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Mendel’s Experiments and Laws of Inheritance

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  • Last Updated : 24 Jul, 2022
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Gregor Johann Mendel (1822-1884) was an Austrian scientist, teacher, and Augustinian prelate who lived in the 1800s.  He was educated in a monastery and went on to study science and mathematics at the University of Vienna. He failed an exam for a teaching certificate but did not suppress his zeal for the scientific quest. He went back to his monastery and started growing peas. Mendel blended his knowledge of science and mathematics and became the first one to keep count of individuals exhibiting a particular trait in each generation. This helped him to arise the laws of inheritance.

Characters of peas 

He reveals that there are 7 different traits 

  • Pea shape (round and wrinkle)
  • Pea color (green and yellow)
  • Pod color (green and yellow)
  • Pod shape (constricted and inflated)
  • Flower color (purple and white)
  • Plant size (tall and dwarf)
  • Position of flowers (axial and terminal)

Characteristics of Mendel experiments 

Mendel explains the concept of dominant and recessive alleles. The following table shows each of the traits and which traits are dominant and which are recessive.

S.no. Character Dominant Recessive
1 Seed color Yellow Green
2 Pod shape Inflated Constricted
3 Pod color Green Yellow
4 Seed shape Round Wrinkle
5 Stem height Tall  Dwarf
6 Flower color Violet White
7 Flower position Axial Terminal

Mendel’s experiment 

Mendel worked on inheritance. Inheritance is genetic qualities that transfer from parent to offspring. Mendel took pea plants with different characteristics example-tall/short plants, white/violet flowers, etc. First, he took short (tt) and tall plants (TT) and produced progeny from them by self-pollination. He observed in the F1 generation, all tall offspring (Tt). Then he took two tall plants (Tt ) from the F1 generation and produced progeny then he observed the result of three tall plants (Tt) and one short plant (tt).

tall plant (TT) * short plant (tt)   —>all tall offspring (Tt)           <F1 generation>

Tall pant (Tt) * tall plant (Tt)     —-> tall (TT)  tall (Tt) tall (tt) short (tt)              <F2 generation>

He concludes that transmission of genes in the generation.

In Mendel’s experiment with pea plants, when he cross-bred a tall pea plant with a short plant, he found that the first generation (F1) was of only tall plants. In the F1 progeny, no short plants were obtained. However, in the F2 generation, both tall and short plants were obtained in the ratio 3:1 respectively. In the F2 progeny, both traits of tallness and shortness were inherited where the tall character was the dominant trait and short character was the dominant trait and short character was the recessive trait.

Dominant and Recessive gene – A gene that expresses itself in the presence of its contrasting gene in a hybrid is termed a dominant gene. A recessive gene is that whose expression is suppressed in the presence of a dominant gene e.g. in a hybrid (Tt) tall plant, the t gene for dwarfness is recessive and T gene for tallness is dominant.

  • Filial generation – The generation of offspring is termed filial generation. 
  • First Filial generation (F1) – The first generation of offspring produced from the parent generation.
  • Second Filial generation (F2) – The second generation of offspring.

Monohybrid cross 

Mendel Cross (Monophybrid)

 

It is a cross in which only one character is considered at a time, e.g. in a cross between a tall and dwarf plant, the size of the stem is considered. Mendel made a cross between a pure tall (TT) and a pure dwarf (tt) pea plant. He obtained all tall (hybrid) plants in the F1 generation. On self, these plants produced tall and dwarf in the ratio 3:1 The genotypic ratio of 1:2:1 and the phenotypic ratio of 3:1 is termed the monohybrid ratio. It is a single cross between two organisms of a species that is made to study the inheritance of single pairs of genes or factors. This cross helps to study the principle of dominance given by Mendel

Dihybrid cross 

Dihybrid Cross

 

It is a cross between two individuals taking two contrasting traits at a time. It helps to study the inheritance of two pairs of alleles. The genotypic ratio in the F2 generation is 1:2:2:4:1:2:1:2:1 and the phenotypic ratio in the F2 generation is 9:3:3:1 This cross helps to study the principle of Independent assortment given by Mendel. For example – the cross between pea plants having yellow wrinkled seeds with those having green round seeds is a dihybrid cross.

