Genetics: Mendalism versus meiosis: Mendal’s inheritance, Genes and their interaction, Polygenic inheritance
Introduction:-
> Genetics:- a branch of biology that deals with the heredity and variation of organisms.
> Heredity:- It is the passing of genetic information and traits from parents to offspring.
> Gregor Johann Mendel is considered the father of genetics.
> Father of modern genetis is Bateson who gave the term genetics to Mendelian experiment.
Mendalism versus meiosis:-
> Gregor Mendel was a 19th century Moravian monk who demonstrated that the inheritance of traits (i.e. genes) follow particular findings:
i. In pairs of alleles that are different, one allele will mask the effect of the other allele. This describes how homologous chromosomes (and hence allele pairs) are separated in meiosis I.
ii. Each hereditary characteristic is controlled by two alleles which separate into different gametes. This describes how homologous chromosomes (and hence allele pairs) are separated in meiosis I.
iii. The separation of alleles for one gene is independent to allele separation for another gene. This describes how homologous pairs align randomly (as bivalents) during metaphase I.
> Through the elucidation of the process of meiosis, we now know that there are certain exceptions to Mendel’s laws:
i. Genes that are on the same chromosome (linked genes) will not undergo independent assortment (unless recombination occurs)
ii. Not all genes display a dominance hierarchy – certain traits may display codominance or incomplete. dominance
Mendal’s inheritance:-
Terminology:-
1. Gene:- The factor that is responsible for the expression of a genetic trait is called a gene. It is called the unit of heredity. Mendel used the term factor for a gene. Johansen coined the term gene in 1909.
2. Alleles:- The alternative forms of a gene present at the same locus on homologous chromosomes are called alleles.
3. Hybrid:- The progeny produced by crossing between two plants having different alternative traits, is called hybrid.
i. Monohybrid:- When only one genetic trait is considered while crossing between two plants, it is called monohybrid cross. Example:- TT x tt
ii. Dihybrid:- When two genetic traits are considered while crossing between two plants, it is called dihybrid cross. Example:- TTRR x ttrr
4. Reciprocal hybridization:- When two plants are crossed by changing in the form of male and female, it is called reciprocal hybridization.
5. Back cross:- When a F1 hybrid is crossed with its any one parent, it is called back cross.
6. Test Cross:- When a F1 hybrid is crossed with its recessive parent, it is called test cross.
7. Phenotype:- The external characters and morphology of a plant are called phenotype. This term was given by Johansen in 1909.
8. Genotype:- The complete genetic constitution of a plant is called a genotype. This term was given by Johansen in 1909.
9. Dominance:- When a cross is done between plants having different alternative traits, one of the two alternative traits is expressed and the other trait is hidden. Expressed trait is called dominant trait while hidden trait is called recessive trait.
10. Homozygous:- When both the alleles are same for a trait in a plant, it is called homozygous stage. This term was given by Bateson in 1902. Example – TT, tt, RR, rr
11. Heterozygous:- When both the alleles for a trait are different in a plant, it is called heterozygous stage. This term was given by Bateson in 1902. Example – Tt, Rr
Mendel’s Laws of Inheritance:- The year 1900 is called 'the year of rediscovery of Mendelism'. Karl Correns established Mendel's discovery as the laws of heredity.
1. Law of Dominance
2. Law of Purity of Gametes
3. Law of Segregation
4. Law of Independent Assortment
1. Law of Dominance:- According to this law, when a cross is made between plants having different alternative traits, only one trait is expressed and the other trait is hidden. Expressed trait is called dominant trait while hidden trait is called recessive trait.
Exceptions - Co-dominance, Incomplete dominance
2. Law of Purity of Gametes:- According to this law the gametes formed from each plant are 100% pure. Both alleles of a gene can never remain together in the same gamete.
3. Law of Segregation:- According to this law, during the formation of gametes in F1 progeny, both the homologous chromosomes move to different gametes, due to which both the alleles of the same gene get separated from each other and move to different gametes. This process is called segregation.
