Have you ever wondered how you came to be? How did you come to this very planet at this moment in time to be able to read this article? You are probably thinking; I came from my parents, who came from their parents, and so on. That's correct, but have you wondered how our Cells know what to do? Why do we look like what we look like? Why do you have a certain number of fingers, toes, and even eyes? What does it mean to look human? How exactly are these traits passed on, and why? Confused? It's all right. This is the topic we will examine the units of Inheritance and answer in this article today.
Units of inheritance definition
Genes are the units of Heredity inherited from parent to offspring.
The Genome is the complete set of Genes in a living organism.
Genes carry Genetic Information from generation to generation that determines an individual's traits.
When we talk about discrete units, we mean discontinuous traits or characteristics controlled by usually only one gene. Discrete genes usually only have two Alleles, each of which comes from one parent.
Before particulate inheritance, people believed in many other things, including a disproven theory called blending inheritance.
Blending inheritance states that offspring inherit traits that are averaged between their parents from the "blending" of parental genes.
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.
The anther of a flower is where the male reproductive parts or gametes are located, while the stigma of a flower is where the female reproductive parts of gametes are located.
Mendel also had to remove the anthers from the flowers to which he transferred pollen to prevent self-fertilization.
Self-fertilization features the mating of male and female gametes from the same individual or, in this case, plants.
Hybridization features the mating of male and female gametes from different individuals or, in this case, plants.
The pea plants Mendel originally pollinated can be referred to as the parental generation or \(P_0\).
After this, Mendel used the seeds from the \(P_0\) pea plants and grew them; this generation can be referred to as the \(F_1\) or filial generation. Filial generation, in this case, means offspring.
The second filial generation or \(F_2\) was allowed to self-fertilize and came from \(F_1\) seeds.
Mendel also did experiments extending to the third, fourth, etc. filial generation, but only the first three contributed to the ratio characteristics we're interested in today.
After several more experiments, Mendel could categorize traits into dominant and recessive traits.
For example, in Figure 1, the dominant trait is the purple flower.
For example, in Figure 1, the recessive trait is the white flower.
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.
Understand that not all genes are controlled by a single gene with two Alleles. In other words, not every gene can be explained by Mendelian inheritance, but it's an excellent place to start.
Although Mendel is still the father of Genetics, he discovered essential inheritance laws such as genes being inherited discretely (particular inheritance) and that genes are inherited in pairs from each parent.
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.
Thus, the closer we are to an organism, the more similarities there are in our respective genomes.
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.
The sequence of nucleotides determines a gene's information.
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.
Proteins are organic compounds that consist of building blocks called amino acids. They perform essential functions such as speeding up reactions, repairing tissues, cell signaling, etc.
Organic compounds are chemical compounds that involve carbon bonded to usually hydrogen, oxygen, or nitrogen. Carbon is a vital part of life because it creates stable bonds with various elements, allowing it to form many complex molecules such as proteins, carbohydrates, etc.
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.
Scientists have found that non-coding DNA sequences can regulate when specific genes are turned on or off.
Non-coding DNA sequences can act as:
- Promoters are regions in DNA that allow Transcription of a gene to start. Transcription is the process of transcribing DNA to RNA.
- Enhancers are regions in DNA that "enhance" or activate transcription at higher levels.
- Silencers are regions in DNA that do the opposite of enhancers and repress transcription.
- Non-coding DNA sequences can make transfer RNAs (tRNAs), ribosomal RNAs (rRNAs), etc. tRNAs and rRNAs have significant roles in Protein Synthesis.
- Non-coding DNA sequences also make up the ends of telomeres. Telomeres serve as protection for the ends of chromosomes when DNA is replicated.
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:
| The pattern of inheritance | Examples | Explanation |
| X-linked Dominant | Fragile X Syndrome | Individuals with this disorder have delayed speech and language. |
| X-linked Recessive | Hemophilia | Individuals with this disorder have slower Blood clotting. This can result in prolonged bleeding, even from minor injuries. |
| Autosomal Dominant | Huntington Disease | Individuals with this disorder have a loss of cognition, involuntary movements, and an overall decline in motor skills. |
| Autosomal Recessive | Sickle Cell | Individuals with this disorder have their hemoglobin affected into an S shape. Hemoglobin is a protein found in red Blood cells (RBCs) that binds to oxygen to deliver it throughout the body—the S shape results in hemoglobin breakdown, leading to anemia. Anemia can result in fatigue and delayed development. |
| Mitochondrial | Kearns-Sayre syndrome | Individuals with this disease gain paralysis of eye muscles until they eventually become immobile. |
So how are these genetic diseases transmitted, or what are the patterns of inheritance?
X-linked Dominant:
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:
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.
As mentioned in the section above, understanding how genes work enables you to understand how genetic diseases are inherited.
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.
Knowing your family health history and how Genetic Disorders work can also help you identify genetic health risks and, therefore, how you can make better lifestyle choices to mitigate risks.
Units of Inheritance - Key takeaways
- Genes are the units of heredity inherited from parent to offspring.
- The genome is the complete set of genes in a living organism.
- Genes carry genetic information from generation to generation that determines an individual's traits. 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 are located on threadlike structures called chromosomes and are found in the nuclei of most organisms. Chromosomes are made of nucleic acids and proteins; most living beings have multiple chromosomes.
References
- Mary Ann Clark Matthew Douglas Jung Choi, Mendel's Experiments and Heredity, Biology 2e, 2018.
- Cooper GM, Heredity, Genes, and DNA, The Cell: A Molecular Approach, 2000.
- National Geographic Society, Genes, 2022.
- Genetic Alliance, Classic Mendelian Genetics, Understanding Genetics: A District of Columbia Guide for Patients and Health Professionals, 2010.
- MedlinePlus, What are the different ways a genetic condition can be inherited?, 2021.
- MedlinePlus, What is noncoding DNA?, 2021.
- Genetic Alliance, Why is genetics important to my family and me?, A Guide to Genetics and Health, 2006.
- Terese Winslow, National Cancer Institute, 2015.
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