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Molecular Genetics

Difference Between Homozygous & Heterozygous

The human DNA is a complex aspect of living organisms. Understanding the difference between homozygous and heterozygous is an important part of the study of DNA and genetics. Students can learn more about homozygous and heterozygous here. Most noteworthy, students will understand the difference between homozygous and heterozygous here.

difference between homozygous and heterozygous

Table of content

1 What is the Difference Between Homozygous and Heterozygous?

1.1 Chromosomes

1.2 Homozygous and Heterozygous

1.3 Dominant and Recessive Inheritance

1.4 Solved Questions on Homozygous and Heterozygous

What is the Difference Between Homozygous and Heterozygous?

Homozygous, simply speaking, means having identical alleles for a single trait. Furthermore, alleles are known to exist in various different forms.

The diploid organisms almost always have two alleles for a particular trait. Homozygous means a particular r gene that has alleles of an identical nature on both the homologous chromosomes.

The situation of heterozygous stands in contrast to that of homozygous. This is because, in heterozygous inheritance, the genes come from two cells which are parent cells.

When it comes to diploid organisms, heterozygous happens to be an organism that has two different alleles for a certain trait. This article discusses the difference between homozygous and heterozygous in detail.

Chromosomes

Humans are certainly diploid organisms. What this means is that each cell contains two copies of each and every chromosome. For the purpose of conservation of diploidy, the sperm and egg cells each contribute only one copy of each chromosome at the time of conception.

This causes the offspring to receive the full diploid complement. Meiosis is the process by which sperm and egg cells facilitate the splitting of chromosomal copies in order to become haploid.

Homozygous and Heterozygous

When we talk about genetic traits, experts look at genes. Furthermore, they also look at the locus where that trait or gene encodes on the chromosome. Also, humans certainly have two copies of each chromosome. Similarly, they also possess copies of each gene and locus on such chromosomes.

Allele refers to each of these trait-encoding genes (or loci). In case the alleles match, for that particular trait the person is homozygous. In contrast, if the alleles are different, then for that particular trait the person is heterozygous.

Dominant and Recessive Inheritance

For this type of inheritance, one must look at alleles using letters where a capital letter shows a dominant allele, and a lowercase letter is representative of a recessive allele: AA, Aa, and aa.

Examples of dominant traits can be like the ability to roll the tongue or have a genetic likelihood of developing Huntington’s disease. Moreover, such traits only require one dominant allele for the purpose of expression.

This means that people or organisms with homozygous dominant alleles (AA) and also the heterozygous alleles (Aa) express such traits. However, people with homozygous recessive alleles (aa) certainly do not express such traits.

Recessive traits, in contrast, are like having straight thumbs or cystic fibrosis. Furthermore, such recessive traits require two recessive alleles for the purpose of expression.

This means that only people who are characterized by the homozygous recessive alleles (aa) express the trait. People with homozygous dominant alleles (AA) will not carry the trait or express it. Finally, there is no expression of the trait from individuals with heterozygous alleles (Aa) but they happen to be carriers for it.

Solved Questions on Homozygous and Heterozygous

Q1 Which of the following statements with regards to homozygous and heterozygous is wrong?

A. Homozygous means having identical alleles for a single trait
B. In heterozygous inheritance, the genes come from two cells which are parent cells
C. Humans are not diploid organisms
D. The diploid organisms almost always have two alleles for a particular trait

A1 The correct option is C., which is “humans are not diploid organisms.” This is because the correct statement is “humans are certainly diploid organisms.” The other three options are definitely true with regards to homozygous and heterozygous.

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Molecular Genetics

What Is a Chromosome?

Living organisms are defined by the ability to pass on their genetic information to the next generation of offspring. Chromosomes provide organisms with the means by which this information can be transcribed and replicated for inheritance by daughter cells or offspring.

A chromosome represents a molecule containing millions of base pairs that form the genetic code for a living organism. Chromosomes are found within the nucleus of eukaryotic cells. They are thin, thread-like, and consist of proteins and DNA arranged into specific genes coding for specific traits.

Because of the length of chromosomes in complex organisms, this genetic material needs to be compacted in order to fit within the nucleus of a cell. Chromosomes accomplish this by compacting DNA into distinct units. A chromosome is formed when DNA wraps around a protein know as a histone. The histone can be thought of as a spool and the DNA as the thread. The DNA wraps around the histone much like a spool of thread. Eight histones together form a complex known as a nucleosome. The nucleosomes then wrap around each other to form chromatin. Nucleosomes can be thought of as the beads found on a necklace. The necklace in this case is the chromatin. The chromatin then becomes further condensed until it forms an individual chromosome.

Humans have 46 chromosomes in total. The chromosomes form 22 pairs of numbered autosomal chromosomes and one pair of sex chromosomes that determines the biological gender of an individual. These chromosomes can be divided into four main types depending on the location of the centromere, which links sister chromatids together and serves as the site for spindle fiber attachment during mitosis. The four types include:

In this lesson, these four chromosome types will be further explored.


