Punnett Squares are used in genetics to determine the possible combinations of genes that will occur at fertilization.A Punnett square is made from a grid of 2x2 spaces.With the knowledge of both parents' genes, scientists can find potential combinations of genes for the offspring.
Step 1: Understand the concept of genes.
It's important to get some important basics out of the way before learning how to make Punnett squares.The idea that all living things have genes is the first.Almost every single cell in an animal's body has instructions in its genes.Genes are responsible for virtually every aspect of an organisms life, including the way it looks, how it behaves, and much, much more.When working with Punnett squares, it's important to understand that living things get their genes from their parents.You are aware of it subconsciously.Do the people you know look and act similar to their parents?
Step 2: Sexual reproduction is a concept to understand.
Most of the organisms you are aware of are used to make children.A female parent and a male parent each give half of their genes to their child.A Punnett square is a way of showing the different possibilities that can occur from this half-and-half exchange of genes.Sexual reproduction is only one form of reproduction.Asexual reproduction is when one parent makes a child all by themselves.The child is more or less a copy of its parent in asexual reproduction.
Step 3: Understand the idea of alleles.
An organisms genes are a set of instructions that tell every cell in the body how to live.Just like an instruction manual is divided into different chapters, sections, and subsections, different parts of an organisms genes tell it how to do different things.One person with black hair and another with blonde hair may be the result of genetic differences between the two organisms.There are different forms of the same gene.A child gets two sets of genes, one from each parent.
Step 4: Understand the concept of dominant and recessive all genes.
A child's genetic power is not always shared.By default, dominant alleles will manifest in the child's looks and behavior.If they're not coupled with a dominant allele which can "override" them, others will only be expressed.Punnett squares can be used to determine how likely a child is to have a dominant or recessive allele.They can be "overridden" by dominant alleles, which makes them less likely to be expressed.A child will have to get an allele from both parents for it to be expressed.A blood condition called sickle-cell anemia is an example of a recessive trait that isn't "bad" by definition.
Step 5: A 2x2 grid is needed.
Punnett squares are easy to set up.Divide a square into four even boxes by drawing a good-sized square.There should be two squares in each row and column.
Step 6: Each row and column can be represented by letters.
Columns are assigned to the mother and the father on a Punnett square.Each row and column has a letter next to it that represents the mother and father's alleles.Capital letters are used for dominant alleles.This can be understood with an example.Let's say you want to know if the child of a couple will be able to roll its tongue.This can be represented with the letters and the first and last letters of the dominant and recessive genes.If both parents have one copy of each allele, we would write.
Step 7: Write the letters in the row and column.
It's easy to fill in your Punnett square if you know the alleles of your parents.Write the two-letter genes combination on each square.Write the letters from the space's column and row in the same place.In our example, we would fill in our squares with capital letters first.
Step 8: Determine the potential offspring's genetics.
The children that the two parents can have are represented by each square of a filled-in Punnett square.On a 2x2 grid, there is a 1/2 chance for each of the four possibilities.The Punnett square has different combinations of alleles.The offspring won't necessarily turn out differently for each square because of genetic differences.There are two squares with the same genotype in our example Punnett square.
Step 9: Determine the potential offspring's characteristics.
The physical trait that an organisms displays is based on its genetics.There are a few examples of physical characteristics that are determined by genes, but not the actual genes themselves.The characteristics of the genes determine the phenotype a potential offspring will have.Different genes have different rules for how they manifest.Let's say that the gene that allows someone to roll their tongue is dominant.Even if only one of their alleles is dominant, any offspring will be able to roll their tongue.The potential offspring are: Top left, Top right, and Bottom left.
Step 10: The squares can be used to determine the probability of different things.
Punnett squares can be used to determine how likely offspring will have certain characteristics.Punnett square tells us that there are four possible gene combinations for any offspring from these parents.Three combinations make an offspring that can roll its tongue, while one does not.Offspring can roll its tongue, but offspring cannot.
Step 11: The basic 2x2 grid should be doubled for each additional gene.
Some gene combinations are more complex than the one-gene cross from the section above.Some phenotypes are determined by more than one gene.You have to account for every possible combination in order to draw a bigger grid.The rule for Punnett squares when it comes to more than one gene is this: In other words, since a one-gene grid is 2x2, a two-Gene grid can be 4x4 and so on.Let's follow along with a two-gene example problem to make these concepts easier to understand.We should draw a grid.The problems that require larger grids and more work are the same for three or more genes.
Step 12: Determine if the parents' genes are being contributed.
You should find the genes that both parents have.If you're dealing with multiple genes, each parent's genotype will have an additional two letters for each gene beyond the first letter.It is helpful to write the mother's and fathers' names above the top of the grid as a visual reminder.Let's use a classic example problem to show the conflicts.Pea plants can have peas that are smooth or wrinkled.The dominant traits are smooth and yellow.S and s are used to represent dominant genes for smoothness and Y and y for yellowness.Let's say that the mother and father have the same genetics.
Step 13: Different combinations of genes are written along the top and left sides.
Below the top row of squares in the grid and to the left of the column, write the different alleles that can be contributed by each parent.Each allele is likely to be passed on.Each row and column will have multiple letters: two for two genes, three for three genes and so on.We need to write down the different combinations of genes that each parent can contribute to.Across the top and the father's SsYY genes on the left are the alleles for each genes.
Step 14: The spaces should be filled with the combinations of alleles.
When dealing with a single gene, fill the spaces in the grid.Each space will have two extra letters for each gene beyond the first: four for two genes and six for three genes.The number of letters in each space should match the number in the parent's genes.We would fill in our spaces like this.
Step 15: You can find the phenotypes for each potential offspring.
When dealing with multiple genes, each space in the Punnett square still represents the genotype for each potential offspring, with a greater number of choices than there are with one gene.The exact genes being dealt with are what determines the phenotypes for each square.According to a general rule, dominant traits only need one dominant allele to be expressed.Since smooth and yellowness are the dominant traits for our peas, any square with at least one capital S is a plant with a smooth phenotype.Green plants need two lowercase ys.We get top row, second row and third row.
Step 16: The squares can be used to determine the likelihood of each phenotype.
The same techniques can be used to find out if any offspring from the two parents can have different characteristics.The number of squares with the phenotype divided by the total number equals the probability for each one.Offspring is smooth and yellow, and wrinkled and green, but it's impossible for any of them.