Sunday,26 September 2021, 5 : 34 AM

Fish Genetics-II

Animal Cell
Cell & Chromosome
Image Credit: Wikipedia


This is Second Part of Series of articles on Genetics & Inheritance, for first part click Genetics, Inheritance and Fish -I

Dominant & Recessive Alleles

Normally every gene has two alternative forms for a character which produces different effects.
For example for a character "Colour of eye" there can be two possibilities "Brown" or "Blue".
These alternative forms of a gene are referred as "alleles".
Out of the two alleles, one is Dominant and the other one is recessive.
The dominant allele is denoted by capital letter and the recessive is denoted by small letter.

In our example of eye colour "Brown" colour of eye is "dominant" allele and is expressed as "B", while the "Blue"colour is recessive and is expressed as "b".
Now an individual can have following combination :

  • BB (Both dominant) - Brown colour.
  • Bb ( One dominant, One recessive) - Brown Colour.
  • bb ( Both recessive) - Blue colour.
  • When both alleles are same it is called "homozygous" and when the alleles are different it is called "heterozygous".
In the above example 1 and 3 are homozygous and 2 is heterozygous.
Thus, dominant allele can express both in homozygous (both allele same) and heterozygous (different alleles), whereas recessive allele can only express in homozygous condition.

Cell Division :

Cell division is one of the most fundamental characteristics of life.
This is the method which enables life to perpetuate generation after generation.
This is true for all life forms, whether a simple organism like bacteria or ourselves.
New cells are needed to be produced for growth, repair or replacement.

There are two types of cell division, Mitosis & Meiosis.

  • Mitosis: This is the type of division where two daughter cells are produced by division of one parent cell.
    The most important part of mitosis is that after division same chromosome number is retained.
    This type of division leads to growth and development.

  • Meiosis : This is the type of cell division that produces the sex cells called "gametes".
    It takes place in reproductive organs.
    In case of animals it takes place in "testis"(male) and "ovary"(female) and produces "Semen" and "Ova" respectively.
    In case of flowering plants it takes place in "anthers" and "ovary" and produces pollen grain and ova.
    The most important part of Meiosis is that the number of chromosomes is halved.
    For example in case of humans out of 23 pairs of chromosomes, only single chromosome, i.e. one of each pair is passed on sex cells.
    So the semen and ova carries only one chromosome from each pair of chromosomes.
    During fertilization when sperm and ova combine the chromosome pair is completed and life begins.
    This is how variability is generated since after fertilization the embryo has chromosome pairs in which one chromosome is from male and other from female parent.


  • Each organism is composed of small structures called cells.
  • Each cell has a Nucleus, Cytoplasm and a membrane which encircles everything.
  • Each Nucleus has thread like structures called chromosomes, these are always present in pairs, and their number is constant for an organism.
  • The Chromosome is made up of DNA and histones (which are proteins).
  • DNA has a helix like structure which is wound around histone.
  • There are specific parts of DNA segments which are responsible for a particular characteristic.
    These are known as "genes".
  • As Chromosome occur in pair there are two possible forms of every gene out of which only one (mostly) is expressed.


We have so far discussed the various aspects of cell, life etc.
Now we bring them all together and discuss how life begins and other related aspects.
As discussed above Sperm is produced by Males by process of "Meiosis" and it contains one chromosome from each pair of the chromosomes.
Eggs are produces by females again by process of "Meiosis" and contains other chromosome from each pair of the chromosomes.
Conception takes place when the sperm combines with egg of the female and they combine together to form a cell which has again a pair of chromosome, one from father and other from mother.
It is now clear that the new baby cell has one chromosome from father and one from mother.
Each pair contains information about a particular trait from both mother and father, however one characteristic which is "Dominant" is expressed.
This fertilized cell again divides into two identical cells having same number of chromosomes into 2 cells by the process of "Mitosis" as described earlier.
These 2 cells divide to form 4 cells and the process continues.


Laws of inheritance were first proposed by an Austrian Scientist Gregor Mendel who is also regarded as Father of genetics.
He experimented extensively on peas and propounded following laws:

1. Law of Dominance:-

Out of a pair of contrasting characters present together, only one is able to express itself while the other one remains suppressed.
The one that expresses itself is called "dominant" character and the one which remains unexpressed is called "recessive".
The dominant character will express if both pair are dominant or even if one is dominant, however the recessive character can express itself only when the pair consists of both recessive alleles.

2. Law of Segregation:

When two members of a pair of factors separate during formation of Gametes or sex cells, they do not blend but are separated into different gametes.
These gametes fuse randomly during fertilization.

3. Law of independent Assortment:

When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the other pair.

Punnet´s square:

Punnet square is a simple diagram in which different types of gametes of one parent are put on one side and and those of other are put on other side.
The possible combinations are then written in the squares.

We now take up a example one Male black fish and Female White fish.
The Male black fish has dominant black allele represented by "BB".
The female has recessive white allele represented by "bb".

Now we create a Punnet square for breeding of this fish.


Genetics table

These offspring are referred here as F1 generation, the offspring of F1 generation will be referred as F2 and so on.
We get 4 combinations in first generation (F1)which are all "Black" in colour because it is the dominant trait, the recessive trait cannot express unless both contain recessive trait.
This is a very important observation, we crossed a black fish with a white fish and all offspring (100%) are black in colour.
However the trait for white colour has not disappeared, it is present but it is not able to express itself because it is recessive.
The offspring from above generation are not pure black they contain white gene as recessive trait.

Now let us see what happens when we allow second generation to breed amongst themselves.
Remember all of them have "Bb" allele.
Now we allow the offspring to breed amongst themselves, the result will be as follows


Genetics table2

These offspring are now referred as F2.
Now we get:-

  • One BB- Pure Black
  • Two Bb- Hybrid Black
  • One bb- Pure white

Thus we see that in second generation there is a 25% probability of getting pure white strains, and 75% chance of getting black fish.

Continuing the above example we can build following scenarios:

Scenario 1

Male in original breeding pair(BB) to mate with one of the offspring in generation F1(Bb).

Genetics table3

Now we get

  • Two BB- Pure Black
  • Two Bb- Hybrid Black.

All progenies from this pairing will be black, there will be no white, as it is a recessive trait. As mentioned earlier an recessive trait can appear only when it is not present with dominant trait.

Scenario 2

Female in original breeding pair(bb) to mate with one of the offspring in generation F1(Bb).

Genetics table5

Now we get
  • Two bB- Hybrid Black
  • Two bb- Pure white.
The recessive trait is now able to express itself. and we have 50% chance of getting black and 50% chance of getting white progenies.


Genotype is the set of genes present in the cells of organism, in the example used above there are three genotypes "BB", "Bb" and "bb".

Phenotype is the observable characteristic which is primarily genetically controlled, in the example used above there are two phenotypes, Black and white.

In our above examples the Color Black is expressed in two genetic combinations, BB and bB, thus we say the that the Phenotype "Black" has two genotypes "BB" and "bB".
The colour white is expressed in only genetic combination "bb", thus we can say that Phenotype "White" has one genotype "bb".
Please note that these are very simple examples where only one trait is considered, in actual life there are many other factors which will be taken up in subsequent articles.
Moreover it is based on the fact that a particular trait is primarily controlled by a single gene.
Several traits are governed by more than one gene in which case the genetics will be more complicated.