Sex-Linked Traits & Reading Pedigrees
Learn how X-linked traits pass from parents to offspring and how to read a pedigree to decide if a trait is dominant or recessive, autosomal or X-linked.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Sex-Linked Traits & Reading Pedigrees, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Colorblindness, hemophilia, and Duchenne muscular dystrophy all share a strange pattern: they show up far more often in males, and an affected boy usually has unaffected parents. Mendel's rules still apply here — but the gene sits on the X chromosome, and males carry only one X. That single fact changes every prediction you make.
In this lesson you will set up Punnett squares using sex-chromosome notation, predict phenotype ratios separately for sons and daughters, and then work backwards: given a family tree with squares, circles, and shaded shapes, you will decide whether the trait is dominant or recessive and whether the gene is on an autosome or on the X. Working backwards from a pedigree is the harder skill, so we will build a short set of decision rules you can apply to almost any family chart.
In this lesson you will set up Punnett squares using sex-chromosome notation, predict phenotype ratios separately for sons and daughters, and then work backwards: given a family tree with squares, circles, and shaded shapes, you will decide whether the trait is dominant or recessive and whether the gene is on an autosome or on the X. Working backwards from a pedigree is the harder skill, so we will build a short set of decision rules you can apply to almost any family chart.
Sex Chromosomes and Why Males Are Hemizygous
Humans have 46 chromosomes: 22 pairs of autosomes plus one pair of sex chromosomes. Females are typically XX, males XY. The X chromosome is large and carries over a thousand genes; the Y is small and carries few genes besides those that trigger male development.
Because a male has only one X, he has only one copy of every X-linked gene. He is hemizygous — not homozygous, not heterozygous, just single-copy. This is the key to the whole topic. A female with one recessive allele on one X and a normal allele on the other X is a carrier: she has the allele but not the phenotype. A male with that same single recessive allele has no second copy to mask it, so he shows the trait. That is why X-linked recessive conditions appear much more often in males.
Notation matters. Write the chromosome as the base and the allele as a superscript: for a carrier female, for an affected male, for an unaffected male. Never write a male genotype as or as a bare — you must show the Y, because the Y is what makes the inheritance pattern lopsided.
One more consequence: a father passes his X to every daughter and his Y to every son. So an X-linked allele can never travel from father to son. A boy always gets his X-linked alleles from his mother. This father-to-daughter-to-grandson route is called criss-cross inheritance.
Because a male has only one X, he has only one copy of every X-linked gene. He is hemizygous — not homozygous, not heterozygous, just single-copy. This is the key to the whole topic. A female with one recessive allele on one X and a normal allele on the other X is a carrier: she has the allele but not the phenotype. A male with that same single recessive allele has no second copy to mask it, so he shows the trait. That is why X-linked recessive conditions appear much more often in males.
Notation matters. Write the chromosome as the base and the allele as a superscript: for a carrier female, for an affected male, for an unaffected male. Never write a male genotype as or as a bare — you must show the Y, because the Y is what makes the inheritance pattern lopsided.
One more consequence: a father passes his X to every daughter and his Y to every son. So an X-linked allele can never travel from father to son. A boy always gets his X-linked alleles from his mother. This father-to-daughter-to-grandson route is called criss-cross inheritance.
Predicting X-Linked Crosses
Set up an X-linked Punnett square exactly like a monohybrid square, but list the mother's two X gametes and the father's and gametes. Then report results by sex, because sons and daughters get different answers.
Consider red-green colorblindness, an X-linked recessive trait where is normal vision and is colorblind. Cross a carrier mother with an unaffected father :
Overall, 1/4 of all children are colorblind, but the sex-specific statement is more informative: half the daughters are carriers and none are colorblind, while half the sons are colorblind. Saying "50% colorblind" without specifying sons is a common error on homework.
Now flip it. An affected mother with an unaffected father produces daughters (all carriers, all unaffected) and sons (all colorblind). Every son of an affected mother inherits the trait, because his only X came from her.
For X-linked dominant traits the logic reverses: an affected father passes the trait to all of his daughters and none of his sons. That single pattern is often enough to identify X-linked dominance in a pedigree.