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What Is the Function of Chromosomes?

The main purpose of chromosomes is to transmit genetic information from generation to the next. Another important chromosome function is to help maintain the integrity of the DNA by condensing this genetic material in order to prevent DNA from getting tangled or damaged. Chromosomes contain the DNA which is not only responsible for the observed traits, or phenotype, of an individual, but also for processes such as gene regulation, protein synthesis, and cellular replication.

Complex organisms like humans tend to be characterized by having multiple pairs of chromosomes that are linear in structure. However, other organisms, like bacteria, possess circular chromosomes. The mitochondria of human cells likewise contain its own circular chromosome, reflecting the early relationship between eukaryotic cells and mitochondria as symbionts that eventually formed a single unit.

Chromosome structure.


Different Types of Chromosomes

Each chromosome consists of several structures: the p arm, the q arm, and the centromere. The p arm refers to the shorter arm of the chromosome, while the q arm represents the longer one. In the constricted portion of the chromosome, sister chromatids are joined by the centromere. The location of the centromere provides the criteria for the four different types of chromosomes- metacentric, submetacentric, acrocentric, and telocentric. These chromosome types are important when mapping out the chromosomes of an organism.

Metacentric Chromosome

In metacentric chromosomes, the p and q arms are of roughly equal length and the centromere is located at the center of the chromosome. Metacentric chromosomes can be thought of as having a V-shape. In humans, chromosome pairs 1, 3, and 16 are described as metacentric.

Submetacentric Chromosome

Submetacentric chromosomes are characterized by a placement of the centromere which is slightly off-center. This produces p and q arms that possess slight differences in length from each other. This produces a J- or L- shaped chromosome. Chromosomes 2, 6, and 10 are considered submetacentric in humans.

Question Video: Defining the Function of ChromosomesBiology

Chromosomes are formed from the nucleic acid DNA. What is the primary function of these chromosomes? [A] To keep the contents of the nucleus and the cytoplasm separate. [B] To act as the site of cellular respiration. [C] To provide essential nutrition for the cell. [D] To carry genetic information and allow it to be passed from parent to offspring.

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Video Transcript

Chromosomes are formed from the nucleic acid DNA. What is the primary function of these chromosomes? To keep the contents of the nucleus and the cytoplasm separate. To act as the site of cellular respiration. To provide essential nutrition for the cell. Or to carry genetic information and allow it to be passed from parent to offspring.

Let’s start by deciphering the information in the question and what we already know about the structure and function of chromosomes. Each chromosome is a large unit used by cells to organize and store their DNA. You can see a chromosome on the diagram here. Chromosomes can be made up of one or two chromatids. In this case, it consists of two chromatids. If this chromosome was to unravel and if we had a very sophisticated microscope so we could zoom in really close on it, we might be able to see the DNA like it’s shown in the side of the diagram here.

DNA is the molecule that carries the genetic instructions for life. It can be passed on from a parent to their offspring. DNA takes the shape of a double helix. DNA is found in the form of these chromosomes in the nucleus of almost all eukaryotic cells. This nucleus is surrounded by a membrane called the nuclear envelope, which controls what can enter and leave the nucleus. As the nuclear envelope is a membrane, it also acts to separate the nucleus and its contents from that of the cytoplasm. Now that we know a little bit more about chromosomes and their function, let’s try to work out the correct option using the process of elimination.

When a question asks for the primary function of something, several of the answers may seemingly be correct. We are looking for the option which defines the function of chromosomes best. So all answers must be carefully scrutinized. One of our options refers to keeping the contents of the nucleus and the cytoplasm separate. This is a role of a membrane, and in this specific case, it is referring to the nuclear envelope. Chromosomes, however, are not membranes, and therefore this option can be eliminated.

Another of our options refers to the site of cellular respiration. Cellular respiration does not occur in the nucleus of eukaryotic cells but in a different organelle called a mitochondrion, which is pictured here. This is, therefore, not a function of a chromosome. The next option refers to another key life process other than respiration, which is obtaining nutrition. Some eukaryotic organisms can obtain nutrition from structures within their own cells, such as plants, which have chloroplasts in their cells, like the one pictured here.

Plants carry out a process called photosynthesis, using light energy to synthesize glucose in these chloroplasts. Not all eukaryotes have chloroplasts, so not all of them can obtain nutrition within their own cells, however. This cannot therefore be a role of a structure common to all eukaryotic organisms, such as the chromosome, so this option is also incorrect.

This leaves us with the option that defines the two main aspects required when defining the function of chromosomes that they hold genetic information in the form of DNA and that this genetic information is heritable. So it could be passed from a parent onto their offspring. So, the primary function of chromosomes is to carry genetic information and allow it to be passed from a parent to offspring.