TEAS 7 Science Punnett Squares
Punnet Squares
Punnet squares are a diagram used to predict the genotype of offspring in hybridization experiments.
- They work under the assumption that the gene of interest follows Mendel's laws of inheritance. On TEAS test we can assume the conditions for Punnet square are met if they are asking us to produce one.
- For those with a burning desire to learn more about Mendelian genetics you can read THIS Links to an external site. Wikipedia article. Placing emphasis on sections 2.1 thru 2.3
- On the TEAS test most crosses will look at one gene (mono hybrid) , and occasionally look at two genes (dihybrid). TEAS will not ask past dihybrid crosses.
- The dihybrid cross on TEAS will look at crossing two offspring that are heterozygous in both genes of interest (example AaBb x AaBb).
- Make sure to not confuse the terms below with each other:
- homozygous/heterozygous
- alleles/genes
- phenotype/genotype
- If a phenotype is showing the dominant trait the genotype can be either homozygous dominant (AA) or heterozygous (Aa).
- If a phenotype is showing the recessive trait the genotype can only be recessive (aa). We never have to say a genotype is recessive and also homozygous as that is implied.
- The website Science Primer has a Useful Calculator Links to an external site. for calculating Punnett square ratios if you need to check your work.
Results of common one gene crosses
Assume A = dominant allele and a= recessive
Example 1. One parent homozygous dominant and one heterozygous
| A | a | |
|---|---|---|
| A | AA | Aa |
| A | AA | Aa |
Genotype: half will be homozygous dominant. Half will be heterozygous.
Phenotype: All offspring will show dominant phenotype
Example 2. One parent heterozygous and one recessive
| A | a | |
|---|---|---|
| a | Aa | aa |
| a | Aa | aa |
Genotype: Half will be recessive. Half will be heterozygous dominant
Phenotype: Half will show recessive pheno. Half will show dominant phenotype
Example 3. Both parents heterozygous
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
This will produce the most diverse distribution of offspring.
Genotype: 1/4 homozygous dominant, 1/2 heterozygous, and 1/4 recessive
Phenotype: 3/4 will show dominant phenotype. 1/4 will show recessive phenotype
Other crosses:
-When both parent AA all offspring AA
-When both parent aa all offspring aa
Results of the dihybrid cross
No lesson in genetics is complete without a obligatory diagram about pea plants.
Assume gene 1 Y= dominant (yellow color), y= recessive (no yellow color and becomes green)
Assume gene 2 R= dominant (round pea), r= recessive (wrinkled pea)
We will perform the following cross YyRr x YyRr:
Image from Openstax biology book
image source:http://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction
Note that in typical dihybrid crosses we produce a 9:3:3:1 ratio. It is unrealistic for the TEAS test to ask the student to produce a 4x4 Punnett during the exam so more likely they expect the student to understand the general outcome of the dihybrid cross. That is
- 9/16 of the offspring will have a phenotype that shows both dominant traits
- 3/16 of the offspring will have a phenotype that shows one of the recessive traits (in the table above that is being green)
- 3/16 of the offspring will show the other recessive trait (in the table above that is being round)
- 1/16 will show BOTH recessive traits at same time (being green AND round)