Biology MENDEL’S LAWS OF INHERITANCE

### KEY TOPICS

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

### MENDEL’S LAWS OF INHERITANCE

● 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".

### LAW OF DOMINANCE

(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.

### LAW OF SEGREGATION

● 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.

### INCOMPLETE DOMINANCE

● 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).

### EXPLANATION OF THE CONCEPT OF DOMINANCE

● SOME IMPORTANT QUESTIONS:

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.