Medallion Inheritance – Monohybrid and Dihybrid cross

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MENDELIAN INHERITENCE

Mendelian inheritance is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900. These laws were initially very controversial. When Mendel’s theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher later combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics and the modern evolutionary synthesis.

Mendel’s laws

Mendel discovered that, when he crossed purebred white flower and purple flower pea plants (the parental or P generation), the result was not a blend. Rather than being a mix of the two, the offspring (known as the F1 generation) was purple-flowered. When Mendel self-fertilized the F1 generation pea plants, he obtained a purple flower to white flower ratio in the F2 generation of 3 to 1. The results of this cross are tabulated in the Punnett square to the right.

Law of Segregation of genes: The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles. An offspring thus receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms: one allele for each trait from each parent.[6] Molecular proof of this principle was subsequently found through observation of meiosis by two scientists independently, the German botanist Oscar Hertwig in 1876, and the Belgian zoologist Edouard Van Beneden in 1883. Paternal and maternal chromosomes get separated in meiosis and the alleles with the traits of a character are segregated into two different gametes. Each parent contributes a single gamete, and thus a single, randomly successful allele copy to their offspring and fertilization.

Law of Independent Assortment

Mendel’s law of independent assortment, states that allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another.  Mendel formulated this principle after performing dihybrid crosses between plants that differed in two traits, such as seed color and pod color.   After these plants were allowed to self pollinate, he noticed that the same ratio of 9:3:3:1 appeared among the offspring. Mendel concluded that traits are transmitted to offspring independently.

Law of Dominance

Mendel’s Law of Dominance states that recessive alleles will always be masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. Law of Dominance can be explained easily with the help of a mono hybrid cross experiment:- In a cross between two organisms pure for any pair (or pairs) of contrasting traits (characters), the character that appears in the F1 generation is called “dominant” and the one which is suppressed (not expressed) is called “recessive.” Each character is controlled by a pair of dissimilar factors. Only one of the characters expresses. The one which expresses in the F1 generation is called Dominant. It is important to note however, that the law of dominance is significant and true but is not universally applicable. According to the latest revisions, only two of these rules are considered to be laws. The third one is considered as a basic principle but not a genetic law of Mendel.

 

Monohybrid cross

A monohybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in a single given trait. The P generation organisms are homozygous for the given trait, however each parent possesses different alleles for that particular trait. A Punnett square may be used to predict the possible genetic outcomes of a monohybrid cross based on probability. This type of genetic analysis can also be performed in a dihybrid cross, a genetic cross between parental generations that differ in two traits.   Traits are characteristics that are determined by discrete segments of DNA called genes. Individuals typically inherit two alleles for each gene. An allele is an alternate version of a gene that is inherited (one from each parent) during sexual reproduction. Male and female gametes, produced by meiosis, have a single allele for each trait. These alleles are randomly united at fertilization.  Example: In the image above, the single trait being observed is pod color. The organisms in this monohybrid cross are true-breeding for pod color. True-breeding organisms have homozygous alleles for specific traits. In this cross, the allele for green pod color (G) is completely dominant over the recessive allele for yellow pod color (g). The genotype for the green pod plant is (GG) and the genotype for the yellow pod plant is (gg). Cross-pollination between the true-breeding homozygous dominant green pod plant and the true-breeding homozygous recessive yellow pod plant results in offspring with phenotypes of green pod color.  All genotypes are (Gg). The offspring or F1 generation are all green because the dominant green pod color obscures the recessive yellow pod color in the heterozygous genotype.

Monohybrid cross: F2 generation

The F2 generation would have genotypes of (GG, Gg, and gg) and a genotypic ratio of 1:2:1. One-fourth of the F2 generation would be homozygous dominant (GG), one-half would be heterozygous (Gg), and one-fourth would be homozygous recessive (gg). The phenotypic ratio would be 3:1, with three-fourths having green pod color (GG and Gg) and one-fourth having yellow pod color (gg).

Dihybrid cross

A dihybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in two traits. The individuals in this type of cross are homozygous for a specific trait. Traits are characteristics that are determined by segments of DNA called genes. Diploid organisms inherit two alleles for each gene. An allele is an alternate version of a gene that is inherited (one from each parent) during sexual reproduction.   In a dihybrid cross, the parent organisms have different pairs of alleles for each trait being studied. One parent possesses homozygous dominant alleles and the other possesses homozygous recessive alleles. The offspring, or F1 generation, produced from the genetic cross of such individuals are all heterozygous for the specific traits. This means that all of the F1 individuals possess a hybrid genotype and express the dominant phenotypes for each trait.

 

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