Mendel’s Laws
1. the Law of Dominance
2. the Law of Segregation
3. the Law of Independent Assortment
The 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.
- 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.
The 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.
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).
