Chapter 03 Lecture Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cytogenetics
•  The field of genetics that involves the
microscopic examination of chromosomes
•  A cytogeneticist typically examines the
chromosomal composition of a particular
cell or organism
–  This allows the detection of individuals with
abnormal chromosome number or structure
–  This also provides a way to distinguish between
two closely-related species
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-9
Cytogenetics
•  Animal cells are of two types
–  Somatic cells
•  Body cells, other than gametes
–  Blood cells, for example
–  Germ cells
•  Gametes
–  Sperm and egg cells
•  Figure 3.2 shows the general procedure for
viewing chromosomes from a human
(somatic) cell
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-10
Cytogenetics
•  In a cytogenetics laboratory, the microscopes are
equipped with a camera
•  Microscopic images of chromosomes can now be
visualized using a computer
•  There, chromosomes can be organized in a
standard way, usually from largest to smallest
•  A karyotype is an organized representation of the
chromosomes within a cell
•  Refer to Figure 3.2(c)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-12
A karyotype of a diploid human cell
1
6
7
13
19
2
3
8
14
9
15
20
4
5
10
11
16
17
21
12
18
22
XY
© Leonard Lessin/Peter Arnold
11 µm
(c) For a diploid human cell, two complete sets of chromosomes from!
a single cell constitute a karyotype of that cell.
Figure 3.2
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-13
Eukaryotic Chromosomes Are
Inherited in Sets
•  Most eukaryotic species are diploid
–  Have two sets of chromosomes
•  For example
–  Humans
•  46 total chromosomes (23 per set)
–  Dogs
•  78 total chromosomes (39 per set)
–  Fruit fly
•  8 total chromosomes (4 per set)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-14
Eukaryotic Chromosomes Are
Inherited in Sets
•  Members of a pair of chromosomes are called
homologs
–  The two homologs form a homologous pair
•  The two chromosomes in a homologous pair
–  Are nearly identical in size
–  Have the same banding pattern and centromere
location
–  Have the same genes
•  But not necessarily the same alleles
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-15
Eukaryotic Chromosomes Are
Inherited in Sets
•  The DNA sequences on homologous
chromosomes are also very similar
–  There is usually less than 1% difference between
homologs
•  Nevertheless, these slight differences in DNA
sequence provide the allelic differences in
genes
–  Eye color gene
•  Blue allele vs. brown allele
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-16
Eukaryotic Chromosomes Are
Inherited in Sets
•  The sex chromosomes (X and Y) are not
homologous
–  They differ in size and genetic composition
–  However, they do have short regions of homology
•  Figure 3.3 considers two homologous
chromosomes labeled with 3 different genes
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-17
The physical location of a gene on a
chromosome is called its locus.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Gene loci (location)
Figure 3.3
A
b
c
A
B
c
Homologous"
pair of"
chromosomes
Genotype:
AA
Bb
Homozygous" Heterozygous
for the"
dominant"
allele
cc
Homozygous"
for the"
recessive"
allele
3-18
3.2 CELLULAR DIVISION
•  One purpose of cell division is asexual
reproduction
–  This is the means by which some unicellular
organisms produce new individuals
–  Examples
•  Bacteria
•  Amoeba
•  Yeast
–  Saccharomyces cerevisiae (Baker s yeast)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-19
3.2 CELLULAR DIVISION
•  A second important reason for cell division is
multicellularity
–  Plants, animals and certain fungi are derived from
a single cell that has undergone repeated cell
divisions
–  For example
•  Humans start out as a single fertilized egg
•  End up as an adult with several trillion cells
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-20
MITOSIS
•  Cell division in eukaryotes requires a
replication and sorting process that is more
complicated than simple binary fission
•  Eukaryotic cells that are destined to divide
progress through a series of stages known as
the cell cycle
–  Refer to Figure 3.5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-24
Figure 3.5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Interphase
Mother cell
S
Synthesis
Chromosome
Nucleolus
e
G2
Gap 2
ase
G0
Metaph
Gap 1
M
Mitosis
Anapha
s
G1
(Nondividing cell)
Two"
daughter"
cells
3-25
MITOSIS
•  In actively dividing cells, G1 (Gap 1), S and G2
are collectively know as interphase
•  A cell may remain for long periods of time in
the G0 phase
–  A cell in this phase is in a quiescent state and has
•  Either postponed progression through the cell cycle
