Chromosomes and Inheritance

Human body cells normally contain:

46 chromosomes.

These chromosomes are arranged into:

23 pairs.

Of these:

22 pairs are autosomes

and:

1 pair consists of sex chromosomes.

The sex chromosomes are particularly important because they are involved in biological sex determination and also carry genes that can produce distinctive:

inheritance patterns.

---

Autosomes

Autosomes are chromosomes that are not sex chromosomes.

Humans normally have:

44 autosomes

arranged into:

22 pairs.

Autosomes contain thousands of genes involved in characteristics such as:

• growth
• metabolism
• development
• cell function
• many inherited characteristics

Autosomal genes are inherited from:

both biological parents.

---

Sex Chromosomes

The remaining pair of chromosomes are called:

sex chromosomes.

The two main human sex chromosomes are:

X

and:

Y.

In the usual chromosomal pattern:

XX is associated with female development

XY is associated with male development.

These chromosomes also contain genes unrelated to sex determination.

---

Autosomes vs Sex Chromosomes

Together:

44 autosomes + 2 sex chromosomes = 46 chromosomes.

---

Human Sex Determination

In the usual XX/XY system, biological sex is determined by which sex chromosome is contributed by the:

sperm.

An egg normally contains:

one X chromosome.

A sperm normally contains either:

X

or:

Y.

Therefore:

X egg + X sperm → XX

X egg + Y sperm → XY.

---

Why Eggs Carry X

A person with an XX chromosome pattern produces eggs through:

meiosis.

Each egg receives one chromosome from the sex-chromosome pair.

Because both chromosomes are X chromosomes, an egg normally receives:

X.

Therefore:

Egg → X.

---

Why Sperm Can Carry X or Y

A person with an XY chromosome pattern produces sperm through:

meiosis.

The X and Y chromosomes separate.

Some sperm receive:

X.

Other sperm receive:

Y.

Therefore:

Sperm → X or Y.

---

A Punnett Square for Sex Determination

We can represent this using a Punnett square:

The expected probabilities are:

50% XX

and:

50% XY.

This is an expected probability, not a guarantee that every family will have equal numbers of XX and XY children.

---

Which Parent Determines XX or XY?

In the usual XX/XY system, the egg always contributes:

X.

The sperm contributes:

X or Y.

Therefore, whether fertilization produces XX or XY depends on which type of sperm fertilizes the:

egg.

This process is based on chance.

---

The Y Chromosome and SRY

The Y chromosome contains a gene called:

SRY.

SRY plays an important role in initiating the developmental pathway that usually leads to formation of:

testes.

These then produce hormones that influence further sexual:

development.

Biological sex development is more complex than simply writing XX or XY, but the XX/XY model provides the foundation for understanding typical human sex-chromosome inheritance.

---

Chromosomal Variations

Not every person has an XX or XY chromosome:

pattern.

Naturally occurring variations include chromosome patterns such as:

XXY

X0

and:

XYY.

Differences can also occur in genes or hormone responses involved in sexual development.

Therefore, XX and XY describe the typical chromosomal system used in introductory genetics, rather than every possible pattern of human biological development.

---

Genes on the Sex Chromosomes

The X and Y chromosomes are not identical.

The:

X chromosome

is much larger and contains many more genes than the:

Y chromosome.

Some of these genes have nothing directly to do with biological sex.

When a gene is located on a sex chromosome, its inheritance may be described as:

sex-linked inheritance.

---

What Is a Sex-Linked Trait?

A sex-linked trait is a characteristic controlled by a gene located on a sex:

chromosome.

Most introductory examples involve genes located on the:

X chromosome.

These are called:

X-linked traits.

Because XX and XY individuals have different numbers of X chromosomes, X-linked alleles can show distinctive inheritance patterns.

---

X-Linked Inheritance

An XX individual normally has:

two X chromosomes.

Therefore, they normally possess two copies of genes located on the X chromosome.

An XY individual normally has:

one X chromosome.

Therefore, for many X-linked genes, an XY individual possesses only:

one copy.

This has important consequences for recessive X-linked traits.

---

Writing X-Linked Alleles

For X-linked traits, we usually write the allele as a superscript on the:

X chromosome.

