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.
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:
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Parental (P) |
Green
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Yellow
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|
 |
x
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 |
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aa
 |
AA
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Gametes
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 |
 |
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a
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A
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1st Generation subsidiary (F1) |
 |
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| ZIGOTS | |||
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Aa
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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.
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:
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Gametes
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TO
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to
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TO
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AA
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Aa
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to
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Aa
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aa
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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:
Therefore, phenotypically, 75% would be green and 25% yellow.
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:
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AB
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Ab
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aB
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ab
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AB
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AABB
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AABb
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AaBB
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AaBb
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Ab
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AABb
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AAbb
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AaBb
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Aabb
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aB
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AaBB
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AaBb
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aaBB
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aaBb
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ab
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AaBb
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Aabb
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aaBb
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aabb
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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:
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16 types of zygotes
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9 genotypes
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4 phenotypes
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Proportion
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TOTAL
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AABB
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AABB
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plain green
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1/16
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9/16
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AABb
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AABb
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plain green
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2/16
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AABb
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||||
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AaBB
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AaBB
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plain green
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2/16
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AaBB
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||||
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AaBb
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AaBb
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plain green
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4/16
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AaBb
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||||
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AaBb
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||||
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AaBb
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||||
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AAbb
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AAbb
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rough green
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1/16
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3/16
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Aabb
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Aabb
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rough green
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2/16
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Aabb
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||||
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aaBB
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aaBB
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yellow-smooth
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1/16
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3/16
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aaBb
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aaBb
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yellow-smooth
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2/16
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aaBb
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||||
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aabb
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aabb
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yellow-rough
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1/16
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1/16
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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.
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