LECTURE 1: Mendel's Law

Many students fear when they start learning Genetics. Well, the fact is that genetics is one of the most fascinating subjects I have ever seen in my life. In order to understand genetics, you need to understand the basics. Students often fail to understand the difference between genes and alleles, gene locus, and other such terms, which are often repeated in the topic.

My purpose of discussing this chapter is to let you know the following things:

1. Mendel's Profile
2. Why Mendel chose Pea plant?
3. Mendel's Law
4. Why Mendel was lucky?
5. Discussion of important questions that have been asked in various medical entrance tests such as CBSE, AIIMS, CPMT, JIPMER, etc.

 Mendel's Brief Profile:

- Born in Moravia in 1822
- Joined Augustinian Monsatry at Brunn in Austria (now Brno in Czechoslovakia) where he took holy orders
- He went to the university of Vienna where he spent two years studying Natural History and Mathematics before he returned to the monastery in 1853.

Why Mendel Chose Pea Plant?

Well, many books say that when Mendel went to the Augustinian Monastery, he saw a Pea garden in the backyard, and that influenced him to work on that plant, which is absolutely wrong. He went to Vienna to study and he didn't have Facebook to connect with friends :) He spent his two valuable years in studying research papers and that made him realize that Pea plant would be the right choice. What conclusion he made from his research?

- There are several varieties of pea plants
-Pea plants are easy to cultivate
- Reproductive structures of the flower were completely enclosed by the petals, and therefore, self-pollinating
- Artificial cross breeding was possible

What were the seven characters Mendel Chose?

1. Stem height (Tall, Dwarf)
2. Flower color (Violet, White)
3. Flower Position ( Terminal, Axial)
4. Pod Shape (Inflated, Constricted)
5. Pod Color (Green, Yellow)
6. Seed Shape (Round, Wrinkled)
7. Seed Color (Yellow, Green)

Note: When Mendel was working on his experiments, Chromosomes and Genes were not discovered yet. He called gene as factor.

ALLELE: It is the two contrasting characters of a trait. For example the trait is Height and its contrasting characters are Tall and Dwarf. One allele comes from dad and the other comes from mom. The dominant allele always shows in the first generation.

- Mendel used true breeding lines means, the character is pure and will show the same feature in every generation. For example, if you cross purple flower with purple flower for many generations, it will be pure to purple color and it will not show any other color in the offspring.

MONOHYBRID CROSS (Mono = one, hybrid = mixture): Mendel considered one character at a time, and that is why it is called Monohybrid cross. Now the question is how this cross pollination is done? Well, if he considered white flower as female then he removed the anther from the white flower, which is called EMASCULTION, and pollinated the stigma with purple anther. Such crossed flowers are often covered with a polyethylene bag, and the procedure is called BAGGING.

The above picture shows the following steps:

1. Crossed true breeding purple flower (male) and white flower (female) in the parental generation.
2. It is evident from the picture that the first generation flowers (F1) are all purple colored.
3. F1 generation flowers are self-pollinated
4. In the F2 generation, both purple and white flowers are shown. However, out of 4 flowers, 3 flowers are purple colored and one is white colored.


If you see the picture on the right, I have shown alleles ( P, p) on chromosomes.

- Purple flower is having PP alleles and white flower is having pp alleles.

-when we say P/P, it is the genotype of the purple plant and the color purple, which we see by the naked eye is called the phenotype. There are other related terms such as:

Homozygous: P/P, or p/p (individual with two identical alleles)

Heterozygous: P/p (individual with two different alleles)

So, the above picture shows what exactly happens when purple flower is crossed with white flower. Look at the genotype of the parent flower. It is P/P and p/p.

What would be the gamete of the purple flower? Well, if you remember meiosis, which is a reduction division, it forms 4 haploid gametes. In this case only P gamete will be formed from the purple flower and similarly p egg from the female white flower. When they cross, they form P/p in the F1 generation (Since purple is dominant over white flower, P shows the character in the F1 generation).

NOTE: Segregation of gamete takes place during Anaphase I of meiosis.

If you see the picture on the right, the F1 generation (having a genotype P/p) is selfed, it forms one pure purple flower plant, 2 hybrid consisiting of both P/p genotype and 1 white flower with genotyple (p/p).

Therefore, the genotypic ratio is:

P/P (1): P/p (2): p/p (1) = 1:2:1

However, the phenotypic ratio is:

3 purple: 1 white = 3:1


Well, before I finish this section, I would like to discuss one of the interesting things with my students, which I would not reveal at this time. My question is can anyone of you tell me how would you know that a purple flowered plant in the garden is homozygous or heterozygous?

Well, Mendel also had this question in his mind and what he did is quite appreciable. He crossed F1 plant with the homozygous recessive parent.

So, why wait... let's do it:

P/p x p/p

Use Punnel Square and find out how many flowers are purple and how many are white. It would be interesting to note that 50% of the plants bear purple flowers and 50% bears white flowers. If you get this ratio then you can say that the purple flowered plant in the garden is having Pp genotype, which is heterozygous. 

Note: What we did, we crossed, F1 plant with homozygous recessive parent -- this is called TEST CROSS because we wanted to know the genotype of the purple flowered plant. 

When will you get 100% purple flowered plant? Think about it and you will surely get the answer. 


This finishes the Mendel's first Law. We'll continue our discussion of the Mendel's Second law in the next post.