recessive yellow

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

Parents (P):  TT x tt

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).


Narusaku precious child

Okay call me nerdy but I did the punnett square thing with Naruto dominant for yellow but has a recessive red from his mom and Sakura dominant for pink but has a recessive of yellow from her mom and then I did the thing and my results were precious. Take a look at this!

50% chance for pink
25% chance for yellow
25% chance for orange


Bless you all xD