Info > Genetics > Mendel's genetic laws

Goes like so: 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.

The way I figure it, Mendel probably got really bored crossing pure dominant trait pea plants with pure recessive trait pea plants (over & over & over again) & getting nothing but pea plants with the dominant trait as a result. Except for gaining more & more evidence for his Law of Dominance, this probably grew tiresome. So, at one point he takes the offspring of a previous cross & crosses them. Ooooooooh ............

Recall that his original cross for the tall & short pea plants was:

	Parents	F1 Offspring
Genotype(s)	TT x tt	100% Tt
Phenotype(s)	tall x short	100% tall

So, he takes two of the "F1" generation (which are tall) & crosses them. I would think that he is figuring that he's gonna get all tall again (since tall is dominant). But no! Low & behold he gets some short plants from this cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall & 1/4 short. So he says to himself,

"Greg ol' boy, the 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, otherwise a plant with just short factors couldn't be produced. The factors must SEGREGATE themselves somewhere between the production of sex cells & fertilization."

I think it's easier to picture this law by using a p-square. Our cross is two hybrid parents, Tt x Tt.
The punnet square would look like this:

Now, when completing a Punnet Square, we model this "Law of Segregation" every time. 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. Sometimes the term "GAMETOGENESIS" is used instead of meiosis. Scientists love vocabulary (sorry).
You can see from the p-square that any time you cross two hybrids, 3 of the 4 boxes will produce an organism with the dominant trait (in this example "TT", "Tt", & "Tt"), and 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype ("tt" in this example).

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	Phenotypes: 75% tall 25% short

A helpful thing to recognize:
Any time two parents have the same phenotype for a trait but some of their offspring look different with respect to that trait, the parents must be hybrid for that trait.

Ma-Ma-Ma-Ma-Mendel's Third Law

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

OK. So far we've been dealing with one trait at a time. For example, height (tall or short), seed shape (round or wrinkled), pod color (green or yellow), etc. Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another. In other words, being tall didn't automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY.

Please note my emphasis on the word "different". Nine times out of ten, in a question involving two different traits, your answer will be "independent assortment". There is a big ugly punnet square that illustrates this law so I guess we should take a look at it. It involves what's known as a "dihybrid cross", meaning that the parents are hybrid for two different traits.

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

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).

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. 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").

This is what it looks like:

	RG	Rg	rG	rg
RG	RRGG round	RRGg round	RrGG round	RrGg round
Rg	RRGg round	RRgg round	RrGg round	Rrgg round
rG	RrGG round	RrGg round	rrGG wrinkled	rrGr wrinkled
rg	RrGg round	Rrgg round	rrGg wrinkled	rrgg wrinkled

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 (wrinled & yellow).

So, as you can see from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod. The different traits do not influence the inheritance of each other. They are inherited INDEPENDENTLY.

Interesting to note is that if you consider one trait at a time, we get "the usual" 3:1 ratio of a single hybrid cross (like we did for the LAw of Segregation). For example, just compare the color trait in the offspring; 12 green & 4 yellow (3:1 dominant:recessive). Same deal with the seed texture; 12 round & 4 wrinkled (3:1 ratio). The traits are inherited INDEPENDENTLY of eachother --- Mendel's 3rd Law.

Summary:

I would like to summarize Mendel's Laws by listing the cross that illustrates each.

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

There you have them, Mendel's huge contributions to the world of science. A very smart cookie. His work has stood the test of time, even as the discovery & understanding of chromosomes & genes has developed in the 140 years after he published his findings. New discoveries have found "exceptions" to Mendel's basic laws, but none of Mendel's things have been proven to be flat-out wrong.

Hail to the "Father of Genetics" !

Part I Part II Part III