`star` Mendel’s Laws of Inheritance
`star` Law of Dominance
`star` Law of Seggregation
`star` Incomplete Dominance
`star` Explanation of the concept of dominance


● Based on his observations on monohybrid crosses Mendel `color{Violet}"proposed two general rules"` to consolidate his understanding of inheritance in monohybrid crosses.

● Today these rules are called the `color{Violet}"Principles or Laws of Inheritance"`:

● The `color{Violet}"First"` Law or `color{Violet}"Law of Dominance"`

● The `color{Violet}"Second"` Law or `color{Violet}"Law of Segregation"`.


(i) Characters are controlled by `color{Violet}"discrete units"` called `color{Violet}"factors"`.

(ii) Factors occur in `color{Violet}"pairs"`.

(iii) In a dissimilar pair of factors one member of the pair dominates (`color{Violet}"dominant"`) the other (`color{Violet}"recessive"`).

● The law of dominance is used to explain the expression of only `color{Violet}"one of the parental characters"` in a monohybrid cross in the F1 and the expression of both in the F2.

● It also explains the proportion of `color{Violet}"3:1"` obtained at the F2.


● This law is based on the fact that the alleles `color{Violet}"do not show any blending"` and that both the characters are recovered as such in the `color{Violet}"F2 generation"` though one of these is not seen at the F1 stage.

● Though the parents contain two alleles during `color{Violet}"gamete formation"`, the factors or alleles of a `color{Violet}"pair segregate"` from each other such that a gamete receives only one of the two factors.

● A `color{Violet}"homozygous parent"` produces all gametes that are similar while a `color{Violet}"heterozygous"` one produces two kinds of gametes each having one allele with equal proportion.


● When experiments on peas were `color{Violet}"repeated using other traits"` in other plants, it was found that sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two.

● The inheritance of flower colour in the `color{Violet}"dog flower"` (`color{Violet}"snapdragon"` or `color{Violet}"Antirrhinum sp"`.) is a good example to understand `color{Violet}"incomplete dominance"`.

● In a cross between `color{Violet}"true-breeding"` red-flowered (`color{Violet}"RR"`) and true breeding white-flowered plants (`color{Violet}"rr"`), the F1 (`color{Violet}"Rr"`) was pink.

● When the F1 was self-pollinated, the F2 resulted in the following ratio `color{Violet}"1 (RR) Red: 2 (Rr) Pink : 1 (rr) White"`.

● Here the genotype ratios were exactly as we would expect in any mendelian monohybrid cross, but the phenotype ratios had changed from the `color{Violet}"3:1 dominant : recessive ratio"`.

● What happened was that R was `color{Violet}"not completely dominant"` over r and this made it possible to distinguish `color{Violet}"Rr as pink"` from RR (red) and rr (white).



`star` What exactly is `color{Violet}"dominance"`?

`star` Why are some alleles `color{Violet}"dominant and some recessive"`?

● Every gene, contains the information to `color{Violet}"express a particular trait"`.

● In a diploid organism, there are `color{Violet}"two copies of each gene"`, i.e., as a pair of alleles.

● Now, these two alleles need not always be `color{Violet}"identical"`, as in a heterozygote.

● One of them may be different due to some `color{Violet}"changes that it has undergone"` which modifies the information that particular allele contains.

● Let’s take an example of a gene that contains the `color{Violet}"information"` for producing an `color{Violet}"enzyme"`.

● Now there are two copies of this gene, the two `color{Violet}"allelic forms"`.

● Let us assume (as is more common) that the `color{Violet}"normal allele"` produces the `color{Violet}"normal enzyme"` that is needed for the transformation of a `color{Violet}"substrate S"`.

● Theoretically, the `color{Violet}"modified allele"` could be responsible for production of –
(i) the `color{Violet}"normal/less efficient"` enzyme, or

(ii) a `color{Violet}"non-functional enzyme"`, or

(iii) `color{Violet}"no enzyme"` at all

● In the first case, the modified allele is equivalent to the unmodified allele, i.e., it will produce the `color{Violet}"same phenotype/trait"`, i.e., result in the transformation of substrate S. Such equivalent allele pairs are very common.

● But, if the allele produces a `color{Violet}"non-functional enzyme"` or no enzyme, the phenotype may be affected.

● The phenotype/trait will only be dependent on the functioning of the `color{Violet}"unmodified allele"`.

● The unmodified (functioning) allele, which represents the `color{Violet}"original phenotype"` is the dominant allele and the modified allele is generally the recessive allele.

● Hence, in the example above the recessive trait is seen due to `color{Violet}"non-functional enzyme"` or because no enzyme is produced.