Genes, Heredity & Punnett Squares

Mendel’s units of heredity are now called genes. A gene is a piece of DNA that provides a set of instructions to a cell.

Each gene will have a specific locus; a specific location on a chromosome.

Think of a locus as the gene’s address; it tells where a gene is located on a chromosome.

Mendel’s Units of Heredity

An allele is any of the alternative forms of a gene that may occur at a specific locus.

Each cell has two alleles for each gene. Homozygous alleles are two alleles that are the same at one locus; i.e. they both code for blue eyes.

Heterozygous alleles are two alleles that are different at a given locus; i.e. one codes for red flowers while the other codes for pink flowers.

Alleles

Genotype & Phenotype

Genotype Phenotype

Genotype is the genetic makeup of a specific set of genes.

For every trait there is a genotype.

Example: the genotype for blue eyes is “bb.”

Phenotype is the physical makeup or characteristics of an organism.

Depending on the genotype, people express a certain phenotype.

Example: a person with genotype BB has brown eyes.

You have two genes for every trait that you have. One gene comes from your mother, and the other gene comes from your father.

Therefore, genotypes are always expressed by two letters. Each letter represents a gene. We use different letters to represent different genes.

The blue eyed gene is symbolized by a little b. A person with blue eyes has twoof them, and their genotype is bb.

Important Point!

When two different alleles are present in a genotype, one of them tends to dominate the other.

Example: If someone receives an allele symbolized by “B” for eye colour from their mother and an allele symbolized by “b” for eye colour from their father, one dominates the other. The allele represented by the capital letter is always dominant.

In this case, the person would have the physical trait represented by the “B”, which is brown eyes.

Dominant and Recessive Alleles

Definitions:

A dominant allele is expressed when two different alleles or two dominant alleles are present.

A recessive allele is expressed only when two are present in the genotype.

Dominant and Recessive Alleles

Using the following information, answer the next questions:

Put it All Together:

Gene TraitB Brown eyes

b Blue eyes

T Tall

t Short

H Blonde hair

h Black hair

U Hitchhiker’s thumb

u Straight thumb

1. A person has the genotype Bb for eye colour. What colour are the person’s eyes?

2. Someone has blonde hair. What are their possible genotypes for hair colour?

3. John goes hitchhiking, and notices that he has the thumb for it. What genotype does he have for this trait?

Questions

Punnett SquaresIf you’ve ever wondered if two blue eyed people could have a brown-eyed kid, now you’ll know

Punnett Squares are a method of predicting the genotypes of the offspring between two parent organisms.

If we know the genotypes of the parents for a particular trait, we can predict the frequency of different genotypes in the offspring.

We can perform both monohybrid crosses (one trait) and dihybrid (two traits) crosses using Punnett Squares.

Punnett Squares

One allele from each parent is written in one grid box. Each possible genetic combination is shown and has a specific probability.

Punnett Squares in a Nutshell

1. Determine (if not given) the genotypes of each parent for the trait in question.

2. Place the parent genotypes on either side of the Punnett Square. Each allele must stay with its respective parent.

3. Fill in the square with the various combinations of alleles that are possible between the parents.

4. Interpret the square to determine the probability of the different gene combinations in the offspring.

Steps to Using a Punnett Square

A green flower (GG) is crossed with a white flower (gg). Determine the probability that the offspring will be green.

The allele that represents eye colour is symbolized by the letter “b,” and the dominant allele codes for brown eyes. If two heterozygous (for eye colour) humans mate and produce offspring, what is the probability of each possible genotype?

Examples