B6 More Genetic Diagrams (page 73)

Question 1

Look at the homozygous dominant hamster (BB) diagram 1 on page 73, explain the cross diagram?

(you got to be able to predict and explain the outcomes of crosses between individuals for each possible combination of dominant and recessive alleles of a gene. You should be able to draw a genetic diagram and work it out - but it’ll be easier if you’ve seen them all before. You also need to know how to interpret another type of genetic diagram called a family tree)

Answer 1

In this cross, a Homozygous dominant hamster (BB) is crossed with a Homozygous recessive hamster (bb). All the offspring are normal (boring).

Question 2

Look at diagram 2 on page 73, and explain the cross diagram?

Answer 2

If you crossed a Homozygous dominant hamster (BB) with a heterozygous hamster (Bb), you would also get all normal (boring offspring).

To find out which it was you’d have to breed the offspriing together and see what kind of ratio you got that time - then you’d have a good idea. If it was 3:, it’s likely that you orginally had BB and bb.

Question 3

Look at diagram 3 on page 73, there is a 1:1 Ratio in the offspring. Explain this?

Answer 3

A cat with long hair was bred with another cat with short hair. The long hair is caused by a dominant allele ‘H’, and the short hair by a recessive allele ‘h’.

They had 8 kittens - 4 with long hair and 4 with short hair. This is a 1:1 ratio - it’s what you’d expect when a parent with only one dominant allele (heterozygous - Hh) is crossed with a parent with two recessive alleles (homozygous recessive - hh).

Question 4

You need to be able to Interpret Family Trees.

Knowing how interitance works can help you interpret a family tree - this is one for cystic fibrosis (page 74).

Look at diagram 4 on page 73, the family tree, can you tell if the allele for cystic fibrosis is or isn’t dominant and why?

Answer 4

No, cystic fibrosis isn’t dominant because plenty of the family carry the allele but don’t have the disorder.

Question 5

Look at diagram 4 on page 73.

Explain what percentage that the new baby will have cystic fibrosis?

Answer 5

There is a 25% chance that the new baby will have the disorder, and a 50% chance that it will be a carrier, as both of its parents are carriers but are unaffected. The case of the new baby is just the same as in the genetic diagram on page 72 - so the baby could be unaffected (FF), a carrier (Ff) or have cystic fibrosis (ff).

(in the exam, you might get a family tree showing the inheritance of a dominant allele - in this case, there won’t be any carriers shown)

Question 6

Round peas are caused by the dominant allele, R. The allele for wrinkly peas, r, is recessive. Using a Punnett square, predict the ratio of plants with round peas to plants with a wrinkly peas for a cross between a heterozygous pea plant and a pea plant that is homozygous recessive (3 marks)

Answer 6

R r

r Rr rr

r Rr rr

round peas : wrinkly peas
1:1

(1 mark for correct gametes, 1 mark for correct offspring genotypes and 1 mark for correct ratio).