1. Monohybrid crosses

2. By the end of this lesson:
You should be able to:
Know what a monohybrid cross
Know how to draw out a monohybrid cross
Predict the proportions of phenotypes of
the F2 offspring in a monohybrid cross
Know why there is a difference between
observed and predicted ratios in monohybrid
crosses

3. F1 and F2 generations
When 2 parents are crossed they produce offspring.
We refer to these offspring as the First Filial generation (F1)
When we do genetics experiments, we often take individuals from the F1 generation, and cross them
Their offspring are called the Second Filial generation (F2 )

4. Then we cross two members of the F1 generationX
Parents F1
generation
Then we cross two members of the F1
generation

5. F1
generation X F2
generation

6. Monohybrid Crosses
In standard grade we have to work out theoretical crosses.
When we do this we only consider one characteristic at a time
This is called a monohybrid cross

7. An example The gene for coat colour in mice has 2 alleles
Brown which is dominant
White which is recessive
What GENOTYPE would a HOMOZYGOUS brown mouse have? BB
What GENOTYPE would a HOMOZYGOUS white mouse have? bb

8. Brown mouse
White mouse
Parents phenotype X BB bb
Parents genotype
All b
All B
Gametes
F1 genotype
All Bb
F1 phenotype
All heterozygous brown
Now we cross 2 individuals from the F1 generation

9. Brown mouse
Brown mouse
F1 phenotype X Bb Bb
F1 genotype
B or b
B or b
Gametes
F2 genotype
Aargghh, how do we work this out?????
Use a punnett square

10. B b Punnett square BB (Brown) Bb bb (White) Gametes from one parent
Gametes from other parent

11. Results! So all the possible F2 generation genotypes are BB (Brown)
bb (white)
The F2 phenotypic ratio would be
3 brown : 1 white
The F2 genotypic ratio would be
1 BB: 2 Bb : 1 bb
1:2:1

12. Does this happen in nature?
It is important to remember that when we are working out ratios, that it is only probable as fertilisation is a random process, and the true ratio is a matter of chance.

13. Work
Insert and complete the “Genetics- Monohybrid Cross” sheet from your diagram pack.

14. Work
Calculate the F2 generation phenotypic and genotypic ratios of a cross between these flies- homozygous straight- winged and curved-winged fruit fly
Straight wing is dominant

15. Work
Once you are confident that you understand how to write out a monohybrid cross, ask your teacher for a question sheet.
Remember to set our your answers in the correct format, as you have been shown!

16. By the end of this lesson:
Can you do the following?
Do you know what a monohybrid cross is?
Can you draw out a monohybrid cross?
Can you predict the proportions of
phenotypes of the F2 offspring in a
monohybrid cross?
Explain why there could be a difference
between the observed and predicted ratio