Genetic Crosses

Monohybrid Crosses and Test Crossing

Using Punnett Squares

The work of Gregor Mendel

Considered the father of genetics, Mendel correctly deduced the patterns of inheritance that we know know as dominant and recessive traits, using pea plants.

A recap of alleles

• Dominant traits are expressed if at least one dominant allele is present, and are represented with a capital letter

• Recessive traits are expressed only if both alleles are recessive, and are represented with a lower case letter

• Homozygous means to have matching alleles

• Heterozygous means to have alternate alleles

Monohybrid Crosses

These are crosses between two organisms that have a trait controlled by a single gene locus. They can be used to determine the likelhood of a trait being passed on to offspring, or to determine the unknown genotype of the parent.

When two organisms that are homozygous for the same trait breed, this is called 'true-breeding'.

Multiple Choice

A dominant allele is represented by a:

• Capital letter

• Lower-case letter

Multiple Choice

True-breeding is:

• Where members of the same species reproduce

• Where parents with matching homozygous alleles reproduce

• Where parents with matching heterozygous alleles reproduce

Testing for Dominance

An organism displaying a known dominant trait cannot be 100% known to be either homozygous dominant or heterozygous for the trait. 

To figure this out, test-crossing can be performed.

Test Crossing

To figure out the genotype of an organism displaying a dominant phenotype, it can be crossed with an organism displaying a recessive phenotype.
If ALL offspring are displaying the dominant phenotype, then the unknown genotype must be homozygous dominant.
If ANY offspring display the recessive trait, then the parent must be heterozygoug.

Multiple Choice

A test-cross involves:

• An organism displaying a dominant phenotype and an organism displaying a recessive phenotype

• Two organisms displaying dominant phenotypes

• Two organisms displaying recessive phenotypes

Multiple Choice

A test-cross occurs to determine the genotype of an organism displaying a dominant phenotype. If alleles for the trait are A and a, what is the genotype of the parent if all offspring display the dominant trait?

• AA

• Aa

• aa

Multiple Choice

A test-cross occurs to determine the genotype of an organism displaying a dominant phenotype. If alleles for the trait are A and a, what is the genotype of the parent if one offspring displays the recessive trait?

• AA

• Aa

• aa

Multiple Choice

A true-breeding dominant parent breeds with a true-breeding recessive parent. If alleles for the given trait are B and b, 100% of the offspring will have the genotype of:

• BB

• bB

• Bb

• bb

Your job now:

• Familiarise yourself with the SAC advice sheet if you haven't already.

• Continue adding glossary terms from Chapter 11 to your Marzano glossary.

• Complete the interactive in the lesson plan.

Multiple Choice

True-breeding is:

• Where members of the same species reproduce

• Where parents with matching homozygous alleles reproduce

• Where parents with matching heterozygous alleles reproduce