What is sex-linked inheritance? What influence does it have on human development? What does it mean if a trait is polygenic? Learn the answers to these questions as this lesson explores this area of genetic inheritance.
What is Genetic Inheritance?
This is Bobby and his parents:
Bobby has inherited traits from his mother and father.
When we look at Bobby, it is easy to tell that he has inherited his father's green eyes and his mother's blond hair. Bobby has inherited all of his characteristic traits from his parents.
Genetic inheritance is the process by which genes are passed down from parents to their offspring.
This can occur in a relatively simple way if traits are controlled by single gene pair inheritance. In this case, one gene controls the trait, and you receive half of that gene's makeup from each parent. Sometimes things can get a bit more complex. This lesson will focus on two other types of genetic inheritance: sex-linked inheritance and polygenic inheritance.
Sex-linked inheritance occurs when a trait is linked to one of the sex chromosomes. This is a particularly important category of genetic linkage. Humans have 46 chromosomes that are arranged in 23 pairs. The 23rd pair of chromosomes are the sex chromosomes. If a person has two X-shaped sex chromosomes, she is a female. If a person has one X-shaped sex chromosome and one Y-shaped sex chromosome, he is a male.
These chromosomes carry the genes that determine if a person will be male or female, but they carry other genes as well. These other genes that are found on sex chromosomes are said to be sex-linked.
First, let's look at Y-linked traits. Only men will inherit these traits since only men inherit Y chromosomes.
A father has only one Y chromosome to pass on through inheritance. He will pass on this Y chromosome to all of his sons. Since there is only one, there is no corresponding information on the other allele (the X chromosome passed on from the mother) to override the traits that occur on the Y chromosome. This means that all Y-linked traits will be expressed and will be expressed in all males who have the same father and inherited his Y chromosome.
Very few human traits are known to be Y-linked. Because of this, it might be easier to visualize a Y-linked trait if we think about birds. Most male birds are more colorful than their female counterparts. This makes them more attractive as mates and also more visible to predators than the female bird, who sits on the nest. This trait is Y-linked. It is passed down to only male birds, it will always be expressed, and it will be passed down to future generations.
Now let's look at X-linked traits. Both men and women can inherit X-linked traits because both inherit X chromosomes. However, X-linked recessive traits are primarily expressed in men. Since females have two X chromosomes, any recessive traits are usually suppressed by a dominant trait found on the other X chromosome. Since men only have one X chromosome, if they inherit the recessive trait, there will be no corresponding dominant trait to override its expression.
These X-linked recessive traits are often responsible for abnormal or harmful conditions. Hemophilia is one well-known X-linked recessive trait, and red-green color blindness is one of the most common.
Polygenic inheritance occurs when traits are controlled by more than one gene. In fact, the name polygenic literally translates to 'many genes.'
Let's look at height as an example of this. We will assume height is determined by four different gene pairs, each made up of two alleles. Each allele is represented by a letter, and uppercase letters represent dominant alleles for height. A very tall person may have a genotype, or a combination of these gene pairs, that looks something like this: AaBBCCDD. A very short person may have a genotype that looks something like this: aaBbccdd. The combination of these gene pairs determines each person's degree of height. Do you notice how the tall person has more dominant alleles than the short person?
This doesn't seem too complicated with only four different gene pairs involved, but imagine if there are 10, or 20, or even more genes involved in determining a trait. It could start to get a bit tricky.
It is estimated that each person has about 25,000 genes. The huge number of possible gene combinations that could be related to a specific trait can make the genes difficult to identify. It can also be difficult to understand how these genes interact with one another.
Imagine a huge pile of sand. Now imagine you are looking for 20 specific grains of sand. You take out your magnifying glass and start hunting away, even though you aren't exactly sure what the different grains of sand will look like. This is a bit like trying to discover new types of polygenic inheritance.
Genetic inheritance is the process by which genes are passed down from parents to their offspring. Two types of genetic inheritance discussed in this lesson are sex-linked inheritance and polygenic inheritance.
Sex-linked inheritance occurs when a trait is linked to one of the sex chromosomes. Traits can be X-linked or Y-linked. Y-linked traits are passed down only to men because only men have a Y chromosome. X-linked traits are passed down to both men and women because both have an X chromosome. Recessive X-linked traits are typically expressed in men and are often harmful.
Polygenic inheritance occurs when traits are controlled by more than one gene. Since each person has about 25,000 genes, there are a huge number of possible gene combinations that could be related to a specific trait. It can be difficult to understand how these genes interact with one another. The genes responsible for a specific trait can also be difficult to identify.
After reviewing this lesson, you'll have the ability to:
- Define genetic inheritance
- Describe what sex-linked inheritance is and differentiate between X-linked and Y-linked
- Explain why it is difficult to discover new types of polygenic inheritance