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.
