Arriving at a crime scene with a dead body, Shawn sees another victim that he connects to an unsolved case of his father's from 20 years ago. Henry works with Shawn and Gus. As the murders are solved, Shawn realizes his father is in danger. Written by
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Spot the Pineapple: The candy dish is pineapple shaped. See more
Thia determines that Ellis is her biological father (and thus her mother's killer) due to his colorblindness, a trait she shares. During Shawn's speech at the reveal, Shawn states that "Color blindness is a genetic trait that is almost always passed on from father to daughter." In fact, though the daughter will most likely inherit the colorblind gene from her father, the chance of a daughter actually being colorblind is very slim. Thia's mother would also have to have been a carrier for Thia to even have a chance at also being so. In general, colorblindness is extremely rare in females. According to the NIH, only about 0.5% of women of Northern European ancestry suffer from it. A quick internet search will be able to explain the genetics behind colorblindness far better than is possible in this little blurb. See more
References NYPD Blue