4. Law of Independent Assortment:- According to this law, the segregation of both the alleles of one gene is completely independent of the segregation of both the alleles of the other gene. Thus new combinations of alleles can be formed in the second generation. This is due to crossing over.
Exception - Linkage
Genes and their interaction:-
Gene Interactions:- The interaction between alleles and non-alleles, which changes the phenotypic ratio, are called as gene interactions. These are of two types –
1. Intra-genic (Inter - allelic)
2. Inter-genic (Non - allelic)
1. Intra-genic (Inter - allelic):- Interactions occur between two alleles of the same gene. 3 types -
a. Complete Dominance
b. Incomplete Dominance
c. Co - dominance
a. Complete Dominance:- When a cross is made between two pure parents, the character of one parent appear in the progenies of the F1 generation and the character of the other parent is completely hidden. This process is called complete dominance.
Example:- Plant height in peas
b. Incomplete Dominance:- When a cross is made between two pure parents, the character of both the parents mixed up to produce a new character in the offspring of the F1 generation. Due to which the phenotypic ratio changes in the F2 generation. This is called incomplete dominance.
Example:- Colour of flowers in the following plants-
i. Mirabilis jalapa (4 ‘O’ clock plant)
ii. Antirrhinum majus (Dog flower or Snapdragon)
In both the above plants there are 3 coloured flowers –
i. Red
ii. Pink
iii. White
c. Co - dominance:- When a cross is made between two pure parents, the character of both the parents are appeared separately and equally in the offspring of the F1 generation to produce a new character. This changes the phenotypic ratio in the F2 generation. This is called co-dominance.
Examples:-
i. Colour of skin in cattles:-
ii. AB blood group in humans:-
2. Inter-genic (Non - allelic):- Interactions between alleles of two different genes. Many types -
a. Complementary Genes:- Here, the two dominant non-alleles are complementary to each other. The character appears only when these two occur together. In this case the phenotypic ratio changes to 9 : 7.
Example:- Color of flowers in sweet pea (Lathyrus odoratus)
b. Supplementary Genes:- When two non-allelic genes interact with each other in such a way that the recessive allele suppresses the effect of the other non-allele, it is called as supplementary gene. It is also called as recessive epistasis. In this, the phenotypic ratio changed to 9 : 3 : 4.
Examples:-
i. Colour of skin in mice:-
ii. Colour of glume in sorghum:-
c. Duplicate Genes:- When a character is controlled by two non-allelic genes and the dominant alleles of both the genes give the same character either singly or together, they are called Duplicate Genes. In this the phenotypic ratio changes to 15 : 1.
Example:- Fruit shape in Capsella
d. Masking Genes:- When two non-allelic genes interact with each other in such a way that the dominant allele suppresses the effect of the other non-allele, it is called as masking gene. It is also called as dominant epistasis. In this, the phenotypic ratio is changed to 12 : 3 : 1.
Example:- Color of fruit in Summer squash
(Summer squash or Chappan Kaddu = Cucurbita pepo)
e. Inhibitory Genes:- It is a dominant allele which has no character of its own. But inhibit the expression of the another non-allele. In this, the phenotypic ratio is changed to 13 : 3.
Example:- Color of leaves in paddy
f. Polymeric Genes:- A dominant allele which has its own character. But it increases the intensity of the trait when combined with another dominant allele, called a polymeric gene. It is also called Additive Gene or Cumulative Gene. In this, the phenotypic ratio is changed to 9 : 6 : 1.
Example:- Colour of seeds in wheat
Polygenic inheritance:-
Polygenes (Multiple Genes):- When many genes together control a genetic trait, they are called Polygenes.
Quantitative Traits:-
Ø These are expressed as quantities.
Ø They are polygenetic i.e. one trait is controlled by multiple genes.
Ø The environment has a great influence on traits.
Ø They show continuous variation.
Ø These traits are normally distributed in the population.
Ø Example:- Animal metabolism is controlled by a number of genes. The end product of metabolism is a good example of this type of trait. E.g. milk production, growth rate
Other Examples:- Skin colour, Eye colour, Height, Weight
Variations:- The difference in the characters found in the organisms is called variations. These are of 2 types –