Consider red-green colorblindness, an X-linked recessive trait where is normal vision and is colorblind. Cross a carrier mother with an unaffected father :
| (mom) | (mom) | |
|---|---|---|
| (dad) | ||
| (dad) |
Now flip it. An affected mother with an unaffected father produces daughters (all carriers, all unaffected) and sons (all colorblind). Every son of an affected mother inherits the trait, because his only X came from her.
For X-linked dominant traits the logic reverses: an affected father passes the trait to all of his daughters and none of his sons. That single pattern is often enough to identify X-linked dominance in a pedigree.
Reading a Pedigree: Symbols and Structure
A pedigree is a standardized family diagram. Squares are males, circles are females. A horizontal line between two shapes is a mating; a vertical line drops to their children, who hang from a horizontal sibship line, usually oldest on the left. Generations are numbered with Roman numerals (I, II, III) and individuals within a generation with Arabic numbers, so "II-3" means the third person in generation II.
Shaded (filled) shapes show the trait; unshaded shapes do not. A half-shaded shape or a shape with a dot in the center is sometimes used for a known carrier, and a diamond marks a person of unspecified sex. A double horizontal line indicates a consanguineous mating (related parents), which raises the chance of two copies of a rare recessive allele.
When you analyze a pedigree, do not guess from the overall look of it. Work individual by individual, writing possible genotypes next to each shape and using underscores for unknown alleles, such as for someone who shows a dominant phenotype but whose second allele is undetermined.
One fact does most of the work: an affected child of two unaffected parents means the trait is recessive, because a dominant allele must show itself in whoever carries it. From there the parents' genotypes follow from where the allele could have come. For an autosomal recessive trait both parents must be heterozygous carriers. For an X-linked recessive trait an affected son needs only a carrier mother, since his father hands him a and no allele at all. Anchor your reasoning to those parent-child contradictions rather than to how many people are shaded.
Shaded (filled) shapes show the trait; unshaded shapes do not. A half-shaded shape or a shape with a dot in the center is sometimes used for a known carrier, and a diamond marks a person of unspecified sex. A double horizontal line indicates a consanguineous mating (related parents), which raises the chance of two copies of a rare recessive allele.
When you analyze a pedigree, do not guess from the overall look of it. Work individual by individual, writing possible genotypes next to each shape and using underscores for unknown alleles, such as for someone who shows a dominant phenotype but whose second allele is undetermined.
One fact does most of the work: an affected child of two unaffected parents means the trait is recessive, because a dominant allele must show itself in whoever carries it. From there the parents' genotypes follow from where the allele could have come. For an autosomal recessive trait both parents must be heterozygous carriers. For an X-linked recessive trait an affected son needs only a carrier mother, since his father hands him a and no allele at all. Anchor your reasoning to those parent-child contradictions rather than to how many people are shaded.
Deciding Dominant or Recessive, Autosomal or X-Linked
Use a fixed order: first settle dominant versus recessive, then autosomal versus X-linked.
After narrowing the possibilities, test your hypothesis by assigning genotypes to every person. If any individual would need an impossible genotype, the hypothesis fails. For example, suppose you propose X-linked recessive and the pedigree shows an affected female whose father is unaffected. She would have to be , meaning her father gave her an — but then he would be and affected. Contradiction, so the trait is autosomal recessive instead.
Two cautions. Pedigrees are small, so many are consistent with more than one mode of inheritance; the correct answer is often "most likely X-linked recessive, and here is the evidence that rules out the alternatives." And absence of the trait in females does not prove X-linkage — with a rare autosomal recessive allele in a small family, you might see only affected males by chance. Always cite a specific individual as your evidence.
| Observation in the pedigree | Conclusion |
|---|---|
| Two unaffected parents have an affected child | Recessive (trait can skip generations) |
| Every affected person has at least one affected parent, trait in every generation | Likely dominant |
| Trait appears mostly in males, often through carrier mothers | Likely X-linked recessive |
| An affected female has an unaffected father | Not X-linked recessive |
| An affected father has an unaffected daughter | Not X-linked dominant |
| A trait that looks dominant passes from an affected father to his son | Not X-linked (fathers give sons a , not an ) |
Two cautions. Pedigrees are small, so many are consistent with more than one mode of inheritance; the correct answer is often "most likely X-linked recessive, and here is the evidence that rules out the alternatives." And absence of the trait in females does not prove X-linkage — with a rare autosomal recessive allele in a small family, you might see only affected males by chance. Always cite a specific individual as your evidence.
