Genetics

What is the difference between genes and alleles?

All living things contain DNA packaged into chromosomes. A chromosome is formed when strands of DNA are tightly wrapped around histones (proteins found in chromosomes that help control actvity of the genes) to form a chromatid (identical halves of the replicated chromosome). Two chromatids join to make a chromosome. Humans have 46 chromosomes whereas some species can have over 100 chromosomes.

Each chromosome is made of a single, long, tightly compressed DNA molecule that houses a number of genes. A gene is a section of DNA, or a sequence of nucleotide bases, that usually codes for a particular polypeptide; they may also codehi for ribosomal RNAs or transfer RNAs. The exact position of the gene on a DNA molecule is known as its locus.

Humans have two copies of each gene – one set from their mother and one from their father. Genes can exist in two or more different forms that occupy the same locus but have slightly different nucleotide bases. These different forms are known as alleles. All individuals of a species possess the same genes, but they may not all have the same alleles. Only one allele can occur at a gene's locus on any chromosome; this allele is often represented by letters.

Fig. 2 - Representation of a gene

Gregor Mendel

It wasn’t until the 19th century that we began to really learn more about genetics and the mechanism of hereditary traits. Gregor Mendel performed experiments on pea plants and discovered how traits are passed on from parent to offspring. He found that when he bred red-flowered plants with white-flowered plants, all of the offspring had red flowers. This went against his prediction of pink flowers being produced. When the offspring were bred, he got a mix of red and white flowers. We now know that this is due to dominant and recessive alleles. The modern age of genetics is based on the re-discovery of his work early in the 20th century. Because of this, Gregor Mendel is known as the father of modern genetics.

What is Mendelian inheritance?

In Mendelian inheritance, offspring inherit genes from their parents in a way that follows the principles initially proposed by Gregor Mendel. This includes monohybrid (characteristics controlled by a single gene) and dihybrid (characteristics controlled by two genes) inheritance. Mendelian inheritance refers to the inheritance that is the consequence of a single gene.

What is non-Mendelian inheritance?

Non-Mendelian inheritance is any pattern of inheritance that does not occur as predicted by Mendel’s laws. There are several reasons why offspring might not follow Mendelian patterns. For instance, genes might exhibit linkage, or they might not follow Mendelian patterns of dominance. There might also be additional interactions between genes that affect the resulting phenotype.

Fig. 3 - Drawing of Mendel

Genetics Key Terms

Gregor Mendel paved the way to us understanding more about genes. We will cover some of the key vocabulary we use in genetics in the following sections.

Genotypes, phenotypes, and the environment

The alleles that an individual has make up its genotype or genetic constitution. You can think of the genotype as the blueprint for the organism's characteristics, such as its maximum height or the intensity of its colour.

However, environmental factors may also affect the expression of an individual's genes. The phenotype refers to the organism’s observed characteristics, which result from the expression of its genotype and its interaction with the environment. For instance, while a plant may have alleles that allow it to grow tall, it may remain quite small if it is growing in a nutrient-poor or excessively shady environment.

What is dominance in genetics?

As we said above, the pea plant experiment that Mendel did was a perfect example of dominant and recessive alleles.

There are usually two alleles present in an individual. They can either be dominant or recessive. A dominant allele is always expressed whereas a recessive allele will not be expressed in the presence of the dominant allele. A recessive gene will remain dormant unless it is paired with another recessive gene.

Homozygous and heterozygous

In diploid organisms (organisms that contain a complete set of chromosomes), chromosomes occur in homologous pairs. In other words, cells of diploid organisms contain two copies of every chromosome; this means that an individual carries two loci (position of an allele on the chromosome) each with one allele of a gene.

Fig. 4 - Heterozygous and homozygous alleles

There can, therefore, be different allele combinations in an individual. When the two alleles on each locus are the same, the individual is homozygous for a particular trait. When the two alleles are different, the individual is heterozygous for that trait.

Codominance

Many genes have different dominant traits. Sometimes two dominant genes are expressed at the same time. We call this codominance.

Fig. 5 - Codominance in coat colours of the cows

What is a test cross?

Phenotype does not always clearly reflect the genotype of an individual. For instance, if the short phenotype in a plant can only be expressed if the individual is homozygous recessive, one can assume that its genotype is tt. However, if the plant's phenotype is tall, it may still be either homozygous dominant (TT) or heterozygous (Tt).

A test cross is a method used to deduce the unknown genotype of an individual expressing the dominant phenotype. The unknown individual is crossed with a homozygous recessive individual. If the resulting offspring all express the dominant phenotype, then the unknown parent is homozygous dominant. If some offspring express the recessive phenotype, the unknown parent is heterozygous. For more in-depth explanation, see our Inheritance and Linkage articles.

Fig. 6 - Test cross example

What is linkage in genetics?

Autosomal linkage occurs when two or more genes on the same autosome (non-sex chromosome) do not assort independently during meiosis. When two genes are autosomally linked, they stay together in the original combination inherited from parents.

Sex linkage, on the other hand, occurs when genes are found on a region of a sex chromosome that is not present on the other sex chromosome. The inheritance of these genes is therefore dependent on the sex of the individual.

Why are genetics important?

As the basis of all life, genetics is important for many reasons. It has many significant applications.

It allows us to study:

• Hereditary diseases
• Pathogens
• Cancers
• Crime scenes

It allows us to create:

• Vaccines
• Antibiotics
• Genetically modified organisms
• Large quantities of organic molecules