Coupon Accepted Successfully!


Contributions Of Mendel


Gregor Johann Mendel is considered as the father of modern genetics. He laid a firm foundation for the study of inheritance though he had no information of meiosis, structure of chromosome and gene. Even then, he could formulate certain principles and laws in genetics based on the results of his experiments on pea plants. Genetists who conducted similar experiments arrived at the same conclusion.

  • Mendel’s experimental organism is a common garden pea (Pisum sativum). He selected garden pea for the following reasons:
  1. The pea plant is small and easy to grow.
  2. It reproduces a large number of offsprings and completes its life cycle in one season.
  3. A large number of true breeding varieties of pea are available.
  4. The plant is naturally self-pollinating but can be easily cross-pollinated experimentally.
  5. The plant has a number of contrasting characters, e.g. red versus white flowers, round versus wrinkled seeds etc.
  • Mendel’s methodology involved:
  1. Obtaining seeds which were true to their types, i.e. the plants with red flowers always produced generations of red flower plants.

Trait Studied

Dominant Trait

Recessive Trait

Stem height



Flower position



Pod shape



Pod colour



Seed shape



Seed colour



Seed coat colour



  1. Counting all seeds and grouping them on the basis of characters that they represent. This method thus was quantitative and he kept the track of characters of plants of several generations after a particular cross.
  2. Drawing generalisations on the basis of numerical relationships between the hybrids having contrasting characters.
  • Mendel selected seven visible characters, each with contrasting traits (see Table 12.1). He also kept records of his experiments, giving all the details of number and type of individuals which are necessary in the genetic studies.
  • Concept of dominance: Mendel called the substance or agent responsible for each trait a ‘factor’. According to Mendel, the trait that appeared in first filial generation (F1) was dominant and the one which did not appear was recessive.
  • Concept of unit character: Mendel explained that each genetic character was represented or controlled by a pair of unit factors or elements. One of the factors came from one parent and the other from the other parent.
  • Law of segregation: When the tall plants in F1 were crossed among themselves, the F2 generation had 75% tall plants and 25% dwarf plants (ratio 3:1) (see Figure 12.1). Hence, Mendel concluded that the alleles representing dwarfness were intact and were neither lost nor contaminated. Mendel’s study with one character (monohybrid cross) led to the formulation of the law or principle of segregation. This means that although the alleles of a character remain together, they are separated in subsequent generation.

F2 Generation

  •  Law of independent assortment: After studying the inheritance of one pair of contrasting characters, Mendel went on to take two characters (dihybrid cross) into account. He crossed a plant having round and yellow seeds with a plant having wrinkled and green seeds. All F1 plants had round and yellow seeds. When a certain number of F1plants were grown and self-crossed, they gave rise to four types of seeds: 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green. When its ratio is worked out, it comes to be 9:3:3:1.

This shows that the chances for the pea seeds to be round or wrinkled has no bearing on their chances to be yellow or green. In other words, each pair of alleles is independently separate of the other pair. This is the principle of independent assortment.

A breakthrough in our knowledge of heredity and pattern of inheritance came from the studies of Mendel. While formulating the laws of heredity, Mendel stated the units of inheritance controlled the inheritance of characters. Now, we call these unit factors ‘genes’.


Generation of Dihybrid Cross


Genetics is a science of chance. Any male gamete can fuse with any female gamete. It is known as random mating. Totally 16 offsprings are produced in the ratio of 9 yellow round, 3 yellow wrinkled, 3 green round and 1 green wrinkled (see Figure).

F2: 9 Yellow round:3 Yellow wrinkled:3 Green round:1 Green wrinkled

In a dihybrid cross the phenotypic ratio is 9:3:3:1

Random Mating

What about the genotypic ratio? 
Let us see:

  1. Homozygous for yellow and homozygous for round (YYRR): 1
  2. Homozygous for yellow and heterozygous for round (YYRr): 2
  3. Heterozygous for yellow and homozygous for round (YyRR): 2

The genotypic ratio is 1:2:2:4:1:2:1:2:1 (easy to remember the genotypic ratio—1:2:1:2:4:2:1:2:1).

Back cross

Back cross: Back cross is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent.

Test cross: 
A test cross is a back cross if the F1 offspring is crossed to either of the parent. A cross between the F1 hybrid and a homozygous recessive plant is called a test cross. Hence, all test crosses are back crosses but all back crosses are not test crosses. Such a cross, involving only one pair of contrasting characters, is called a monohybrid test cross. If two pairs of characters are involved, it is called a dihybrid test cross.

Significance of a test cross

  • It is conducted to make sure of the genotype of the F1 generation.
  • It helps to determine whether the dominant character is due to homozygous or heterozygous condition.
  • It always produces only two types of offsprings. 50% of them show dominant character and the other 50 % will show the recessive character (1:1).

Appearance of recessive character shows that the organism in question is heterozygous.


Monohybrid Test Cross


A test cross conducted for the monohybrid inheritance results in the two opposite characters expressing in a ratio of 1:1 as shown Figure 12.4.

Similarly, a test cross conducted for the dihybrid inheritance results in the expression of the two parental combinations and the two new combinations in the ratio 1:1:1:1 as shown in Figure 12.5.


Description: 27650.png

Dihybrid Test Cross

Test Your Skills Now!
Take a Quiz now
Reviewer Name