Where Students Actually Go Wrong
The most frequent mistake is dropping the Y. Writing a male's genotype as alone, or as , hides the fact that he passes no X to his sons, and students who do this often conclude a father transmitted colorblindness to his son. He cannot.
Second, students report a single ratio for all offspring when the question asks about one sex. If a cross yields , , , and , the probability that a randomly chosen child is colorblind is 1/4, but the probability that a son is colorblind is 1/2. Read the question carefully: "of their sons" changes the denominator.
Third, calling a male a "carrier." A carrier has the allele without the phenotype, which requires a second, masking allele. A hemizygous male with a recessive X-linked allele expresses it, so he is affected, not a carrier.
Fourth, treating shading counts as evidence. Whether 3 or 8 people are shaded tells you almost nothing. Parent-child relationships tell you everything.
Finally, students forget that unaffected parents of an affected child are obligate carriers — you can fill in their genotypes with certainty even though nothing is shaded. In an X-linked recessive pedigree, the mother of an affected son must carry the allele; in an autosomal recessive pedigree, both parents of an affected child must be heterozygous. Filling in those certainties first usually unlocks the rest of the chart.
Second, students report a single ratio for all offspring when the question asks about one sex. If a cross yields , , , and , the probability that a randomly chosen child is colorblind is 1/4, but the probability that a son is colorblind is 1/2. Read the question carefully: "of their sons" changes the denominator.
Third, calling a male a "carrier." A carrier has the allele without the phenotype, which requires a second, masking allele. A hemizygous male with a recessive X-linked allele expresses it, so he is affected, not a carrier.
Fourth, treating shading counts as evidence. Whether 3 or 8 people are shaded tells you almost nothing. Parent-child relationships tell you everything.
Finally, students forget that unaffected parents of an affected child are obligate carriers — you can fill in their genotypes with certainty even though nothing is shaded. In an X-linked recessive pedigree, the mother of an affected son must carry the allele; in an autosomal recessive pedigree, both parents of an affected child must be heterozygous. Filling in those certainties first usually unlocks the rest of the chart.
Key terms
- Autosome.
- Any chromosome that is not a sex chromosome; humans have 22 pairs. Autosomal traits appear with roughly equal frequency in males and females.
- Sex chromosome.
- The X or Y chromosome, which determines sex in humans. Females are typically XX and males XY.
- Hemizygous.
- Having only one copy of a gene rather than a pair. Human males are hemizygous for X-linked genes, so a single recessive allele is expressed.
- Carrier.
- A heterozygous individual who has a recessive allele but does not show the trait. For X-linked recessive traits, only females can be carriers.
- X-linked recessive trait.
- A trait caused by a recessive allele on the X chromosome. It appears more often in males, can skip generations, and is never passed father to son.
- Criss-cross inheritance.
- The pattern in which an X-linked allele passes from a father to all of his daughters and then from a carrier daughter to about half of her sons.
- Pedigree.
- A standardized diagram of a family across generations using squares for males, circles for females, and shading for individuals showing the trait.
- Obligate carrier.
- A person whose genotype must include the recessive allele based on their relatives' phenotypes, even though they show no trait themselves.
Worked example
Hemophilia A is X-linked recessive ( = normal clotting, = hemophilia). A woman with normal clotting whose father had hemophilia marries a man with hemophilia. Determine the woman's genotype, then predict the genotypes and phenotypes of their children, separated by sex.
Step 1 — Deduce the mother's genotype. She does not have hemophilia, so she has at least one . Her father was , and a father gives his only X to every daughter. Therefore she must have received from him. She is , an obligate carrier.
Step 2 — Write the father's genotype. He has hemophilia and is male, so he is .
Step 3 — List gametes. Mother: or . Father: or .
Step 4 — Build the square.
Step 5 — Interpret by sex. Daughters: 1/2 are carriers with normal clotting, and 1/2 are and have hemophilia. Sons: 1/2 are unaffected and 1/2 are affected. Note that the affected sons got from their mother, not their father — the father contributed only the Y.
