Biology INHERITANCE OF ONE GENE

### KEY TOPICS

star Experiment with F_1 generation
star Experiment with F_2 generation
star Units of Inheritance
star Dominant and Recessive traits

### EXPERIMENT WITH FI GENERATION

● Let us take the example of one such color{violet}"hybridisation experiment" carried out by Mendel where he crossed tall and dwarf pea plants to study the color{violet}"inheritance of one gene".

● He collected the seeds produced as a result of this cross and grew them to generate plants of the color{violet}"first hybrid generation".

● This generation is also called the color{violet}"Filial 1 progeny" or the color{violet}(F_1).

● Mendel observed that all the F_1 progeny plants were tall, like color{violet}"one of its parents"; none were dwarf .

● He made similar observations for the other pairs of traits – he found that the F_1always resembled either color{violet}"one of the parents", and that the trait of the other parent was not seen in them.

### EXPERIMENT WITH F2 GENERATION

● Mendel then color{violet}"self-pollinated the tall" F_1 plants and to his surprise found that in the color{violet}"Filial2 generation" some of the offspring were color{violet}"dwarf" ; the character that was not seen in the F_1 generation was now expressed.

● The proportion of plants that were dwarf were color{violet}(1/4th of the F_2) plants while color{violet}(3/4th of the F_2 plants) were tall.

● The tall and dwarf traits were color{violet}"identical to their parental" type and did not show any color{violet}"blending"d, that is all the offspring were either tall or dwarf, none were of in between height.

● Similar results were obtained with the color{violet}"other traits" that he studied: only one of the parental traits was expressed in the F_1 color{violet}"generation" while at the F_2 stage both the traits were expressed in the proportion 3:1.

● The contrasting traits color{violet}"did not show" any color{violet}"blending" at eitherF_1 or F_2 stage.

### UNITS OF INHERITANCE

● Based on these observations, Mendel proposed that something was being color{violet}"stably passed down", unchanged, from parent to offspring through the gametes, color{violet}"over successive generations".

● He called these things as color{violet}"factors" Nowadays, we call them as color{violet}"genes".

● Genes, therefore, are the color{violet}"units of inheritance".

● They contain the information that is required to color{violet}"express a particular trait", in an organism.

● Genes which code for a color{violet}"pair of contrasting traits" are known as color{violet}"alleles", i.e., they are slightly different forms of the same gene.

● If we use color{violet}"alphabetical symbols" for each gene, then the capital letter is used for the trait expressed at the F_1 stage and the small alphabet for the other trait.

● For example, in case of the character of height, color{violet}"T is used for the Tall" trait and color{violet}"t for the dwarf", and T and t are alleles of each other.

● Hence, in plants the pair of alleles for height would be color{violet}"TT, Tt or tt".

### DOMINANT AND RECESSIVE TRAITS

● Mendel also proposed that in a color{violet}"true breeding", tall or dwarf pea variety, the color{violet}"allelic pair" of genes for height are color{violet}"identical or homozygous", color{violet}"TT and tt", respectively.

● TT and tt are called the color{violet}"genotype" of the plant while the color{violet}"descriptive terms" tall and dwarf are the color{violet}"phenotype".

● As Mendel found the phenotype of the F_1 color{violet}"heterozygote" Tt to be exactly like the TT parent in appearance, he proposed that in a color{violet}"pair of dissimilar factors", one color{violet}"dominates the other" (as in the F_1 ) and hence is called the color{violet}"dominant factor" while the other factor is color{violet}"recessive" .

● In this case color{violet}"T" (for tallness) is color{violet}"dominant" over color{violet}"t" (for dwarfness), that is recessive.

● He observed color{violet}"identical behaviour" for all the other color{violet}"characters/trait-pairs" that he studied.

● It is convenient (and logical) to use the color{violet}"capital and lower case" of an alphabetical symbol to remember this concept of dominance and recessiveness.

● NOTE: (We should color{brown}"not use" color{violet}"T for tall" and color{violet}"d for dwarf" because you will find it difficult to remember whether T and d are alleles of the same gene/character or not).

● Alleles can be similar as in the case of color{violet}"homozygotes TT and tt" or can be dissimilar as in the case of the color{violet}"heterozygote Tt".

● Since the Tt plant is color{violet}"heterozygous" for genes controlling one character (height), it is a color{violet}"monohybrid" and the cross between TT and tt is a color{violet}"monohybrid cross".