Lesson Video: Inheritance of Genes | Nagwa Lesson Video: Inheritance of Genes | Nagwa

Lesson Video: Inheritance of Genes Science

In this video, we will learn how to describe the inheritance of chromosomes and genes from parent to offspring and explain the difference between dominant and recessive genes.


Video Transcript

In this video, we’ll learn about how genes are passed from parents to offspring by genetic inheritance. We’ll also explain the difference between dominant and recessive genes. Let’s get started!

DNA, which stands for deoxyribonucleic acid, is a molecule found in almost every cell of a living organism. It contains all the information necessary for the organism to function. DNA is a very long helical molecule which contains genes. Genes are sections of DNA that provide instructions for making specific molecules and proteins. If you imagine your DNA as a unique cookbook that’s used to make you who you are, then your genes would be like the recipes contained within the cookbook.

As humans, we have around 30000 genes that carry all the instructions needed for building every cell in our bodies. That’s a pretty big cookbook. We receive half of our genes from each of our biological parents. To continue the cookbook analogy, this means we have two versions of every recipe, because we have two copies of every gene. So we make unique versions of all our molecules and proteins. So how do we inherit our genes?

When an organism reproduces, they pass down genes to their children, which are also known as their offspring. This is why we say that the offspring inherit their parents’ genes. Genes usually code for proteins, which in turn produce characteristics that can be transmitted from parents to their children. A characteristic is an observable feature, such as eye color, or a quality that can be determined by our DNA. As we’ve already said, DNA molecules are incredibly long. If we took all the DNA from a single human cell and laid it out end to end, it would be between two and three meters long.

Because human cells are microscopic, they must package this long DNA into incredibly compact thread-like structures called chromosomes, like the ones you can see here. These chromosomes are small enough to fit inside the nucleus, the compartment of the cell where all the DNA is found. Because offspring inherit their genes from their biological mother and father, they have a unique combination of their parents’ characteristics.

Sexual reproduction plays a really important role in genetic inheritance. A typical human body cell has 46 chromosomes in its nucleus, consisting of 23 chromosome pairs. However, humans also produce special reproductive cells, which are known as gametes. In biological males, the gametes are called sperm. And in biological females, they’re called eggs. These gametes contain half the number of chromosomes of a body cell, meaning they have 23 chromosomes in total.

When two individuals reproduce sexually, their gametes combine in a process called fertilization. This is where a sperm cell from the father fuses with an egg cell from the mother to produce a new cell called a zygote. The zygote has a full set of 46 chromosomes. Once the zygote has formed, it divides into many more cells, which eventually develop into a brand new human being.

During this process, each new cell that’s produced also carries 46 chromosomes, which is why almost every body cell contains 46 chromosomes in its nucleus. Because each pair of chromosomes contains one from each parent, an individual will inherit a unique mix of genes from their mother and father. Therefore, as the individual grows and matures, they develop a unique blend of both parents’ characteristics. This is why we might appear more similar to one or both of our parents at different points in our lives.

When a human reaches sexual maturity, they start producing their own gametes from specialized body cells. Each gamete randomly keeps one chromosome from each of the 23 chromosome pairs. And it’s these chromosomes that the organism could pass on to any future offspring they have. The random nature of genetic inheritance is what leads to the variation we see within a species such as humans. In other words, it’s what makes us all unique.

Not all of our characteristics are caused by our genes, however. Instead of being inherited, some characteristics are picked up over the course of an organism’s lifetime. We call these acquired characteristics. Acquired characteristics result from interactions with the environment. You can also think of them as features that we as individuals have some level of control over. Let’s have a look at some examples.

Blood group, eye color, and bone structure are all inherited characteristics. For example, if a child has blue eyes, it doesn’t necessarily mean that both their parents will have blue eyes. But they must both carry the gene for blue eyes in order to have passed it on to their offspring. It also means that the child may pass down the blue eyes gene to their own children in the future.

Piercings, scars, and tattoos are all examples of acquired characteristics. For example, if a child develops a scar after scraping their knee playing rugby, the scar is the result of an interaction with their environment, not their genetics. So, if the child went on to have children in the future, the scar would not be passed on to them.

