You've learnt that in the case of sexual reproduction, the offspring inherits DNA from the male and female parent.
This means that each trait in the offspring can be affected by the maternal and paternal DNA.
How is it decided which trait gets expressed?
The way Mendel figured this out was through conducting experiments with some garden peas!
He chose a sample of garden peas which had a variety of different traits.
Some were tall and some were short.
He then crossed these two plants and observed the characteristics of the offspring.
In the first generation of offsprings between a short and a tall pea plant, there were no short plants.
This means plants in the first generation of offsprings, atleast on surface did not display the short trait.
But does that mean the tall plants did not inherit the short trait?
To answer this question ,Mendel crossed two of the tall plants from the first generation.
Surpisingly, what he observed was the 1/4th (1 in 4)of the offsprings in the second generation were short!
For the sake of explanation, lets say T is the trait of being a tall plant and t is the trait of being a short plant.
Then in the above example the T gene is the dominant trait, while t is the recessive trait.
A dominant gene is a gene which expresses itself more frequently/strongly than other genes.
Mendel pushed his experiment further to see how different trait get expressed across generations.
Say that there are four traits we observe in pea plants. The color can be either Yellow(Y) or green(y) and the seed can be either rounded (R) or wrinkled(r).
If we know that yellow color and roundness of seeds are the dominant genes then we can see the possible combinations of traits in the offsprings.
Below you can see the parent plants. One is yellow and has rounded seeds. The other is green and it has wrinkled seeds.
When these two plants cross they will create an offspring, which will still be yellow and have rounded seeds but now the genetic composition of the offspring will have the green color and wrinkled seeds as latent traits.
Now what happens if this plant self pollinates?
Well because we have 4 different traits, there are 16 distinct combinations that can arise from the combination of these 4 traits.
the table below shows the 16 possible combinations.
In 12 out of the 16 possibilities, the pea plant will have yellow color. Out of these 12, 9 will have round seeds.
Only 1/16 plants will have both the recessive traits of being green in color and having wrinkled seeds.
Traits like height, hair color eye color etc are controlled by genes.
Each gene is present not as a single long sequence of DNA, but as seperate independent pieces- each of which is called a chromosome.
Each offspring cell will this have two copies of each chromosome, one from the female parent and one from the male parent.
When the male and the female gamete combine they form complete pairs of genes, thus ensuring the stability of the DNA in the species.
Genetic variation is an important force in evolution as it allows for natural selection to increase or decrease the frequency of certain genes in the population. The variation and accumulation of certain genes is what allows organisms to evolve and survive in their changing environment.