CHAPTER 20
REVIEW QUESTIONS
20. 1
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
Allele: an alternative expression for a gene.
Gene: a small segment of a DNA molecule which determines a specific characteristic.
Genotype: the alleles actually present on the two homologous chromosomes.
Phenotype: the physical/chemical expression of the genotype.
Monohybrid cross: a cross between two heterozygotes when only one characteristic is
being examined.
Dihybrid cross: a cross between two heterozygotes when two characteristics are being
examined simultaneously.
Hybrid: a heterozygote for the characteristic(s) under examination.
Pure breeding: a homozygote for the characteristic(s) under examination.
Heterozygous: an individual displaying two of the possible alleles for the trait.
Homozygous: both alleles for the trait are exactly the same.
20.2
Law Of Dominance: when homozygous parents with contrasting expressions for a given trait
are crossed, only one expression, the dominant allele, will be seen in the offspring.
When Mendel crossed pure breeding pea plants for yellow seeds with those for green seeds,
the offspring always had yellow seeds. Similar results were found for crosses between other
traits.
Law Of Segregation: hereditary factors are determined by pairs of alleles, one inherited from
each parent, which are separated when the gametes are formed, one going to each gamete.
When Mendel allowed the offspring between two pure breeding plants for alternative
expressions of the trait, all of which exhibited the dominant phenotype, to breed, some of their
offspring in this F2 generation displayed the recessive genotype. This could only be explained
if the F1 generation were also carrying the recessive allele from the original recessive parent.
Law Of Independent Assortment: each allele is an independent unit which is inherited on its
own.
When Mendel examined two characteristics simultaneously he found that there were four
possible phenotypes in the F2 generation which could only be explained by each of the four
possible types of alleles acting as independent units.
20.3
Mendel chose to investigate garden peas which :
 were readily avaiolable and easily cultivated
 are found in different varieties with clearly different traits for each characteristic
 are self-pollinating and thus pure strains can be bred
 allow different strains to be deliberately cross-pollinated
 can produce a large number of offspring in a short time
 can produce several generations in a few growing seasons.
He conducted carefully controlled experiments.
He restricted his study to a few characteristics.
By chance he selected characteristics which were inherited, or behaved as though they were,
on different chromosomes.
He collected a vast amount of data which gave statistically reliable results.
20.4
Genes are found on chromosomes. Each chromosome in the normal cell has two of each type
of chromosome – homologous chromosomes – each of which carries the same kinds of genes
e.g. the gene to determine the seed colour in peas. The exact information about the trait may
be different, i.e. different alleles for the gene may be present on each of the homologous
chromosomes. During meiosis I the homologous pairs of chromosomes link to the same
spindle and the first division brings about separation of these so that one member of the pair
goes into each new cell. Thus in a situation where where than one pair of alleles is being
examined, and where each pair is on different homologous chromosomes, the alleles involved
will finish up in the two new cells in a completely random fashion. Since meiosis II is a
division which separates the chromatids in each cell, and since both chromatids of each
chromosome involved will be exactly the same (one is a copy of the original), then of the four
gametes produced, there will be two types when one pair of alleles is being examined. Because
of the randomness of separation, however, if all gametes formed from a dihybrid cross
(genotype for both characteristics heterozygous) are examined there would be an equal chance
of four different types of gametes,
e.g. DdGg individual in which Dd and Gg are carried on different homologous chromosomes
would result in an equal chance of the following gametes:
DG
Dg
dG
dg
The number of different types of gametes, therefore, is determined by the number of different
pairs of homologous chromosomes involved.
1 pair of chromosomes
21 possible gamete types
= 2
2 pairs of chromosomes
22 possible gamete types
=4
3 pairs of chromosomes
23 possible gamete types
=8
4 pairs of chromosomes
24 possible gamete types
= 16 etc
20.5
The product rule is a statistical principle which states that the chance of two independent
events occurring together is equal to the chance of one event occurring alone multiplied by the
chance of the other event occurrring alone.
