Units of Inheritance

Units of inheritance in living organisms

So how did we determine how genes were first inherited in living organisms and disprove the theory of blending inheritance?

Well, a monk named Gregor Mendel did. In 1865, Mendel performed a series of experiments involving the breeding of pea Plants, and by studying traits such as seed color, he was able to find out how genes are passed on.

Mendel did something called hybridizations, which involve mating two true-breeding Plants or pea Plants. He did this in pea Plants by transferring pollen from the anther of one pea plant to the stigma of another pea plant.

Mendel also had to remove the anthers from the flowers to which he transferred pollen to prevent self-fertilization.

After several more experiments, Mendel could categorize traits into dominant and recessive traits.

• Dominant traits are traits that don't change when inherited.

• Recessive traits are traits that, when inherited, disappear in the \(F_1\) generation but show back up in the \(F_2\) generation.

These observations disproved blended inheritance as the recessive trait, white flower, showed back up in the \(F_2\) generation instead of blending with the dominant trait or purple flower with a three purple: one white ratio.

Figure 1: Mendel's Experiments. Daniela Lin, Study Smarter Originals.

Now that we understand that genes are the units of inheritance in living organisms and that Mendel first discovered them, we can move on to understanding some practical terms for this article in the next section.

Units of inheritance: chromosomes

The Human Genome Project has estimated that the human genome is around 20,000-25,000 genes. We, humans, look like each other because our genes are more similar to each other than we are to other organisms. Our closest living relatives are chimpanzees, as we share 99% of our DNA with them.

This means that your genomes are most similar to your family, then other humans, then chimpanzees, and finally other organisms or:

Family > other humans > chimpanzees > other organisms

Genes consist of short strands of DNA or deoxyribonucleic acid, while some viruses can use RNA or ribonucleic acid instead.

DNA and RNA are both Nucleic Acids consisting of small molecules called nucleotides. DNA is double-stranded, while RNA is single-stranded. Most of the DNA is located inside the cell's nucleus in eukaryotes or multicellular organisms with membrane-bound organelles.

Nucleotides can have four bases: cytosine (C), guanine (G), adenine (A), and thymine (T). In RNA, thymine (T) is substituted by uracil (U). These bases are shown as different colors on the DNA helix in Figure 2.

Genes are located on threadlike structures called Chromosomes and are found in the nuclei of most organisms. Chromosomes are made of Nucleic Acids and Proteins, otherwise known as chromatin; most living beings have multiple chromosomes.

Nucleosomes are subunits of chromatin, consisting of around two turns of DNA enveloped around eight Proteins called histones. The organization levels of the units of inheritance mentioned above are all illustrated in Figure 2 for clarity purposes.

Figure 2: Units of Inheritance structure illustrated. Daniela Lin, Study Smarter Originals.

To synthesize proteins from a gene, Cells must transcribe a gene from DNA to RNA and then translate the gene into proteins.

About 1% of our DNA is in the protein-coding genes, and the other 99% is in non-coding DNA! While scientists don't understand precisely why DNA that doesn't have instructions for making proteins or non-coding DNA exists, they believe it's essential for controlling gene activity.

Units of inheritance examples

In the earlier parts of the article, we went over what units of inheritance are, who discovered them, and the associated terms. Now we'll be going over examples of Mendelian inheritance, particularly the patterns of diseases controlled by one gene with two alleles.

The five common types of patterns of inheritance for Mendelian genes are:

So how are these genetic diseases transmitted, or what are the patterns of inheritance?

X-linked Dominant:

• They are seen more commonly in females because it's thought that the males are affected so much that they don't survive.

• Afflicted males and females can be in the same generation.

X-linked Recessive:

• They are seen more commonly in males because they only have one X chromosome, unlike females with two. Thus, a single recessive gene on the X chromosome in males will cause the X-linked recessive genetic disease.

• Afflicted males often show up in each generation.

Autosomal Dominant:

• Autosomes are any chromosomes that are not sex chromosomes.

• You only need to get the defective gene from one parent to have the disease.

• Usually occurs in each generation.

Autosomal Recessive:

• You need to get the defective from two parents to have the disease. Usually making both parents carriers.

• Usually not seen in each generation.

Mitochondrial:

• Disorders that affect the mitochondria and, therefore, the ability of a cell to produce and provide energy.

• Usually can occur in each generation.

The importance of units of inheritance

Genes are essential because they make up our Genetic Information, which living organisms inherit from generation to generation. Genes also contain information to make proteins, which perform various vital roles in our bodies.

Genes also control your appearance and how your body functions making you unique. They do this by carrying the genetic information to enable your cells to function. Cells act as the smallest functional unit of living organisms or the building blocks of life; billions work together to keep you alive!

Genes are organized into threadlike structures called chromosomes and are made up of DNA. DNA serves as our genetic code and enables us to transcribe into RNA to synthesize proteins.