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Next Generation: HS.LS3.3 MS.LS3.2 HS.LS3.3 MS.LS3.2

# Punnett Squares

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Author: Sophia Tutorial
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This lesson will demonstrate how to use a punnett square to determine the genetic probability of a child inheriting a trait from the parents.

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Tutorial

This lesson is going cover Punnett squares and their use by looking at:

1. Punnett Squares
2. Independent Assortment

## 1. Punnett Squares

Punnett squares are tools that are used to determine the probability or the chance that a trait will show up in an offspring. Genetic traits are characteristics of an organism that are determined by their genes, and are inherited.

An example of a trait could be eye color, height, hair color, or a certain genetic disorder.

• Probability
• A measure of the chance of an outcome.
• Punnett Square
• A tool used to determine the probability of offspring inheriting a trait based on the genotypes of both parents.
• Genetic Trait
• A characteristic of an organism such as eye color, skin color, hair color, gender, body type, etc.

To see how Punnett squares are used, look at this square crossing male and female sex chromosomes:

A Punnett square helps determine the odds of an offspring’s genotype given the parent’s genotype, ultimately genotype will determine the phenotype.

IN CONTEXT
The Punnett square crosses the sex alleles of a male and a female. A female will have two X chromosome, and a male will have an X and a Y chromosome. The mother’s alleles are listed on top and the fathers are listed on the side. Using the square above what are the odds an offspring will be female?
You can see that there is a 50% chance that any offspring would have two XX chromosomes, making the child female.

• Phenotype
• The physical manifestation of the expression of an organism's genes.
• Genotype
• The genes inherited by an organism which represent a trait.

Let’s try another one:

This is a Punnett square related to hairline showing whether a person is going to have a straight hairline or a widow's peak; a widow's peak is a dominant trait. If a person has at least one dominant allele, they will have the widow's peak.

The mom in this example is homozygous dominant for widow's peak. This means that she has the same alleles and both are dominate. Her phenotype would be the dominate trait, which is this case means she has a widow’s peak. The father is heterozygous for the trait, meaning that he has one dominant allele and one recessive allele. Since he has at least one dominant allele, it masks that recessive allele. The dad will also have a widow's peak as his phenotype.

How many of the offspring from this example are going to have a widow’s peak?

We can see that because each box has at least one big W, or dominant allele, 100% of their offspring will have a widow's peak.

## 2. Independent Assortment

The law of independent assortment is a law that states that the vast majority of traits are inherited independently of one another. This means that the inheritance of one trait is not influenced by the inheritance of another trait.

The inheritance of a Y chromosome does not influence, or is not dependent on, the inheritance of a widow's peak, for example. So traits are inherited independently of one another.

• Independent Assortment
• A law that states traits are inherited independently of one another; the inheritance of one trait does not influence the inheritance of another trait.

Punnett squares are used to determine the probability of inheriting a trait based on the parent’s genotype. The dominate trait will always determine the phenotype of the offspring. The law of Independent Assortment says that traits are inherited independently of one another.

Keep up the learning and have a great day!

Source: SOURCE: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND

Terms to Know
Genetic Trait

A characteristic of an organism such as eye color, skin color, hair color, gender, body type, etc.

Genotype

The genes inherited by an organism which represent a trait.

Independent Assortment

A law that states traits are inherited independently of one another; the inheritance of one trait does not influence the inheritance of another trait.

Phenotype

The physical manifestation of the expression of an organism's genes.

Probability

A measure of the chance of an outcome.

Punnett Square

A tool used to determine the probability of offspring inheriting a trait based on the genotypes of both parents.