Step 6 — Overall probability. Each child has a 1/2 chance of having hemophilia, and unlike most X-linked cases, affected daughters are expected here because the father contributes an to every daughter.
Step 2 — Write the father's genotype. He has hemophilia and is male, so he is .
Step 3 — List gametes. Mother: or . Father: or .
Step 4 — Build the square.
| (mother) | (mother) | |
|---|---|---|
| (father) | ||
| (father) |
Step 6 — Overall probability. Each child has a 1/2 chance of having hemophilia, and unlike most X-linked cases, affected daughters are expected here because the father contributes an to every daughter.
Practice questions
A colorblind woman () has children with a man who has normal color vision (). Which statement correctly describes their children?
- All daughters are colorblind and all sons have normal vision
- All sons are colorblind and all daughters are unaffected carriers
- Half the sons and half the daughters are colorblind
- No children are colorblind, but all are carriers
Answer: All sons are colorblind and all daughters are unaffected carriers
Each son receives his single X from his mother, and she can only give , so every son is and colorblind. Each daughter receives from her mother and from her father, making her — a carrier with normal vision. The wrong answers come from forgetting that sons get no X from their father, or from applying a 50 percent heterozygous-cross ratio to a cross where the mother is homozygous.
In a three-generation pedigree, individual III-2 is an affected female. Her father, II-3, is unaffected, and her mother, II-4, is unaffected. Her affected brother III-1 also appears. What mode of inheritance does this pattern support, and what evidence rules out the alternatives?
Answer: Autosomal recessive; two unaffected parents having affected children rules out dominance, and an affected daughter with an unaffected father rules out X-linked recessive.
Start with dominant versus recessive. A dominant allele always shows in whoever carries it, so two unaffected parents could not produce affected children if the trait were dominant. The trait must be recessive, and both parents must be heterozygous. Next test X-linkage. If the trait were X-linked recessive, affected daughter III-2 would be , meaning her father gave her an . He would then be and would show the trait — but he is unaffected. That contradiction eliminates X-linked recessive, leaving autosomal recessive with genotypes for the parents and for both affected children.
Explain why a father with an X-linked recessive condition can never pass that condition to his sons, but is certain to pass the allele to all of his daughters.
Answer: Sons inherit the father's Y chromosome, which carries no copy of the gene, while daughters must inherit the father's single X, which carries the recessive allele.
Sex is determined by which sex chromosome the father contributes. A son by definition received the Y, so he received none of his father's X-linked alleles; his X came from his mother, making her the source of any X-linked recessive condition he has. A daughter by definition received the father's X, and since an affected father is , that X carries the recessive allele. She therefore has at least one and is a carrier at minimum — affected only if her mother also contributed . This asymmetry is why the trait appears to skip a generation and reappear in grandsons through carrier daughters.
FAQ
- Can a female be a carrier of an X-linked dominant trait?
- No. A carrier by definition has an allele without expressing it, which requires the allele to be recessive. A female with one X-linked dominant allele shows the trait, so she is affected, not a carrier. For X-linked dominant conditions, look for affected fathers passing the trait to all daughters and no sons.
- Why do X-linked recessive traits appear more often in males?
- Males have one X, so a single recessive allele is expressed with nothing to mask it. A female needs two copies — one from each parent — to show the trait, which is much less likely when the allele is rare. Hemizygosity in males is the entire reason for the sex bias.
- How do I tell autosomal recessive from X-linked recessive when both seem to fit a pedigree?
- Hunt for a decisive individual. An affected female with an unaffected father rules out X-linked recessive. For a trait that looks dominant, an affected father with an affected son rules out X-linkage, because fathers pass sons a . Careful: for a recessive trait that same picture proves nothing, since an affected son can inherit the recessive allele from a carrier mother while his father is affected independently. If no such individual exists, the pedigree is consistent with both, and a strong male bias among affected individuals makes X-linked recessive the more likely explanation — state it as most likely, not certain.
- Do I include the Y chromosome in a Punnett square?
- Yes. The father's two gamete types are his X and his Y, so one column or row of the square is labeled . Leaving the Y out makes every male genotype look female and destroys the pattern of sex-specific inheritance.
Learn this with a teacher, not a page
The Crimsora tutor teaches Sex-Linked Traits & Reading Pedigrees live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.