Crandall: If I let you do the [paternity test], it means I don't trust her.

House: No, it means I don't trust her.

―Who's Your Daddy?

Who's Your Daddy is a second season episode of House which first aired on May 16, 2006. An old friend of House shows up with his newly found daughter, who has survived Hurricane Katrina. House agrees to take the case only to try to convince his friend that the young woman is lying to him.

This episode finished 3rd in Facebook's poll of the best episodes of the series which was completed in April, 2012.

One of the hidden themes at this point in the series (which is finally revealed in Birthmarks, but is hinted at as early as Season 1 's Paternity ) is House's uncertainty about his own paternity. Whether the plight of the patient in this episode was a conscious effort to play up this theme for it's final reveal or just a lucky happenstance, it's arguable that the part of House represented by the patient is this very same uncertainty.

However, the argument that this theme was known to the writers and producer at this point but remained intentionally hidden is bolstered by House's attempts to convince Cuddy not to accept an anonymous sperm donor. Having been uncertain about his own paternity, in retrospect, he's desperate to avoid another child being born into this world not knowing for certain anything important about his father.

Although this episode received mixed reviews on its premiere, in retrospect it provides two other important parts of the development of House's character (as well as developing an important aspect of Cuddy's character). First, House's backstory to this point is largely clouded in mystery. Although we have met his parents and his ex-girlfriend, at this point we still don't know much about House's life prior to the series. In this episode, for the first time, we meet someone other than Wilson who thinks of House as his friend and, in some way, House also thinks of as a friend. As they interact, we once again learn that House's personality now is pretty much as it has always been despite the intervention of disability and drugs. His old friend Dylan is used to how House acts and how he's supposed to act around House and their nearly 30 year separation has not affected their ability to relate to each other in the least. For his part, House still thinks of Dylan as the perfect fool, and as the episode progresses, we see that House is right that nothing much has changed. Despite this, and House's noted failure to suffer fools gladly, we still see House being willing to protect his old friend, if in his own unique way.

Perhaps more importantly we see that House is capable of putting aside his sarcastic self to really relate to another human being, in this case Cuddy. Despite numerous opportunities to have fun at her expense, House seems genuinely concerned about her welfare and even the welfare of her yet to be born baby. By the end of their interaction, we see both parties coming very close to letting Huddy finally take its course - Cuddy figures that perhaps the reason House doesn't want her to have a child with another man is because he wants her instead. Despite this opportunity, House realizes it's the wrong moment and reminds her that relationships should be started between people who like each other, and that's not him.

While on a flight with her newly found father, a girl who survived Hurricane Katrina starts hallucinating that the plane is filling up with water. She then collapses.

House is trying to deal with his leg pain on his day off and starts off the day with Vicodin. He tries to massage and exercise his leg to relieve the pain. Just as he's about to inject himself with morphine, Cuddy leaves a message that he has a new case. The patient has cardiogenic shock, but has not had a heart attack.

When House gets to the hospital, Cuddy briefs him on the case. There's nothing wrong with her heart, no infection and no drugs. It turns out that the girl's father is an old friend of House. House pretends not to know him, but quickly admits he is putting on the most gullible guy on the planet. He tells House that the girl is the granddaughter of a famous jazz pianist, Jesse Baker. He tells House how he just found that she was his daughter, but House thinks the girl's lying. He agrees to treat her anyway.

The team does a differential. House decides that it might be an arrhythmia, and he decides to induce one even though it might be dangerous. He also decides to get the informed consent from the father himself. He talks his friend into consenting, then admits it's a dangerous test, but still tells him to sign the form. House also suggests a paternity test. His friend feels such a test will show he doesn't trust his daughter.

They proceed to induce an arrhythmia. The patient starts having a heart attack, but doesn't hallucinate. They keep proceeding with the test on House's instructions. She finally starts hallucinating and they freeze the affected muscle in her heart, expecting that will cure the problem.

Cuddy asks House to review her sperm donor candidates. House calls them both losers. Cuddy tells House to find a suitable donor himself.

The patient awakens and seems to hear someone asking for water. However, she's just hallucinating again.

They realize that the heart wasn't the problem. House thinks it might have been an atypical seizure. He thinks it's unlikely that the heart problem caused the hallucination. He starts thinking that pain may trigger the hallucinations. He decides to put the patient in pain. House sticks the patient with a needle, but she doesn't hallucinate. House also confronts her about lying to his friend about being his daughter, and she finally starts hallucinating when he bends her finger which has her believing that House's face is dissolving. The hallucinations under pain indicate an autoimmune disorder, but they don't know which one.

To treat her, they need to destroy her bone marrow and replace it. When her father finds out, he demands a bone marrow test, but still no paternity test. He's afraid that the test will either show he didn't trust her or that she isn't his daughter. Either way, he loses.

Cuddy asks House if he told anyone else she was looking for a sperm donor. House denies it. Cuddy just wanted to see if she could ask anyone else to help her. House agrees to help Cuddy with a shot into her gluteus maximus.

Wilson asks House why the patient's father is a friend. House admits that he was only his friend because he had a car. They have found a bone marrow donor for the patient, but while they're irradiating her, she starts leaking stool out of her mouth.

The reverse peristalsis shifts the team's diagnosis away from an autoimmune disease. House orders a liver biopsy.

