Mendel's Pea experiments.
Genotype.
Alleles.
Inheriting genes.
Homozygotes.
Cross breeding - First generation.
Cross breeding - Second generation.
Even before DNA was invented, Mendel developed the concept of genes.
He was experimenting by breeding pea plants.
We can start with one of the simplest, but most profound experiments.
Some pea plants, that he bred, were pure green pod peas.
Some pea plants, that he bred, were pure yellow pod peas.
When he bred pure green pea plants with each other,
he always got green pea pod offspring.
When he bred pure yellow pea plants with each other,
he always got yellow pea pod offspring.
When he interbred, the green and the yellow pea plants,
he got the surprise, and shock of his life.
The resultant pea plant, where all green pod, pea plants.
With centuries of thinking, which revolved over blending,
he was expecting a sort of greenish blue pea pods.
That is, he was expecting the green and blue colours to blend,
in the offspring.
No wonder, he was surprised and shocked.
He cross bred, the resulting pea plants, again.
This gave him a bigger surprise.
Some of the offspring pea plants, had green colour pea pods.
Some of the offspring, had yellow colour pea pods.
None of them, were a blend of green and yellow.
He repeated these experiments, over and over again.
He was amazed to find, that the results were consistent.
To understand these results, first and foremost,
he had to get rid of his mindset of “blending”.
He carefully and consistently observed, that the yellow colour,
always disappeared in the second generation,
and reappeared in the third generation.
Mendel made a profound conceptual breakthrough.
He said, that a trait is passed down, from the first generation,
to the next generation.
We now call this, as genotype.
This trait, he said, does not physically manifest itself, in the second generation.
We now call this phenomena, as phenotype.
Mendel proposed that there were discrete units of heredity.
We now call them “genes”.
Mendel proposed that, there could be different forms, of a gene.
For example, a gene, that determines colour, of the pea pod,
could have two variants.
One variant gives it a green colour.
Another variant gives it a yellow colour.
We now call these variants, of a gene, as alleles.
Dominant and recessive alleles.
The second profound concept, that Mendel discovered was,
the concept of dominant and recessive alleles.
Dominant alleles express themselves.
Recessive alleles, do not express themselves,
when a dominant allele is present.
However, if the dominant allele is not present,
the recessive allele, can express itself.
In the experiment we discussed, the colour gene, had two alleles.
A green allele,
and a yellow allele.
The green allele was dominant.
The yellow allele was recessive.
Depending on what alleles are expressed, it is a phenotype.
In the first generation phenotype, the green allele was dominant.
This explains how, in the first generation, all the offspring,
had green pods.
We will now see, how this happens, from a genetic perspective.
We will represent the dominant green allele, as capital ‘G’.
We will represent the recessive yellow allele, as lower case ‘g’.
Dominant genes, by convention, is represented in capital letters.
Recessive genes, by convention, is represented in lower case letters.
We know that chromosomes, come as pairs.
Genes also come as pairs.
The offspring, get one copy of each gene, from each parent.
One randomly chosen copy of each gene, from each parent,
is passed on to the offspring.
This is called as the law of segregation.
Example case.
Let us start with the pure bred, pea plants that Mendel started with.
The pure green pea plants, had two capital ‘G’ alleles.
We will call them capital ‘GG’ types.
This is also known as the capital ’GG’ genotype.
The pure yellow pea plants, had two lower case ‘g’ alleles.
We will call them lower case, ‘gg’ genotype.
When two alleles, of a gene, are the same we call them homozygotes.
The capital ‘GG’ genotype is a homozygote.
The lower case ‘gg’ genotype is a homozygote.
When a capital ‘GG’ homozygote, is bred, with another capital ‘GG’ homozygote,
the resulting offspring, inherits one capital ‘G’ allele, from each parent.
This results in a offspring, with capital ‘GG’ homozygote.
This results in green pea pod plants.
When a lower case ‘gg’ homozygote, is bred, with another lower case,
‘gg’ homozygote,
the resulting offspring, inherits one lower case ‘g’ allele from each parent.
This results in a offspring, with lower case ‘gg’ homozygote.
This results in yellow pea pod plants.
Now we will see, what will happen, if we inter breed,
a capital ‘GG’ homozygote,
with a lower case ‘gg’ homozygote.
The resulting offspring ,
will inherit one capital ‘G’ allele from one parent,
and one lower case ‘g’ allele from one parent.
This will result in a capital ‘G’, lower case ‘g’ genotype.
What colour will the pea pod be?
This is where Mendel gave us the insight, that we never had before.
It will have the colour, of the dominant allele.
Since the green allele is dominant,
all the offspring, will have green colour pea pod plants.
We can note the fact, that all the four possibilities,
of crossing a capital ‘GG’ homozygote, with a lower case ‘gg’ homozygote,
will be four instances of capital ‘G’, lower case ‘g’ genotype.
In this case, the genotype, has different alleles.
This kind of genotype is called heterozygous.
All the heterozygous possibilities, in this example,
carry a recessive lower case ‘g’ allele.
But, since all of them also carry a dominant, capital ‘G’ allele,
none of the lower case ‘g’ allele, can express themselves.
All the off spring, have green colour, but all of them are carriers,
of the lower case ‘g’ allele.
Mendel, we recollect, again cross bred, the second generation, pea plants.
Let us examine, what would happen, from a genetic view point.
Each of the second generation pea plants, are heterozygous.
We are now crossing, a capital ‘G’, lower case ‘g’,
with a capital ‘G’, lower case ‘g’.
The offspring pea plants, will inherit, one allele from each parent.
There are four possible offsprings.
1. Capital ‘G’, capital ‘G’.
2. Capital ‘G’ , lower case ‘g’.
3. Lower case ‘g’, capital ‘G’.
4. Lower case ‘g’, lower case ‘g’.
How will each of these genotypes, express themselves?
In other words, what will be their phenotype?
Case-1.
Capital ‘G’ capital ‘G’.
Only dominant alleles are present.
It will express itself as green.
Case-2.
Capital ‘G’ , lower case ‘g’.
Dominant ‘G’ is present.
It will express itself as green.
Case-3.
Lower case ‘g’, capital ‘G’
Dominant ‘G’ is present.
It will express itself as green.
Case-4.
Lower case ‘g’, lower case ‘g’.
Both the alleles are lower case ‘g’.
Both are yellow alleles.
There is no dominant capital ’G’.
It will express itself as yellow.
Three of the pea plants will be green.
One of the pea plant will be yellow.
In other words, green has a 75% probability.
Yellow will have a 25% probability.
In general, a recessive gene, will have lower probability,
of expressing itself.