8A Monohybrid Crosses

Question 1

What are monohybrid crosses?

Answer 1

• Monohybrid crosses show how certain traits are passed down from parents to their offspring. By completing a Punnett square, we can predict the genotypes and phenotypes of offspring resulting from a monohybrid cross, for both autosomal and sex-linked traits.
• A monohybrid cross explores how alleles are passed down from parents to their children.

Question 2

What are the steps to completing monohybrid crosses?

Answer 2

1. Assign letter to each allele eg W = white, w = black (dominance can be assumed to be a capital letter)
2. Draw a 2 x 2 grid
3. Complete cross by filling in the grid
4. Calculate proportions / ratios.
5. Calculate phenotype proportions / ratios

Question 3

How does the process differ if there are sex linked alleles?

Answer 3

You need to use the correct allelic notation using superscript, ie XB (superscript) and Xb (superscript).

Question 4

How does the process differ if there is autosomal codominance?

Answer 4

You need to use the correct allelic notation using superscript, ie IA (superscript) and IB (superscript).

Question 5

What is a test cross and what is it used for?

Answer 5

To identify the genotype of an organism with the dominant phenotype but unknown genotype, either homozygous dominant or heterozygous.

Question 6

How do you do a test cross?

Answer 6

First, you cross the possible genotypes for the organism with the unknown genotype with another organism that has the recessive phenotype. Secondly, you calculate the genotype and phenotype ratios. Lastly, you identify which genotype the unknown is.

Question 7

Have a go at some examples to practice completing monohybrid crosses

Answer 7

3. Have a go at some examples to practice completing monohybrid crosses