•  Or made the decision to never divide again
–  Terminally differentiated cells (e.g. nerve cells)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-26
MITOSIS
•  During the G1 phase, a cell prepares to divide
•  The cell reaches a restriction point and is
committed on a pathway to cell division
•  Then the cell advances to the S phase, where
chromosomes are replicated
–  The two copies of a replicated chromosome are
termed chromatids
–  They are joined at the centromere to form a pair of
sister chromatids
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-27
Figure 3.6 (b)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A pair of sister chromatids
Centromere"
(DNA that is"
hidden beneath"
the kinetochore"
proteins)
One"
chromatid
Kinetochore"
(proteins attached"
to the centromere)
One"
chromatid
(b) Schematic drawing of sister chromatids
3-28
•  Note that at the end of S phase, a cell has
twice as many chromatids as there are
chromosomes in the G1 phase
–  A human cell for example has
•  46 distinct chromosomes in G1 phase
•  46 pairs of sister chromatids in S phase
•  Therefore the term chromosome is relative
–  In G1 and late in the M phase, it refers to the
equivalent of one chromatid
–  In G2 and early in the M phase, it refers to a pair
of sister chromatids
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-29
•  During the G2 phase, the cell accumulates the materials
that are necessary for nuclear and cell division
•  It then progresses into the M phase of the cycle where
mitosis occurs
•  The primary purpose of mitosis is to distribute the
replicated chromosomes to the two daughter cells
–  In humans for example,
•  The 46 pairs of sister chromatids are separated and sorted
•  Each daughter cell receives the same complement of
chromosomes
•  Each daughter cell thus receives 46 chromosomes
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-30
•  Mitosis was first observed microscopically in
the 1870s by the German biologist, Walter
Flemming
–  He coined the term mitosis
•  From the Greek mitos, meaning thread
•  The process of mitosis is shown in Figure 3.8
•  The original mother cell is diploid (2n)
–  It contains a total of six chromosomes
–  Three per set (n = 3)
•  One set is shown in blue and the homologous set in red
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-31
•  Mitosis is subdivided into five phases
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
•  Following are the stages of mitosis from
Figure 3.8
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-32
3.3 SEXUAL REPRODUCTION
•  Sexual reproduction is the most common
way for eukaryotic organisms to produce
offspring
–  Parents make gametes with half the amount of
genetic material
•  These gametes fuse with each other during
fertilization to begin the life of a new organism
•  The process of forming gametes is called
gametogenesis
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-43
•  Some simple eukaryotic species are
isogamous
–  They produce gametes that are morphologically
similar
•  Example: Many species of fungi and algae
•  Most eukaryotic species are heterogamous
–  These produce gametes that are morphologically
different
•  Sperm cells (male gametes)
–  Relatively small and mobile
•  Egg cell or ovum (female gametes)
–  Usually large and nonmotile
–  Stores a large amount of nutrients (animal species)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-44
•  Gametes are typically haploid
–  They contain a single set of chromosomes
•  Gametes are 1n, while diploid cells are 2n
–  A diploid human cell contains 46 chromosomes
–  A human gamete only contains 23 chromosomes
•  During meiosis, haploid cells are produced
from diploid cells
–  Thus, the chromosomes must be correctly sorted
and distributed to reduce the chromosome
number to half its original value
•  In humans, for example, a gamete must receive one
chromosome from each of the 23 pairs
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-45
MEIOSIS
•  Like mitosis, meiosis begins after a cell has
progressed through interphase of the cell cycle
•  Unlike mitosis, meiosis involves two successive
divisions
–  These are termed Meiosis I and Meiosis II
–  Each meiotic division is subdivided into
•
•
•
•
•
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-46
MEIOSIS
•  Prophase I is further subdivided into five
stages known as
–  Leptotene
–  Zygotene
–  Pachytene
–  Diplotene
–  Diakinesis
–  Refer to Figure 3.10
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-47
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
STAGES OF PROPHASE OF MEIOSIS I
LEPTOTENE
Nuclear"
membrane
ZYGOTENE
PACHYTENE
DIPLOTENE
Bivalent"
forming
Chiasma
DIAKINESIS
Nuclear membrane"
fragmenting
tetrad
Synaptonemal"
complex forming
Replicated chromosomes"
condense.
