Lecture 4.6 Study Guide – Sex Linkage
Assigned Reading: Chapter 12.4
Learning Objectives – At the conclusion of this lecture, you should be able to:
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State the Chromosomal Theory of Inheritance.
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Compare the means of sex determination in different organisms.
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Describe the molecular basis of sex determination in humans.
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Proficiently use Drosophila genetic nomenclature.
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Explain how the Chromosome Theory of Inheritance was supported by the discovery of a sexlinked gene in Drosophila.
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Perform genetic crosses involving the inheritance of sex-linked genes.
Key Terms You Should Be Able to Use & Apply –
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Chromosomal determination
Chromosome theory of inheritance
Crisscross inheritance
Hemizygous
Heterogametic
Homogametic
Law of independent assortment
Law of segregation
Mutant
Pseudoautosomal region (PAR)
Reciprocal cross
Sex
Sex determination
Sex linkage
Sex reversal
Sexual reproduction
SRY
Testcross
Wild type
X-linked inheritance
Y-linked inheritance
Key Concepts You Should Be Able to Use & Apply –
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The chromosome theory of inheritance states that genes are located on chromosomes. The
chromosome theory of inheritance was supported by the discovery of sex linkage.
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Mendel’s Law of Segregation and Law of Independent Assortment can be explained by events in
meiosis.
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In most eukaryotic organisms, sexual reproduction consists of an alteration between haploid
(1n) and diploid (2n) cells.
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Lecture 4.6 Study Guide
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Mechanisms of sex determination differ between species. Inheritance of sex in organisms with
X and Y chromosomes results in equal numbers of male and female offspring.
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In humans, phenotypic sex is an inherited trait that is determined by the presence or absence of
a Y chromosome containing the SRY gene. The importance of the SRY gene in sex determination
is demonstrated by sex-reversal (XX males and XY females).
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Sex-linked characteristics are determined by genes located on the sex chromosomes.
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Thomas Hunt Morgan was the first person to explain sex-linked inheritance. Morgan used the
fruit fly Drosophila melanogaster as a model organism. Morgan’s experiments showed a
correlation between a genetic trait and the inheritance of a sex chromosome in Drosophila.
Examples of Skills You Should Be Able to Demonstrate –
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Predict the sex of humans with different complements of X & Y chromosomes.
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Determine the expected phenotypic and genotypic outcomes for the F1 and F2 generations of a
given sex-linked cross.
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Determine the genotypes of the original parents in a sex-linked cross based on the progeny.
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Compare and contrast the offspring from a set of reciprocal crosses involving an X-linked gene.
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Complete a monohybrid or dihybrid cross in Drosophila using proper fly nomenclature for genes
on an autosome or X chromosome.
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Infer whether or not a given trait is sex-linked based on the results of a series of crosses.
Recommended Chapter Questions –
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12.4 Recap: Questions 3-4.
Figure Questions: Figure 12.18.
Additional Practice Problems –
1. Complete each statement with the correct Key Term:
A. ____________________ is the phenomenon whereby males are XX or females are XY.
B. ____________________ are homologous regions at both ends of the X and Y chromosomes.
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Lecture 4.6 Study Guide
QUESTION 1 (continued)
C. The two sex chromosomes are dissimilar in the ____________________ sex, where they are
identical in the ____________________ sex.
D. ____________________ refers to an inheritance pattern in which males inherit a trait from their
mothers, while daughters inherit the trait from their fathers.
E. ____________________ describes the genotype for genes present in only one copy in an
otherwise diploid organism.
F. The ____________________ phenotype is the most common phenotype observed in a given
population, where a ____________________ phenotype arises due to heritable change in a
gene.
2. Human males with two X chromosomes are sex-reversed if they have a mutant X
chromosome as depicted to the right. Sex-reversed males often learn of their condition
when they want to have children and discover that they are sterile. Provide an
explanation for the observed sterility.
3. Having scalloped (sd) wings is an X-linked, recessive characteristic in fruit flies. If a female having
scalloped wings is crossed with a wild-type male, what percentage of the F1 males will have
scalloped wings?
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Lecture 4.6 Study Guide
4. A Drosophila mutation called singed (s) causes the bristles to be bent and misshapen. A mutation
called purple (p) causes the fly’s eyes to be purple in color instead of the normal red. A series of
reciprocal crosses was performed, where flies homozygous for singed and purple were crossed with
flies that were homozygous for normal bristles and red eyes. The F1 were intercrossed to produce
the F2, and the following results were obtained.
