Mendel's Law: When Independent Assortment Fails

Gregor Mendel's Law of Independent Assortment, also known as his second law of inheritance, states that the alleles of two or more genes are sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene. However, this law does not always apply. An exception to the law is when genes are located very close to one another on the same chromosome, due to genetic linkage.

Genetic linkage

Mendel's Law of Independent Assortment states that every trait is inherited independently of every other trait. However, this law does not always apply, with genetic linkage being its most prominent exception.

Genes on separate chromosomes are never linked. During the formation of gametes, chromosomes undergo homologous recombination, where they first make identical copies of themselves, called sister chromatids, which remain attached. Next, the four chromatids line up and swap large sections of DNA. Finally, the chromatids are divided so that each gamete receives one copy of each chromosome. While each gamete ends up with one copy of every gene, they have different combinations of alleles for those genes.

The typical unit of genetic linkage is the centimorgan (cM). A distance of 1 cM between two markers means that the markers are separated onto different chromosomes on average once per 50 meiotic products, or once per 50 meioses.

Homologous chromosomes

Mendel's 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 experiment always showed that the combinations of traits of the offspring were different from their parents.

Chromosomes are linear arrangements of condensed deoxyribonucleic acid (DNA) and histone proteins, forming a complex called chromatin. Homologous chromosomes are made up of chromosome pairs of approximately the same length, centromere position, and staining pattern, for genes with the same corresponding loci. One homologous chromosome is inherited from the mother, and the other from the father. After mitosis occurs within the daughter cells, they have the correct number of genes, a mix of the two parents' genes.

The two main properties of homologous chromosomes are the length of chromosomal arms and the placement of the centromere. The actual length of the arm is critical for proper alignment, and the placement of the centromere can be characterised by four main arrangements: metacentric, submetacentric, acrocentric, or telocentric. Humans have a total of 46 chromosomes, with 22 pairs of homologous autosomal chromosomes. The additional 23rd pair is the sex chromosomes, X and Y.

Meiosis

Gregor Mendel's discovery of the Law of Independent Assortment was a turning point in the history of genetics. Mendel's Law of Independent Assortment states that the alleles of two or more genes are sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene. Mendel's experiments on pea plants showed that the combinations of traits in the offspring were always different from their parental traits.

The Law of Independent Assortment occurs during the process of meiosis. Meiosis is a type of cell division that reduces the number of chromosomes in a parent cell by half to produce four reproductive cells called gametes. In humans, diploid cells contain 46 chromosomes, with 23 chromosomes inherited from each parent. These pairs of similar chromosomes are called homologous chromosomes. During meiosis, the pairs of homologous chromosomes are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is random. This means that all of the maternal chromosomes will not be separated into one cell, while all the paternal chromosomes are separated into another. Instead, after meiosis occurs, each haploid cell contains a mixture of genes from the mother and the father.

Another feature of the Law of Independent Assortment is recombination. Recombination occurs during meiosis and is a process that breaks and recombines pieces of DNA to produce new combinations of genes. Recombination scrambles pieces of maternal and paternal genes, ensuring that genes assort independently of one another.

However, it is important to note that there is an exception to the Law of Independent Assortment for genes that are located very close to one another on the same chromosome due to genetic linkage. In this case, the genes do not display independent assortment and are said to be linked.

Recombination

Mendel's Law of Independent Assortment states that the alleles of two or more genes are sorted into gametes independently of one another. In other words, the alleles received for one gene do not influence the alleles received for another gene. This law was formulated by Gregor Mendel in the mid-19th century, based on his experiments with pea plants.

However, this law does not always apply. There is an exception to the law of independent assortment for genes that are located very close to one another on the same chromosome. In this case, genetic linkage can occur, where the alleles on the same chromosome tend to be inherited as a unit, rather than assorting independently.

The reason for this exception lies in the process of recombination, which occurs during meiosis. Recombination is a process that breaks and recombines pieces of DNA to produce new combinations of genes. It ensures that genes assort independently from one another by scrambling pieces of maternal and paternal genes.

During meiosis, the pairs of homologous chromosomes are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is random. However, when genes are located very close together on the same chromosome, they may not have the opportunity to recombine and may, therefore, be inherited as a unit.

In summary, while Mendel's Law of Independent Assortment holds true for genes that are located on different chromosomes or far apart on the same chromosome, it does not always apply to genes that are in close proximity on the same chromosome due to the phenomenon of genetic linkage.

Penetrance and expressivity

Mendel's Law of Independent Assortment states that the alleles of two or more genes are sorted into gametes independently of one another. 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.

Mendel's discovery of this law was based on experiments that followed the segregation of two genes. He performed a dihybrid cross between two parents that differed by two pairs of alleles (AABB x aabb). The dominance relationship between alleles for each trait was already known to Mendel, and he wanted to determine if any relationship existed between different allelic pairs.

The results of Mendel's dihybrid cross experiment led him to formulate his second law, the Law of Independent Assortment. This law 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. Mendel confirmed the results of his second law by performing a backcross, where the F1 dihybrid was crossed with a recessive parent.

The Law of Independent Assortment is a fundamental principle of inheritance in genetics, and it gives each pair of characters a chance of expression. However, it is important to note that this law only applies when there is independent assortment during the process of meiosis, where the chromosomes are halved and are known as haploid.

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