Mendelian Principles

Key Concepts

• Inheritance
• Monohybrid cross
• Law of dominance
• Law of segregation
• Phenotype and genotype
• Homozygous and heterozygous alleles
• Dihybrid cross
• Law of independent assortment

Introduction: 

Genetics is the field of biology which involves the understanding of how characteristics are transmitted from parents to offspring.  

All organisms reproduce for the continuity of their species. Except for a specific difference, the offspring shows remarkable similarities with their parents. 

Organisms that are produced by asexual reproduction show minute variations. In contrast, offspring produced through sexual mode of reproduction show more significant variations. 

Heredity is defined as the transfer of characters from parents to offspring. This transfer of characters takes place at the time of reproduction. These characters can be our skin color, height, eye color, etc. 

We inherit these characters from both of our parents. This means that half the characters are inherited from our mother and the other half from our father. As a result of this inheritance, similarities and differences exist between different individuals.  

These inheritable characters are called ‘traits.’ These characters are determined by our DNA which comprises genes. Chromosomes carry the DNA and are situated in the nucleus of the cell. 

Explanation: 

Gregor Mendel: 

The journey of genetics began with the discovery of work done by Gregor Johann Mendel. He was an Austrian monk who discovered important facts about heredity.  

Mendel was the first person to predict which traits would be passed from parents to offspring. The study of heredity is called genetics. He laid the basic principles of genetics and is popularly known as ‘The Father of Genetics.’ 

Mendel’s garden peas: 

Between 1856-1863, Mendel conducted hybridization experiments on the garden peas (Pisum sativum). During that period, he chose some distinct characteristics of the peas. Garden peas are self-pollinators, meaning the pollen from a flower pollinates the female sex cells within that same flower. As a result, the seeds that develop carry the traits of that plant. When he wanted to have the gametes of different plants unite, he performed cross-pollination. Cross-pollination occurs between flowers of two plants. The seeds that develop from cross-pollination have traits of two different plants. 

Mendel prevented self-pollination by removing all of the anthers from the flowers of a plant. Then, he performed cross-pollination by manually transferring pollen from the flower of a second plant to the stigma of the antherless plant. By preventing self pollination and manually cross-pollinating pea plants, Mendel selected parent plants that had specific traits and observed the traits that appeared in the offspring. 

Mendel carried out his experiments on inheritance with the pea plant by taking seven contrasting characters of the plant that included: 

1. Stem height. 
2. Pod shape. 
3. Pod color. 
4. Flower position. 
5. Flower color. 
6. Seed color. 
7. Seed shape. 

Mendel selected pea plants for his experiments because pea plants can be grown quickly and maintained easily. They are naturally self-pollinating but can also be cross-pollinated manually. It is an annual plant, therefore, it is possible to study several generations in a short period. It has several contrasting characters. 

Mendel’s monohybrid cross: 

When a tall pea plant is crossed with a short pea plant, hybrids are produced. A hybrid is the offspring of parents that have different forms of a trait, such as tall and short height. The first hybrids that Mendel produced are known as monohybrid crosses. Since Mendel used parent plants that differed from each other by only one trait, the offspring are called monohybrids.  

Mendel carried out the monohybrid cross by crossing a tall pea plant with a short pea plant. The results of Mendel’s experiment were very interesting.  

Mendel cross-pollinated a six-foot-tall pea plant with a pea plant that was less than two feet. After he planted seeds from this cross, all the offspring grew tall like the taller parent plant. Not even a single plant was short. The short trait disappeared completely. 

After that, Mendel allowed the offspring to self-pollinate. These offspring are known as the ‘first generation.’ When Mendel planted the seeds from the first generation, three-fourths of the plants were like the tall parent plant. At the same time, one-fourth of the plants resembled the short parent plant. The short trait had reappeared.  

The ratio of the tall to short plants in the second generation was 3:1. 

How to identify the generations? 

In genetics, the generations are denoted by abbreviations, which are as follows: 

• P1 generation: The original parents are the P1 generation. P stands for ‘parent.’  
• F1 generation: The offspring are called the F1 generation. The F here stands for ‘filial,’ which means son or daughter.  
• F2 generation: When two F1 plants are crossed, their offspring are the F2 generation, i.e., the second filial generation. 

Mendel did similar monohybrid crosses with all seven pairs of traits. He used traits such as whether the seed was wrinkled or round, yellow or green. In each case, he found that out of the two, one trait would disappear in the F1 generation. Then the trait would reappear in one-fourth of the F2 plants.  

From these results, Mendel concluded that factors which control the inheritance of characteristics are present in pairs. Today we call these factors genes. Genes exist in pairs, like tall and short, round and wrinkled. The alternative forms of genes are known as alleles. Mendel’s pea plants had two alleles for height. A plant can have a different combination of alleles, such as two alleles for tallness, two alleles for shortness, or one allele for tallness and one allele for shortness. The organism always inherits one allele from the female parent and the other from the male parent. 

