AP Biology

## Learning Outcomes

- Distinguish autosomal inheritance from sex-linked inheritance.
- Explain why X-linked recessive traits are expressed more often in males than in females.
- Predict offspring genotypes and phenotypes for X-linked recessive and X-linked dominant crosses.
- Interpret pedigrees that suggest sex-linked inheritance.
- Compare sex-linked inheritance with nonnuclear maternal inheritance.
- Describe the role of X-inactivation and Barr bodies in females.
- Avoid common AP Biology mistakes involving carriers, affected males, and affected fathers.

## AP Biology Syllabus

Sex-linked inheritance belongs in Unit 5 because it tests how genetic information is transmitted across generations when genes are found on sex chromosomes rather than autosomes. AP Biology questions may ask you to analyze Punnett squares, pedigrees, probability, or claims about inheritance patterns. The focus is usually on X-linked inheritance, especially X-linked recessive traits such as red-green color blindness and hemophilia.

- Sex-linked genes are located on sex chromosomes, most commonly the X chromosome.
- Humans are typically described in AP Biology as XX female and XY male for chromosomal sex.
- Males are more likely to express X-linked recessive traits because they have only one X chromosome.
- Females can be carriers for X-linked recessive traits if they have one recessive allele and one normal allele.
- X-linked dominant traits have different pedigree patterns from X-linked recessive traits.
- Pedigrees can help distinguish autosomal, sex-linked, and nonnuclear inheritance.
- Nonnuclear maternal inheritance involves mitochondrial or chloroplast DNA, not sex chromosomes.
- X-inactivation produces Barr bodies and affects expression of some X-linked genes in females.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why are X-linked recessive traits more commonly expressed in males than in females?
2. A carrier female for an X-linked recessive disorder has children with an unaffected male. What fraction of sons are expected to express the disorder?
3. In an X-linked dominant trait, why does an affected father pass the trait to all daughters but no sons?
4. How can you tell the difference between X-linked inheritance and mitochondrial maternal inheritance in a pedigree?
5. What is a Barr body, and why does it form in female cells?

## Introduction

Many Mendelian genetics problems assume that genes are located on autosomes, the chromosomes that are not directly involved in determining biological sex. In those cases, males and females usually have the same chance of inheriting and expressing a trait. Sex-linked inheritance is different because the gene is located on a sex chromosome, usually the X chromosome. Since males and females do not have the same sex chromosome pair, the inheritance pattern changes.

This subtopic is a common source of AP Biology mistakes because students often use ordinary autosomal Punnett squares without tracking whether an allele is attached to an X or Y chromosome. To solve sex-linked problems, always write the allele as part of the sex chromosome symbol, such as $$X^N$$ for a normal allele or $$X^n$$ for a recessive disease allele.

> **Key Term: sex-linked gene**
> A gene located on a sex chromosome, most often the X chromosome in humans.

### Sex chromosomes and why X-linked traits are different

Humans have 23 pairs of chromosomes. Twenty-two pairs are autosomes. The remaining pair consists of sex chromosomes. In the AP Biology course, biological sex is usually treated using the chromosomal pattern XX for female and XY for male.

Females typically inherit one X chromosome from each parent. Males typically inherit an X chromosome from their mother and a Y chromosome from their father. This means that fathers pass their X chromosome to daughters and their Y chromosome to sons. Mothers can pass either of their X chromosomes to sons or daughters.

This difference is the basis of sex-linked inheritance. A male has only one X chromosome, so any allele on that X chromosome is expressed if there is no matching allele on the Y chromosome to mask it. For many X-linked genes, the Y chromosome does not carry the same gene. This makes males hemizygous for many X-linked traits.

> **Key Term: autosome**
> A chromosome that is not directly involved in determining biological sex.
>
> **Key Term: sex chromosome**
> A chromosome involved in determining biological sex, such as the X or Y chromosome in humans.
>
> **Key Term: hemizygous**
> Having only one copy of a gene in a diploid organism, as males do for many genes on the X chromosome.

