1. the Law of Dominance
2. the Law of Segregation
3. the Law of Independent Assortment
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.
Mendel crossed many different combinations of pea plants
When pure tall plants crossed with pure short plants, all the new pea plants (referred to as the F1 generation) were tall.
Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants.
Instead of creating medium height plants or yellowy-green seeds that might have been expected, one trait came out as dominant
Ie there is a gene that codes for height. One allele (form of the gene) codes tall and another short. In this case, the tall is dominant
The dominant is represented with a capital letter (eg T for tall) while the recessive is lower case (t)
The cross Mendel performed was
where T = the dominant allele for tall stems
& t = recessive allele for short stems
The punnet square looks like:
A plant that contains the dominant T will be tall, explaining why 100% of the plants he crossed came out tall.
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.
Now, Mendel decides to cross the offspring from the above experiment - all Tt
Two of the “F1” generation (tall) are crossed
Would assume to get all tall again as tall is dominant
HOWEVER some come out short
“F2″ generation is about ¾ tall & ¼ short
Parent plants for this cross each have one tall factor that dominates the short factor & causes them to grow tall.
To get short plants from these parents, the tall & short factors must separate (allowing the possibility of 2 short factors coming together without a dominant tall) otherwise a plant with just short factors couldn’t be produced
The factors must SEGREGATE themselves somewhere between the production of sex cells & fertilization
Two hybrid parents, Tt x Tt.
The punnet square would look like this:
This splitting happens during meiosis.
The Law of Independent Assortment
Alleles for different traits are distributed to sex cells (& offspring) independently of one another.
Previously Mendel addressed one trait at a time.
He noticed that different traits had no effect on each other, eg being tall didn’t automatically mean the plants had to have green pods
The different traits seem to be inherited INDEPENDENTLY.
The genotypes of our parent pea plants will be:
RrGg x RrGg where
"R” = dominant allele for round seeds
“r” = recessive allele for wrinkled seeds
“G” = dominant allele for green pods
“g” = recessive allele for yellow pods
The results from a dihybrid cross are always the same:
9/16 boxes (offspring) 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).