The observations from the monohybrid cross gave rise to two of Mendel’s laws of genetics: 

Law of dominance: 

It is also known as Mendel’s first law of inheritance. The law of dominance states that hybrid offspring will only acquire the dominant trait in their phenotype. The alleles that get suppressed are called recessive traits, while the alleles that decide the trait are known as the dominant traits. 

The allele for tall plants is dominant to the allele for short plants. The plants that had both alleles were tall because the tall allele is dominant over the short allele.  

As a result, the plants with two alleles for tallness were tall, and the plants with two alleles for shortness were short. In the F1 generation, each plant had one tall and one short allele. That is why all the offspring were tall. 

Dominant alleles are denoted by capital letters, whereas recessive alleles are denoted by small letters. The dominant allele is always written first. So, while writing down the results, T is used for the dominant allele, i.e., tallness, while t is used for the recessive allele, i.e., shortness. 

The Law of Segregation: 

It is Mendel’s second law of inheritance. As per the law of segregation, every organism has two alleles of each gene in a cell, and when gametes are produced, the alleles separate. Each gamete receives only a single copy of these alleles. During fertilization, these gametes randomly pair to produce four combinations of alleles. As per this law, the offspring will acquire one gene copy from each parent.  

Phenotypes and Genotypes 

By looking at an organism or a plant, we cannot tell what genes it might pass on. In the monohybrid cross, we have seen that sometimes when tall plants are crossed with each other, they produce both tall and short offspring. Whereas, sometimes a short plant and tall plants produce all tall offspring.  

Two organisms can look similar but have different allelic combinations.  

The way an organism looks or behaves is called its phenotype. The phenotype of a tall plant is tall, but the same plant could have an allelic combination of TT or Tt.  

Genotype is known as the allele combination of an organism. The genotype cannot always be determined even if we know the phenotype. 

Homozygous and heterozygous alleles: 

An organism is said to be homozygous for a trait if the two alleles for the trait are identical. They can have either dominant or recessive pairs of alleles.  

A plant having both alleles for tallness (TT) would be homozygous for the trait of height. As tallness is dominant, a TT plant is homozygous dominant for height. A short plant will always have two alleles for shortness (tt). Therefore, a short plant would be homozygous recessive for height. 

Mendel’s dihybrid cross: 

In a dihybrid cross, Mendel used pea plants that differed from each other in two traits. A cross that involves two different traits is called a dihybrid cross. 

Mendel was curious whether, in a dihybrid cross, the two different traits would stay together or would they be passed on independently of each other in the next generation.  

In this experiment, Mendel observed the traits for seed color and seed shape. He cross-pollinated true-breeding round, yellow seeds (RRYY ) with true-breeding wrinkled, green seeds (rryy). This created a dihybrid cross. Mendel discovered that all F1 plants produced round yellow seeds which was not surprising since round and yellow are dominant traits. 

When the F1 plants self-pollinated and produced offspring, there were plants with round yellow seeds and plants with wrinkled green seeds. Mendel also found plants producing two other seed types, round green and wrinkled yellow.  

Mendel found out that the ratio of phenotypes of the F2 generation was—9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. 

Law of independent assortment: 

With the observations of the dihybrid cross, Mendel stated the law of independent assortment, the third law of inheritance. This law states that genes for different traits are inherited independently of each other.  In a dihybrid cross, we can see that the alleles R and r separate from each other in the F1 generation. This explains the law of segregation. The R and r alleles will also separate from the Y and y alleles. That is the law of independent assortment at work. If the alleles for seed shape and color were inherited together, only two kinds of seeds would have been produced. Instead, four different kinds of seeds were produced. 

Summary:

• Heredity is defined as the transfer of characters from parents to offspring. These characters can be our skin color, height, eye color, etc.
• These characters are also known as traits, and the study of heredity is called genetics. As a result of this inheritance, similarities and differences exist between different individuals.
• Our DNA comprises of genes that are present on the chromosomes that are present in the nucleus of our cells.
• Gregor Johan Mendel discovered the basic principles of genetics and is popularly known as ‘The Father of Genetics.’
• He performed his experiments on garden pea (Pisum sativum). He studied seven different contrasting traits of the pea plant. He used the pea plant because it was an annual plant and easy to maintain.
• He performed his experiments by cross-pollinating true-breeding plants.
• Mendel performed the monohybrid cross by taking only a single pair of contrasting characters (tall x dwarf). From the results of this experiment, he gave the first two laws of inheritance.
• The first law, i.e., the law of dominance, states that an organism with two different allele of a gene will express the allele that is dominant.
• The second law, i.e., the law of segregation, states that alleles are separated so that each gamete carries only a single allele for each gene.
• Phenotype is the external appearance and behavior of an organism, whereas genotype is the genetic makeup.
• An organism is said to be homozygous for a trait when it has identical alleles for the same trait (both recessive or both dominant). Example TT or tt.
• Organisms are said to be heterozygous for a trait when they have two different alleles for one trait. Example Tt.
• A dihybrid cross involves a cross with two different traits. Mendel performed the dihybrid cross because he wanted to know if the two traits were inherited together or independently.
• He later discovered that segregation of alleles for one gene occurs independently of another gene. This is the law of independent assortment.