20.6
A Punnet Square is a method of calculating the possible offspring from a particular parental
cross. The possible gametes produced by one parent are listed on the top, and the posssible
gametes from the other parent on the side. The middle squares represent the offspring which
would be formed when a gamete from one of the rows unites with a particular gamete from the
side.
hybrid tall
X
dwarf
Genotypes
Tt
tt
Possible gametes
T and t
t
T
t
t
20.7
20.8
Tt
tt
TtPp
x
TtPp
9 tall purple : 3 tall white : 3 short purple : 1 short white
(Dihybrid ratio)
Black, rough parent (a) could have possible genotypes:
BBRR
BbRr
BBRr
BbRR
Since white and smooth are both recessive traits then white smooth parent (b) must have
genotype bbrr.
Thus:
B?R?
x
bbrr
Off spring
1/4 black smooth (B?rr)
must get b from parent b
since rr then must get r from parent b.
offspring Bbrr
1/4 white rough
bbrr
1/4 black rough
BbRr
Parent genotypes BbRr and bbrr.
20.9
a.
b.
c.
RRBB x
rrbb
all offspring RrBb
rough black
F2
dihybrid ratio
9 rough black : 3 rough white : 3 smooth black : 1 smooth white
i.
RrBb
x
rrbb
rb
Thus
RB
Rb
rB
rb
RrBb
Rrbb
rrBb
rrbb
1 rough black : 1 rough white : 1 smooth black : 1 smooth white
ii.
Rrbb
x
RrBb
RB
Rb
rB
rb
Rb
RRBb
RRbb
RrBb
Rrbb
rb
RrBb
Rrbb
rrBb
rrbb
Thus
iii.
3 rough black : 3 rough white : 1 smooth black : 1 smooth white
RRbb
x
rrBB
Rb
rB
Thus
RrBb
all rough black.
20.10
RRBB
x
rrbb
All offpring RrBb. Allow these individuals to interbreed.
a. Want population rrbb
thus only breed smooth white furred individuals produced
from F2.
b. Want population RRBB
Select F2 individuals that are rough black – breed with
smooth white to establish genotype; cull any which produce smooth or white or smooth
and white progeny.
20.11
a.
b.
c.
d.
e.
f.
g.
20.12
Continuous variation: a trait controlled by more than one gene (= mulitple gene
inheritance), each allele of which may contribute to the characteristic.
Intermediate dominance: a pattern of inheritance in which neither allele for a
characteristic completely masks the effects of the other; results in a belnding of traits for
the characteristic.
Co-dominance: genetic inheritance of two or more traits of a characteristic, each of which
is expressed in the phenotype.
Modifier gene: genes which influence the expression of another gene.
Mulitple alleles: a gene which has more than two alleles although only two will be
present at any one time in an individual.
Universal donor: a person with blood type O which, although it has antibodies to antigens
A and B, can be donated in small amounts to individuals of all other blood groups with
little agglutination.
Universal recipient: a person with blood type AB which has both A and B antigens, and
so can receive small quantities of all other types of blood.
dwarf red
tall white
=
=
ttRR
TTrr
a.
tR
Tr
TtRr
i.e. all tall pink.
b.
20.13
a.
ttRR and TTrr
Case of codominance – neither red nor white exert dominance over each other and in
heterozygotes, some skin cells produce red hairs and some produce white hairs, resulting
in the roan appearance.
b.
Roan (RW)
x
Roan (RW)
R
W
R
RR
RW
W
RW
WW
of 300 hundred offspring, therefore, 150 (half) will be roan (RW).
c.
Red calf may have both parents, roan both parents red, or one parent red and the other
roan.
20.14
short tail
X
3 no tail
short tail
2 long tail
6 short tail
Case of intermediate dominance:
Short Tail
20.15
parent a
parent b
Long tail LL
No tail
NN
LN
O therefore OO
B therefore BB or BO but has one parent O therefore must be BO
X
BO
AB
BB
AB
AO
BO
i.e. 50 % children could be type B.
20.16
Mr Middleduck: A
Mrs Middleduck: B
Thus children could be AB, BO, AO or OO
Mr Spiros: A
Mrs Spiros: A
Thus children could be AA, AO or OO
Jemima was type OO:
could be a Middleduck or a Spiros.
Letitia was type B:
could not be a Spiros but could be a Middleduck.
Thus a mishap has occurred!
20.17
a.
b.
c.
d.
e.
f.
g.
20.18
Barr body: inactivated X chromosome in the cells of female vertebrates.
Carrier: an individual (female in humans) who is heterozygous for a sex-linked dominant
gene.
Holandric: gene found on the Y chromosome only.
