1. Calculating the percent of offspring with different traits…
Punnett Squares
Calculating the percent of offspring with different traits…

2. Punnett Squares are…
A matrix which allows us to calculate the probability of offspring with different genotypes and phenotypes.
Solving Punnett square problems is simple once you know some genetics vocabulary, but it is best to follow a series of steps to make sure you do everything correctly.

3. Genetics Vocab needed to solve Punnett Problems…
Heterozygous
Allele
Parental Generation
Homozygous
Genotype
Dominant
Gene
1st Filial Generation
Gamete
Phenotype
Zygote
2nd Filial Generation
Hybrid
Dihybrid
Recessive

4. Genetics Vocab needed to solve Punnett Problems…
Recessive
1_____________ A gene that is only expressed if 2 copies are inherited.
2_____________ The first cell of a new organism.
3_____________ The common name for an organism with 2 different alleles.
4_____________ A gene that is always expressed.
5_____________ An organism with two of the same alleles.
6 _____________ A piece of DNA with the code for a trait.
7 _____________ The original organisms in a family.
8 _____________ A specific version of a gene.
9 _____________ An organism with two different alleles.
10 _____________ The offspring of the original organisms.
11 _____________ The kinds of traits shown in an organism.
12 _____________ The grandoffspring of the original organisms.
13 _____________ A haploid sex cell.
14 _____________ An organism with 2 different alleles for 2 different traits.
15 _____________ The kinds of genes in an organism.
Zygote
Hybrid
Dominant
Homozygous
Gene
Parental Gen.
Allele
Heterozygous
1st Filial Gen.
Phenotype
2nd Filial Gen.
Gamete
Dihybrid
Genotype

5. Six steps to solve Punnett problems
Example problem: Meerkats often stand on their hind legs, putting stress on their spines. They can have either thick disks between vertebrae, or thin disks. The thick disks give them extra padding, while the thin disks allow them to be more flexible. The thick disk is the dominant trait. A hybrid male mates with a female with thin disks. What percent of their offspring will have each kind of disk?

6. Step One: Define the alleles.
Choose any letter to represent the trait. Only choose ONE letter!!!
The dominant allele MUST be capitalized.
The recessive allele MUST be the SAME letter, but lower case.
In meerkats, thick vertebral disks are dominant over thin disks. A hybrid male mates with a female with thin disks. What percent of their offspring will have each kind of disk?
D = thick disks d = thin disks
(dominant) (recessive)

7. Step Two: Find parental genotypes & phenotypes
Genotypes = the kind of genes, shown by the letters you chose in step one. You MUST use two letters for every trait.
Phenotypes = the kind of traits, shown in a written description.
In meerkats, thick vertebral disks are dominant over thin disks. A hybrid male mates with a female with thin disks. What percent of their offspring will have each kind of disk? Dd dd
Male genotype = Female genotype =
Male phenotype = Female phenotype =
Thick disks
Thin disks

8. Step Three: Find the possible gametes for each parent.
Use the genotypes to figure out what kinds of gametes the parents are able to make. (What kinds of alleles can they contribute to their offspring?)
Gametes MUST have only ONE letter for each trait, since they are haploid.
In meerkats, thick vertebral disks are dominant over thin disks. A hybrid male mates with a female with thin disks. What percent of their offspring will have each kind of disk?
Male genotype = Dd Female genotype = dd
Male gametes = Female gametes =
D or d
d or d

9. How about a shortcut that combines steps 2 & 3?!!!
In meerkats, thick vertebral disks are dominant over thin disks. A hybrid male mates with a female with thin disks. What percent of their offspring will have each kind of disk?
Set up a chart like this, and start with what you know from the description:
Male
Female
Phenotype
Genotype
Gametes
Use these to set up your Punnett square!
Thick
Thin Dd dd
D or d
d or d

10. Step Four: Put gametes on the OUTSIDE of the Punnett square.
Male gametes = D or d Female gametes = d or d d d D d
It doesn’t matter if the eggs are across the top and the sperm are along the side or vice versa.

11. Step Five: Fill in the Punnett square to get the offspring genotypes.d d D D d D d d d d d d
Always copy whatever letter is along the side in every box next to it, and always copy whatever letter is on top in every box under it. This gives you diploid zygotes!

12. Step Six: Calculate the percentage of each genotype and phenotype.
Offspring genotypes: 50% Dd (hybrid) 50% dd (homo. rec.) Offspring phenotypes: 50% Thick disks 50% Thin disks
In meerkats, thick vertebral disks are dominant over thin disks. A hybrid male mates with a female with thin disks. What percent of their offspring will have each kind of disk?

13. Anyone for a shortcut? D d D d
Since these are the same, you can skip the second column! D d D d
You still get the same answer of 50 / 50!

14. Basic Ideas………..
Parents
Offspring
1. Organisms (both ____________ and ____________) always have ____ letters for each trait, because their cells are _____________.
2. Gametes ( both ___________ and __________ cells) always have ____ letters for each trait, because their cells are _____________.
3. The Punnett Square shows the __________ on the outside and the __________ on the inside.
Two
Diploid
Egg
Sperm
One
Haploid
Parent Gametes
Zygote Offspring
Two
Offspring
Parents
Egg
Zygote Offspring
Sperm
Diploid
Parent Gametes
One
Haploid

15. Try another problem!
Scarlet Macaws (a type of large parrot) can either have smooth faces (a recessive trait) or feathered faces. A heterozygous female mates with a smooth-faced male. Find the percent of offspring with smooth and feathered faces.

16. Practice makes perfect!
Clownfish can have sticky mucus or slick mucus on their skin. The sticky mucus is dominant over the slick mucus. Find the genotype and phenotype percentages of the offspring if a homozygous dominant male mates with a hybrid female.