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The Law of Dominance

Stated "simply" it goes like so:

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.

While Mendel was crossing (reproducing) his pea plants (over & over & over again), he noticed something interesting. When he crossed pure tall plants 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. The same was true for other pea traits:

Parent Pea Plants	F1 Pea Plants
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

So, what he noticed was that when the parent plants had contrasting forms of a trait (tall vs short, green vs yellow, etc.) the phenotypes of the offspring resembled only one of the parent plants with respect to that trait.

So, he said to himself,

"Greg, there is a factor that makes pea plants tall, and another factor that makes pea plants short. Furthermore Greg ol' boy, when the factors are mixed, the tall factor seems to DOMINATE the short factor".

Now, in our modern wisdom, we use "allele" or "gene" instead of what Mendel called "factors". There is a gene in the DNA of pea plants that controls plant height (makes them either tall or short). One form of the gene (allele) codes for tall, and the other allele for plant height codes for short. For abbreviations, we use the capital "T" for the dominant tall allele, and the lowercase "t" for the recessive short allele.

Let's revisit the three possible genotypes for pea plant height & add some MORE VOCABULARY.

Genotype Symbol	Genotype Vocab	Phenotype
TT	homozygous DOMINANT or pure tall	tall
Tt	heterozygous or hybrid	tall
tt	homozygous RECESSIVE or pure short	short

Note: the only way the recessive trait shows-up in the phenotype is if the geneotype has 2 lowercase letters (i.e. is homozygous recessive).
Also note: hybrids always show the dominant trait in their phenotype (that, by the way, is Mendel's Law of Dominance in a nutshell).

The PUNNETT SQUARE (P-Square for short)

OK, now is as good of time as any to introduce you to a new friend, the Punnett Square. This little thing helps us illustrate the crosses Mendel did, and will assist you in figuring out a multitude of genetics problems.

We will start by using a P-Square to illustrate Mendels Law of Dominance. Recall that he "discovered" this law by crossing a pure tall pea plant & a pure short pea plant. In symbols, that cross looks like this:

Parents (P): TT x tt

where T = the dominant allele for tall stems
& t = recessive allele for short stems

The P-Square for such a cross looks like this:

Inside the 4 boxes are the possible genotypes (with respect to plant height) of the offspring from these parent pea plants. In this case, the only possible genotype is Tt (heterozygous). In hybrids, the dominant trait (whatever the capital letter stands for) is the one that appears in the phenotype, so all the offspring from this cross will have tall stems.

To "fill in the boxes" of the Punnett Square, say to yourself "letter from the left & letter from the top". The "t" from the left is partnered with the "T" from the top to complete each of the four squares.

A summary of this cross would be:

Parent Pea Plants (P Generation		Offspring (F1 Generation)
Genotypes: TT x tt	Phenotypes: tall x short	Genotypes: 100% Tt	Phenotypes: 100% tall

Now, a helpful thing to recognize is this:

ANY TIME TWO PARENT ORGANISMS LOOK DIFFERENT FOR A TRAIT,
AND ALL THEIR OFFSPRING RESEMBLE ONLY ONE OF THE PARENTS,
YOU ARE DEALING WITH MEDEL'S LAW OF DOMINANCE.

All the offspring are heterozygous for the trait, one parent is homozygous dominant, and the other is homozygous recessive.

Does setting up & using the Punnett Square confuse you? Would you like to see a step-by-step "how to" about the good ol' p-square?

If you said "yes", then check this out: "The Punnet Square (in baby steps)".

For some practice Punnett Square problems visit : "P-Square Practice Page".

Don't forget to come back & learn more about Mendel!

Part I Part II Part III