Mendelian Laws

Introduction

Based on the pea plant experiments, Mendel has proposed 3 laws, the law of segregation, the law of independent assortment and the law of dominance. Mendelian laws were formulated based on the probabilities and the percentage of expression of traits in the subsequent generations after they cross the parental alleles. The following table briefs the 3 laws of inheritance. 

1. Introduction

2. Definitions of laws of inheritance 

3. Explanation for the law of segregation

4. Explanation for the law of dominance

 

5. Incomplete dominance

6. Co-dominance

7. Steps involved in the Inheritance of 2 genes or dihybrid crossing

8. Explanation for  the law of Independent Assortment

 

 

 

 

Law

 

Definition                                                                           

Principle of segregation

The principle of segregation is the 1st law. According to this, during gamete formation, the alleles for each gene will get separated (segregated) from each other among which 50% of the gametes carry one allele, and the rest will carry the other allele.

Principle  of independent assortment

According to this law, alleles of two or more different genes will get sorted independently into separate gametes of one another. Which means the allele received from one gene doesn’t influence on the allele received by another gene.

Law of dominance

The traits are expressed among the inherited pair of heterozygous alleles. If the traits are not expressed phenotypically after crossing then they are called recessive. The law of dominance will help to explain the expression of any one of the parental traits in a monohybrid cross in the F1 and the expression of both traits in the F2, in addition, it is useful to explain the proportion of 3:1 ratio obtained at the F2 after self-pollination

 

 

 

Law of seggregation

Law of segregation from Google

 

Explanation for the law of segregation

Pair of alleles do not influence each other. Each trait will be controlled by the alleles that are responsible for that trait. For example, when a crossing is done between round seeded pea plant with a tall pea plant, offspring receives either one or both traits, in this case, an allele that controls the seed shape will not control the height and vice-versa, which means the two characters do not mix instead they act independently to each other.  According to Mendel’s law of segregation, the phenotypic ratio of 3:1(see picture) where heterozygous offspring show dominant traits while the homozygous recessive shows the recessive trait.

 

Explanation for the law of dominance

It is simply the ability of a gene to express its true character phenotypically (externally). Dominance can be complete dominance, codominance, and incomplete dominance. Now the question arises why do some are dominant and why not all? In fact, it is part of evolution and differentiation. Every gene has some information to express a particular trait and it is expressed under certain circumstances. As we know that, in a diploid organism, alleles are in pairs that need not be the same, instead they can vary as in a heterozygote.

Parent Pea Plants

F1 Pea Plants

Tall stem X Short stem

All tall stems

Yellow seeds X Green seeds

All yellow seeds

Green pea pods X Yellow pea pods

All green pea pods

Round seeds x Wrinkled seeds

All the round seeds

Axial flowers x Terminal flowers

All axial flowers

Due to the influence of modifiers, a slight change can occur in the composition of a  gene which could either turn dominant or recessive. A particular modifying factor alters the information that a particular allele contains. There is also an assumption that one gene may suppress the other to make it no to express its true nature. Some of the examples of dominant traits after crossing are as follows. In the table above, although tall and short were crossed, F1 was all tall, similarly, both the yellow seeds and green seeds were crossed, only yellow were seen which means only one character was expressed and that character or trait is dominant. If you take an example of colour of hair, straight and black as a dominant combination, there is a gene that contains the information for the production of such normal hair. This gene is in allelic form (2 copies), if we assume that the normal allele produces the normal hair with no modifying factor then the law of dominance here means this normal genes dominated. Supposing the hair turned curly and brownish, then some third factor suppressed the allele which we mentioned as normal. So, now this allele got manipulated and became recessive to give rise to different traits ( curly, brown).

 

Incomplete dominance

It is also known as the partial dominance, semi-dominance or intermediate inheritance. Incomplete dominance is when one allele for a specific trait fails to express completely against its counterpart. This results in the appearance of a third phenotype having the combination of phenotypes of both alleles. This occurs in the polygenic inheritance of traits such as eye colour and skin colour. Some of the examples of incomplete dominance are;

  • Crossing of red rose and white rose will produce offspring having pink colour meaning, none of the parental colours have appeared in the progeny becasue of the incomplete dominance.

