Mendel’s laws of inheritance and Chromosomal theory of inheritance

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Mendel’s laws of inheritance :-

Mendel’s present his paper on principles of heredity at the annual conventy on natural history society at Brun ( Austria ) in 1895. This paper use published in the proceeding on the conference in 1866. Tittle of this paper was experiment in plant hybridization.

  • Before garden pea Mendel his worked on the garden plant Heracium which is a cross pollinating plant.
  • Mendel choose garden pea for his experiment mainly because, it is self pollinating plant.
  • Based on his observations on monohybrid and dihybrid crosses Mendel proposed three general rules, these rules are called principles or Mendel’s laws of inheritance.

These principles are – 1. Law of dominance. 2. Law of segregation and 3. Law of independent assortment.

Law of dominance :-

It is a Mendel’s law of inheritance ,

According to this law there are two factors togather for any character in organism they may be similar or contrasting ( RR or rr, Rr ), one factor of dominant alleles other is recessive, dominant factor togather dominant character is made and recessive factor togather recessive character to gives – RR = Red ( Dominant )

rr = white ( recessive )

When gone crossing togather dominant factor suppress to the recessive factor and dominant character is made.

This law is based on monohybrid characters.

Fig. 1. Unknown flower is heterozygous dominant.

The law of dominance is used to explain the expression of only one of the parental characters in monohybrid cross in the F₁ and the expression of both in the F₂. It also explains the proportion of 3 : 1 obtained at the F₂.

Law of segregation :-

According to this Mendel’s law- Two factors which are togather and which any characters segregate for each other because gamete formation. Any gamete contains only one factor for each character therefore gametes are always pure.

Due to segregation of factor, F₁ hybrid of an offspring would produce to takes of male and female gamete respectively, their fertilization leads to 3 : 1 ratio of dominants and recessive characters in F₂ generation.

This law is also called law of purity of gamete.

  • Law of segregation is based on the ratio of 3 : 1 ( monohybrid ).
Fig. 2. Table showing The Law of Segregation.

Law of independent assortment :-

According to this law – pair factors segregation independently from all other of factors. In dihybrid cross both the contrasting pair segregate independently to give 3 : 1 ratio characters separately.

Seed coat.

Round or smooth = 12

Wrinkle = 4

Ratio = 3 : 1

Colour of seeds

Yellow = 12

Green = 4

Ratio = 3 : 1

The Punnet square can be effectively use to understand the independent segregation of the two pairs of genes during meiosis, production of eggs and pollen in the F₁ RrYy plant.

Figure 12 03 02 1
Fig. 3. Punnet square for dihybrid cross.

The important thing to remember here is that segregation of 50% R and 50% r is independent from the segregation of 50% Y and 50% y.

Chromosomal theory of inheritance :-

Boveri and Sutton’s chromosome theory of inheritance states that genes are found at specific locations on chromosomes and that the behaviour of chromosomes during meiosis can explain Mendel’s laws of inheritance.

  • Thomas Hunt Morgan, who studied fruit flies ( Drosophila ) provided the first strong confirmation of the chromosome theory.
  • Morgan discovered a mutation that affected fly eye colour. He observed that mutation was inherited differently by male and female flies.
  • Based on the inheritance pattern, Morgan concluded that the eye colour gene must be located on the x-chromosome.
  • The chromosomal theory of inheritance was consistent with Mendel’s laws and was supported by the following observations – ( 1 ) During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs. ( 2 ) The shorting of chromosomes from each homologous pair into pre-gametes appears to be random. ( 3 ) Each parent synthesizes gametes that contain only half of their Chromosomal complement. ( 4 ) Even through male & female gametes differ in size & morphology, they have the same number of chromosomes suggesting equal genetic contributions form each parent. ( 5 ) The gamete chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.
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Fig. 4. Crossing Over and Separation of Chromosomes.

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