Intro to Punnett Squares

Homework

Objectives Get familiar with more complicated allele formats Be able to construct and interpret a monohybrid cross (basic Punnett Square) Use a Punnett Square to predict genotypes and phenotypes Know what each part of the Punnett Square represents

Allele Formats Most of the time in this class, we’ll use the allele convention you’ve learned already, where one letter = one allele. But most of the time in real-world science, scientists (naturally!) use a more complicated route.

Allele Formats There are SO MANY organisms, so many genes, and only 26 letters in the alphabet. To be specific, scientists usually use more than one letter for an allele. They typically use a normal capital letter to stand for the gene, and then a superscript big or little letter for the allele. Like this:

Allele Conventions The gene for coat color may have the dominant allele CB and the recessive allele Cb. The gene for tail curling may have the dominant allele TC and the recessive allele Tc. The gene for coat consistency may be BCH or BCh. I’ll be sticking to just the normal allele convention to help you learn. But this more accurate convention is one that will come back a bit next week, and MCAS may choose to use it. So just be aware that these are alleles just like any other, they still stand for different base sequences coding for different versions of a protein, they’re just written by scientists to be more specific.

Punnett Squares A cool skill to have is the ability to predict the base sequences of relatives and future offspring. Knowing the alleles of living organisms can tell you about the genes and alleles of organisms that have long been dead! For simplicity, we’ll stick with one or two generations rather than thousands or millions. Here is how it works.

Punnett Squares In The Dark Knight, Harvey Dent had a coin with the same thing on both sides, heads and heads (H and H). When he flipped that coin, what were the chances that it would come up heads (H)?

Punnett Squares He acquired a coin that was heads (H) on one side, and damaged (we’ll call it h) on the other side. NOW what were the odds that, on one flip, it would come up heads (H)?

Punnett Squares There is a scene where he flips it twice. The other two people in the scene are really hoping that it comes up heads (H) both times. What are the odds that it would? Here’s how to set it up…

Punnett Squares The first flip will be either heads (H) or damaged heads (h):

Punnett Squares The second flip will also be either heads (H) or damaged heads (h):

Punnett Squares These are the possible combinations that he could have produced:

Punnett Squares 1 in 4 possible outcomes would be both heads (HH). 1/4 = 25% = .25 h H H H H H h H h h h h

Punnett Squares The actual outcome was one heads and one damaged heads (Hh). What were the odds? h H H H H H h H h h h h

Punnett Squares 2 of 4 possible outcomes = 1/2 = 50% = .5 h H H H H

Punnett Squares What if he’d had two coins somehow, one with two normal heads (HH) and one with one normal and one damaged side (Hh). Then, he flipped both at the same time. What are the odds that both coins would land with normal heads up (HH)?

Punnett Squares H H H H H H H H h H h h This is called a Punnett Square. It’s a way of mathematically determining the probability of an outcome with some randomness (which side will land up?) and some non-randomness (which sides are possible?) What could this have to do with gametes and genes?

Punnett Squares Mom has two of every kind of chromosome, so, two alleles for every gene. Each egg has only of each kind of chromosome, so, one allele. Half of her eggs have allele 1, half have allele 2. Dad has two of every kind of chromosome, so, two alleles for every gene. Each sperm has only one of each kind of chromosome, so, one allele. Half of his sperm have allele 1, half have allele 2. And, it’s random which egg and which sperm fuse to create the offspring.

Punnett Squares If you know Mom’s genotype, and Dad’s genotype, you can predict the likelihood that their offspring would have a certain genotype just like you could predict the likelihood of some coin tosses! Punnett Squares are simple once you’ve got the idea, you just need to adhere to the conventions.

Punnett Squares It doesn’t matter whether Mom or Dad is on the side, just keep both eggs together and both sperm together.

Punnett Squares Just like before, if something is heterozygous, write the dominant allele (capital letter) first. a a A a A A a A

Punnett Squares Take out your color-coded Meiosis Guides, and with your partner, try to answer this question: If Meiosis makes four gametes for each parent, why do we make a Punnett Square with only two gametes per parent?

Punnett Squares Results can be in two formats. Genotype Fractions or Probabilities = The likelihood of offspring with different genotypes. You and your partner, make a Punnett Square for this cross: Tt x tt

Punnett Squares Phenotype Fractions or Probabilities: Same thing, but with phenotypes. In the example you just tried, suppose that the T allele in spiders codes for long legs and the t allele codes for short legs.

Practice Whiteboards out, one per table pair. Take turns writing, be kind to the markers. Construct a monohybrid cross for YY x YY. Give genotype fractions and probabilities.

Practice Construct a monohybrid cross for HH x Hh. Give genotype fractions and probabilities.

Practice Construct a monohybrid cross for Rr x rr. Give genotype fractions and probabilities.

Practice Construct a monohybrid cross for Dd x Dd. Give genotype fractions and probabilities.

Practice Construct a monohybrid cross for kk x kk. Give genotype fractions and probabilities.

Practice You cross a homozygous dominant tall pea plant (TT) with a homozygous recessive short pea plant (tt). What are the odds of an offspring being a short plant?

Practice You cross a homozygous white flower with a homozygous red flower. Red is dominant to white. What are the odds of the offspring being a red flower? (note: when the problem doesn’t tell you what letters to use for the genotype, feel free to use any letter you want)

Practice A male human is crossed with a female human. Males are XY, females are XX. What are the odds of a child being a girl?

Practice You cross two heterozygous tall pea plants (Tt x Tt). What will be the phenotype probabilities for these offspring?

Practice You cross two tall pea plants with each other, a homozygous dominant (TT) with a heterozygous (Tt). The pea plants produce 1000 offspring. How many of those offspring do you predict will be heterozygous?

Punnett Squares Use the remaining time to work on your homework.