1. 6.2 Principles of Inheritance

2. You’ll be able to: To explain the chromosome theory of heredity
To summarize Mendel’s 3 laws

3. Walter S. Sutton Born 1877, Utica, N.Y., U.S. geneticist
1902 Demonstrated Gregor Mendel’s concept of heredity.
In 1903 concluded that chromosomes contain units of heredity

4. Chromosomal Theory of Heredity
Inheritance of traits is controlled by genes
Genes are located on chromosome
Each color represents a different gene

6. Representing Alleles Designated by capital and lower cases pairs
Capital letters = dominant allele
Lowercase letters = recessive alleles
A = dominant
a = recessive

7. Genotype vs. Phenotype Genotype
Describes the genes inherited by an organism.
Represented by two letters
Examples
AA, Aa, aa
Phenotype
An individual's anatomical structure, physiology and behavior.
Refers to everything that can be easily observed and measured about an plant, animal or human being.

8. Homozygous vs. heterozygous
Same alleles
Purebred
Genotypes: AA or aa
Heterozygous
Different alleles
Hybrid
Genotypes: Aa

9. Genotype Symbol Genotype Vocab PhenotypeTT
homozygous DOMINANT or pure tall
tall Tt
heterozygous or hybrid tt
homozygous RECESSIVE  or pure short
short

10. Mendel’s 3 Laws the Law of Dominance the Law of Segregation
the Law of Independent Assortment

11. Law of Dominance
In a cross of parents that are pure for contrasting traits
only one form of the trait will appear in the next generation. 
Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

12. Parent Pea Plants F1 Pea Plants
(Purebred parents with opposite traits)
F1 Pea Plants
(their offspring)
tall stem x short stem
all tall stems
yellow seeds x green seeds
all yellow seeds
green pea pods x yellow pea pods
all green pea pods
round seeds x wrinkled seeds
all round seeds
axial flowers x terminal flowers
all axial flowers
   
All offspring express the dominant allele in their phenotype!!!!!

13. Punnett Square Proof Parents (P): TT x tt
T = the dominant allele for tall stems
t = recessive allele for short stems
Parent Pea Plants  (P Generation)
Offspring  (F1 Generation)
Genotypes: TT x tt
Phenotypes: tall x short
Genotypes: 100% Tt
Phenotypes: 100% tall

14. Law of Segregation During the formation of gametes (eggs or sperm),
the two alleles responsible for a trait separate from each other. 
Alleles for a trait are then "recombined" at fertilization,
producing the genotype for the traits of the offspring.

15. Example
When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait.
In real life this happens during a process of cell division called "MEIOSIS". 
Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm.. 

16. If you cross two hybrid:
3/4 boxes will produce an organism with the dominant trait ("TT", "Tt", & "Tt")
1/4 boxes ends up homozygous recessive ("tt").
Parent Pea Plants  (Two Members of F1 Generation)
Offspring  (F2 Generation)
Genotypes:
Tt x Tt
Phenotypes:
tall x tall
Genotypes: 25% TT 50% Tt 25% tt
75% tall 25% short

17. Law of Independent Assortment
Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

18. The genotypes of our parent pea plants will be:
RrGg x RrGg
"R" = dominant allele for round seeds
"r" = recessive allele for wrinkled seeds
"G" = dominant allele for green pods
“g" = recessive allele for yellow pods

19. RrGg x RrGg Notice that we are dealing with two different traits:
(1) seed texture (round or wrinkled)
(2) pod color (green or yellow). 
Notice also that each parent is hybrid for each trait (one dominant & one recessive allele for each trait).

20. We need to "split" the genotype letters & come up with the possible gametes for each parent. 
Keep in mind that a gamete (sex cell) should get half as many total letters (alleles) as the parent and only one of each letter.
So each gamete should have one "are" and one "gee" for a total of two letters. 

21. There are four possible letter combinations:
RG, Rg, rG, and rg.
These gametes are going "outside" the p-square, above 4 columns & in front of 4 rows. 
We fill things in just like before --- "letters from the left, letters from the top". When we finish each box gets four letters total (two "are's" & two "gees").

22. The results from a dihybrid cross are always the same: RG Rg rG rg
RRGG round
RRGg round
RrGG round
RrGg round
RRgg round
Rrgg round
rrGG wrinkled
rrGr wrinkled
rrGg wrinkled
rrgg wrinkled
The results from a dihybrid cross are always the same:
9/16 boxes show dominant phenotype for both traits (round & green)
3/16 show dominant phenotype for first trait & recessive for second (round & yellow)
3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green)
1/16 show recessive form of both traits (wrinkled & yellow).

23. Results a green pod can have round or wrinkled seeds
a yellow pod can have round or wrinkled seeds
The different traits do not influence the inheritance of each other. 
They are inherited INDEPENDENTLY.

25. INDEPENDENT ASSORTMENT
Summary
LAW
PARENT CROSS
OFFSPRING
DOMINANCE
TT x tt tall x short
100% Tt tall
SEGREGATION
Tt x Tt tall x tall
75% tall 25% short
INDEPENDENT ASSORTMENT
RrGg x RrGg round & green x round & green
9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods