Notes from Dr. David A. Jones - January 27, 1997
Genetics is largely a problem solving discipline. Obviously there are basic facts that need
to be known and understood before one can begin to solve genetical problems, but what we have
to do is to take the fullest advantage of what is known, or given as fact when answering a
genetical problem, and then develop our answer from that basic information. The best advice I
can give is - write down what is known and take the problem forward from there.
Those of you who have attended the discussion sessions will understand that this is
precisely the way Ann has been tackling' the problems. She is not writing things down just so
that you can take notes - she is showing you how she sets about genetic problems and how we
both hope you will deal with them.
With your first test not all that far away, I have been preparing it using both my own notes
and Campbell'. To help you with your revision I will highlight the sections of Chapters 11
through 16 that you should study and those that you can either ignore or not study in detail.
In 4th edition of Campbell each section begins with a blue square. Folks with the 3rd
Edition of Campbell should see below.
Chapter ll. Good revision should cover the sections beginning on pages 204, 205 (for the
second test, not for the first), 206, 207, 217 (the last section is general knowledge with which you
should all be familiar anyway).
You need not review, for the tests, sections beginning on page 201, 212, 214, 216. They
do contain, however, points that will help your understanding of the whole process of nuclear and
cell division.
Chapter 12. Study all of it.
Chapter 13. Ignore 1) the trihybrid problem on page 247 and 2) the polygenic inheritance
on page 251 (I'll deal with this later in the course). I expect you will find the last 2+ pages (end of
256, 257 and 258) interesting, but I will not examine your knowledge of them.
Chapter 14. Study all except 1) the section the phenotypic effects of some genes depend
on whether they were inherited from the mother or the father' and ii) the section on Extranuclear
inheritance.
Chapter 15. Study all of it.
Chapter 16. We will not be able to deal with all of this by the end of Thursday's lecture.
There is a lot of words, but nothing particularly difficult in principle in this chapter. I do not
expect you to memorize Fig. 16.5. Even if I get beyond the first part of page 304, I will not ask
you questions on the later part of this chapter in the first test.
My main purpose in talking about the 2 test was to familiarize you with the test, and
some of the background, before you need to use it in your lab classes. I hope you are able to see
how we can do some hypothesis testing and use the results of the analysis to help us interpret the
data. Although I had to go into some detail in order to illustrate the principles, it is the principles
that are more important at this stage. Thus I expect you to know that 1) you use the 2 test to
analyze discontinuous variation i.e. 3:1, 9:3:3:1 types of situation, 2) the threshold level of
probability is P = 0.05 below which we prefer to reject the null hypothesis and 3) the null
hypothesis is the basis for calculating the expected values.
3rd Edition
Chapter 11 pp 221-222 (omit Bacterial reproduction at this stage), pp 223-242 (omit 225,
228-235 and Fig. 11.16). Abnormal Cell Division' to the end of the chapter is largely general
knowledge with which you should all be familiar anyway. Again page 225, 228-235 will help
your understanding, but you will not be tested on the details on this page.
Chapter 12. Study all of it.
Chapter 13. Study all of it except for polygenic inheritance (page 270). I will not be
testing the Genetic Screening and Counseling and the other sections between there and the end of
the chapter, although I expect you will find them interesting.
Chapter 14. Study all except Parental Imprinting of Genes and Extra Nuclear Inheritance.
Chapter 15. Study all of it.
Chapter 16. Study up to page 322. Stop before A Closer Look at Transcription. I may
get further than this in the lectures, but I will not ask questions on the latter part of the chapter in
the first test. I do not expect you to memorize Fig. 16.5.
Additional Genetical Problems - January 28, 1997
For those of you who like genetical problems, or feel you need
more practice, here are 22 that have been around for a long time
but are as relevant today as they were nearly 40 years ago when
the list was first compiled. I have some more that I will add
later when I have finished transcribing them from an old
duplicator-produced copy.
1. Amongst a family of plants uniformly about one metre tall,
two individuals were observed which were only 0.5 metres
tall. Two of the tall plants and both of the short plants
were self pollinated and their progeny sown out the
following year, when all the plants turned out to be one
metre tall. What do you conclude from this result?
2. A rat from a pure-breeding strain with wild fur colour
(i.e.agouti) was crossed to one from a pure-breeding strain
with black fur. All F1's had agouti fur. Assuming this
colour difference to be controlled by two alleles of a
single gene, give the genotypes of the two parents and of
the F1's. The F1 rats were then intercrossed and gave a
total of 48 F2 offspring. How many of these would you
expect to have agouti and how many black fur?
3. Six of the F2 with agouti phenotype were backcrossed to the
pure breeding black strain. Four of the families contained 3
and 2, 3 and 3, 2 and 4, and 2 and 1 agouti/black offspring
respectively; the other two matings gave respectively 5 and 6
offspring all of which were agouti. What do you conclude from
these results?
4. Mendel showed that in the garden pea grey seed colour is
dominant to white seed colour. Suggest the probable genotypes
of the parents in each of the following crosses: (Use G for
grey; g for white.)
Parents Progeny
Grey White
(a) grey x grey 217 69
(b) grey x white 153 0
(c) grey x white 184 191
(d)white x white 0 133
(e) grey x grey 118 0
5. Three stocks of Drosophila "A", "B" and "C" all breed true for
eyes which are reduced in size and irregular in shape when
compared to wild-type stocks. Furthermore, the stocks cannot
be distinguished by inspection. On crossing "A" with a true-breeding wild-type stock all the F1 had reduced, irregular
eyes like stock "A". On crossing "B" with wild-type all the F1
had large regular eyes like wild-type. On crossing "C" with
wild-type all the F1 had large regular eyes like wild-type.
What can you deduce from these results? Are the mutant
allelomorphs in these stocks dominant or recessive to their
corresponding wild-type allelomorphs?
6. Both parents of a blue-eyed man are brown-eyed. He marries a
brown-eyed woman, one of whose parents was brown-eyed, the
other blue, and who has a blue-eyed brother. The man and
woman in question have a brown-eyed child. Give the genotypes
of the members of this pedigree assuming brown eyes to be
dominant to blue eyes.
7. The human blood phenotypes A,B and 0 are determined by a single
gene I with three allelomorphs: Ia, Ib and Io. Ia and IB are
codominant, but both behave as though fully dominant over Io.
Thus:- Genotype Phenotypes
IAIA A
IAIO A
IBIB B
IBIO B
IAIB AB
IOIO 0
If a woman with an AB phenotype marries a man of blood group 0, what phenotypes are possible among their children, and what
will be their genotypes?
Similarly if an A woman marries a B man, what are the possible
phenotypes and genotypes of their offspring?
8. The capacity to taste phenylthlocarbamide (P.T.C.) is determined by a single gene; to those carrying the dominant allelomorph T a solution of P.T.C. has a bitter taste, while
homozygous recessive tt individuals find that the solution tastes only of brackish water. A man of blood group A who was a "taster" and whose wife was also a taster had children
of the following phenotypes:-
B taster : 0 non-taster : AB taster
What were the genotypes of the man, wife and children?
9. In Mirabilis (four o'clock plant) the allele for red
flowers (w+) is incompletely dominant to that for white
flowers (w) (i.e.individuals with the genotype w+/w are
pink). One pink plant is crossed to a red, another to a
white one. Give the genotypic and phenotypic ratios in which
the offspring are expected to be produced in each case.
10. Which of the following men would be excluded as a possible
father of a child whose phenotype is 0 Rh+ MN and whose
mother's phenotype is 0 rh- MN?
(a) AB Rh+ M (b) A Rh + MN (c) B rh- MN (d) 0 rh- N
11. In the broad bean a variety with green seeds and white hilum
(point of attachment of seed to the placenta) was crossed
with a variety having yellow seeds and a black hilum. The F1
plants all carried yellow seeds with black hilums. The
F2 plants were classified as follows:-
93 carried yellow seeds with black hilums
31 carried yellow seeds with white hilums
28 carried green seeds with black hilums
and 8 carried green seeds with white hilums
What can you deduce about the genes controlling these
characters? (Note that the seed colors here are properties of the seed coat and are determined by the genotype of the maternal tissue. What complications does this present for scoring the characters.)
12. In tomatoes the allele for purple stem (P) is dominant to
green stem (p) and that for "cut" leaves (C)is dominant to
"potato" leaves (c). The following results were obtained in
a series of crosses.
Parents Progeny
Purple Purple Green Green
cut potato cut potato
(a) purple cut x green cut 321 101 310 107
(b) purple cut x purple potato 219 207 64 71
(c) purple cut x green cut 722 231 0 0
(D) purple cut x green potato 404 0 387 0
(e) purple potato x green cut 70 91 86 77
What are the probable genotypes of the parents?
13. The stocks of Drosophila "B" and "C" mentioned in problem 5 were intercrossed; all the F1 had large and regular eyes
like wild-type. What conclusion can you draw from this?
14. With reference to problems 5 and 13 a fourth stock "D" was
available which phenotypically was like "B" and "C", but
rather more extreme in expression. "D" behaved like "B" and
"C" on crossing to wild-type; on crossing with "B" it gave
progeny with reduced and irregular eyes; and also on crossing
with "C". What is the genotype of "D"?
15. What will be the proportions of phenotyps and genotypes in
the F2 of the cross between "D" and wild-type? What will be
the corresponding proportions if we find that tho difference
between the phenotypes of stocks "B", "C" and "D" are too
slight to allow accurate classification?
16. Four individuals with reduced and irregular eyes from the F2
in the last problem (call them e, f, g, and h) were crossed
to the stocks "B" and "C" with the following results:-
Eyes Eyes
Wild Reduced Wild Reduced
"B" x e 0 51 "C" x e 30 22
"B" x f 62 0 "C" x f 0 41
"B" x g 0 65 "C" x g 0 69
"B" x h 0 43 "C" x h 52 0
What are the genotypes of e, f, g and h respectively?
17. In a cross between two types of oat, one with black hulled seeds,
the other with white hulled seeds, the F1 all had black-hulled
seeds. Inter-crossing the F1 gave an F2 as follows:- 418 black-
hulled, 106 grey-hulled and 36 white-hulled. Explain the
inheritance of hull-colour in these crosses.
18. Madill et al. (Can. J. Genet. Cytol.6: 467-471, 1964) studied the
inheritance of immunity to flax rust (Melampsora lini) in two
lines of flax, F 8-2 and F 13-4. Both lines were crossed with the
variety Bison which is susceptible to all known North American
races of flax rust, and the following results were obtained.
Bison x F 8-2 F2 157 immune 45 susceptible
Bison x F 13-4 F2 699 immune 149 susceptible
Analyse and interpret these results.
Suggest any further crosses which should be made to complete this
study and help to decide between the alternative results.
19. The presence of glands containing the toxic substance gossypol on
the leaves and cotyledons of cotton lowers the economic value of
the crop. (Rhyne, J. Hered, 53 115, 1962).
Three varieties of cotton, Empire, Acala and BBR, each having
glanded leaves, were crossed with a glandless (homozygous)inbred
line L 7-14 and were lntercrossed. The following results were obtained.
Empire x L 7-14 F1 glanded F2 93 glanded 5 glandless
Acala x L 7-14 F1 glanded F2 80 glanded 25 glandless
BBR x L 7-14 F1 glanded F2 49 glanded 16 glandless
Empire x Acala F1 glanded F2 all glanded ----
Empire x BBR F1 glanded F2 all glanded ----
BBR x Acala F1 glanded F2 78 glanded 6 glandless
Analyse and interpret these results.
Outline a method for the breeding of glandless plants in the
3 varieties Empire, Acala and BBR.
20. Of two pure-breeding stocks of a small mammal one had long and
the other had short fur. Reciprocal crosses were made between the
two stocks and the F1's and F2's gave the following results:-
Parents Female Long x Short Male Female Short x Long Male
Phenotypes long short long short
F1 Females 36 0 49 0
Males 32 0 0 52
F2 Females 47 0 26 25
Males 21 24 23 22
What can you say about the genetic control of this character?
21. One of the most difficult problems is choosing the right
material to carry out genetic experiments. Having chosen
the material it is often necessary to do some pilot breeding
in order to obtain pure breeding strains or special stocks.
The tests carried out in questions 16 and 19 are of this
kind. This problem and the next one are concerned with
stock building.
Suppose in question 11 we had started with one variety with
green seeds and black hilum and another with yellow seeds
and white hilum, and we wanted a pure breeding plant with
yellow seeds and black hilum. How can this be done in the
shortest time? (Beware, the note at the end of question 11
is important here.)
22. Starting with three stocks of flies, one homozygous for
vestigial wing, another homozygous for scariet eye and the
third homozygous for ebony body, how can one obtain the
triple recessive homozygote? Assume that the three loci are
not genetically linked.
Answers to Supplementary Genetical Problems
Study Guide to Test 2
Notes for February 26 Test