For an autosomal recessive trait, an individual usually needs two recessive alleles to show the phenotype. For an X-linked recessive trait, a male needs only one recessive allele on his single X chromosome to show the trait. This is why X-linked recessive disorders are more common in males.

Examples often used in AP Biology include red-green color blindness and hemophilia. Both are commonly treated as X-linked recessive traits in genetics problems.

Use notation carefully:

- Unaffected male: $$X^N Y$$
- Affected male with an X-linked recessive trait: $$X^n Y$$
- Unaffected female, not a carrier: $$X^N X^N$$
- Carrier female: $$X^N X^n$$
- Affected female with an X-linked recessive trait: $$X^n X^n$$

> **Key Term: X-linked recessive trait**
> A trait caused by a recessive allele on the X chromosome; it is expressed in males with one copy and in females with two copies.
>
> **Key Term: carrier**
> An individual who has a recessive allele but does not express the recessive phenotype; for X-linked recessive traits, this often refers to a heterozygous female.
>
> **Test Tip:**
> In sex-linked problems, do not write genotypes as simple letters such as $$AA$$ or $$Aa$$. Attach each allele to the chromosome: $$X^A X^a$$, $$X^aY$$, and so on. This helps you track whether a child is male or female and whether the allele came from the mother or father.

### Predicting X-linked recessive inheritance

A classic AP Biology cross involves an unaffected male and a carrier female for an X-linked recessive trait.

Suppose:

- The normal allele is $$X^N$$.
- The recessive disease allele is $$X^n$$.
- The mother is a carrier: $$X^N X^n$$.
- The father is unaffected: $$X^N Y$$.

The mother can produce eggs carrying either $$X^N$$ or $$X^n$$. The father can produce sperm carrying either $$X^N$$ or $$Y$$.

The possible children are:

- $$X^N X^N$$: unaffected daughter, not a carrier
- $$X^N X^n$$: unaffected carrier daughter
- $$X^N Y$$: unaffected son
- $$X^n Y$$: affected son

So, among all children, the expected fraction affected is $$1/4$$. Among sons only, the expected fraction affected is $$1/2$$. Among daughters only, the expected fraction affected is $$0$$, although half the daughters are expected to be carriers.

This distinction matters. AP questions may ask about “all offspring,” “sons,” “daughters,” or “children who inherit the allele.” Read the wording exactly.

Now consider an affected male and an unaffected noncarrier female:

- Mother: $$X^N X^N$$
- Father: $$X^nY$$

All daughters inherit the father’s X chromosome, so all daughters receive $$X^n$$. They also receive $$X^N$$ from the mother, making them carriers: $$X^N X^n$$. All sons inherit the father’s Y chromosome and an $$X^N$$ from the mother, making them unaffected: $$X^N Y$$.

So an affected father with an X-linked recessive trait passes the recessive allele to all daughters and no sons. His sons inherit his Y chromosome, not his X chromosome.

> **Exam Warning:**
> A father does not pass his X chromosome to his sons. If a pedigree shows father-to-son transmission of a trait, that pattern is not typical X-linked inheritance.

For a female to express an X-linked recessive trait, she must usually inherit the recessive allele from both parents. That means her father must be affected, and her mother must be either a carrier or affected. This is why affected females are less common for X-linked recessive conditions.

A useful pattern for X-linked recessive pedigrees is:

- More males than females are affected.
- Affected males may have carrier daughters.
- Carrier females may have affected sons.
- Father-to-son transmission is absent for X-linked traits.
- The trait can appear to skip generations through carrier females.

### X-linked dominant inheritance

Although AP Biology questions more often use X-linked recessive examples, sex-linked traits can also be dominant. For an X-linked dominant trait, only one copy of the dominant allele on the X chromosome is needed for expression in either sex.

> **Key Term: X-linked dominant trait**
> A trait caused by a dominant allele on the X chromosome; one copy can produce the phenotype.

Let $$X^D$$ represent an X-linked dominant disease allele and $$X^d$$ represent the normal allele.