P
F1
F2
Cross 1
singed bristles, purple eyes ♂ ×
normal bristles, red eyes ♀
420 normal bristles, red eyes ♀
426 normal bristles, red eyes ♂
337 normal bristles, red eyes ♀
113 normal bristles, purple eyes ♀
168 normal bristles, red eyes ♂
170 singed bristles, red eyes ♂
56 normal bristles, purple eyes ♂
58 singed bristles, purple eyes ♂
Cross 2
singed bristles, purple eyes ♀ ×
normal bristles, red eyes ♂
504 normal bristles, red eyes ♀
498 singed bristles, red eyes ♂
227 normal bristles, red eyes ♀
223 singed bristles, red eyes ♀
225 normal bristles, red eyes ♂
225 singed bristles, red eyes ♂
78 normal bristles, purple eyes ♀
76 singed bristles, purple eyes ♀
74 normal bristles, purple eyes ♂
72 singed bristles, purple eyes ♂
A. What is the mode of inheritance for each gene?
singed (select one):
purple (select one):
autosomal
autosomal
X-linked
X-linked
B. What are the genotypes of the parental flies in Cross 1? …in Cross 2? Be sure to use proper
Drosophila nomenclature.
Cross 1: _________________________
Cross 2: _________________________
5. In the mosquito Anopheles culicifacies, the wild-type body color is tan and the wild-type eye color is
black. golden (go) body is a recessive X-linked mutation, and brown (bw) eyes is a recessive
autosomal mutation. A pure-breeding female with a golden body and black eyes (who is
homozygous at both loci) is mated to a male with a tan body and brown eyes. Note: You can use the
same sex chromosome and nomenclature systems as are used for Drosophila for this mosquito.
A. What is the phenotypic ratio expected amongst the F1 offspring?
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Lecture 4.6 Study Guide
QUESTION 5 (continued)
B. What is the phenotypic ratio expected amongst the F2 offspring resulting from an intercross of
F1 mosquitos? HINT: Try using a branch diagram to solve this problem.
6. Lutinos are considered the albino version of the green series of parakeets. A lutino features lush,
buttercup-yellow feathers on the body and head, silver-white cheek patches and paler yellow
coloring on the tail and primary flight feathers. The lutino phenotype is inherited in a sex-linked
recessive manner, with the green (ZB) allele being dominant to the lutino (Zb) allele. Which of the
following crosses would result in 50% of the offspring being lutino females?
a.
b.
c.
d.
e.
ZBZB × ZBW.
ZBZB × ZbW.
ZbZb × ZBW.
ZBZb × ZBW.
ZBZb × ZbW.
7. In Drosophila, the gene for bobbed (bb) is located on the pseudoautosomal region of the X
chromosome. What are the expected genotypes and associated phenotypes of the offspring
resulting from the following crosses?
A. Xbb Xbb × Xbb Y+
B. X+ Xbb × X+ Ybb
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Lecture 4.6 Study Guide
Lecture 4.6 –Sex Linkage
Lecture 4.6.1
Reading: Chapter 12.4
Sex Determination
Learning Objectives:
• State the Chromosomal Theory of Inheritance.
• Compare the means of sex determination in different organisms.
• Describe the molecular basis of sex determination in humans.
Kathryn Gardner, Ph.D.
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A brief timeline in the early history of genetics
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1856-1863
Mendel conducted his experimental work with pea plants.
1866
Mendel published the results of his work on his breeding experiments.
1900
Mendel’s work was rediscovered.
1902
Sutton & Boveri proposed the Chromosome Theory of Inheritance.
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Through careful studies of meiosis in grasshoppers, Walter Sutton demonstrated that each pair
of homologous chromosomes consists of one maternal and one paternal chromosome
image from: The Adapa Project
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Each pair of homologous chromosomes separates independently during meiosis, so
genes located on different pairs of chromosomes assort independently
FIGURE 12.5
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Pairs of alleles resides on homologous chromosomes, and that those alleles segregate
during meiosis
FIGURE 12.2
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The chromosome theory of inheritance was supported by the discovery of sex linkage,
the inheritance of genes on either sex chromosome
image from: J.B. Reece, et al. Campbell Biology, 10th ed. (2013)
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SEXUAL REPRODUCTION results in the formation of offspring that are genetically
distinct from their parents
Meiosis
In the language of genetics, an individual’s sex is defined in reference to its genotype
and corresponding phenotype
Fertilization
Mitosis
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FIGURE 11.14
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SEX DETERMINATION is the mechanism by which sex is established,
and differ between species
Inheritance of sex in organisms with X and Y chromosomes results
in equal numbers of male and female offspring
♀
♂
×
XX (homogametic)
XY (heterogametic)
ZW (heterogametic)
ZZ (homogametic)
Chromosomal Determination
Humans & Drosophila
Birds & Butterflies
♀ XX
Environmental Determination
Lizards & Alligators
Cool temperature
Warm temperature
Tortoises & Turtles
Warm temperature
Cool temperature
♂ XY
X
X
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X
Y
♀ XX
♂ XY
♀ XX
♂ XY
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Sex chromosomes pair during meiosis I and then segregate in males
to form X- and Y-bearing gametes
X
Human sex chromosomes have both shared and unique genes
Y
Meiosis I
Meiosis II
50% of sperm contain
X chromosome
50% of sperm contain
Y chromosome
image from: S. Freeman, et al. Biological Science, 5th ed. (2013)
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image from: L.H. Hartwell, et al. Genetics: From Genes to Genomes (6th ed.).