Sex-influenced: an autosomal gene, the expression of which is influenced by the sex of
the individual
Sex – limited: an autosomal gene, the expressionof which is limited to a particular
gender.
Sex-linked: a gene found on the X chromosome only.
X-inactivation: the condensation and thus inhibition of one of the pairs of X
chromosomes in each cell of the female vertebrate.
a. For two normal parents to have a colour blind son, the mother must have been a carrier
(heterozygous).
XCXc
X
XCY
Since the mother was a carrier then she must have inherited the gene from her mother
(also a carrier XCXc), since the father was normal (i.e. can't carry the recessive) XCY.
b. Therefore the allele must have come from the boy's maternal grandmother.
20.19
Bb
(blue)
Bb
(blue)
bb
(brown)
BB or Bb
(blue)
Bb
(blue)
bb
(brown)
bb
(brown)
20.20
Tortoiseshell is a case of X inactivation – approximately half of the X chromosomes
throughout the body cells are randomly inactivated, thus about half of the epidermal cells will
produce orange hair whilst the other half will produce black hairs giving a tortoiseshell effect.
XO
XB
XB
XBXO
XBXB
Y
X OY
XBY
Thus 1 tortoiseshell female : 1 black female : 1 ginger male : 1 black male
XOXB LL
20.21
20.22
20.23
X
XBY SS
XO L
XB L
XB S
XBXO LS
XBXB LS
YS
XOY LS
XBY LS
Thus same colours but all medium length hair.
Since the son gets his one X chromosome from his mother then she must have been carrying
the haemophiliac allele for him to express it:
Mother
X HX h
Father
XhY
50% chance that daughters could be haemophiliac, since they must get a Xh from the father
and have a 50% chance of getting it from the mother.
XH
Xh
Xh
X HX h
XhXh
Y
X HY
XhY
20.24
a.
b.
c.
Non tasting is autosomal, because female 7 is a non-taster but her son (13) is a taster.
Recessive, because female 7 is a non-taster but both her parents were tasters.
Tt
1, 2, 3, 4, 12, 13
probably TT (but could be Tt)
8. 11
TT or Tt
5, 9, 10, 14, 15, 16, 17, 18, 19
tt
6, 7
20.25
a.
b.
Chromosome map: a map of a chromosome showing its gene loci.
Crossing over: breaking and rejoining, with exchange of material, of adjacent chromatids
of homologous chromosomes during meiosis I.
Gene linkage: location of genes on the same chromosome.
Recombinant gametes: gametes produced as a result of crossing over of the chromatids of
homologous chromosomes during meiosis I.
c.
d.
20.26
The genes are very close together.
23.27
Crossing over only occurs between the inner two chromatids in the tetrad of homologous
chromosomes. Thus one of each chromatid pair is not involved in the process.
20.28
a.
b.
The observation that where a particular disease occurs in a family, there is an increased
occurrence when closely related people marry. An increased knowledge of the way in
which genetic diseases are trasmitted. The greater mobility of people also means that
small communities are less isolated, increasing the chances of marriages between nonrelated individuals.
The proportion of the popualtion with recessive abnormalities should decrease.
20.29
The disease is believed to be caused by several recessive genes. Factors such as age, weight,
sex and diet of the individual can determine the onset of the disease. Thus there is no single
factor responsible for its occurrence.
20.30
a.
b.
c.
20.31
An autosomal disease results from a gene located on an autosome, whereas a sex-linked
disease results from a gene carried on the X chromosome.
Unless the autosomal gene is sex influenced or sex limited, the gender of the child has no
effect upon expession of the disease. In a sex-linked recessive disease there is a greater
chance of the male exhibiting the disease since he has only one X chromosome, and thus
there can be no masking by the alternative dominant allele. If the disease is dominant
there will be an equal chance of males and females exhibiting the disease.
Sex-linked human genetic disease: haemophilia.
Autosomal human genetic disease: sickle-cell anaemia.
Gene tracking is a method of determining the position of a particular gene on the chromosome.
In gene tracking an easily identified linked gene, such as blood group (the genetic marker), is
correlated with the incidence of the allele being tracked. In fruit flies this information can be
more easily gained by selective crossing. The method cannot be used in humans from both an
ethical and practical (few young are produced and the reproductive time scale is too great)
stance.