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### Course: High school biology>Unit 5

Lesson 1: Introduction to heredity

# Worked example: Punnett squares

Punnett squares help predict offspring traits by showing possible gene combinations from parents. In addition, Punnett squares can illustrate trends among dominant and recessive traits, incomplete dominance, codominance, and dihybrid crosses. Punnett squares are useful for understanding genetics and inheritance patterns. Created by Sal Khan.

## Want to join the conversation?

• Wasn't the punnett square in fact named after the british geneticist Reginald Punnett, who came up with the approach?
• . Geneticist Reginald C. Punnet wanted a more efficient way of representing genetics, so he used a grid to show heredity. In his honor, these are called Punett Squares.
My eyes are brown and green, but there is more brown than green... How is that possible? Isn't there supposed to be an equal amount?
(My mom's eyes are green and my dad's are brown)
• Punnett squares are very basic, simple ways to express genetics. It's actually a much more complicated than that. There may be multiple alleles involved and both traits can be present.

In terms of calculating probabilities, you just need to have an understanding of that (refer above). If your mother is heterozygous with Brown eyes (Bb), and your father is homozygous blue eyes (bb), the probability that their child (you) would have blue eyes is only dependent on your mother. Since your father can only pass a "b", your eye color will be completely determined by whether your mom gives you her "B" or her "b". Mendel's laws dictate that it will be random, and therefor, you have a 50% chance of brown eyes (Bb), and 50% blue eyes (bb). It gets a little more complicated as you trace generations, but it's the same idea. Again your mother is heterozygous Brown eyed (Bb), and your father is (bb).

EXAMPLE: You don't know genotype, but your father had brown eyes, and no history of blue eyes (you can assume BB). Your mother has brown eyes, but your grandmother(mom's mom) had blue eyes. What are the chances of you having a child with blue eyes if you marry a blue-eyed woman?

Well the woman has 100% chance of donating "b" --> blue. but you don't know your genotype, so you trace the pedigree.

Grandmother (bb) x grandfather (BB) (parental)
Mother (Bb) X Father (BB)
You = 50% chance of (Bb), or 50% chance that you are (BB).
So what is the probability of your child having blue eyes? completely dependent on what allele you pass down. So the math would go
(1/2)(1/2) = 1/4 chance your child will have blue eyes
The first 1/2 is the probability that your mother gave YOU a little b, the second 1/2 is the probability that you would give that little b on if you had it. Sorry it's so long, hope it helped
• At , why is it that the red and white flowers produce a pink flower? Shouldn't the flower be either red or white? Since both of the "parent" flowers are hybrids, why aren't they pink, like their offspring, instead of red and white.
• No. Sal is talking out how both dominant alleles combine to make a new allele. They will transfer as a heterozygous gene and may possibly create more pink offspring.
• I met a person, who's parents both had brown eyes, but ther son had dark brown? What causes that?
• Parents have DNA similar to their parents or siblings, but their body design is not exactly as their parents or kin.. So, the son could have inherited those dark brownm eyes from someone from his parents' relatives.
• What makes an allele dominant or recessive?
• There isn't any one single reason. In fact, many alleles are partly dominant, partly recessive rather than it being the simple dominant/recessive that you are taught at the introductory level.

One, but certainly not the only, reason for dominance or recessiveness is because one of the alleles doesn't work -- that is, it has had a mutation that prevents it from making the protein the other allele can make (it may be so broken it doesn't do anything at all or it may produced a malformed protein that doesn't do what it is supposed to do). This will typically result in one trait if you have a functioning allele and a different trait if you don't have a functioning allele. So, the dominant allele is the allele that works and the recessive is the allele that does not work. However, sometimes it is the other way around and the defective gene is dominant because it malformed protein will block the action of the correctly formed protein (if you have the recessive allele that works).

This is just one example. There are many reasons for recessive or dominant alleles. For many traits, probably most, there are multiple genes involved in producing the trait so there is not a simple dominance/recessiveness relationship.
• how would a person have eyes that are half one color and half another?
• Very rare but possible. Called a genetic mosaic. It can occur in persons with two different alleles coding for different colours, and then differential lyonisation (inactivation of X chromosome ) in different cells will produce the mosaic pattern, In simpler words, when there are two different genes, different cells will select different genes to express and that can produce a mosaic appearance.
• What is the difference between hybrids and clean lines?
• hybrids are the result of combining two relatively similar species.
clean lines refer to pure breeds which havent been combined with any other species other than their own