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TOPHAT L4.6 Q1
Hemizygosity is used to describe X-linked genes in males who have
only one X chromosome
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TOPHAT L4.6 Q2
Lecture 4.6.2
Drosophila Genetics
Learning Objectives:
• Proficiently use Drosophila genetic nomenclature.
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Thomas Hunt Morgan and colleagues used the fruit fly Drosophila melanogaster
as a model organism
Thomas Hunt Morgan was the first person to explain sex-linked inheritance
“For each chromosome contributed by the sperm
there is a corresponding chromosome contributed by
the egg, there are two chromosomes of each kind,
which together constitute a pair.”
- Thomas Hunt Morgan
Wild type
image from: The Nobel Prize
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Mutant
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TOPHAT L4.6 Q3
Genetic nomenclature in Drosophila melanogaster
Recessive mutant allele
white (w)
Dominant mutant allele
Bar (B)
Gene
w+
brick-red
WT
B+
oval
w
white
Mutant
B
Bar
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TOPHAT L4.6 Q4
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TOPHAT L4.6 Q5
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Eye color in Drosophila is an X-linked trait
Lecture 4.6.3
P
red-eyed female × white-eyed male
F1
all red-eyed
Sex-Linked Inheritance
XwY
Learning Objectives:
♂ gametes
• Explain how the Chromosome Theory of Inheritance was supported by the
discovery of a sex-linked gene in Drosophila.
♀ gametes
• Perform genetic crosses involving the inheritance of sex-linked genes.
X+X+
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X+
Symbols:
Xw
Y
X+Xw
X+Y
♀
♂
X+
Xw
adapted from FIGURE 12.18
red eye allele on X chromosome
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white eye allele on X chromosome
Let’s Think About That!
Let’s Think About That!
What is the expected F2 phenotypic ratio if two F1 flies are crossed with one another?
What is the expected phenotypic ratio of the progeny from a testcross of a F1 female?
F1
F2
Red-eyed ♀
X+Xw
×
Red-eyed ♂
X+Y
♀ X+X+
♂ X+Y
♀ X+Xw
♂ XwY
F1
F2
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Red-eyed ♀
X+Xw
×
♀ X+Xw
♂ X+Y
♀ XwXw
♂ XwY
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Morgan’s experiments showed a correlation between a genetic trait and the
inheritance of a sex chromosome in Drosophila
Eye color in Drosophila is an X-linked trait
RECIPROCAL CROSS:
X+Y
XwXw
adapted from FIGURE 12.18
♀ gametes
♂ gametes
X+Xw
XwY
♀
♂
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In-Text Art, p. 258
Let’s Practice!
Suppose that a gene affecting pigmentation is found on the X chromosome in
mammals or the Z chromosome in birds (but is not on the Y or W chromosome). It is
found on an autosome in bees. This gene exists in two alleles, where dark (D) is
dominant to light (d). What would be the phenotypic results of crosses between pure
breeding dark females and pure breeding light males for each of the following
species?
SPECIES (♀×♂)
A
B
C
Birds
ZDW × ZdZd
Bees
DD × dd
Humans
XDXD × XdY
♀PROGENY
♂ PROGENY
Light (ZdW)
Dark (ZDZd)
All dark (Dd)
All dark (XDXd & XDY)
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Let’s Practice!
TOPHAT L4.6 Q6
Coat color in cats is determined by genes at several different loci. At one locus on the
X chromosome, the X+ allele codes for black fur, where the Xo allele encodes orange
fur. Females can be black (X+X+), orange (XoXo) or tortoiseshell (a mixture of orange
and black; X+Xo). Males are either black (X+Y) or orange (XoY). If a tortoiseshell female
gives birth gives to one orange male, one black male, two tortoiseshell females, and
one orange female, what is the genotype of the cat with which it was mated?
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TOPHAT L4.6 Q7