For example:

Xᴺ = normal allele

Xⁿ = recessive disorder-associated allele

The Y chromosome is written:

Y.

Therefore, possible genotypes include:

XᴺXᴺ

XᴺXⁿ

XⁿXⁿ

XᴺY

XⁿY.

---

Why Not Use Nn?

Writing:

Nn

would not show that the gene is located on the:

X chromosome.

Using:

XᴺXⁿ

makes the chromosome location clear.

This becomes essential when predicting:

sex-linked inheritance.

---

X-Linked Recessive Traits

Many commonly studied sex-linked conditions are:

X-linked recessive.

Consider:

Xᴺ = unaffected allele

Xⁿ = recessive condition-associated allele.

For an XX individual:

XᴺXᴺ → unaffected

XᴺXⁿ → usually unaffected carrier

XⁿXⁿ → affected

For an XY individual:

XᴺY → unaffected

XⁿY → affected.

---

Why Are X-Linked Recessive Traits Often More Common in XY Individuals?

An XY individual has only:

one X chromosome.

If that X carries the recessive condition-associated allele:

XⁿY

there is no second copy of that X-linked gene on the Y chromosome to mask its:

effect.

Therefore, one recessive X-linked allele can be sufficient for the phenotype to appear.

An XX individual usually needs:

two recessive copies

for an X-linked recessive phenotype.

---

Carrier

For an X-linked recessive condition, an individual with:

XᴺXⁿ

is commonly described as a:

carrier.

The individual has one unaffected allele and one recessive condition-associated:

allele.

The recessive allele can be passed to:

offspring.

---

Example: Red-Green Colour Vision Deficiency

Red-green colour vision deficiency is commonly used as an example of:

X-linked inheritance.

Several genes involved in red and green colour vision are located on the:

X chromosome.

Certain variants can result in difficulty distinguishing particular:

colours.

Because these genes are X-linked, red-green colour vision deficiency is more common among XY individuals than among XX individuals.

---

Example: Hemophilia

Some forms of hemophilia, including hemophilia A and B, commonly show:

X-linked recessive inheritance.

Hemophilia affects the body's ability to form blood clots normally.

The genes involved in these forms of hemophilia are located on the:

X chromosome.

---

Predicting an X-Linked Cross

Suppose a mother is a carrier of an X-linked recessive allele.

Her genotype is:

XᴺXⁿ.

Suppose the father does not have the condition:

XᴺY.

The cross is:

XᴺXⁿ × XᴺY.

---

Determine the Gametes

The mother can produce eggs containing:

Xᴺ

or:

Xⁿ.

The father can produce sperm containing:

Xᴺ

or:

Y.

Now we can construct the Punnett square.

---

Punnett Square

The four possibilities are:

XᴺXᴺ

XᴺXⁿ

XᴺY

XⁿY.

---

Interpreting the Cross

The predicted outcomes are:

25% XᴺXᴺ — unaffected XX

25% XᴺXⁿ — carrier XX

25% XᴺY — unaffected XY

25% XⁿY — affected XY.

Across all possible offspring, the probability of an affected offspring is:

25%.

But we can also analyze XX and XY offspring separately.

---

Looking Only at XY Offspring

The possible XY offspring are:

XᴺY

and:

XⁿY.

Therefore:

50% of XY offspring are predicted to be unaffected

and:

50% of XY offspring are predicted to be affected.

This is different from saying 50% of all offspring are affected.

---

Looking Only at XX Offspring

The possible XX offspring are:

XᴺXᴺ

and:

XᴺXⁿ.

Therefore:

50% of XX offspring are predicted to be non-carriers

and:

50% of XX offspring are predicted to be carriers.

In this particular cross, none of the XX offspring are predicted to have the recessive condition.

---

An Important Probability Distinction

Always pay attention to what the question asks.

For the cross:

XᴺXⁿ × XᴺY

Probability of an affected child:

25%

Probability of an affected XY child among all children:

25%

Probability that an XY child is affected:

50%.

These statements sound similar but answer different:

questions.

---

Cross: Carrier Mother × Affected Father

Consider:

XᴺXⁿ × XⁿY.

The Punnett square is:

Possible offspring include:

XᴺXⁿ — carrier XX

XⁿXⁿ — affected XX

XᴺY — unaffected XY

XⁿY — affected XY.

