If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Pedigree for determining probability of exhibiting sex linked recessive trait

Pedigree charts can be used to determine the probability of offspring exhibiting an X-linked recessive trait, such as color blindness. By scrutinizing genotypes and utilizing a Punnett square, the probability of color blindness in offspring can be calculated. Created by Sal Khan.

## Want to join the conversation?

• At the end of the video, Sal says there is 50% chance that the next child of Tom and Barbara will be colorblind. Does it matter that the couple already have 3 children and one of them is color blind? Will that mene that the next child (according to Punnet square) will definitely be colorblind?
• no.. it isnt a set thing. the punnet square shows the probability of the offspring getting those specific genes
• is there any trick to solve pedigree problems in MCAT?
• IF the parents do not show the trait, but some of their children have that trait, that means the trait is recessive. Does that help?
• At the end of the video, Sal says there is 50% chance that the next child of Tom and Barbara will be colorblind. Does it matter that the couple already have 3 children and one of them is color blind
• Lets look at this a different way, when you flip a coin it is 50/50 for you to get heads or tails. If you have flipped it 3 times before and got heads each time it is still a 50/50 chance on the 4th flip. There is no direct causal effect on the next flip from the previous flips.
• How does one decide which parent to start off with? For example, in the video, Sal uses the mother's parents as a starting point. But in real-life examples, how do you decide? Or is it always started with the mother's side?
• It starts with whichever parent has the pedigree. Hopefully, the parents aren't siblings, and therefore you can trace whichever parent is traced in the pedigree you're given.
• Is there a pedigree for codominance?
• Yes, try making one with B for black coloring, W for white coloring. BB would be black, WW would be white, and BW would be gray.
• Let's say the mother is a carrier XB-Xb and the father is healthy XB-Y. They'd have 1 healthy son, 1 affected son, 1 carrier daughter and 1 healthy daughter. If they were to have another daughter, is she more likely to be a carrier or not?
• There is a 50/50 chance of her being a carrier or not
• Sorry I might be confused, but are Tom and Barbara siblings?