Genetic Basis of Inheritance with handwritten notes


We have seen variations in the nature whether it be babies of dogs, cats or humans. It's like we are different from each other but there are some similarities between you and your parents, these similarities are carried by the genes of both the parents and then it has been transferred to you which also is defined as heredity. I have also provided my personal handwritten notes at the end of this post so keep reading :)

Now, this is not as easy as it seems. Actually there was this one person who had this question that how is it inherited and hence he ended up performing various experiments on his own which we will be reading about further.

Before heading to know who was that man who decided to conduct all this experiments and to read about them first let's understand what genetic and inheritance mean.

What is genetics?

Genetics is a branch of biology that studies genes which are responsible for heredity and variations in living organisms. The word was coined by William Bateson in 1906.

What is inheritance?

Inheritance means when genes are transferred to the offspring from their parents and with genes comes traits, characteristics, etc. which are said to be inherited from the parents. 

What is Mendelian Inheritance ?

Gregor Johann Mendel was the one to conduct several experiments to understand how are these variations seen and from where do these come from ? These questions made him very curious. At first he noticed there are some factors being transferred from one generation to the other and they kept moving forward which now are known as genes.

The understanding or experiments he conducted is known as Mendelism and the laws which were derived from these experiments is known as Mendel's law of inheritance.

Mendelian experiment

Now, to experiment he had to select a material. He won't try this with humans obviously because he needed the results in no time so what he did was he carried out these experiments with a garden pea (Pisum sativum) and there are two reasons why he chose garden pea:-
  1. Garden pea has 7 pairs of contrasting traits. 
  2. Garden pea met the requirements of the experiment.
Seven pairs of contrasting traits
Seven pairs of contrasting traits

Characteristic of garden pea: 

Garden pea was a simple plant which met the needs of Mendel and the characteristics are as follows:-
  • Garden pea is an annual plant that means it is available at all the time of the year. It completes is life cycle within 3-4 months so it was easy for him to grow many generations just in a year.
  • It produces many seeds and is a herbaceous plant. 
  • It can conduct self pollination naturally and they are large in size so it was easy for emasculation which were required for artificial cross (this method is used in the experiments we will be reading about ahead). 
 Garden pea plant
Garden pea plant

Procedure of the experiment:

Mendel was smart, instead of trying with all the traits in one go he decided to go with one trait first then with the second then third and so on. These are known as monohybird, dihybrid and trihybrid crosses. 

He started the experiment with pure lines or true breeding plants that means they only produce the offspring of the traits they possessed and Mendel wanted to see. The steps of the experiment are as follows:-

Step 1 - Selection of parents and obtaining pure lines:

Just like we read earlier that at first Mendel only worked with true breeding plants and this the way he selected his parent plants.

Step 2 - Artificial cross of the selected parents to raise F1 generation:

After he was done selecting true breeding plants (female and male) he emasculated the male flower. Emasculation is a technique of removal of stamens from the flower before it matures so once it is in a mature state it does on go under self pollination. After emasculation, he dusted the pollen on the female flower and this is know as the artificial cross

He then waited for the flowers to mature and once they were mature he collected the seeds that were produced and raised hybrids which was his F1 generation

Step 3 - Selfing of F1 hybrids to F2 generation:

Mendel let self-pollination take place in the F1 hybrids, collected the seeds and raised F2 generation ans F3 generation was also obtained by selfing of the F2 hybrids.

Monohybrid and dihybrid cross experiment and ratio:

When Mendel studied one trait at a time that is known as monohybrid cross and when he studied two traits it is known as dihybrid cross and the ratios obtained from both is called the monohybrid or dihybrid ratio.

He conducted two different experiments for both the crosses and he obtained different ratios for both the experiments.

Monohybrid cross:

In monohybrid cross as you know Mendel only studied with one trait. Here he crossed two pure parents and the offspring that he obtained one possessed one pair of character.

Procedure of the experiment:

The steps of the experiment remain the same just as I mentioned above

Step 1 - Selection of parents and obtaining pure lines:

Mendel chose tall pea plant as his female parent and dwarf pea plant as his male parent.

