Study questions for lectures 4, 5 and 6.


1) A, B and C are independently assorting genes controlling the production of a black pigment. The genes act in a biochemical pathway as follows:



Black can be produced only if both red pigments are present; that is, C converts the two red pigments together into a black pigment. Complete loss-of-function alleles of the genes are designated a, b, c. A homozygous black individual is crossed to a homozygous colorless individual to produce a black F1. The F1 is selfed. For the F2 generation,


a) what proportion are colorless?

 b) what proportion are red?

c) what proportion are black?


2) Huntington's chorea is a fatal disease leading to the degeneration of the nervous system. It is found in persons heterozygous for the allele Ht, but has variable expressivity in the sense that a person may develop symptoms as early as childhood or as late as 60 years old, with the mean age of onset at about 40 years.

(a) A man with four children (3 female, 1 male) died of Huntington's chorea. The disease is not known in his widow's family. What are the chances that:

 i) All four children carry the disease?

ii) Only the three daughters carry the disease?

iii) Only two children carry the disease?

iv) Only one daughter carries the disease?

   (b) In certain families known to carry the Ht allele, the disease is found to "skip" several generations before reappearing. How might this be explained?


3) For this problem you may need the following background information on Drosophila:


cm = carmine is an eye color mutation that causes dull red "carmine" eyes. cm is located at 18.0 mu on the X chromosome

v = vermilion is an eye color mutation that causes bright red "vermilion" eyes.

v is located at 33 mu on the X. The double mutant cm v has orange eyes.

eag = ether-a-go-go causes flies to shake their legs like crazy when under ether anaesthesia. eag is located at 50 mu on the X.

The telomere of the X is located at 0 mu and the centromere at 68 mu.

An interesting X-linked recessive lethal mutation was recently isolated in Drosophila. The new mutation, call it lox, for "lethal on the X", originated in a wild-type genetic background. To map lox females heterozygous for lox were crossed to males whose X chromosome is marked with the recessive mutations cm, v, and eag to generate heterozygous females of genotype: lox +++/ + cm v eag. The heterozygous females were crossed to wild-type males with the following results:


Phenotypically wild-type females


males of genotypes:


cm v eag


+ + +


cm v +


+ + eag


cm + +


+ v eag


cm + eag


+ v +



a.) Which gene is lox closest to?

  b.) In which interval does lox map? (i.e. Is it between the telomere and cm, between cm and v, between v and eag, or between eag and the centromere?) Explain your reasoning.

  c.) Calculate the map position of lox.


  4) Explain how balancer chromosomes could be used to stably propagate an autosomal recessive lethal mutation if the balancer chromosome is homozygous viable but carries a recessive sterile allele that eliminates both ovaries and testes. Assume the balancer is marked with a dominant mutation Ha that produces flies with many extra hairs. What phenotypes, and in what proportions, would you expect in the stable stock?


(b) If you want to stably propagate an X-linked lethal mutation with a balancer chromosome containing a recessive sterile allele, would you use a female-specific sterile, a male-specific sterile, or a non sex-specific sterile mutation?

  5) Both hemophilia A h(A) and red-shift color blindness (r) are X-linked recessive traits. The h(A) and r genes are separated by 4 cM. In the pedigree shown, what is the probability that the individual male (IV-1) is color blind but not affected by hemophilia?