An affected male has genotype $$X^D Y$$. He passes his X chromosome to all daughters and his Y chromosome to all sons. Therefore:

- All daughters inherit $$X^D$$ and express the trait.
- No sons inherit the father’s X chromosome, so none receive the trait from him.

If an affected heterozygous female, $$X^D X^d$$, has children with an unaffected male, $$X^dY$$, each child has a 50% chance of inheriting $$X^D$$ from the mother. Both sons and daughters can be affected.

Common X-linked dominant pedigree clues include:

- An affected father passes the trait to all daughters.
- An affected father does not pass the trait to sons.
- A heterozygous affected mother can pass the trait to sons or daughters.
- The trait often appears in every generation because it is dominant.

Be cautious: if the trait is rare, an affected female in a pedigree may often be heterozygous, but the problem should give enough information for a clear answer.

> **Test Tip:**
> For X-linked dominant problems, first ask whether the affected parent is the father or the mother. An affected father gives his X chromosome to every daughter but to none of his sons.

### Reading pedigrees for sex-linked inheritance

Pedigrees are diagrams showing inheritance through a family. In standard AP Biology pedigree symbols, squares represent males, circles represent females, and shaded symbols represent individuals who express the trait.

> **Key Term: pedigree**
> A family tree diagram used to track the inheritance of a trait across generations.

When interpreting a pedigree, do not jump to a conclusion from a single affected individual. Look for patterns across multiple generations.

For an X-linked recessive trait, the key pattern is that males are affected more often than females. A female has two X chromosomes, so she usually needs two recessive alleles to express the trait. A male has one X chromosome, so one recessive allele is enough.

A common X-linked recessive pedigree pattern looks like this in words:

- An unaffected mother has an affected son.
- The mother is likely a carrier.
- The affected son’s father may be unaffected.
- The affected male’s daughters are not necessarily affected, but they may be carriers.
- The trait may reappear later in a grandson through a carrier daughter.

For an X-linked dominant trait, look for affected fathers passing the trait to all daughters and no sons. If an affected father has an unaffected daughter, the trait is probably not X-linked dominant, unless there is unusual information in the question.

Autosomal inheritance differs. In autosomal traits, males and females are generally affected at similar frequencies. Father-to-son transmission can occur because fathers pass autosomes to sons. If a pedigree shows many affected males and father-to-son transmission, think carefully: a trait cannot be ordinary X-linked if fathers pass it directly to sons.

AP Biology questions may also ask you to compare sex-linked inheritance to nonnuclear inheritance. This is a separate pattern.

> **Key Term: nonnuclear inheritance**
> Inheritance of genes located outside the nucleus, such as genes in mitochondria or chloroplasts.

Mitochondria have their own DNA. In plants, chloroplasts also have their own DNA. Eggs and ovules contribute much more cytoplasm and organelle DNA than sperm or pollen. As a result, mitochondrial and chloroplast traits often show maternal inheritance.

> **Key Term: maternal inheritance**
> A nonnuclear inheritance pattern in which traits are passed through the mother because eggs or ovules contribute most organelle DNA.

In maternal inheritance:

- A mother with the trait can pass it to sons and daughters.
- A father with the trait does not pass it to his offspring through mitochondrial or chloroplast DNA.
- The genes are not on the X chromosome; they are in mitochondria or chloroplasts.

This differs from X-linked inheritance. A father can pass an X-linked allele to all daughters, because daughters inherit his X chromosome. He cannot pass mitochondrial DNA in the same way.

> **Exam Warning:**
> Do not confuse X-linked inheritance with mitochondrial maternal inheritance. X-linked genes are nuclear genes on sex chromosomes and can be inherited from either parent. Mitochondrial and chloroplast genes are nonnuclear and are usually inherited from the mother.

### X-inactivation and Barr bodies

Females typically have two X chromosomes, while males have one. If both X chromosomes in every female cell were fully active, females would produce about twice as much of many X-linked gene products as males. Cells avoid this imbalance through X-inactivation.