House's friend comes to see him again. House tells him that the patient's liver has failed and her digestive system is running in reverse. He needs consent for the biopsy because the patient could die during the procedure. House once again confronts his friend about only knowing the patient for a few days.

House starts listening to a tape of the patient's grandfather. Wilson comes in with his lunch. He thinks it's great that House is standing up for an old friend, and that the old friend is taking responsibility. He tells Wilson about the stool coming out of the patient's mouth. House tells Wilson that he once broke up one of the old friend's relationships, supposedly for his benefit.

In his office, House is playing a musical number by Jesse Baker, the patient's grandfather, when he realizes that the piano that Baker was playing was perfectly in tune, even though Baker didn't seem to think so. House subsequently calls off the liver biopsy, and orders his team to his office. They go to see the patient, whose skin is getting darker. This indicates hemochromatosis.

House calls in one of the sperm donors, ostensibly for a job interview. He doesn't come off well in front of Cuddy. House makes his point about being acquainted with a baby's father before commencing IVF treatments.

They start treatment for hemochromatosis, but the patient goes into respiratory arrest. They figure that the chelating agent they gave her may have attached the iron in her lungs to a fungus. However, they can't treat her unless they know what fungus it is. They treat for aspergillus because it's the most common.

Cuddy confronts House about the privacy of sperm donors. House confronts Cuddy about accepting sperm from someone she doesn't like.

Cameron tells House that the patient doesn't have aspergillus. They have to figure out what kind of fungus it is, and they have to ask the patient. She can only communicate through blinking. She admits she lied to House's friend. House asks her to write down where she stayed during the flooding. It turns out she was in a recording studio, which is a breeding ground for zygomycosis, which only breeds in very wet conditions - such as flooding of studio sound insulation.

The patient responds to the treatment for zygomycosis. House tells his patient he ran a paternity test and she really is his friend's daughter and she begins to cry.

Later on, we see House lying on his couch while he's listening to a message sent by Wilson to his answering machine. Furthermore, it is revealed that the patient is not the biological daughter of House's friend. House places the paternity test down on a table, next to an empty syringe, implying that he finally injected himself with morphine.

Zygomycosis infections are rare because the fungus is not common and most healthy people can resist the infection. However, people with iron rich blood are more susceptible to most fungal diseases and some infections. While hemochromatosis is more rare in Black individuals than White ones (she inherited the gene from her African-American grandfather), it does have an incidence rate of ~1/25, 000.

• "Who's Your Daddy?" is a common English idiom to express dominance over another person. It's used by Eric Foreman in this fashion in The Down Low when he tricks the rest of the team into taking a pay cut so he can have a raise. In the episode, it refers both to Leona's paternity and to Cuddy's choice of a sperm donor.
• The tune Leona is listening to on her headphones on the airplane is the famous hymn When the Saints Go Marching In. It has become a jazz standard and is associated with New Orleans. However, New Orleans request bands usually ask for a premium to play it rather than accept playing it several times a night.
• Hurricane Katrina was a category 3 hurricane that struck the City of New Orleans in August 2005. As a result of the surge from the storm, the protective levees around New Orleans were breached, resulting in widespread flooding of the city.
• House's answering machine claims his telephone number is disconnected.
• Twister is a popular party-game which was first released in the 1960s.
• A "'fro" is an Afro - One of many popular African-American hair styles which has also includes shorter cuts and box braiding and many other styles.
• Square dancing is a form of folk dancing. Although highly associated with the United States, similar forms are found in Europe and Asia.
• Mozart was an 18th-century composer whose music is still among the most commonly performed of the classical repertoire.
• The idea the Nazis had about breeding 60 years ago was the now discredited practice of Eugenics.
• Chicken mole (moe-LAY) is a Mexican chicken dish. "Mole" is a generic term for several types of sauce in Mexico, but outside of Mexico it usually refers to a type made primarily from chili peppers and chocolate.
• House implying that Crandall's girlfriend "blew him" was, of course, a reference to fellatio.
• Miles Davis is widely believed to be the best jazz trumpeter of all time.
• Swiss cheese is any of several types of American cheese that resemble Swiss Emmental cheese. They usually are cultured to have bacteria that create large pockets of carbon dioxide within the cheese which remain after the cheese cures and hardens.
• Al-Qaeda is a terrorist organization.
• When Cuddy says she's leaning towards sperm donor 613, House replies, "Oh, sure, go with the Jewish number." House is referring to the 613 mitzvot, the canon of Jewish biblical law. Ironically, and suitably for the episode, the first of these is "be fruitful and multiply".
• In most House episodes, the main patient has one disease, and when the patient has more than one diseases, there's usually a causal relationship (Disease A causes Disease B) and coincidence is frowned upon. This episode is one of the rare instances when a patient has three separate, coincidental diseases: Abnormal electrical pathway in the heart resulting in cardiac arrhythmia; genetic hemochromatosis; and zygomycosis.
• House keeps making incorrect assumptions about when the patient will be well enough to leave the hospital, all tied to a meal time. He does much the same thing in the Season 4 episode Alone.
• Like some other characters on the series, Dylan Crandall was named after a crew member, in this case, costume designer Cathy Crandall.
• Bill Nye the Science Guy was a children's science television program hosted by William "Bill" Nye from 1993 to 1998.

A young boy comes in with what appears to be a bright orange rash. House deduces that the boy was lying on a bright red couch, which transferred its dye onto the boy's torso. House mockingly prescribes the patient a wet rag.