Therefore:

50% of all offspring are predicted to be affected.

---

Cross: Non-Carrier Mother × Affected Father

Consider:

XᴺXᴺ × XⁿY.

The mother produces only:

Xᴺ eggs.

The father produces:

Xⁿ or Y sperm.

Possible offspring are:

XᴺXⁿ

and:

XᴺY.

Therefore:

all XX offspring are carriers

and:

all XY offspring are unaffected.

---

Why Fathers Do Not Pass Their X Chromosome to Sons

In the usual XX/XY system, a father contributes either:

X

or:

Y.

An XY son receives:

Y from the father

and:

X from the mother.

Therefore, a father does not pass an X-linked allele directly to his:

son.

This is one of the most important clues when identifying an X-linked inheritance pattern.

---

Fathers Pass Their X Chromosome to Daughters

An XX daughter receives:

one X from the mother

and:

one X from the father.

Therefore, a father passes his X chromosome to:

all of his daughters.

If the father carries an X-linked allele, every daughter receives that:

allele.

Whether the daughter shows the phenotype depends on the allele and the X chromosome inherited from the mother.

---

A Useful Inheritance Pattern

For X-linked traits:

Father → X to daughters

Father → Y to sons

Mother → X to daughters

Mother → X to sons

This pattern is extremely useful when analyzing:

pedigrees.

---

Sex-Linked Traits in Pedigrees

A pedigree can show how a trait passes through several generations of a:

family.

Certain patterns can provide evidence for X-linked inheritance.

For an X-linked recessive trait, we may observe:

• more affected XY individuals
• carrier XX individuals
• transmission through unaffected carriers
• no direct father-to-son transmission of an X-linked allele

These patterns can help scientists infer possible:

genotypes.

---

Analyzing a Family Pattern

Suppose an unaffected mother and unaffected father have an affected:

son.

If the condition is X-linked recessive:

The son must be:

XⁿY.

The Y chromosome came from his:

father.

Therefore, his Xⁿ must have come from his:

mother.

If the mother is unaffected, a likely genotype is:

XᴺXⁿ.

She is a:

carrier.

---

Using Evidence to Determine Genotypes

Consider an affected father:

XⁿY.

He has an unaffected daughter.

What do we know?

The daughter must receive:

Xⁿ from her father.

If she is unaffected under a simple X-linked recessive model, she must also have received:

Xᴺ from her mother.

Therefore, her genotype is:

XᴺXⁿ.

She is a carrier.

---

X-Linked Dominant Traits

Not all X-linked traits are:

recessive.

Some traits follow:

X-linked dominant inheritance.

For an X-linked dominant allele, one copy can be sufficient for the phenotype to:

appear.

This produces different family inheritance patterns from X-linked recessive traits.

---

Affected Father in X-Linked Dominant Inheritance

Suppose:

Xᴰ = dominant condition-associated allele

and the father is:

XᴰY.

He passes:

Xᴰ to every daughter

and:

Y to every son.

Therefore, under a simple X-linked dominant model with an unaffected mother:

all daughters inherit the trait

and:

none of the sons inherit it from their father.

This father-to-daughter but not father-to-son pattern can provide useful genetic evidence.

---

What About Y-Linked Traits?

Genes located on the Y chromosome can produce:

Y-linked inheritance.

Because the Y chromosome normally passes:

from father to son,

a Y-linked allele follows a very different inheritance pattern.

Only individuals with a Y chromosome can inherit such an allele, and it passes through the:

paternal line.

Y-linked traits are much less commonly used in introductory genetics problems than X-linked traits.

---

Sex-Linked Does Not Mean Sex-Limited

A sex-linked trait is controlled by a gene located on a:

sex chromosome.

This does not necessarily mean that the characteristic can occur in only one:

sex.

For example, an X-linked recessive condition can occur in both XX and XY individuals.

However, the probability may differ because of their different sex-chromosome:

combinations.

---

Worked Problem 1

A woman is a carrier for an X-linked recessive condition:

XᴺXⁿ.

The father is unaffected:

XᴺY.

What is the probability that an XY child will have the condition?

Possible XY genotypes:

XᴺY

XⁿY.

Therefore:

50% of XY offspring are predicted to be affected.

