Main Difference – Homologous vs Non-homologous Chromosomes

Main Difference – Homologous vs Non-homologous Chromosomes

Chromosomes in the nucleus are the arrangement of genetic material of a particular cell. Homologous and non-homologous chromosomes are the two types chromosomes identified based on the pairing pattern of chromosomes during the metaphase 1 of meiosis. Homologous chromosomes refer to the chromosomes in the same pair while non-homologous chromosomes refer to the chromosomes in different pairs. The main difference between homologous and non-homologous chromosomes is that homologous chromosomes consist of alleles of the same type of genes in the same loci whereas non-homologous chromosomes consist of alleles of different types of genes.

Key Areas Covered

1. What are Homologous Chromosomes
– Definition, Characteristics, Examples
2. What are Non-homologous Chromosomes
– Definition, Characteristics, Examples
3. What are the Similarities Between Homologous and Non-homologous Chromosomes
– Outline of Common Features
4. What is the Difference Between Homologous and Non-homologous Chromosomes
– Comparison of Key Differences

Key Terms: Alleles, Centromere Position, Homologous Chromosomes, Locus, Non-homologous Chromosomes, Recombination, Translocation

What are Homologous Chromosomes

Homologous chromosomes are chromosomes with the same gene sequences that are paired during meiosis. The number of chromosomes in the pair is determined by ploidy of that particular organism. Ploidy refers to the number of chromosome sets present in the organism. Generally, most organisms are diploid, consisting of two chromosomes in the homologous pair, one with the maternal origin and the other with paternal origin. One chromosome of the homologous pair is referred to as a homologue. The length of the arms and the position of the centromere are the same in the chromosomes in the homologous pair. Each homologue of the pair consists of alleles of the same genes in the same loci (order). Therefore, the banding pattern of the homologues is similar. The human genome consists of 22 pairs of autosomal chromosomes and 2 sex chromosomes. The autosomal chromosomes are homologous. Only the female sex chromosomes, X and X are homologous; male sex chromosomes, X and Y are not truly homologous. One of the most significant consequences of homologous chromosomes is the genetic recombination. Genetic recombination of homologous chromosomes is shown in figure 1.

Figure 1: Genetic Recombination in Homologous Chromosomes

Genetic recombination occurs during the metaphase 1 of meiosis 1 where the exchange of chromosome parts of the homologous chromosomes occurs. It leads to genetic variation in the offspring. The non-disjunction of homologous chromosomes during meiosis leads to chromosomal abnormalities such as monosomy and trisomy.

What are Non-homologous Chromosomes

Non-homologous chromosomes are chromosomes that do not belong to the same pair. Generally, the shape of the chromosome, that is, the length of the arms and the position of the centromere, is different in non-homologous chromosomes. Therefore, non-homologous chromosomes do not pair during meiosis. Each chromosome of a particular organism only pairs with its homologue. This means homologous pairs segregate from other chromosomes of the nucleus as described by the Law of segregation. The chromosomes of a diploid human cell are shown in figure 2.

Figure 2: Human Chromosomes

The autosomal chromosome pairs are shown from 1 – 22. Each chromosome in the pair are homologous chromosomes since the banding pattern and the position of the centromere are the same in each chromosome. But, the individual chromosomes of the different pairs are non-homologous chromosomes. They consist of different banding patterns as well as different centromere positions. The female sex chromosomes (XX) are also homologous. But male sex chromosomes are non-homologous. The size, banding pattern (genetic composition), and the position of the centromere differ in X and Y chromosomes. Translocations are the most significant consequences of non-homologous chromosomes; here, parts of different chromosomes are exchanged between each other. Translocations are a type of chromosomal mutations that sometimes produce lethal conditions in individuals.

Similarities Between Homologous and Non-homologous Chromosomes

• Both homologous and non-homologous chromosomes are made up of DNA.
• The chromosome structures such as a centromere, long and short arms are common in both homologous and non-homologous chromosomes.
• Both homologous and non-homologous chromosomes consist of alleles.

Difference Between Homologous and Non-homologous Chromosomes

Definition

Homologous Chromosomes: Homologous chromosomes refer to a pair of chromosomes having the same gene sequences, each derived from one parent.

Non-Homologous Chromosomes: Non-homologous chromosomes are chromosomes that do not belong to the same pair.

Significance

Homologous Chromosomes: Homologous chromosomes belong to the same pair of chromosomes consisting of maternal and paternal chromosomes.

Non-Homologous Chromosomes: Non-homologous chromosomes are the chromosomes that belong to the different homologous pairs.

Types of Alleles

Homologous Chromosomes: Homologous chromosomes consist of alleles of same genes located in the same loci.

Non-Homologous Chromosomes: Non-homologous chromosomes consist of alleles of different genes.

Pairing Pattern

Homologous Chromosomes: Homologous chromosomes pair during meiosis 1.

Non-Homologous Chromosomes: Non-homologous chromosomes do not pair during meiosis 1.

Structure

Homologous Chromosomes: Chromosomal arms and the centromere positions are the same in homologous chromosomes.

Non-Homologous Chromosomes: Non-homologous chromosomes have different chromosomal arm lengths and centromere positions.

Consequences

Homologous Chromosomes: The parts of the homologous chromosomes can be exchanged during recombination.

Non-Homologous Chromosomes: The parts of non-homologous chromosomes can be exchanged during translocations.

Examples

Homologous Chromosomes: The 22 autosomal chromosomes are homologous in humans.

Non-Homologous Chromosomes: The X and Y chromosomes are non-homologous.

Conclusion

Homologous and non-homologous chromosomes are the two types of chromosomes found in the genome. Homologous chromosomes pair during meiosis. The homologous pair consists of alleles of the same genes in the same loci in both chromosomes. But, non-homologous chromosomes consist of alleles of different genes. The main difference between homologous and non-homologous chromosomes is the relationship of alleles.

Reference:

1. Bailey, Regina. “What Are Homologous Chromosomes?” ThoughtCo, Available here.
2. “Nonhomologous chromosomes.” The Free Dictionary, Farlex, Available here.

Image Courtesy:

1. “Figure 11 01 02” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Karyotype” By Courtesy: National Human Genome Research Institute – Modified from Human Genome ProjectFrom en: with same file name, contributor: en:User:TedE (Public Domain) via Commons Wikimedia