If you’re looking at a hemophilia pedigree for a biology or genetics class, the individuals who have hemophilia are the ones shown as filled-in (solid black) symbols. Males appear as solid black squares, and females appear as solid black circles. Without seeing your specific pedigree, the most useful thing you can do is learn exactly how to read the symbols and apply the inheritance rules so you can confidently identify every affected person, carrier, and unaffected individual on your own.
How to Read Pedigree Symbols
Pedigrees use a consistent set of shapes. Squares represent males. Circles represent females. Diamonds are used when sex is unknown. A horizontal line connecting two symbols means those individuals are mated or married, and vertical lines drop down to their offspring.
Shading tells you about disease status:
- Completely filled (solid black): This person is affected, meaning they have hemophilia.
- Half-shaded circle: This female is a carrier. She has one copy of the hemophilia gene but typically shows no serious bleeding symptoms herself.
- Circle with a dot in the center: Another way some pedigrees mark an obligate carrier (explained below).
- Clear/empty symbol: This person is unaffected and, if male, does not carry the gene. If female, she may or may not be a carrier depending on her parents.
So the short answer to “who has hemophilia” is: look for every solid black square and solid black circle on the chart. Those are your affected individuals.
Why Males Are Almost Always the Affected Ones
Hemophilia is X-linked recessive. The gene for the clotting factor sits on the X chromosome, and there is no matching gene on the Y chromosome. Males have only one X chromosome (XY), so if that single X carries the hemophilia gene, they have no backup copy to produce normal clotting factor. They will have hemophilia.
Females have two X chromosomes (XX). If one X carries the hemophilia gene, the other X usually produces enough clotting factor to prevent serious symptoms. That makes her a carrier rather than affected. For a female to actually have hemophilia, she would need to inherit the gene from both parents, which requires an affected father and a carrier (or affected) mother. This is rare, which is why most filled-in symbols on a hemophilia pedigree are squares.
How to Spot Carriers Who Aren’t Shaded
Some of the most important people in a hemophilia pedigree look perfectly healthy on the chart. These are obligate carriers: females who must carry the gene based on their position in the family, even if they show no symptoms. Identifying them is often part of the question you’re being asked.
A woman is an obligate carrier if any of the following are true:
- Her father has hemophilia. He has only one X chromosome, and he gave it to her. She definitely has one copy of the gene.
- She has an affected son. He got his X from her, so she must carry the gene on at least one of her X chromosomes.
- She has both an affected brother and an affected son. Two independent new mutations in the same family would be extraordinarily unlikely, so she inherited the gene from her mother and passed it to her son.
On many pedigrees these carriers are shown as half-shaded circles or circles with a center dot. On simpler diagrams, they may appear as clear circles, and you’re expected to figure out their carrier status from context.
The “Skipping Generations” Pattern
A classic feature of hemophilia pedigrees is that the condition seems to skip a generation. An affected grandfather in generation I may have no affected sons (he can’t pass it to sons, because sons get his Y chromosome). Instead, his daughters become carriers. Then in the next generation, those carrier daughters can have affected sons. The result is affected males in generations I and III with an apparently unaffected generation II in between.
This pattern is one of the strongest clues that a pedigree shows X-linked recessive inheritance. If you see affected males on both sides of a generation gap, connected through unaffected females, you’re almost certainly looking at hemophilia or a similar X-linked condition.
One Rule That Eliminates Guesswork
An affected father cannot pass hemophilia to his sons. Ever. He gives his sons a Y chromosome, not his X. This means if you see a father-son pair who are both affected, the son did not get the gene from his father. He got it from his mother, who must be a carrier (or affected herself). This single rule helps you trace the gene through the family tree with confidence: follow the X chromosomes, not just the parent-child lines.
Putting It All Together
To answer “who has hemophilia” on your specific pedigree, work through these steps. First, find every filled-in symbol. Those individuals are affected. Second, identify obligate carriers by checking whether any unaffected females have an affected father, an affected son, or both an affected brother and an affected son. Third, confirm the pattern makes sense: affected males should trace their hemophilia gene back through carrier mothers, never directly from fathers. If the inheritance path breaks that rule, re-check your work.
In most textbook pedigrees for hemophilia, you will find two or three affected males (solid black squares) scattered across generations, connected by carrier females (half-shaded circles) who bridge the generational gaps. Affected females are possible but uncommon and only show up when the pedigree is specifically designed to illustrate that scenario.