  • A snapdragon flower in pink is the result of cross-pollination between a red flower and a white flower.

  • A brown fur coat is appeared in a rabbit due to the failure of the dominance of any one of the parental traits that had red and white allele.

  • A child getting a wavy hair when one of the parents had curly hair and the other one had straight hair.

 

Co-dominance

In the case of co-dominance, the F1 generation resembles the dominant traits of both parents, in other words, there is a double dominance which is against the normal medallion’s law of dominance because Mendel’s law of dominance says, there is only one phenotype appears as a dominant trait in the F1. Examples of codominance include a person with type AB blood, which means that both the A allele and the B allele are equally expressed. Another example is roan fur in cattle, in which white and red hair is equally expressed.

 

Steps involved in the Inheritance of 2 genes or dihybrid crossing

  • The aim of dihybrid crossing is to bring out a mix of traits with varied futures in the future progeny. It is generally known as dihybrid crossing, in which, crossing is done between 2 parental alleles having a pair of opposing traits, for example, in the case of pea plants, Mendel tried crossing pea plant that has seeds with yellow colour and round shape against a one that had seeds of green colour and wrinkled shape. In this particular case, the end result was yellow-coloured seeds with a round shape, which means this combination is called dominant dihybrid combination according to the law of dominance.

  • The table below will explain what dihybrid crossing is. Based on the Mendelian genetics, if we use 2 opposing traits of pea plants we get some ratios. If you see the image below, it is describing a dihybrid cross between two pea plants, by taking 2 traits such as pod colour and pod shape. Mendel chose to cross a pea plant that has a round shape indicated by RR and the yellow colour of it is indicated by YY. On the other hand, the recessive traits of this is indicated by rr for wrinkled and yy for green seeds. In the next stage, Mendel crossed two plants by taking plants from F1 generation (hence the term dihybrid crossing) I, e RrYy X RrYy.

  • As a result, he found the ratio of   F2 progeny of his dihybrid cross as 9:3:3:1. This means there are 9 plants with round and yellow seeds (RRYY), 3 plants with round and green seeds (Rryy), 3 plants with wrinkled and yellow seeds (rryy). Only 1 plant with wrinkled and green seeds (rryy). From his experiment, Mendel observed that the pairs of traits in the parental generation sorted independently from one another, from one generation to the next. This is very well explained by Punnet`s chart below           

                                                                              

                                 

RY                        

Ry                         

rY                            

ry                               

RY

RRYY

RRYy

RrYY

RrYy

Ry

RRYy

Rryy

RrYy

Rryy

rY

RrYY

RrYy

rrYY

rrYy

ry

RrYy

Rryy

rrYy

rryy

 

 

Dihybrid crossing

 

 

Explanation for  the law of Independent Assortment

Independent assortment states that, although the traits are controlled by alleles in pairs, they get independently assorted into haploid ( single ) units. In other words, when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters’. Punnet`s square will help to explain what exactly mean by independent assortment. An assortment takes its role after the segregation being done at the time of fertilization. During the production of eggs and pollen in the F1 of a combination of  RrYy ( ROUND SEEDED, WRINKLED SEEDED, YELLOW SEEDED, GREEN SEEDED), there is a segregation of one pair of genes into R and r., which means,50% of the gametes have the gene R and the remaining  50% have the gene r. Similarly, Yy becomes independent genes by splitting or assorting into the Y gene and y gene by sharing 50% each. One interesting feature of genes to be noted here is that the segregation of R and r, Y and y  takes place independently without being influenced by each other, so is the reason there is exact sharing of 50% between the 2 gene sets and this is known as independent assortment. One of the best examples for independent assortment is the phenomenon that takes place during fertilization.



img-1

img-1

img-1