| Criteria | Marks |
|---|
| Explains frequency ratios<br>Draws suitable Punnett squares for each trait<br>Links Punnett squares to frequency ratio and type of inheritance<br>Provides a suitable key | 5 |
| Identifies frequency ratios<br>Draws suitable Punnett squares for each trait<br>Provides a suitable key<br>Indicates both types of inheritance correctly | 4 |
| Identifies frequency ratio(s)<br>Draws a suitable Punnett square<br>Indicates one type of inheritance correctly | 3 |
| Demonstrates understanding of a mode(s) of inheritance | 2 |
| Provides some relevant information | 1 |
Sample answer:
Graph A shows a ratio of 3:1 of the two phenotypes. This is typical of dominant/recessive alleles.
The pure breeding parents must have been homozygous for seed shape eg RR for round and rr for wrinkled. The first generation would all have been round heterozygous Rr. When this generation was self-pollinated to provide the second generation the following Punnet square can be produced.
RrRRRRrrRrrr
KEY:
R = Round seed
r = Wrinkled seed
Graph B shows a ratio of 1:2:1 for the three phenotypes. This is typical of co-dominant alleles.
Neither allele is dominant and both are expressed. In the F1 generation, the chickens express both alleles and are heterogeneous. When they breed together the following Punnett square is produced.
FBFWFBFBFBFBFWFWFBFWFWFW
KEY:
FB = Black feathers
FW = White feathers
This results in 50% with black and white feathers, 25% black feathers and 25% white feathers.
Answers could include:
Incomplete dominance for Graph B where both alleles are blended.