Step 2 - Artificial cross of the selected parents to raise F1 generation:

Once Mendel was sure that the selected parents are pure parents he collected their seeds and sowed and waited for the results. He had expected that the progeny obtained would be of intermediate height or they would be 50% tall and 50% dwarf, but to his surprise they were all tall. He wasn't satisfied with his results and so he moved onto step 3.

Step 3 - Selfing of F1 hybrids to get F2 generation:

Mendel let self pollination take place in the F1 hybrids, again collected the seeds, sowed and waited for the results and the results were the same as it was in the F1 generation. Total seeds he had collected were 1064 from that 787 pea plants were tall and 277 were dwarf.

Mendel thought this tallness in the plants came from female dominance, so this time he performed reciprocal cross means he took tall pea plant as his male parent and dwarf pea plant as his female this time. He again carried out the same process, but there was no change in the result that he had obtained in the F1 generation and in the F2 generation the ratio was 3:1 in both the experiments.

He not only did this with one pair of character he did it with all the seven pairs and got the same result that one of the two traits were seen in the offspring.

Explanation of the monohybrid cross experiment result:

Mendel chose tall pea plants which is represented as "T" and dwarf pea plant which is represented as "t". When gamete formation takes place the two factors separate and only one enters the gamete, thus only one type is expressed. Tall parents (T) would produce all tall offspring and dwarf parents (t) would produce only dwarf offspring.

The progeny obtained in the F1 generation was not due to the female dominance but because the tallness was the dominant one.

 Graphic representation of the monohybrid cross
Graphic representation of the monohybrid cross

The capital "T" will always be the dominant one and the small "t" will be the recessive one. This is represented everywhere the same way so don't get confused which it comes to "T" and "t".

As you can see in the image above, Mendel crossed tall plants (TT) with the dwarf plants (tt) as you now know he obtained tall plants (Tt) in the F1 generation and then he let self pollination occur. In F2 generation he got the same results because of tallness being the dominant one and dwarfness the recessive.

When he obtained tall in F2 generation they were all not pure tall only 1/3rd of the progeny was pure tall and 2/3rd were hybrid tall, so the genotypic ratio is 1:2:1 (pure tall:hybrid tall:dwarf) which means the constitution of genes in an individual and the phenotypic ratio is 3:1.

Dihybrid:

When Mendel worked with two pairs of characters at the same time and they were considered simultaneously this is cross. Dihybrid had the same situation as monohybrid Mendel kept on getting the same pattern with all the pairs he worked with. From this particular cross Mendel derived the principle of independent assortment. 

Procedure of the experiment:

Mendel carried out the same steps for all the experiments he performed in order to take genetics to another level. 

Step 1 - Selection of parents and obtaining pure lines:

Just as you know Mendel worked with two pairs of characters in this experiment and this time he selected yellow and round seeds as his female parents and green and wrinkled seeds as male parents. He confirmed that these selected parents are pure lines by selfing them for three generations.

Step 2 - Artificial cross of the selected parents to raise F1 generation:

When Mendel performed artificial cross he had expected for yellow and round seeds and that's what he obtained in the results. The colour yellow is dominant over the green and the round seeds are dominant over wrinkled.

Step 3 - Selfing of F1 hybrids to get F2 generation:

When Mendel came to step 3 he decided to raise the F1 plants and allowed selfing to take place, collected the seeds and sowed them. He had expected for 75% yellow and round and 25% green and winkled but to his surprise the results were a little different this time because they showed four different types of phenotypes - 315 yellow round, 108 yellow wrinkled, 101 green round and 32 green wrinkled. This get us a total of 556 and the phenotypic ratio here is 9:3:3:1.

From the four phenotypes that Mendel obtained form that two were the parental combinations and the rest two were new/recombination which are yellow wrinkled and green round. Mendel was never satisfied with whatever results or patterns he got so again here he performed reciprocal cross and got the same pattern in F1 and F2 generation.

The dihybrid ratio is a product of two monohybrid ratio - (3:1) ✕ (3:1) = 9:3:3:1.
Yellow round 3✕3 = 9
Yellow wrinkled 3✕1 = 3
Green round 1✕3 = 3
Green wrinkled 1✕1= 1
This gets us a ratio of 9:3:3:1 and this is known as the dihybrid ratio.

