Basics of genetics: 

In the previous article, we have gone through the basic terminology of genetics. In this article, we will cover some basic concepts one needs to have in mind before proceeding towards classical Mendelian genetics and even post Mendelian genetics. Make sure to practice the MCQs given at the end.

Cross vs selfing:

In plants, selfing is the fertilization between two plants having the same genotype or the fertilization between flowers of the same plant whereas crossing refers to the fertilization between two plants having different genotypes. 
Examples:
Note that Mendel used to obtain F1 generation by cross-fertilization whereas F2, F3 and so on by self-fertilization.
Mendelian genetics:
Mendel was the pioneer of genetics and is known as the father of modern genetics. Mendel experimented for about 7 years on pea plants interpreting and analyzing the inheritance of seven pairs of traits in them.
Mendel chose the garden pea plant, Pisum satium for his experimentation because:
  1. It had a short life span.
  2. It was easy to grow in the garden and in the fields.
  3. It had self-pollinating flowers.
  4.  Cross-pollination was also possible.
Mendel studied seven pairs of contrasting traits in garden pea: The seven pairs of traits studied by Mendel while experimenting on pea plants are :
  1. Pea shape (round or wrinkled)
  2. Pea color (green or yellow)
  3. Pod shape (constricted or inflated)
  4. Pod color (green or yellow)
  5. Flower color (purple or white)
  6. Plant size (tall or dwarf)
  7. Position of flowers (axial or terminal)
Note that there were seven characters each having two contrasting traits. So overall the traits studied by Mendel in pea were fourteen.

How to represent genotype: We know that genotype is the genetic makeup of an organism. It is represented by alphabetical letters.

For example, If a plant is tall genotypically it is represented by the letter TT or Tt. We write the symbol twice because in diploid organisms each gene has two copies so in the above case the TT or Tt is the genotypic representation of the tallness of the plant.
In gametes, we write a single symbol such as T or t because after meioses each gamete ends up with a single copy of each gene. So gametes of the above genotype TT are T and T and for Tt, they are T and t.

Learning how to make gametes for crosses:

We know that in genetics one is only able to interpret crosses theoretically when he knows how to make gametes. So learning how to make gametes is the primary requirement for understanding the crosses in genetics. There is a simple way of doing it described below.
Simple trick: There is a simple method named as fork line method used to make gametes for crosses. You only need to have two things in mind.
  1. Use a straight line for the homozygous genotype.
  2.  Use fork line for heterozygous genotype.
In a homozygous state, only one type of gamete is formed that in the given example is T. In a heterozygous condition two different gametes are formed that in the given context is T and t.
Examples: A monohybrid cross is one in which only one character is considered. For such crosses some examples are:
    In a dihybrid cross, two characters are considered. Gametes for such crosses are made like this:

A simple trick to find types of gametes from a given genotype:

In MDCAT you will come across such types of questions. You will be given a genotype from which you have to find out the number of different types of gametes.
The trick is 2^n
where n is the no of heterozygous pairs.
Example:
Q1. Find the no of different gametes possible from genotype AaBbCC.
Ans: As there are two pairs of heterozygous genes so n is equal to 2
so 2^2 is equal to 4
so from the above genotype 4 gametes are possible.
Q2. Find the no of gametes from AaBbZzQq
since n is 4 in this case so 2^4 is equal to 16
so 16 types of gametes are possible
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