Synapsis begins.
A bivalent has formed and"
crossing over has occurred.
Synaptonemal complex"
dissociates.
A physical exchange of
chromosome pieces
End of prophase I
Figure 3.10
3-48
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Synaptonemal Complex
Synaptonemal complex
•  Formed between
homologous chromosomes
•  May not be required for pairing
•  Precise role not clearly understood
Bound to
chromosomal
DNA of
homologous
chromatids
Figure 3.11
Lateral " Central "
element" element"
Chromatid
Transverse "
filament"
Provides link between
lateral elements
3-49
Figure 3.12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Spindle apparatus complete
Chromatids attached via
kinetochore microtubules
MEIOSIS I
Centrosomes with centrioles
Mitotic spindle
Sister"
chromatids
Bivalent
Synapsis of"
homologous"
chromatids and"
crossing over
EARLY PROPHASE
LATE PROPHASE
Nuclear"
membrane"
fragmenting
PROMETAPHASE
Cleavage"
furrow
Metaphase"
plate
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
MEIOSIS II
Four haploid daughter cells
PROPHASE
PROMETAPHASE
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
3-50
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
•  Bivalents are organized
along the metaphase plate in
Meiosis I
Metaphase"
plate
–  Pairs of sister chromatids are
aligned in a double row, rather
than a single row (as in
mitosis)
Figure 3.12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The arrangement is random
with regard to the (blue and
red) homologs
Kinetochore
Furthermore
n  One pair of sister chromatids
is linked to one of the poles
n  And the homologous pair is
Figure 3.13
linked to the opposite pole
n
n
3-51
The two pairs of sister chromatids
separate from each other
However, the connection that
holds sister chromatids together
does not break
Sister chromatids reach their
respective poles and decondense
Nuclear envelope reforms to produce
two separate nuclei
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cleavage"
furrow
Metaphase"
plate
METAPHASE
Figure 3.12
ANAPHASE
TELOPHASE AND CYTOKINESIS
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-52
•  Meiosis I is followed by cytokinesis and then
meiosis II
•  The sorting events that occur during meiosis
II are similar to those that occur during
mitosis
•  However the starting point is different
–  For a diploid organism with six chromosomes
•  Mitosis begins with 12 chromatids joined as six pairs
of sister chromatids
•  Meiosis II begins with 6 chromatids joined as three
pairs of sister chromatids
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-53
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Four haploid daughter cells
PROPHASE
PROMETAPHASE
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Figure 3.12
3-54
•  Mitosis vs Meiosis
–  Mitosis produces two diploid daughter cells
–  Meiosis produces four haploid daughter cells
–  Mitosis produces daughter cells that are
genetically identical
–  Meiosis produces daughter cells that are not
genetically identical
•  The daughter cells contain only one homologous
chromosome from each pair
•  The daughter cells contain many different
combinations of the single homologs
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-55
Spermatogenesis
•  The production of sperm
•  In male animals, it occurs in the testes
•  A diploid spermatogonial cell divides
mitotically to produce two cells
–  One remains a spermatogonial cell
–  The other becomes a primary spermatocyte
•  The primary spermatocyte progresses
through meiosis I and II
–  Refer to Figure 3.14a
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-56
Meiois I yields two
haploid secondary
spermatocytes
Each spermatid
matures into a
haploid sperm cell
Meiois II yields four
haploid spermatids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
MEIOSIS I
MEIOSIS II
Primary"
spermatocyte"
(diploid)
Spermatids
(a) Spermatogenesis
Sperm cells"
(haploid)
Figure 3.14 (a)
3-57
•  The structure of a sperm includes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
–  A long flagellum
–  A head
•  The head contains a haploid nucleus
–  Capped by the acrosome
The acrosome contains
digestive enzymes
- Enable the sperm to
penetrate the protective layers
of the egg
n
In human males, spermatogenesis is
a continuous process
n
Sperm cells"
(haploid)
A mature human male produces several
hundred million sperm per day
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-58
Oogenesis
•  The production of egg cells
