Special Inheritance Patterns

Lesson Plan - Get It!

Audio:

You have learned how traits are passed from parents to offspring and can be dominant or recessive.

All genetic information is coded onto chromosomes.

Every human has 23 pairs of chromosomes, so that is 46 total! Notice how the chromosomes are arranged in the image below. This image shows a karyotype, a visual lineup of each pair of chromosomes arranged based on size. The karyotype shows the genetic material of a male, because there is an X and Y chromosome present:

Females have two X chromosomes — notice that the chromosomes are arranged the same way, but there are no Y chromosomes pictured:

The X and Y chromosomes are known as sex chromosomes, because they determine the sex of the individual. You learned in the last Related Lesson, found in the right-hand sidebar, that traits follow patterns that can be predicted using tools like Punnett Squares. There are also traits that follow a special inheritance pattern because they are exclusively located on sex chromosomes. While females get two X chromosomes, males only get one!

We call traits that are passed on by the sex chromosomes "sex-linked." Most sex-linked traits occur on the X chromosome, so you might see X-linked as another name for these inheritance patterns.

_{Image by OpenStax College, Anatomy & Physiology, via Wikimedia Commons, is licensed under the Creative Commons Attribution 3.0 Unported license.}

Observe the image above carefully. Notice how the mother is called a "carrier" because she has one affected X chromosome and one normal. The father only has one normal X chromosome with a Y chromosome. The possible offspring show how sex-linked traits can be passed from parents to children. Sons are more likely to receive X-linked disorders because they only receive one X chromosome. Daughters receive two, meaning that one of the X chromosomes can dominate the affected gene. The daughter would be considered a carrier, but not have symptoms of the disorder.

Males are more likely to receive sex-linked traits than females, but females are more-likely carriers of genetic disorders. Genetic disorders that are sex-linked include:

• color-blindness, where an individual cannot see a difference between red and green colors
• hemophilia, a bleeding disorder where individuals do not clot blood and can bleed excessively
• Duchenne muscular dystrophy, where individuals lose control over muscle groups

Discuss the following questions with a parent or teacher:

• How are male and female karyotypes different?
• What are sex chromosomes?
• Why do more males have sex-linked genetic disorders?
• What is a carrier?

In the Got It? section of this lesson, you will learn more about how to write Punnett Squares for sex-linked traits.