(b) In chickens, a gene on the X chromosome controls the rate of feather production.
The allele for slow feather production, F, is dominant to the allele for rapid feather production, f. The following figure shows the results produced from crosses carried out by a farmer.
(i) Explain one piece of evidence from the figure which shows that the allele for rapid feather production is recessive.
(c) Feather colour in one species of chicken is controlled by a pair of codominant alleles which are not sex-linked. The allele CB codes for black feathers and the allele CW codes for white feathers. Heterozygous chickens are blue-feathered.
On a farm, 4% of the chickens were black-feathered. Use the Hardy-Weinberg equation to calculate the percentage of this population that you would expect to be blue-feathered. Show your working.
Answer ...................................... %
(Total 9 marks) Q2.Researchers investigated some characteristics of people from different parts of England. In the north of England they selected 200 people and recorded their phenotypes for three different characteristics.
Their results are shown in the figure below.
by dominant allele
by recessive allele
(a) Calculate the ratio of straight thumb to hitch-hiker thumb in this study.
Ratio = .......................................................................
(b) The numbers for the tongue rolling and thumb characteristics do not add up to 200.
For each characteristic suggest one reason why the numbers do not add up to 200.
Tongue rolling ................................................................................................
Q3.The fruit fly is a useful organism for studying genetic crosses. Female fruit flies are approximately 2.5 mm long. Males are smaller and possess a distinct black patch on their bodies. Females lay up to 400 eggs which develop into adults in 7 to 14 days. Fruit flies will survive and breed in small flasks containing a simple nutrient medium consisting mainly of sugars.
(a) Use this information to explain two reasons why the fruit fly is a useful organism for studying genetic crosses.
(b) Male fruit flies have the sex chromosomes XY and the females have XX. In the fruit fly, a gene for eye colour is carried on the X chromosome. The allele for red eyes, R, is dominant to the allele for white eyes, r. The genetic diagram shows a cross between two fruit flies.
(ii) The number of red-eyed females and red-eyed males in the offspring was counted. The observed ratio of red-eyed females to red-eyed males was similar to, but not the same as, the expected ratio. Suggest one reason why observed ratios are often not the same as expected ratios.
M1.(a) (Recessive) allele is always expressed in females / females have one
(recessive) allele / males need two recessive alleles / males need to be
homozygous recessive / males could have dominant and recessive alleles /
be heterozygous / carriers;
Accept: Y chromosome does not carry a dominant allele. Other answers must be in context of allele not chromosome or gene.
(b) (i) 1. 1, (2) and 5;
Accept: for 1 mark that 1 and 2 have slow (feather production) but produce one offspring with rapid (feather production).
Neutral: any reference to 3 being offspring of 1.
2. 1 must possess / pass on the recessive allele / 1 must be a carrier / heterozygous / if slow (feather production) is recessive all offspring of (1 and 2) would be slow (feather production) / if rapid (feather production) was dominant 1 would have rapid (feather production);
Reject: both parents must be carriers / possess the recessive allele.
Reject: one of the parents (i.e. not specified) must be a carrier / heterozygous.
(ii) 5 = XfY / XfY- / f / f- / fY ;
7 = XFXfand XFXF (either way round) /
or XfXFand XFXF (either way round) /
or XFXf, XfXFand XFXF(in any order);
Note: allow 5 = XfY, XfY.
Accept: for both 5 and 7 a different letter than F. However, lower case and capital letter must correspond to that shown in the answer. For example accept 7 = XRXr and XRXR.
(iii) XFXfand XfY or XfXFand XfY
or XFXfand XfY- or XfXFand XfY- /
or Ff and fY /
or Ff and fY- /
or Ff and f- /
or Ff and f;
Accept: a different letter than F. However, lower case and capital letter must correspond to that shown in the answer.