Chromosomal basis of Mendel's Laws

Explanation of Mendelian genetics

Mendel did not know what a gamete was, nor did he know how meiosis or sexual reproduction worked. We, with our knowledge, can understand their discoveries and the transmission mechanism of inheritance.

Mendel's 1st Law

When two purebred individuals that differ in one character are crossed, all the descendants of the first filial generation (F₁) are equal to each other, both in genotype and phenotype.

Mendel called hereditary factors what we call alleles of a gene located on homologous chromosomes. The purebreds are individuals homozygous  for a character, and hybrids, heterozygous.

According to Mendel´s First Law, if a homozygous dominant individual is crossed with another homozygous recessive individual, individuals heterozygous for that character will always be obtained .

Let's see what happens when the two gametes unite in fertilization:

Parental (P)	Green		Yellow
		x
	aa		AA
Gametes
	a		A
1st Generation subsidiary (F1)
ZIGOTS		100% yellow
		Aa

Individuals dominates homozygotes have two identical alleles, one from the father and one mother. All gametes that are formed will have the A allele.

The homozygous recessive also have two identical alleles, but in this case, contain the allele. All of your gametes will carry the allele a.

When a dominant homozygous individual is crossed with another recessive one, the gametes will contribute the A and a alleles, respectively, obtaining zygotes with Aa alleles.

Interactive activity: Genes and alleles.

Mendel's 2nd Law

When two individuals of the first filial generation (F₁) are crossed, obtained from the crossing of individuals of two pure races that differ in one character, a second filial generation (F₂) appears formed by two types of phenotypes, which are equal to those of the parents from whom the first filial generation emerged (F₁).

Each of the two alleles is on a different chromosome from the same pair, so after meiosis, only one will go in each gamete.

The Second Law of Mendel explains:

F₁ phenotypes:   Green color.

Genotype:          Aa

Gametes:           A                  a

As the crossing occurs between two equal individuals, they will also produce the same types of gametes that, after fertilization, will give rise to the following types of zygotes:

Gametes	TO	to
TO	AA	Aa
to	Aa	aa

Gametes can combine to produce up to four different types of zygotes, although two of them have the same alleles.

The second generation F₂ would be made up of:

• 1/4 of AA individuals (green phenotype).
• 2/4 of Aa individuals (green phenotype).
• 1/4 of aa individuals (yellow phenotype).

Therefore, phenotypically, 75% would be green and 25% yellow.

Mendel's 3rd Law

If two individuals who differ in more than one character are crossed, these characters are transmitted independently of the rest.

The experiment is the same as with a character, but instead of observing a pair of chromosomes, we will have to do it with two pairs of chromosomes, such as a pair of chromosomes with the alleles that determine the color of the seed, and another pair of chromosomes with the alleles that determine the shape of said seed.

A = yellow seed a = green seed    

B = smooth seed                                   b = rough seed.

F₀                     AABB             *           aabb

Gametes           AB                                  ab

F ₁                                             AaBb

All individuals in the first generation will be diheterozygous (heterozygous for both characters). Its phenotype will be yellow and smooth seed, just like one of its parents.

By crossing the F₁ individuals with each other, they will produce the following types of gametes:

F₁                          AaBb             *          AaBb

                         A   a    B    b             A    a    B    b

Gametes:     AB   Ab    aB    ab        AB   Ab    aB    ab

All the gametes of these individuals have the same possibility of forming, which is why the following zygotes can be obtained after fertilization:

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

In this way, the following phenotypic proportions are obtained:

9/16 A_B_     3/16 A_bb      3/16 aaB_ 1/16 aabb

(9 smooth yellow seeds, 3 rough yellow seeds, 3 smooth green seeds, 1 green rough seed) .

If we analyze the results obtained in the Punnett square, we will observe that there are 16 different possible zygotes, although they only give rise to 9 different genotypes, and these 9 genotypes only give rise to 4 different phenotypes:

16 types of zygotes	9 genotypes	4 phenotypes	Proportion	TOTAL
AABB	AABB	plain green	1/16	9/16
AABb	AABb	plain green	2/16
AABb
AaBB	AaBB	plain green	2/16
AaBB
AaBb	AaBb	plain green	4/16
AaBb
AaBb
AaBb
AAbb	AAbb	rough green	1/16	3/16
Aabb	Aabb	rough green	2/16
Aabb
aaBB	aaBB	yellow-smooth	1/16	3/16
aaBb	aaBb	yellow-smooth	2/16
aaBb
aabb	aabb	yellow-rough	1/16	1/16

That is, as the alleles go on different chromosomes, they separate in meiosis and combine in all possible ways, thus new phenotypes appear, which did not exist before.

Animation : Mendel's three laws.