Mendel's Law: Segregation In Dihybrid And Monohybrid Crosses

Gregor Mendel, the father of genetics, discovered three laws of inheritance through experiments on pea plants: the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. Mendel's Law of Segregation, also known as the Law of Purity of Gametes, states that during the formation of gametes, each gene separates from each other so that each gamete carries only one allele for each gene. This law applies to both monohybrid and dihybrid crosses, where monohybrid refers to a cross between two monohybrid traits, and dihybrid refers to a cross between two parents that differ by two pairs of alleles. In a monohybrid cross, both alleles are expressed in the F2 generation without any blending, resulting in a 3:1 ratio of dominant to recessive traits. In a dihybrid cross, the independent assortment of genes can be observed, where each gamete can contain any combination of paternal and maternal chromosomes.

Mendel's law of segregation and monohybrids

Mendel's Law of Segregation, also known as the Law of Purity of Gametes, is one of three laws of inheritance formulated by Gregor Johann Mendel, the 19th-century founder of genetics. Mendel's laws form the theoretical basis of our understanding of the genetics of inheritance.

Mendel's Law of Segregation states that:

> "During the formation of gamete, each gene separates from each other so that each gamete carries only one allele for each gene."

This law is based on four basic concepts:

• A gene exists in more than one form of an allele.
• When gametes are produced by meiosis, the allelic pairs separate, leaving each gamete with a single allele.
• Every organism inherits two alleles for each trait.
• The two alleles of a pair are different, i.e., one is dominant and one is recessive.

Mendel's Law of Segregation is applicable to both dihybrid and monohybrid crosses. In a monohybrid cross, both the alleles are expressed in the F2 generation without any blending. This is because each gamete contains only one allele, as per the Law of Segregation. Mendel's experiments on pea plants demonstrated this law, as he observed that traits which were absent in the F1 generation reappeared in the F2 generation.

Mendel's three laws of inheritance are:

• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

Mendel's laws of inheritance were initially controversial and were not fully accepted by the scientific community until the early 20th century. Today, they are taught as the foundation of classical genetics, although it is now known that many traits are inherited in a non-Mendelian fashion.

Mendel's law of segregation and dihybrids

Mendel's Law of Segregation, also known as the Law of Purity of Gametes, states that during the formation of gametes, each gene separates from each other so that each gamete carries only one allele for each gene. This law is based on four basic concepts:

• A gene exists in more than one form of an allele.
• When gametes are produced by meiosis, the allelic pairs separate, leaving each gamete with a single allele.
• Every organism inherits two alleles for each trait.
• The two alleles of a pair are different, i.e., one is dominant and one is recessive.

This law is applicable to both monohybrid and dihybrid crosses. During fertilization, each allele in its pair separates and enters a gamete, so each gamete has only one allele of the pair and is thus pure for a trait. This is why a trait that is not expressed in one generation can be expressed in the next.

Mendel's Second Law, the Law of Independent Assortment, states that during gamete formation, the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair. This law can be illustrated by a dihybrid cross, which is a cross between two true-breeding parents that express different traits for two characteristics. For example, consider a dihybrid cross involving seed colour and seed texture for two pea plants, one with green, wrinkled seeds (yyrr) and another with yellow, round seeds (YYRR). The F1 generation of offspring are all heterozygous (YyRr). For the F2 generation, the law of segregation requires that each gamete receives either an R or an r allele, along with either a Y or a y allele. The law of independent assortment states that a gamete with an r allele is equally likely to contain either a Y or a y allele. Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed: YR, Yr, yR, and yr.

Therefore, Mendel's laws of segregation and independent assortment are applicable to both monohybrid and dihybrid crosses.

Mendel's law of independent assortment

Gregor Mendel, in 1865, was the first to observe the independent assortment of genes and their corresponding traits while studying genetics in pea plants. He performed dihybrid crosses, which are crosses between organisms that differ in regard to two traits. Mendel discovered that the combinations of traits in the offspring of his crosses did not always match the combinations of traits in the parental organisms. This led him to formulate the law of independent assortment.

The law of segregation states that each gamete will receive either an R allele or an r allele, along with either a Y allele or a y allele. The law of independent assortment states that a gamete that receives an r allele is equally likely to contain either a Y or a y allele. There are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed: YR, Yr, yR, and yr. These gametes can be arranged in a 4x4 Punnett square, resulting in 16 equally likely genotypic combinations. From these genotypes, a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green can be inferred.

The physical basis for the law of independent assortment lies in meiosis I, during which the different homologous pairs line up in random orientations. Each gamete can contain any combination of paternal and maternal chromosomes, as the orientation of tetrads on the metaphase plane is random.

While the law of independent assortment generally holds true, there is an exception for genes that are located very close to each other on the same chromosome due to genetic linkage. In this case, the alleles tend to be inherited as a unit, and the genes do not assort independently.

Limitations of Mendel's law of segregation

Mendel's Law of Segregation is applicable to both monohybrid and dihybrid crosses. During fertilization, each allele in its pair separates and enters a gamete, meaning that each gamete only carries one allele of the pair. This is known as Mendel's second law, also called the Law of Purity of Gametes.

However, Mendel's Law of Segregation has several limitations. Firstly, it only applies to diploid organisms that are formed from haploid gametes during sexual reproduction. The law is not valid for genes that are collaborative and may vary in expression, nor does it work for complementary genes. It also does not hold true for alleles that exhibit incomplete dominance or codominance, nor for traits encoded by more than one gene pair. Furthermore, it only applies to traits that are completely controlled by a single gene pair, where one of the two alleles is dominant over the other.

Mendel's law of dominance

Each character is controlled by distinct units called factors, which occur in pairs. If the pairs are heterozygous, one will always dominate the other. In a monohybrid cross between a pair of contrasting traits, only one parental character will be expressed in the F1 generation, and both parental characters will be expressed in the F2 generation in a 3:1 ratio.

The trait that is expressed in the phenotype is called the dominant trait, and the one that is suppressed is called the recessive trait. Mendel's experiments with pea plants demonstrated this concept. He crossed tall and dwarf pea plants, resulting in tall offspring (the F1 progeny). When the F1 progeny plants were self-pollinated, this resulted in both tall and short plants in a 3:1 ratio.

Dominant alleles are expressed exclusively in a heterozygote, while recessive traits are expressed only if the organism is homozygous for the recessive allele. However, it is important to note that dominance is not inherent. One allele can be dominant over one allele but recessive to another, and not all traits follow simple dominance as a form of inheritance.

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