Inheritance of one gene; Monohybrid cross; Mendel's laws- Laws of dominance, Laws of segregation; Back cross- Test cross, Outcross; Exceptions to mendelian Principles- Incomplete dominance, Pleiotropy, Codominance. PRINCIPLE OF INHERITANCE AND VARIATION




Inheritance of one gene; Monohybrid cross; Mendel's laws- Laws of dominance, Laws of segregation; Back cross- Test cross, Outcross; Exceptions to mendelian Principles- Incomplete dominance, Pleiotropy, Codominance. PRINCIPLE OF INHERITANCE AND VARIATION 

Inheritance of One Gene/Monohybrid cross

Study of inheritance of single pairs of contrasting traits of a character at a time is called one gene inheritance.

Mendel proposed two laws (Laws of dominance, Law of segregation) on the basis of his observation on one gene inheritance.

  • Cross between homozygous/true-breeding tall and homozygous/true-breeding dwarf pea plants (Monohybrid cross).

Fig.1. Monohybrid cross
  • Monohybrid genotypic ratio- 1:2:1
  • Monohybrid phenotypic ratio- 3:1

On the basis of observation on monohybrid cross, Mendel proposed two laws-
1.     Laws of Dominance
·        This law is not universally applicable.
2.     Laws of Segregation
·        This law is universally applicable.
·        Laws of segregation ensure the purity of gametes.

Exceptions to Mendelian Principles

1.     Incomplete Dominance


·        Proposed by Carl Correns in Mirabilis jalapa.
·        Example: Incomplete dominance in Snapdragon/Dog flower(Antirrhinum majus)
Fig.2. Incomplete dominance in Snapdragon flower


             Here, one allele is incompletely dominant over other so that                 intermediate phenotype is produced by F1 hybrid.
·        Phenotypic ratio- 1:2:1
·        Genotypic ratio- 1:2:1
2.     Pleiotropy


·        When one gene influence two or more seemingly unrelated phenotypic traits.
·        Example:

Phenylketonuria

Causes due to the mutation in the gene that codes for phenylamine hydroxylase.
This result in:
·        Mental retardation.
·        Reduction of hair.
·        Reduction of skin pigment.


                    Drosophila White Eye Mutation

                     Drosophila white eye mutation leads to de-pigmentation in many other parts of the body.

3.     Codominance

·        When cross takes place in between two homozygous individuals for a pair of contrasting characters. In F1 generation both the character appears side by side. This Phenomenon is called codominance.
E.g. 1.Coat color of cattle.

2. Different types of red blood cells that determine ABO blood grouping in humans. ABO blood groups are controlled by the gene I. I gene has three alleles IA, IB, i.  IA and IB are completely dominant over i.

Allele from parent  1
Allele from parent 2
Genotype of offspring
antigen
Blood types of offspring
IA
IA
IA IA
A
A
IA
IB
IA IB
A, B
AB
IA
i
IA i
A
A
IB
IA
IA IB
A, B
AB
IB
IB
IB IB
B
B
IB
i
IB i
B
B
i
i
ii
NIL
O

 Learn More 

  • Mendel called genes as factors. 
  • Gene is defined as the functional unit of in heritance.
  • A segment of D.N.A that codes for a particular character is called gene.
  • Genes that codes for a pair of contrasting traits are known as alleles. E.g.- Pairs of alleles for heights of pea plants- TT/Tt/tt.
  • Term ‘Gene’ was given by Wilhelm Johannsen.
  • Term ‘Allele’ and ‘Genetics’ was given by William Bateson.
  • Punnett Square was developed by a British geneticist Reginald C. Punnett. It is a square diagram that is used to predict the genotypes of a particular cross and to determine the probability of an offspring having a particular genotype.
  • F1 generation- 1st filial generation.
  • Backcrossing is a crossing of a hybrid with one of its parents. There are two types of back cross-


Test cross- F1 (Tt) ×Recessive parent (tt).
Out cross- F1 (Tt) × Dominant parent (TT).
  • Emasculation is the process of removal of anthers from the bisexual flower (male sex organ).
  • According to the laws of segregation two alleles of a gene controlling each character stay together in the individual, but during gamete/spore formation by meiosis, the alleles of a pair segregate (separate) from each other so that a gamete/spore carries only one allele of a character.
  • When true-breeding organisms are self-fertilized they only produce offspring with the same traits. 

Summary

Mendel proposed two laws of inheritance on the basis of his observation on the monohybrid cross. First law- law of dominance is not universally applicable, second law- law of segregation is universally applicable and it ensures purity of gametes. There are exceptions in Mendel's law of inheritance- Incomplete dominance, Codominance.

Comments

Post a Comment