Explanation of the dihybrid cross experiment result:

Mendel started with pure lines parents which yellow round as the female parent which is represented as "YYRR" and green wrinkled the male plant which is represented as "yyrr". Both the parents are homozygous meaning they have the same identical pair of alleles of a particular gene or maybe a bunch of genes.

The F1 generation hybrid seeds will be heterozygous meaning they will have two different alleles of a particular gene or bunch of genes and these will produce traits with genotype of "YyRr" (yellow round) and this is due to yellow round being the dominant one.

 Graphic representation of a dihybrid cross
Graphic representation of a dihybrid cross
In F1 generation, the progeny produced by dihybrid will be having one allele from both the pairs because while gamete formation the alleles in both the pair split or segregate and each is received by an individual. For example, one gamete will receive "Y" for colour and "R" or "r" for shape of the seed. This is give us a result of YR or Yr (YYRr - yellow round) type of formation. There can be different variations as well for this such as, "y" is received for colour and "R" or "r" for shape of the seed. Again this will result in yR or yr (yyRr - green round) type of formation.

Applying this, the F1 dihybrids would produce four different types of gametes in equal proportion (25% each). Four male gametes and four female gametes will randomly fuse whilst selfing is taking place. So now there are 8 gametes in totoal 4 male and 4 female so multiply them 4 ✕ 4 = 16, so there would be 16 combinations. These 16 combinations would fall into 9 categories of possible combinations - YYRR, YYRr, YyRR, YyRr, YYrr, Yyrr, yyRR, yyRr and yyrr.

Hence, we get a genotypic ratio of 1:2:2:4:1:2:1:2:1 and a phenotypic ratio of 9:3:3:1. The phenotypic ratio is your dihybrid ratio.

Mendel's law of inheritance:

Based on the crosses above Mendel derived three laws or principles of inheritance, which are as follows:

1. Law of dominance:

This is the first law of inheritance. This law was derived from the monohybrid cross experiment. Here where the character from two pairs of contrasting characters, which is able to express in the F1 generation is called the dominant and the one which is not able to express itself and is being suppressed is called recessive. Due to this dominance there is uniform expression in the F1 generation. 

There factors which control the pairs. When different factor controls each character then only one character is able to express in the F1 generation which is called the dominant one and the other one which is not able to express itself is called the recessive. 

Some examples of law of dominance - In guinea pigs the the black colour coat is dominant over the white one and similarly in humans, curly hair is dominant over straight hair or brown eyes are dominant over green eyes. 

This law is not universally applicable because sometimes the dominance is not complete or is totally absent in some cases.

2. Law of segregation:

This is the second law of inheritance. Here the alleles which are in a pair stay together with mixing with each other. During gamete formation these allelic pairs separate or segregate and each gamete will receive one trait from of each pair of allele. This law is also known as law of purity of gametes. This happens in dihyrbids because in that experiment we are working with two traits so segregation takes place, but you can explain it with the help of dihybrid cross experiment.

This law is universally applicable because all sexually reproducing organisms are diploid (2n) meaning they have two sets of chromosomes and the gametes are haploid (n) meaning having only one set of chromosome. Since this most of the organisms produce sexually this law is applicable universally.

3. Law of independent assortment:

This is known as the third law of inheritance and also the principle of independent assortment. This law or principle is based on dihybrid cross experiment. Here, the traits of both the parents are expressed simultaneously in the progeny as we have read about it above.

Mendel also performed trihybrid cross ratio but since it is not mentioned in your syllabus we won't be going in detail. After he performed trihybrid cross he came to a conclusion that there is indeed random fusion taking place and each gamete will receive only one allele from each pair of traits and they are independent to go anywhere and enter into any gamete they want. They do not tend to carry their parental combinations and are totally independent hence this is known as the law of independent assortment.

Deviation from Mendelian ratios: 

After Mendel, numerous scientist performed experiments with different types of plants and animals with the help of the checker board method and the crosses mentioned above. These scientist modified the experiments conducted by Mendel and his laws took many turns and hence known as Post-Mendelian genetics or Neo-Mendelian genetics.