---

Worked Problem 2

For the same cross:

XᴺXⁿ × XᴺY

what is the probability that any child will have the condition?

Only one of four Punnett-square outcomes is affected:

XⁿY.

Therefore:

1/4 = 25%.

---

Worked Problem 3

An affected father has genotype:

XⁿY.

The mother is:

XᴺXᴺ.

Can their XY sons inherit the father's X-linked allele?

No.

The sons receive:

Y from their father.

Their X chromosome comes from their:

mother.

---

Worked Problem 4

In the same cross:

XᴺXᴺ × XⁿY

what happens to the daughters?

Every daughter receives:

Xⁿ from the father

and:

Xᴺ from the mother.

Therefore every daughter is:

XᴺXⁿ.

Under the simple recessive model, they are:

carriers.

---

Worked Problem 5

A carrier mother and affected father have children:

XᴺXⁿ × XⁿY.

What is the probability of an affected XX child among all offspring?

One of the four possible outcomes is:

XⁿXⁿ.

Therefore:

25% of all offspring.

Among XX offspring specifically:

50% are predicted to be affected.

---

A Strategy for Solving Sex-Linked Problems

Use this method:

Step 1: Determine whether the trait is X-linked or Y-linked.

Step 2: Identify whether the allele is dominant or recessive.

Step 3: Write the allele as part of the chromosome symbol.

Step 4: Determine the parent genotypes.

Step 5: Identify the possible gametes.

Step 6: Construct the Punnett square.

Step 7: Identify the XX and XY offspring.

Step 8: Determine which offspring show the phenotype.

Step 9: Calculate the requested probability.

Step 10: Check whether the question asks about all offspring or only a particular group.

---

Common Mistake: Writing X-Linked Alleles as Ordinary Alleles

For an X-linked trait, write:

XᴺXⁿ

rather than simply:

Nn.

This keeps track of which allele is located on which:

chromosome.

---

Common Mistake: Giving the Y Chromosome the X-Linked Allele

For a typical X-linked gene, do not write:

Yⁿ.

The gene being studied is located on the:

X chromosome.

Therefore, use:

XⁿY.

---

Common Mistake: Saying the Father Gives His X to His Son

An XY son receives:

Y from the father

and:

X from the mother.

Therefore, an X-linked allele does not pass directly from:

father to son.

---

Common Mistake: Confusing 25% and 50%

Suppose:

XᴺXⁿ × XᴺY.

The probability of an affected child among all offspring is:

25%.

But among XY offspring:

50%

are predicted to be affected.

Always identify the group the probability refers to.

---

Common Mistake: Thinking Carrier Means Affected

For a simple X-linked recessive condition:

XᴺXⁿ

is usually described as a:

carrier genotype.

The person has the recessive allele but usually does not show the full recessive phenotype.

Carrier and affected are therefore not the same:

thing.

---

Common Mistake: Assuming Every Sex-Linked Trait Is X-Linked Recessive

Sex-linked inheritance can include:

X-linked recessive

X-linked dominant

and:

Y-linked inheritance.

X-linked recessive traits are simply the most common examples used in introductory:

genetics.

---

Common Mistake: Treating XX and XY as the Entire Story of Biological Sex

The XX/XY model explains the typical chromosome pattern used in introductory human genetics.

However, biological sex development also involves:

• genes
• hormones
• hormone receptors
• reproductive anatomy
• developmental processes

Chromosomal variations also occur naturally.

Therefore, XX and XY are an important foundation, but human biological development is more complex than a single:

Punnett square.

