Independent Assortment: Monohybrid Crosses And Mendel's Law

Mendel's Law of Independent Assortment is one of the three laws of inheritance formulated by Mendel, alongside the Law of Dominance and the Law of Segregation. Mendel's laws are based on his experiments with pea plants, in which he crossed plants with contrasting traits and observed the phenotypes of the offspring. The Law of Independent Assortment states that the alleles of two or more genes are sorted into gametes independently of each other. In other words, the allele received for one gene does not influence the allele received for another gene. Mendel's experiments always showed that the combinations of traits in the offspring were different from their parents, leading him to formulate the Law of Independent Assortment. This law is fundamental to our understanding of genetics and inheritance.

Mendel's Second Law

The Law of Independent Assortment states that the alleles of two or more genes are sorted into gametes independently of each other. In other words, the allele received for one gene does not influence the allele received for another gene. Mendel's experiments consistently showed that the combinations of traits in the offspring were always different from their parents, supporting this law.

This law can be illustrated by a dihybrid cross, which involves two true-breeding parents expressing different traits for two characteristics. For example, Mendel studied pea plants with green, wrinkled seeds (yyrr) and others with yellow, round seeds (YYRR). The F1 generation of this cross resulted in offspring that were all heterozygotes (YyRr). When these F1 offspring were self-crossed, the F2 generation exhibited four different combinations of seeds in a phenotypic ratio of 9:3:3:1 (round/yellow, round/green, wrinkled/yellow, and wrinkled/green).

The Law of Independent Assortment states that during a dihybrid cross, the assortment of each pair of traits is independent of the other. This means that during gamete formation, one pair of traits segregates independently from another pair, giving each pair of characters a chance of expression. The physical basis for this law lies in meiosis I, where different homologous chromosome pairs line up in random orientations, allowing any combination of paternal and maternal chromosomes in the gametes.

Law of Independent Assortment

Mendel's Law of Independent Assortment is one of the three laws of inheritance formulated by Mendel, the others being the Law of Dominance and the Law of Segregation. Mendel's laws of inheritance are fundamental principles of genetics.

The Law of Independent Assortment states that the alleles of two or more genes are sorted into gametes independently of each other. In other words, the allele received for one gene does not influence the allele received for another gene. Mendel's experiments always showed that the combinations of traits of the offspring were different from their parents. This led him to formulate the Law of Independent Assortment.

The law applies to sexually reproducing organisms. It states that during a dihybrid cross (crossing of two pairs of traits), the assortment of each pair of traits is independent of the other. In other words, during gamete formation, one pair of trait genes segregates from another pair of trait genes independently. This gives each pair of characters a chance of expression.

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

The law of independent assortment can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics. For example, Mendel crossed two pea plants, one with green, wrinkled seeds (yyrr) and another with yellow, round seeds (YYRR). The F1 generation of offspring were all YyRr. For the F2 generation, the law of segregation requires that each gamete 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 in which an r allele is sorted would be equally likely to contain either a Y allele 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.

The law of independent assortment also indicates that a cross between yellow, wrinkled (YYrr) and green, round (yyRR) parents would yield the same F1 and F2 offspring as in the YYRR x yyrr cross.

Monohybrid Crosses

The results of monohybrid crosses also informed Mendel's Law of Independent Assortment, which he tested through dihybrid crosses. This law states that the alleles of two or more genes are sorted into gametes independently of each other. In other words, the allele received for one gene does not influence the allele received for another gene. Mendel's dihybrid crosses produced a 9:3:3:1 phenotypic ratio, which could be calculated using the product rule or a Punnett square.

The law of independent assortment can be illustrated by a dihybrid cross between two true-breeding parents that express different traits for two characteristics. For example, Mendel crossed pea plants with green, wrinkled seeds (yyrr) and yellow, round seeds (YYRR). The F1 generation of offspring were all heterozygotes (YyRr). When these F1 offspring were self-crossed, the F2 generation exhibited four phenotypes: round/yellow, round/green, wrinkled/yellow, and wrinkled/green, in a 9:3:3:1 ratio.

The law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur. This law is based on the random orientation of tetrads on the metaphase plane during meiosis I, allowing each gamete to contain any combination of paternal and maternal chromosomes and the genes they carry.

Dihybrids

Mendel's Second Law, also known as the Law of Independent Assortment, states that the segregation of alleles at one locus will not influence the segregation of alleles at another locus during gamete formation. In other words, the alleles segregate independently.

To test this law, Mendel performed a dihybrid cross, which is a cross between two true-breeding parents that express different traits for two characteristics. For example, he crossed a pure-breeding line of green, wrinkled peas with a pure-breeding line of yellow, round peas. This produced F1 progeny that were all yellow and round, and they were called dihybrids because they carried two alleles at each of the two loci.

The F1 dihybrids were then crossed to produce four phenotypic classes in the F2 generation, which appeared in a 9:3:3:1 ratio. This ratio was calculated using the product rule, which states that the probability of two or more independent events occurring is the product obtained by multiplying the probabilities of the individual events. For example, the probability of a round and yellow phenotype in the offspring is calculated by multiplying the probability of a round phenotype (3/4) by the probability of a yellow phenotype (3/4), giving 9/16.

The same 9:3:3:1 phenotypic ratio can also be obtained using a Punnett Square. In this case, the genotypes of the possible gametes are listed along each axis of the Punnett Square, resulting in 16 equally likely genotypic combinations. By applying our knowledge of the alleles and their dominance relationships, we can assign a phenotype to each of the 16 genotypes.

The 9:3:3:1 phenotypic ratio assumes that:

• Both loci assort independently.
• One allele at each locus is completely dominant.
• Each of the four possible phenotypes can be distinguished unambiguously, with no interactions between the two genes that would interfere with determining the genotype correctly.

Deviations from the 9:3:3:1 phenotypic ratio may indicate that one or more of these conditions have not been met. For example, linkage is one of the most important reasons for distortion of the ratios expected from independent assortment. Two loci show linkage if they are located close together on the same chromosome, altering the frequency of allele combinations in the gametes.

Linked Genes

Genes that are located on separate non-homologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes, and the segregation of alleles into gametes can be influenced by linkage. Genes located physically close to each other on the same chromosome are more likely to be inherited as a pair.

For example, in a dihybrid cross involving flower colour and plant height, if one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, the tall and red alleles will go together into a gamete, and the short and yellow alleles will go into other gametes.

However, it is possible for two genes on the same chromosome to behave independently, or as if they are not linked, due to the process of recombination, or "crossover". Homologous chromosomes possess the same genes in the same linear order, and during meiosis, they replicate and synapse, with like genes on the homologs aligning with each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material in a process called recombination or crossover. This is a common genetic process that results in maternal and paternal alleles being combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.

As the distance between two genes increases, the probability of one or more crossovers between them also increases, and the genes behave more like they are on separate chromosomes. Geneticists have used the proportion of recombinant gametes (those not like the parents) as a measure of how far apart genes are on a chromosome.

Frequently asked questions