In these experiments they influenced or you can say modified the phenotypic expression by the other gene. From these experiments many different patterns of inheritance and gene interactions were to be seen. There are two types of gene interactions which are as follows:

  1. Intragenic gene interaction - This type of interaction takes place between alleles of the same genes. Eg, incomplete dominance, co-dominance and multiple alleles
  2. Intergenic gene interaction - This type of interaction takes place between the alleles of the different genes whether it is present on the same or different chromosomes. Eg, pleiotropy, polygenes, epistasis, supplementary and complementary genes

Incomplete dominance:

Here, the allelomorphic pairs present in both the genes express themselves only to a limited extent in the progeny obtained. This happens because one gene cannot express the other gene totally so there is no dominant and no recessive traits to be found.

When such type of dominance takes place you get an intermediate expression in the F1 hybrids. For eg, in four o' clock plant (Mirabilis jalapa) the red colour is supposed to be dominant over the white but in since it shows incomplete dominance you get an intermediate expression i.e the colour pink which will be the mixture of the colour red and white.

In F2 generation you get a ratio of 1:2:1 i.e red:pink:white. So we can that the factors which are present don't mix with each other to suppress the other and there is segregation taking place. Other example of incomplete dominance is Snapdragon (Antirrhinum majus).

 Graphical representation of incomplete dominance
Graphical representation of incomplete dominance

Co-dominance:

In incomplete dominance the genes express themselves to an extent but in co-dominance they express themselves equally in the F1 hybrids. Therefore, you can say that both can express themselves. 

For eg, the coat colour of a cattle. There are two types of colours which are - red and white. When the red cattle is crossed with white, in the F1 hybrids you would get a roan colour cattle. This is because co-dominance is taking place where they can express themselves equally. 

You get a phenotypic ratio of 1:2:1 i.e red:roan:white. In co-dominance the genotype and the phenotype would both have the same ratios, but in incomplete dominance the ratios won't be the same instead you would get an intermediate progeny. 

 Graphical representation of co-dominance
Graphical representation of co-dominance

Multiple alleles:

A single organism has only two alleles but when there are two alternative forms or maybe more than two this phenomena is known as multiple alleles. Multiple alleles are not present all the time but when there is a mutation taking place in the wild type gene meaning the gene which is supposed to be the dominant one multiple alleles take place.

Multiple alleles cannot undergo crossing over. Eg, in Drosophila which is a small fruit fly there are large number of multiple alleles. For instance the wings of this fruit fly has wings abnormality and this is not just one for two but five. It is like in a form of series. Here is a table for your reference of multiple alleles in Drosophila.

Phenotype Genotype
Normal wings Vg+
Nicked wings vgni
Notched wings vgno
Strap wings vgst
Vestigial wings vg


Just like Drosophila, even in humans the ABO blood group system possess multiple alleles. The "I" gene controls the ABO blood group so it has three alleles i.e IA, IB and i. Here is a table for your reference of multiple alleles in human blood group.

Genotype Phenotype-blood group
IA IA or IA i A
IB IB or IB i B
IA IB AB
i i O


Pleiotropy:

We all know a particular gene controls one particular trait but when one gene controls more than it has to like maybe more than two or more then it is known as pleiotropic gene and this phenomenon is called pleiotropy or pleiotrophism. This this type of gene interaction the ratio is 2:1 instead of 3:1. 

For example, the disease sickle-cell anaemia. This disease is caused by the gene HbS. The HbA gene is dominant. 

 Graphical representation of pleiotropy
Graphical representation of pleiotropy


Polygenes:

Each gene has the ability to control one for two characters, but ever wondered in humans, we show different variations in skin colour, height, intelligence etc.. All these characters are handled by two or more gene pairs and these genes are called polygenes or multiple factors. This factor is also known as polygenic inheritance.

The tend to have a additive or cumulative effect meaning, the genes work in a pair to show the all these characters in the phenotype.

 Graphical representation of polygenic inheritance
Graphical representation of polygenic inheritance

Handwritten notes:

Here are my personal handwritten notes for the chapter Genetic Basis of Inheritance

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