•  In female animals, it occurs in the ovaries
•  Early in development, diploid oogonia
produce diploid primary oocytes
–  In humans, for example, about 1 million primary
oocytes per ovary are produced before birth
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-59
•  The primary oocytes initiate meiosis I
•  However, they enter into a dormant phase
–  They are arrested in prophase I until the female
becomes sexually mature
•  At puberty, primary oocytes are periodically
activated to progress through meiosis I
–  In humans, one oocyte per month is activated
•  The division in meiosis I is asymmetric
producing two haploid cells of unequal size
–  A large secondary oocyte
–  A small polar body
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-60
•  The secondary oocyte enters meiosis II, but is
arrested at metaphase II
•  It is released into the oviduct
–  An event called ovulation
•  If the secondary oocyte is fertilized
–  Meiosis II is completed
–  A haploid egg and a second polar body are produced
•  The haploid egg and sperm nuclei then fuse to
create the diploid nucleus of a new individual
•  Note that only one of the four cells produced in this
meiosis becomes an egg
•  Refer to Figure 3.14b
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-61
Unlike spermatogenesis,
the divisions in oogenesis
are asymmetric
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Secondary oocyte
Second polar body
Primary"
oocyte"
(diploid)
Egg cell"
(haploid)
First polar body
(b) Oogenesis
Figure 3.14 (b)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-62
•  The chromosome theory of inheritance is based on a
few fundamental principles
–  1. Chromosomes contain the genetic material
–  2. Chromosomes are replicated and passed from parent to
offspring
–  3. The nuclei of most eukaryotic cells contain chromosomes
that are found in homologous pairs, they are diploid
•  During meiosis, each homologue segregates into one of the two
daughter nuclei
–  4. During the formation of gametes, different types of
(nonhomologous) chromosomes segregate independently
–  5. Each parent contributes one set of chromosomes to its
offspring
•  The sets are functionally equivalent
–  Each carries a full complement of genes
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-75
•  The chromosome theory of inheritance
allows us to see the relationship between
Mendel s laws and chromosome
transmission
–  Mendel s law of segregation can be
explained by the homologous pairing and
segregation of chromosomes during meiosis
•  Refer to Figure 3.16
–  Mendel s law of independent assortment can
be explained by the relative behavior of
different (nonhomologous) chromosomes
during meiosis
•  Refer to Figure 3.17
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-76
Sex Determination
•  In many animal species, chromosomes play
a role in sex determination
•  Refer to Figure 3.18
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-79
•  Humans have 46 chromosomes
–  44 autosomes
–  2 sex chromosomes
•  Males contain one X and one Y chromosome
–  They are termed heterogametic
•  Females have two X chromosomes
–  They are termed homogametic
•  The Y chromosome determines maleness
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
44 +
XY
(a) X–Y system in mammals"
44 +
XX
3-80
•  In some insects,
–  Males are XO and females are XX
•  In other insects (fruit fly, for example)
–  Males are XY and females are XX
•  The Y chromosome does not determines maleness
•  Rather, it is the ratio between the X chromosomes
and the number of sets of autosomes (X/A)
–  If X/A = 0.5, the fly becomes a male
–  If X/A = 1.0, the fly becomes a female
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
22 +
X
22 +
XX
(b) The X–0 system in certain insects"
3-81
•  The sex chromosomes are designated Z and W to
distinguish them from the X and Y chromosomes of
mammals
•  Males contain two Z chromosomes
–  Hence, they are homogametic
•  Females have one Z and one W chromosome
–  Hence, they are heterogametic
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
76 +
ZZ
(c) The Z–W system in birds!
76 +
ZW
and some fish
3-82
•  Males are known as the drones
–  They are haploid
–  Produced from unfertilized haploid eggs
•  Females include the worker bees and queen bees
–  They are diploid
–  Produced from fertilized eggs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
16
haploid
32
diploid
(d) The haplodiploid system in bees!
