Heredity: Inherited traits, Mendel’s Contribution, and Trait expression.

Keshav Bhatia, Academic Content writer at Edumarz

We are going to cover three topics in this article:

Inherited traits
Mendel’s Contribution and the rules for inheritance
Trait expression

Inherited traits are default traits that you acquire from your parents, they are different from acquired traits in the way that acquired traits are traits that you learn over your lifetime.
An example of an inherited trait is a detached or attached earlobe.

The study of Inheritance in humans is based on the fact that the father and mother of the child contribute practically equal amounts of DNA to the child.
Each trait can be influenced by both Paternal and Maternal DNA.
For each trait, there are two versions in the Child

A. Weissman is the scientist who successfully conducted the mice tail experiment, he cut mice tails in each successive generation surgically, but the mice kept giving birth to tail-bearing progeny.
This experiment disproved the Lamarckian theory of inheritance which said that organisms can pass on traits acquired through use and disuse during their lifetime.
In the beetle example, we see that the generations that lost weight because of starvation did not have underweight progeny, their progeny gained healthy weight when more food became available for them. This means that the change was not inherited over generations.
Any change other than a change in the genes of germ cells of an organism will not pass on.

Inheritance and genetics are key to understanding evolution.

Mendel’s Contribution

Scientist Gregor Mendel did his research on pea plants and was the first one to codify the laws of inheritance.
Mendel crosses pollinated true-breeding lineages of pea plants, a true breeding line shows stable trait inheritance because of the continuous self-pollination it goes through.

His experiments were successful because of the huge sampling size and the confirmation of his inferences from experiments on successive generations of his test plants.
He studied characters that were manifested as two opposing traits.
He set up a basic framework of rules governing inheritance.
He used several contrasting traits of garden peas some of which we’re going to study here:

Wrinkled/Rounded Peas
Tall/Short plants
White/Violet Flowers

Tall/Short plants

Mendel crossed tall and dwarf plants by cross-pollination.
The first hybrid generation(F1) had all Tall plants, none of them was dwarf.
But when he self-pollinated the F1 plants, not all of their progeny F2 generation were tall, 25% or a quarter(1/4th) of these organisms were short.
These traits did not show any mixing.
Mendel concluded that some factors were being passed down from parent to child without being changed.
Mendel observed that in all F1 progenies in experiments observing contrasting features in pea plants that the F1 progeny always resembled one of the parents.
The Law of dominance says that one trait will dominate and express and conceal the other trait.
Like in the pea plants the first generation had all tall plants because the tallness feature was dominant in these plants.
In the second generation, 1/4th of the plants were short because the recessive genes express themselves when the dominant factor is not present. Short plants had both recessive genes.
We have used T as the trait for tallness and t for shortness as T is a dominant trait and t is recessive as per Mendel’s findings.
He made the same observations in all the F2 generations, that there was always a 3:1 ratio of expression of dominant to recessive traits.
He concluded through these experiments that each plant was inheriting one copy of the gene from each parent, and that these gene copies stabilized during gamete fusion. The Law of Segregation states that each offspring inherits a single copy from each parent, resulting in each offspring having a pair for a particular trait.

Wrinkled/Round peas
Mendel observed that only roundness was expressed in the first generation(F1) and concluded that Roundness must be a dominant trait.
He again got the 3:1 ratio in the F2 generation which confirmed his findings, wrinkled seeds made up only 25% or 1/4th of the total seeds.

White/Violet Flowers
Mendel observed that only Violet flowers were there in the first generation(F1) and concluded that violet flowers must be dominant.
He got the 3:1 ratio in the F2 generation again confirming his findings that there are dominant and recessive traits, that F1 progeny always resembles either parent’s characteristic, and those recessive traits are expressed in progeny where dominant genes are not present.

Phenotype is the physical expression of the genes of an organism, while Genotype is the genetic makeup.

All the plants in the F1 generation of the first experiment might be tall but their genotype is Tt that is, they have both the recessive and dominant genes for trait height.

25% of the plants in the F2 generation are short(which is their phenotype) and they have a genotype of tt.

Mendel studied the inheritance of two traits by first crossing Tall plants with round seeds and short plants with Wrinkled Seeds, he observed that the F1 progeny all resembled the traits of one parent, which was the dominant parent.
All F1 progeny were Tall with round seeds
Self Pollinating the F1 progeny resulted in some F2 progeny that was Tall with round seeds, and some were short with wrinkled seeds, but there were new mixtures, some plants were tall but had wrinkled seeds and some were short but had round seeds.
Mendel concluded that the trait for tallness/shortness and the trait for round/wrinkled seeds were being inherited independently of each other; these traits were being inherited independently.

Trait Expression

Cellular DNA is the information source for making proteins in the cell(at the cellular level).
A section of DNA that provides information of one protein is called the gene for that protein.

Let us consider how genes control the tallness trait of a pea plant; genes for plants that are tall might cause more growth by coding for the enzyme that is responsible for tallness.
Genes for tallness will trigger more growth by coding for the normal production of an enzyme that is crucial for plant hormone production.
As plant height depends on hormones this will result in tall plants.
If this gene has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short.
This means that genes control the characteristics or traits of organisms.

This also means:

Both parents contribute equally to the DNA of the progeny.
Both parents contribute a copy of the same gene.
Each pea plant has two sets of genes, one inherited from each parent.
If our experiments are true, each germ cell must have only one gene set.
Germ cells are made by meiosis and have half the number of genes of their parents.
Progeny does not inherit a single whole gene set from each parent, if they did traits would be interlinked and could not be independently inherited.
Traits aren’t interlinked and are independently inherited because the gene set is not present as a single long DNA thread, but as separate independent pieces, each one of which is called a chromosome.

Each cell has two copies of each chromosome, one from the mother and one from the father.
Each Germ cell takes one chromosome from each pair after meiosis, and it can be of paternal or maternal origin.
After gamete fusion, the Chromosome number will be normal again and the DNA stability of the species will be ensured.

Mendel’s experiments confirm these facts about the mechanism of inheritance.
These facts are true in all organisms that reproduce sexually.