Why did Mendel choose only the pea plant?

Mendel selected the pea plant (Pisum sativum) because-

  • Many varieties were available with observable alternative forms for a trait or characteristics.
  • Peas are normally self-pollinated; as their corolla completely encloses the reproductive organs until pollination is completed.
  • peas are easily available.
  • Peas have pure lines for experimental purposes, i.e. they always breed true.
  • Peas have contrasting characters. The trait was seed color, pod color, pod shape, flower shape, the position of the flower, seed shape, and plant height.
  • Its life cycle was short and produced a large number of offspring.
  • The plant is grown easily and does not require care except at the time of pollination.

Laws of Mendel

Mendel had given three laws of inheritance to follow as –

Law of Dominance

The law of dominance states that the expression of only one of the forms of the parental trait in the F1 hybrid. In heterozygous condition i.e. different alleles, the dominant allele get expressed. when two different alleles are present, only one is dominant and will be expressed. F1 generation express dominant alleles.

         TT   ×   tt    (parents)   ——>   Tt    ; F1 generation

Law of Segregation  

The law of segregation states that for any trait, each pair of alleles of a gene split, and one gene passes from each parent to an offspring. Two alleles do not mix when they come together in hybrid pair and are independent of each other.

          TT  ×  tt       (parents)  ——–>   Tt      ; F1 generation 

          Tt  ×   Tt      ———->    TT         Tt         Tt         tt   ; F2 generation 

In the F1 generation, all tall offspring are observed and in F2, gene passes from each parent to offspring i.e. tall tall tall and short plants are observed.

Law of Independent Assortment 

The law of independent assortment states that completely different pairs of alleles are passed on to the offspring independently of each other that is during gametes formation, two genes segregate independently of each other as well as of the other trait. The inheritance of one gene does not affect the inheritance of any other gene. 

Results of Mendel’s experiment with pea plants

  • From the first generation crosses Self-pollinating true breeding plant. The result shows that offspring were identical to the parent plant.
  • From the second generation: The results show traits are transmitted from parents to offspring via three fourth of the plants were tall and one-fourth were short

Mendel concluded that two factors control each inherited trait. He also proposed that sperm and egg; each contribute one factor for each trait i.e. two factors control each inherited trait. Transmission of genes from generation to generation must.

Conceptual Questions

Question 1: What is a gene? What is the nature of genes?

Answer:

Term Gene was given by Jo hansen (1900). It is a hereditary determiner specifying a biological function; a unit of inheritance (DNA) located in a fixed place on the chromosome is called a Gene. Mendel considered every character as a unit which is controlled by a factor, presently called a gene.  Chemically, a gene is a segment of DNA that controls one character and physically is a part of the chromosome.

Question 2: Where are genes located? What is the chemical nature of genes?

Answer:

Genes are located on chromosomes. Chemically, a gene is a segment of DNA (Deoxyribose nucleic acid)

Question 3: Write three basic features of inheritance?

Answer: 

According to Kolreuter, basic features of inheritance are –

  • The traits could be hidden or masked for one or more generations, but ultimately reappear unchanged.
  • Traits may remain together in one generation and separate in subsequent generations.
  • One alternate of a particular trait may be exhibited more frequently than the other alternate form.

Question 4: What are alleles?

Answer: 

Two alternatives of the same genes at the same locus in a pair of homologous chromosomes are termed alleles. It is an abbreviated form of allelomorph.

Question 5: What is a dominant gene? 

Answer: 

A gene that expresses itself in the presence of its contrasting gene in a hybrid is termed a dominant gene.

Question 6: What is a recessive gene?

Answer:

It is that whose expression is suppressed in the presence of the dominant gene. For example—in a hybrid (Tt) tall plant, t gene for dwarfness is recessive and T gene for tallness is dominant.


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