of the combined probabilities of each F2 phenotype for two independently inherited characters. The probability of each plant being yellow or green is independent of the probability of it being round or wrinkled.

23. Fork Diagram for Trihybrid Cross
Figure: 03-09
Caption:
Generation of the F2 trihybrid phenotypic ratio using the forked-line method.