---

Check Your Understanding

1. How many chromosomes are normally present in a human body cell?

2. How many pairs of autosomes are normally present?

3. What are autosomes?

4. What are sex chromosomes?

5. What sex-chromosome combination is typically associated with female development?

6. What combination is typically associated with male development?

7. Which sex chromosome is normally found in every human egg?

8. Which two types of sex chromosome can human sperm normally carry?

9. Explain why the sperm determines whether the usual fertilization produces XX or XY.

10. What is the expected probability of XX and XY offspring?

11. Why does a 50% probability not mean every family will contain equal numbers of each?

12. Define a sex-linked trait.

13. What is an X-linked trait?

14. Why are X-linked recessive traits often more common in XY individuals?

15. What does XᴺXⁿ represent in a simple X-linked recessive model?

16. What does XⁿY represent?

17. Name two examples of X-linked disorders or conditions.

18. Construct a Punnett square for XᴺXⁿ × XᴺY.

19. What percentage of all offspring are predicted to be affected?

20. What percentage of XY offspring are predicted to be affected?

21. What percentage of XX offspring are predicted to be carriers?

22. Why can a father not pass an X-linked allele directly to an XY son?

23. Which chromosome does a father pass to all of his XX daughters?

24. An affected father and non-carrier mother have daughters. What allele must every daughter receive from the father?

25. Explain how an unaffected mother can have an affected son with an X-linked recessive condition.

26. How can pedigrees provide evidence for X-linked inheritance?

27. Describe one important difference between X-linked recessive and X-linked dominant inheritance.

28. What inheritance pattern would you expect for a gene found only on the Y chromosome?

29. Why must you distinguish between "probability among all offspring" and "probability among XY offspring"?

30. Explain how chromosome inheritance, Punnett squares, and family evidence can be combined to predict sex-linked inheritance.

---

Key Terms

• Autosome: Chromosome that is not a sex chromosome.
• Sex chromosome: Chromosome involved in biological sex determination and carrying other genes.
• X chromosome: One of the two main human sex chromosomes.
• Y chromosome: Sex chromosome containing genes including SRY that contribute to typical male development.
• XX: Chromosomal pattern typically associated with female development.
• XY: Chromosomal pattern typically associated with male development.
• SRY: Gene on the Y chromosome that normally helps initiate development of testes.
• Sex-linked trait: Trait influenced by a gene located on a sex chromosome.
• X-linked trait: Trait influenced by a gene located on the X chromosome.
• X-linked recessive: Inheritance pattern in which a recessive allele is located on the X chromosome.
• X-linked dominant: Inheritance pattern involving a dominant allele on the X chromosome.
• Y-linked: Inheritance involving a gene located on the Y chromosome.
• Carrier: Individual who carries a recessive allele without usually showing the full recessive phenotype.
• Hemophilia: Group of inherited bleeding disorders; some forms show X-linked recessive inheritance.
• Red-green colour vision deficiency: Common example of an X-linked inherited characteristic.
• Pedigree: Diagram used to investigate inheritance through generations of a family.

---

Key Takeaways

• Human body cells normally contain 46 chromosomes arranged into 23 pairs.
• 22 pairs are autosomes and one pair consists of sex chromosomes.
• The two main human sex chromosomes are X and Y.
• XX is typically associated with female development.
• XY is typically associated with male development.
• Eggs normally carry an X chromosome.
• Sperm normally carry either X or Y.
• Therefore, the sperm determines whether typical fertilization produces XX or XY.
• The expected probability is approximately 50% XX and 50% XY.
• Individual families do not have to match this ratio exactly.
• The X chromosome contains many genes in addition to genes involved in sex development.
• A trait controlled by a gene on a sex chromosome is called sex-linked.
• Most introductory sex-linked problems involve X-linked genes.
• X-linked alleles should be written as part of the X chromosome symbol, such as Xᴺ or Xⁿ.
• XY individuals normally have only one copy of most X-linked genes.
• This makes X-linked recessive phenotypes more likely to appear in XY individuals.
• An XᴺXⁿ individual can carry a recessive X-linked allele without showing the full recessive phenotype.
• Red-green colour vision deficiency is a common example of X-linked inheritance.
• Hemophilia A and B commonly show X-linked recessive inheritance.
• Fathers pass their X chromosome to daughters.
• Fathers pass their Y chromosome to sons.
• Therefore, X-linked alleles do not pass directly from father to son.
• Mothers can pass X-linked alleles to both XX and XY offspring.
• Punnett squares can predict probabilities for sex-linked traits.
• Always distinguish between probabilities among all offspring and probabilities within XX or XY offspring.
• Pedigrees can provide evidence about sex-linked inheritance.
• X-linked traits can be recessive or dominant.
• Some genes also show Y-linked inheritance.
• The XX/XY system is a useful introductory model, although biological sex development includes additional genetic, hormonal, and developmental factors.
• Understanding sex-linked inheritance connects chromosomes, meiosis, probability, Punnett squares, and family inheritance patterns.