A3. Genetic Diagrams. Multiple Allele and Dihybrid Crosses

Question 1

Multiple allele crosses aren’t much different to the monohybrid crosses you’ve already come across. They still only involve ___ _____, it’s just the gene can have more than ___ _________. If you want to, you can also use genetic diagrams to look at the inheritance of two genes simultaneously this is called a ____________ ______

Answer 1

Multiple allele crosses aren’t much different to the monohybrid crosses you’ve already come across. They still only involve one gene, it’s just the gene can have more than two alleles. If you want to, you can also use genetic diagrams to look at the inheritance of two genes simultaneously this is called a dihybrid cross.

Question 2

Multiple allele crosses - Example
In the ABO blood group system in humans there are three alleles for blood type:
- Iᴼ is the allele for blood group O.
- Iᴬ is the allele for blood group A.
- Iᴮ is the allele for blood group B. Allele 1ᴼ is recessive. Alleles Iᴬ and Iᴮ are codominant-people with genotype I will have blood group AB.
Figure 1 shows a cross between a heterozygous person with blood group A and a heterozygous person with blood group B.
Any offspring could have one of four different blood groups (A, B, O or AB). So the expected phenotypic ratio is …

Answer 2

Any offspring could have one of four different blood groups (A, B, O or AB). So the expected phenotypic ratio is 1:1:1:1.

Question 3

Tip: Monohybrid crosses look at the inheritance of ____characteristic only.

Answer 3

Tip: Monohybrid crosses look at the inheritance of one characteristic only.

Question 4

Dihybrid crosses
Dihybrid inheritance is the inheritance of ____characteristics, which are controlled by different genes. Each of the two genes will have different _________. Dihybrid crosses can be used to show the likelihood of offspring inheriting certain combinations of the two characteristics from particular parents.

Answer 4

Dihybrid inheritance is the inheritance of two characteristics, which are controlled by different genes. Each of the two genes will have different alleles. Dihybrid crosses can be used to show the likelihood of offspring inheriting certain combinations of the two characteristics from particular parents.

Question 5

Dihybrid crosses - example
the gene for seed shape has two alleles. The allele for round seeds (R) is dominant and the allele for wrinkled seeds (r) is recessive. The seed colour gene also has two alleles. The allele for a
yellow seed (Y) is dominant and the allele for a green seed (y) is recessive. The genetic diagram in Figure 3 shows a cross between two heterozygous
parents-both have round and yellow seeds (RrYy). Here’s how to draw a genetic diagram for this cross:
(3 steps)

Answer 5

Step 1: Make sure you’re clear what the letters mean.
Step 2: Work out the alleles the gametes would have.
Step 3: Cross the parents’ gametes to show the possible offspring.

Question 6

Usually, whenever you do a dihybrid cross with two heterozygous parents you get a ::_:_phenotypic ratio- that’s _ __________ _____ : _ ________ _____, __________ _______: _ _________ first, ___________ second: _ ____________ both.

Answer 6

Usually, whenever you do a dihybrid cross with two heterozygous parents you get a 9:3:3:1 phenotypic ratio- that’s 9 dominant both : 3 dominant first, recessive second: 3 recessive first, dominant second: 1 recessive both.

Question 7

Dihybrid crosses and codominance
You can also do _________crosses involving ___________ ________. They work in exactly the same way as the example above, but the ______________ ______produced are quite different (there are _____than four possible phenotypes in the offspring).

Answer 7

You can also do dihybrid crosses involving codominant alleles. They work in exactly the same way as the example above, but the phenotypic ratios produced are quite different (there are more than four possible phenotypes in the offspring).

Question 8

Changes to phenotypic ratios
Even if neither of the genes involved in the ________ ______is ______________, you won’t always get the _________ ::: phenotypic ratio in the offspring of ____ ______________parents. This could be because of _________ or _________

Answer 8

Even if neither of the genes involved in the dihybrid cross is codominant, you won’t always get the expected 9:3:3:1 phenotypic ratio in the offspring of two heterozygous parents. This could be because of linkage or epistasis

Question 9

Tip: Each gamete should have ___ ________to represent each _____in the cross. So in a ___________ ______, each gamete has two letters.

Answer 9

Tip: Each gamete should have one letter to represent each gene in the cross. So in a dihybrid cross, each gamete has two letters.

Question 10

Tip: A dihybrid cross between a ____________ ____________parent and a ____________ ___________ parent (e.g. RRYY x rryy) will produce all _______________offspring in the __ generation.

Answer 10

Tip: A dihybrid cross between a homozygous dominant parent and a homozygous recessive parent (e.g. RRYY x rryy) will produce all heterozygous offspring in the F1 generation.