Early in female embryonic development, one X chromosome in each cell becomes mostly inactive. The inactive X chromosome condenses into a dark-staining structure called a Barr body. Which X chromosome is inactivated is generally random in each cell. Once an X chromosome is inactivated in a cell, the same inactive X is usually maintained in that cell’s descendants.

> **Key Term: X-inactivation**
> The process in female mammals by which one X chromosome in each cell becomes mostly inactive during embryonic development.
>
> **Key Term: Barr body**
> The condensed, mostly inactive X chromosome found in many female mammalian cells.

X-inactivation can produce mosaic expression. This means different cells in the same female may express different X-linked alleles depending on which X chromosome remains active. A classic example outside humans is coat color patterning in some female cats, where different patches of cells express different X-linked coat color alleles.

For AP Biology, you usually do not need to solve advanced X-inactivation problems. You should understand the basic reason Barr bodies exist: they balance X-linked gene expression between XX females and XY males.

X-inactivation also explains why some heterozygous females for X-linked traits may show variable expression. If more cells inactivate the X chromosome carrying the normal allele, expression of a recessive allele may be stronger in some tissues. However, most AP Biology inheritance problems simplify this and treat heterozygous females for X-linked recessive traits as carriers who do not express the trait.

### Worked examples for AP-style reasoning

> **Question:**
> A woman who is a carrier for an X-linked recessive condition has children with a man who does not have the condition. What is the probability that a son will have the condition?

The mother is $$X^N X^n$$. The father is $$X^N Y$$. A son must inherit $$Y$$ from the father and one X chromosome from the mother. The mother has a 50% chance of passing $$X^n$$. Therefore, the probability that a son is affected is:

$$
\frac{1}{2}
$$

The probability that a child of any sex is an affected son is:

$$
\frac{1}{2}\text{ chance of being male} \times \frac{1}{2}\text{ chance of receiving }X^n = \frac{1}{4}
$$

Both answers can be correct depending on whether the question asks about sons only or all children.

> **Question:**
> A man with an X-linked dominant trait has children with an unaffected woman. What pattern is expected?

The father is $$X^D Y$$. The mother is $$X^d X^d$$. Daughters inherit the father’s $$X^D$$ and are affected. Sons inherit the father’s $$Y$$ and the mother’s normal $$X^d$$, so they are unaffected. Therefore, all daughters are expected to express the trait, and no sons are expected to inherit it from the father.

> **Question:**
> In a pedigree, every child of an affected mother has a trait, but none of the children of an affected father have it. Is this more likely X-linked inheritance or maternal inheritance?

This pattern suggests nonnuclear maternal inheritance. A mother can pass mitochondrial DNA to all offspring, while a father generally does not pass mitochondrial DNA to offspring. In X-linked inheritance, an affected father would pass his X chromosome to daughters.

## Key Point Checklist

This article has covered the following key knowledge points:

- Sex-linked genes are located on sex chromosomes, most commonly the X chromosome.
- Autosomal traits usually affect males and females at similar rates; X-linked traits often do not.
- Males are more likely to express X-linked recessive traits because they have only one X chromosome.
- A heterozygous female for an X-linked recessive allele is usually a carrier.
- X-linked recessive traits include classic AP examples such as red-green color blindness and hemophilia.
- An affected father passes his X chromosome to all daughters and his Y chromosome to all sons.
- Father-to-son transmission is not expected for X-linked inheritance.
- X-linked dominant traits are passed from an affected father to all daughters but no sons.
- Pedigrees with more affected males than females often suggest X-linked recessive inheritance.
- Maternal inheritance involves mitochondrial or chloroplast DNA, not sex chromosomes.
- X-inactivation forms Barr bodies and balances X-linked gene expression in female cells.
- Always attach alleles to sex chromosome symbols when solving sex-linked Punnett square problems.

## Key Terms and Concepts

- sex-linked gene
- autosome
- sex chromosome
- hemizygous
- X-linked recessive trait
- carrier
- X-linked dominant trait
- pedigree
- nonnuclear inheritance
- maternal inheritance
- X-inactivation
- Barr body


Non-Mendelian genetics - Sex-linked inheritance