3-83
Morgan s Experiment
•  The chromosome theory of inheritance was
confirmed through studies carried out by
Thomas Hunt Morgan
•  Morgan tried to induce mutations in the fruit fly
Drosophila melanogaster
–  Treatments included
•  Rearing in the dark
•  X-rays
•  Radium
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-84
•  After 2 years, Morgan finally obtained an
interesting result
–  A male fruit fly with white eyes rather than the
normal red eyes
•  Morgan reasoned that this white eyed male must have
arisen from a new mutation that converted a red-eyed
allele into a white-eyed allele
•  Morgan followed Mendel s approach in
studying the inheritance of this white-eyed trait
–  He made crosses then analyzed their outcome
quantitatively
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-85
The Hypothesis
–  A quantitative analysis of genetic crosses may
reveal the inheritance pattern for the white eye
allele
Testing the Hypothesis
n
Refer to Figure 3.19
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-86
Figure 3.19
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Conceptual level
Experimental level
XwY
1. Cross the white-eyed male to a
true-breeding red-eyed female.
Xw+Xw+
x
x
2. Record the results of the F1 generation.
This involves noting the eye color and
sexes of many offspring.
Xw+Y male offspring and Xw+Xw female
offspring, both with red eyes
Xw+Y x Xw+Xw
x
F1 generation
1 Xw+Y : 1 XwY : 1 Xw+Xw+ : Xw+Xw
1 red-eyed male : 1 white-eyed male :
2 red-eyed femles
3. Cross F1 offspring with each other to
obtain F2 offspring. Also record the
eye color and sex of the F2 offspring.
F2 generation
4. In a separate experiment, perform a
testcross between a white-eyed male
and a red-eyed female from the F1
generation. Record the results.
XwY
x
x
Xw+Xw
From
F1 generation
+
+
1 Xw+Y : 1 XwY : 1 Xw+Xw : XwXw
1 red-eyed male : 1 white-eyed male :
1 red-eyed femles : 1 red-eyed femles
3-87
The Data
Cross
Results
Original white eyed-male F1 generation:
to red-eyed females
All red-eyed flies
F1 male to F1 female
F2 generation:
2,459 red-eyed females
1,011 red-eyed males
0 white eyed-females
782 white-eyed males
Test Cross
Results
White-eyed male to
F1 female
129 red-eyed females
132 red-eyed males
88 white eyed-females
86 white-eyed males
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-88
Interpreting the Data
n
The first cross yielded NO white-eyed females
in the F2 generation
n
n
These results indicate that the eye color alleles
are located on the X chromosome
Genes that are physically located on the X
chromosome are called X-linked genes or Xlinked alleles
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-89
n
A Punnett square predicts the absence of
white-eyed females in the F2 generation
w
+
F 1 male is X Y
+
F 1 female is X w X w
Male gametes
X
X
X
w
w
w
+
+
X
w
Y
+
X
w
+
Y
+
Red, female Red, male
X
w
+
Xw
Xw Y
Xw
Red, female White, male
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
3-90
n
Note:
The experimental ratio of red eyes to white eyes
in the F2 generation is (2,459 + 1,011):782
n  This ~ 4.4:1 ratio deviated significantly from the
predicted 3:1 ratio
n  How can the lower-than-expected number of
white-eyed flies be explained?
n
n
Decreased survival of white-eyed flies
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-91
n
The testcross resulted in red-eyed females
and males, and white-eyed females and
males, in approximately equal numbers
n
This is consistent with an X-linked pattern of
inheritance
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Testcross:
Male is Xw Y
+
F1 female is Xw Xw
Male gametes
Xw
Y
+
Xw Xw
Xw
+
Xw Y
+
Red, female
Xw Xw
Red, male
Xw Y
Xw
White, female White, male
3-92
n
Note:
The observed data of the testcross are
129:132:88:86
n  This ~ 1.5:1.5:1:1 ratio deviates from the
predicted 1:1:1:1 ratio
n  Again, the lower-than-expected number of whiteeyed flies can be explained by a lower survival
rate for those flies
n
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
3-93