Prokaryotic gene regulation (basics concept of Operon) for NEET Biology and NET life science students

 

Basic principles of heredity

The principles of heredity explain how genes are passed from generation to generation and how factors such as dominance influence inheritance. Gregor Mendel was the first to demonstrate the basic principles of heredity.

Experiments conducted: 1856 to 1863

Published: 1866

Carl, von and Vries (Botanist) in 1900 recognised Mendel’s principles.

Pea plant characteristics studied by Mendel:

 

 Mendel's success:

1. Choice of experimental subject: Pea plant (Pisum sativum)
2. The plant is easy to cultivate
3. Single growing season
4. Large number of varieties were genetically pure
5. Adopted experimental approach and interpreted the result using mathematics
6. Formulate hypothesis and test it.
7. Seven characteristics chosen for study.
8. Avoid characteristics that display a range of variation.

Some important genetic terms:

The term gene is a word that Mendel never knew. It was coined by Wilhelm Johannsen in 1909.

·       Gene: A genetic factor (region of DNA) that helps to determine a character

·       Allele: One of two or more alternative forms of a gene.

·       Locus: Specific place on a chromosome occupied by an allele.

·       Genotype: Set of alleles possessed by an organism.

·       Phenotype or trait: appearance or manifestation of a character.

·       Heterozygote: an individual organism possessing two different alleles at a locus.

·       Homozygote: an individual organism possessing two of the same allele at a locus.

·       Character: General feature.

An obvious but important concept is that only the alleles of the genotype are inherited. Although the phenotype is determined by genotype, organisms do not transmit their phenotypes to the next generation.

Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance

Mendel began by studying monohybrid crosses—parents differed in a single characteristic. Mendel crossed a pure-breeding (homozygous) pea plant for round seeds with one that was pure-breeding for wrinkled seeds. This first generation of a cross is the P (parental) generation. The offspring from the parents in the P generation are the F1 (filial 1) generation. When Mendel examined the F1 generation of this cross, he found that they expressed only
one of the phenotypes present in the parental generation: all the F1 seeds were round.

Mendel planted the F1 seeds, cultivated the plants that germinated from them, and allowed the plants to self-fertilize, producing a second generation—the F2 (filial 2) generation. About 3/4 of the F2 seeds were round and 1/4 were wrinkled. Mendel reasoned that, although the F1 plants display the phenotype of only one parent, they must inherit genetic
factors from both parents because they transmit both phenotypes to the F2 generation.

The genetic factors (now called alleles) that Mendel discovered the allele for round seeds is usually represented by R, and the allele for wrinkled seeds by r. The plants in the P generation of Mendel’s cross possessed two identical alleles: RR in the round-seeded parent and rr in the wrinkled-seeded parent.

Monohybrid crosses also reveal that the two alleles in each plant separate when gametes are formed, and one allele goes into each gamete. When two gametes (one from each parent) fuse
to produce a zygote, the allele from the male parent unites with the allele from the female parent to produce the genotype of the offspring. Thus, Mendel’s F1 plants inherited an R allele from the round-seeded plant and an r allele from the wrinkled-seeded plant. However, only the trait encoded by the round allele (R) was observed in the F1: all the F1 progeny had round seeds. Those traits that appeared unchanged in the F1 heterozygous offspring Mendel called
dominant, and those traits that disappeared in the F1 heterozygous offspring he called recessive. When dominant and recessive alleles are present together, the recessive allele
is masked, or suppressed, called the concept of dominance.

The common allele for a character—called the wild type because it is the allele usually found in the wild. The mutant phenotype is unusual.

Back cross:

Cross between F1 genotype and either of the parental genotype. F1x homozygous dominant produce dominant phenotype. F1x homozygous recessive produce dominant and recessive phenotype.

Test cross:

One individual of unknown genotype crossed with another individual with a homozygous recessive genotype. If unknown is homozygous recessive, it produces all single phenotype. If unknown is heterozygous, it produces 1:1 phenotype.

 

Dihybrid Crosses Reveal the Principle of Independent Assortment

Mendel crossed varieties of peas that differed in two characteristics—a dihybrid cross.

Mendel carried out a number of dihybrid crosses for pairs of characteristics and always obtained a 9 : 3 : 3 : 1 ratio in the F2. Each plant possesses two alleles encoding each characteristic, and so the parental plants must have had genotypes RR YY and rr yy. The principle of segregation indicates that the alleles for each locus separate, and one
allele for each locus passes to each gamete. Therefore, the gametes produced by the round, yellow parent contain alleles RY, whereas the gametes produced by the wrinkled, green parent contain alleles ry. These two types of gametes unite to produce the F1, all with genotype Rr Yy. Because round is dominant over wrinkled and yellow is dominant over green,
the phenotype of the F1 will be round and yellow. When Mendel self-fertilized the F1 plants to produce the F2, the alleles for each locus separated, with one allele going into each gamete. Both kinds of separation occur equally and all four type of gametes (RY, ry, Ry, and rY) are produced in equal proportions. When these four types of gametes are
combined to produce the F2 generation, the progeny consist of 9/16 round and yellow, 3/16 wrinkled and yellow, 3/16 round and green, and 1/16 wrinkled and green, resulting in a
9 : 3 : 3 : 1 phenotypic ratio.