Some characteristics are considered to be both inherited and acquired. An example of one such characteristic is height. While inheriting a tall gene from one or both parents might contribute to an individual growing tall, the amount and type of food they eat while they’re growing will also influence their final height.

Now, let’s see how an individual’s genes relate to their characteristics. We’ve already said that there are different versions of each gene. These gene versions can either be dominant or recessive. If a gene version is dominant, the characteristic it codes for will always be expressed, whether the individual has one or two copies of it. But if a gene version is recessive, the characteristic it codes for will only be expressed if the individual has two copies of it. If an individual only has one copy of a recessive gene, its characteristic will be hidden.

For example, let’s say we have a gene that determines whether we have freckles or not. The freckles gene version, represented here as an asterisk, is dominant. And the no freckles gene version, represented here as a triangle, is recessive. If a child inherited a freckles gene version from one of their parents and a no freckles gene version from the other, the child will have freckles because the characteristic encoded by their freckles gene version asserts dominance over that of their no freckles gene version.

However, suppose the child inherits a copy of the no freckles gene version from both their parents. In this scenario, the child will not have freckles because they have two copies of the recessive no freckles gene version. And there is no dominant gene version to mask the recessive gene’s characteristic. This is a really important feature of genetic inheritance.

Even though individuals carry genes from both their parents, their characteristics are not necessarily a direct mix of their parents’ features, because one gene version will often assert dominance over the other. The ability of living organisms to pass on genetic information to their offspring is incredibly important, because it allows them to adapt over time to their environment and provide them with characteristics that better equip them for survival and reproduction.

Now we’ve found out all about genetic inheritance, let’s have a go at some practice questions.

Inside the nucleus of your cells, DNA is wound and coiled into long strands. What are these long strands of DNA called? (A) Nucleosomes, (B) ribosomes, (C) chromosomes, (D) helices, or (E) genes.

As you can see in this diagram, DNA is a long helical molecule which is usually double stranded. A section of DNA which codes for a particular protein is known as a gene. Human DNA contains around 30000 genes, which contribute to our characteristics. This is why DNA is a very long molecule. In fact, if you took all the DNA from a single human cell and laid it out end to end, it would be between two and three meters long. And yet it’s all found inside the cellular compartment called the nucleus, which is only about six micrometers in diameter. So how does the DNA fit inside a space which is over 400000 times smaller than it?

The cell packages the DNA into tightly wound and coiled structures called chromosomes, which can then be stored inside the nucleus. Humans have 46 chromosomes, which are found as 23 chromosome pairs, and they appear as thin threads if we look at them under a microscope. The correct answer to the question is therefore (C). The long strands of DNA that are found inside the nucleus of your cells are called chromosomes.

Let’s have a go at another question.

The diagram shows a basic outline of the process of fertilization. After fertilization, a zygote forms. The zygote shown in the diagram has four cells. How many chromosomes will be in the nucleus of each of these cells?

Humans reproduce by sexual reproduction. During this process, a biological female and a biological male both produce sex cells, which are also known as gametes. The female gamete is called an egg cell or ovum, and the male gamete is called a sperm cell. These gametes each contain 23 chromosomes in their nuclei, which is half the number of a regular body cell.

Next, one egg cell and one sperm cell join together in a process called fertilization. During fertilization, the nucleus of the sperm cell combines with the nucleus of the egg cell. This produces a new cell called a zygote. Because the zygote has both sets of chromosomes from the two gametes in its nucleus, it has 46 chromosomes altogether.

Once the zygote has formed, it undergoes cell division so that it can grow. It’s this process of cell division that causes the ball of four cells that you see here to eventually develop into a human baby. Every time a cell divides in this way, it produces genetically identical daughter cells, which therefore also have 46 chromosomes in their nuclei. We have therefore determined that the nucleus of each cell of the zygote will contain 46 chromosomes.

Let’s summarize what we’ve learnt in this video by reviewing the key points. A gene is a DNA sequence which usually codes for a protein and, therefore, determines an organism’s characteristics. Humans inherit one copy of every gene from each of their parents. Regular body cells contain 46 chromosomes in their nuclei, while gametes contain 23 chromosomes. Genetic inheritance is a random process causing individuals to have a unique mix of genes. Characteristics can be inherited, acquired, or both. And finally, the versions of a gene can either be dominant or recessive.

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