1. Genetic Crosses Dihybrid & Test Crosses
Higher Biology
Genetic Crosses
Dihybrid & Test Crosses

2. Genetic Crosses By the end of this lesson you should be able to:
Know what a dihybrid cross is
Know how many alleles for each gene there is in a
diploid organism
Know how to use a Punnett Square for a dihybrid
cross
Know what a testcross is
Know how to carry out a testcross

3. Introduction
The foundations of genetics were laid by an Austrian monk called Gregor Mendel.
He published a paper in 1866, called “Experiments with Plant Hybrids”- but his work was largely ignored until the early 1900s.
Mendel carried out a large number of crosses involving pea plants.

4. Introduction
Mendel investigated the following characteristics in pea plants:
Flower colour (purple & white)
Pod shape (inflated & constricted)
Seed shape (round & wrinkled)
Seed colour (yellow & green)
Stem length (tall & dwarf)

5. Monohybrid Crosses- a reminder
What do the following terms refer to? F1 F2
Genotype
Phenotype
Allele
Homozygous
Heterozygous
Dominant
Recessive

6. Monohybrid Crosses- a reminder
How many traits are involved in a monohybrid cross?
If both parents are true-breeding, one dominant and one recessive, what would be the phenotype of the F1?
If two of the F1 generation from Q3 were crossed, what would be the phenotypic ratio of the F2?
If two of the F1 generation from Q3 were crossed, what would be the genotypic ratio of the F2?
What is a Punnett Square?

7. Dihybrid Crosses
A dihybrid cross is a cross involving two characteristics at the same time.
E.g. in pea plants X
This means that four alleles are involved, two for each gene.
Purple flowers
Short stem
White flowers
Long stem

8. Dihybrid Crosses Mendel carried out these type of crosses.
E.g. in pea plants X
In the F1 he found that all the plants had round yellow seeds.
Which two characteristics do you think are dominant?
What term would be used to describe the other two characteristics?
Round yellow
seeded plant
Wrinkled green
seeded plant

9. Dihybrid Crosses Complete the dihybrid cross from your diagram pack. X
Black hair
Green eyes
Ginger hair
Blue eyes

10. Back Crosses Sometimes called a testcross
This procedure is used to determine the genotype of an individual with the dominant phenotype.
When an organism exhibits a dominant trait (characteristic), it is not obvious whether its genotype is homozygous (true breeding) or heterozygous for that trait.

11. Back Crosses
Example: We have a black mouse (shown below), it could be Bb or BB.
How do we know which genotype it has?
B=Black b=white

12. Back Crosses B=Black b=white
Is this mouse BB (homozygous) or Bb (heterozygous)?
Cross it with a homozygous recessive (bb) and check out the offspring produced.

13. Back Crosses
TEST CROSS 1 X
?? bb

14. Back Crosses ?? must be BB B Why? b BB x bb All the offspring
have the same
phenotype as each
other & the
dominant parent. Bb Bb Bb Bb

15. Back Crosses
TEST CROSS 2 X
?? bb

16. Back Crosses ?? must be Bb Why? Bb x bb B b Half the offspring
have the same
phenotype as
the dominant
parent & half have
the same phenotype
as the recessive
parent. B b Bb bb Bb bb

17. Practice Questions
Complete the “Unlinked Backcross” sheet from your diagram pack. Don’t fill in the boxes in the middle of the page at this stage!
Torrance
TYK page 93 Q2
AYK page 89 Q3
AYK pages 101 Q2.

18. Genetic Crosses Can you do it? Know what a dihybrid cross is
Know how many alleles for each gene there is in a
diploid organism
Know how to use a Punnett Square for a dihybrid
cross
Know what a testcross is
Know how to carry out a testcross