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STUDY UNIT 3
Benjamin A. Pierce
GENETICS
A Conceptual Approach
Sexual reproduction forms offspring that are
genetically unique.
Consists of alteration of haploid (n) and diploid (2n)
cells produced by meiosis and fertilization
respectively.
FIFTH EDITION
CHAPTER 4
Sex Determination and Sex-Linked
Characteristics
© 2014 W. H. Freeman and Company
4.1 Sex Is Determined by a Number of
Different Mechanisms, p 78 [p 74]
Sexual phenotype: male or female.
Mechanisms whereby sex is established is sex
determination.
Chromosomal Sex-Determination Systems
Sex chromosomes: chromosomes which differ in
number and/or morphology between males and
females.
Autosomes (non-sex chromosomes): the same in
males and females.
• Hermaphroditism: both sexes in the same organism.
• Monoecious: both male and female reproductive
structures in the same organism.
• Dioecious: either male or female reproductive
structures in one organism.
Female karyotype
Male karyotype
XX-XO sex determination system:
Homogametic sex: sex that produces gametes that
are all the same with respect to the sex chromosomes.
Heterogametic sex: sex that produces two different
types of gametes with respect to the sex chromosomes.
E.g. grasshoppers:
Females XX, all eggs have haploid set of autosomes
+ 1 X chromosome → gametes all X
Males XO, all sperm have haploid set of autosomes,
single X chromosome segregates to half the sperm
cells in meiosis, the other half receive no sex
chromosome.
→ gametes ½ X : ½
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XX-XY sex determination system:
E.g. all mammals (including humans), some plants,
insects, reptiles.
X and Y chromosomes are not homologous over their
full lengths, but pair during meiosis through small
homologous pseudoautosomal regions at the tips.
Females XX (homogametic)
→ gametes all X
Males XY (heterogametic)
→ gametes ½ X : ½ Y
ZZ-ZW sex determination system:
E.g. all birds, snakes, butterflies, some amphibians and
fishes.
Females ZW (heterogametic) → gametes ½ Z : ½ W
Males ZZ (homogametic) → gametes all Z
Genic Sex Determination
E.g. some plants, fungi, protozoans.
No sex chromosomes.
Genes at one or more autosomal loci determine
sex of individual.
(NB: Even in chromosomal sex-determining
systems sex is actually determined by individual
genes on the sex chromosomes, e.g. SRY gene in
mammals.)
Environmental Sex Determination
Environmental factors determine sex
• Limpet’s position in the stack.
• Temperature in turtles.
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Sex Determination in Drosophila melanogaster
• Genic balance system.
• In Drosophila 2n = 8, with 3 pairs of autosomes and
one pair of sex chromosomes (XX in ♀, XY in ♂).
• Normal fly therefore has two haploid sets of
autosomes (AA) and two sex chromosomes.
• Presence of Y chromosome does not determine
maleness (only male fertility).
♀ somatic cell
AAXX
• X chromosome has genes with female-producing
effects, autosomes have genes with male-producing
effects.
• Sex is determined by X : A ratio
X = number of X chromosomes
A = number of haploid sets of autosomes.
• X:A ratio of 1 → female
X:A ratio of 0.5 → male.
♂ somatic cell
AAXY
Class work
Normal
situation
Sex Determination in Humans XX-XY
• ♀ XX, ♂ XY.
• SRY gene on Y chromosome determines maleness.
• Sex chromosome abnormalities arise when gametes
have unusual numbers of sex chromosomes due to
non-disjunction (abnormal chromosome segregation)
during meiosis.
• Sex chromosome abnormalities illustrate the
importance of the Y chromosome in sex
determination.
Turner syndrome:
Females with XO.
1/3000 female births.
Short, normal intelligence,
usually sterile.
No known cases where a person is missing both X
chromosomes.
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Klinefelter syndrome:
Males with XXY, or XXXY, or XXXXY, or XXYY.
1/1000 male births.
Often tall, normal intelligence,
sterile.
Poly-X females:
Most common is triplo-X syndrome, XXX.
1/1000 female births.
Also occasionally 4 or 5 X chromosomes.
The Male-determining Gene in Humans
• Sex-determining region Y (SRY) gene on Y
chromosome is male-determining gene.
• Activated at 6 weeks of embryonic development.
• Encodes transcription factor that activates testes
differentiation.
• Absence of SRY – neutral embryonic gonads
become ovaries.
4.2 Sex-Linked Characteristics Are Determined
by Genes on the Sex Chromosomes, p 85
[p 77]
The Role of Sex Chromosomes
• The X chromosome contains genetic information
essential for both sexes; at least one copy of an
X is required.
• The male-determining gene is located on the Y
chromosome. A single Y, even in the presence
of several X, still produces a male phenotype.
• The absence of Y results in a female phenotype.
• Genes affecting fertility are located on the X and
Y chromosomes.
• Additional copies of the X chromosome may
affect normal development.
Androgen-insensitivity syndrome
• SRY gene is primary determinant of sex.
• Several other genes (X-linked, Y-linked and
autosomal) influence sexual development.
• Effect of testosterone on embryonic tissues is via
binding to androgen receptor.
• Defective androgen receptor results in female
development, even though person is XY.
X-Linked Characteristics
X-linked white eyes in Drosophila
Sex-linked characteristics are characteristics
encoded by genes on sex chromosomes.
• X-linked
• Z-linked
• Y-linked
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X-linked white eyes in Drosophila
P: ♀ red eyes x ♂ white eyes
↓
F1:
all red eyes
F2:
↓
♀ all red eyes
♂ ½ red : ½ white eyes
Class work
♀ white eyes x ♂ red eyes
↓
♀ all red eyes
♂ all white eyes
↓
♀ ½ red : ½ white eyes
♂ ½ red : ½ white eyes
Reciprocal crosses results in differences.
Can be explained by X-linked inheritance.
Females are XX, so can be homozygous (e.g.
XAXA or XaXa) or heterozygous (e.g. XAXa) for Xlinked genes.
Males are XY, so male cells have only one copy of
any X-linked gene.
Males therefore cannot be homozygous or
heterozygous, but are hemizygous for X-linked
loci, as they only carry one allele for a specific
locus (e.g. XAY or XaY).
X-linked colour blindness in humans
X+ allele (normal) > Xc allele (colour blind)
♀ X+X+
X+Xc
XcXc
normal vision
♂ X+Y normal vision
XcY colour blind
colour blind
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Z-Linked Characteristics
• Reciprocal crosses result in differences.
• Affected woman passes X-linked recessive trait to
her sons but not daughters.
• Affected man passes trait to his grandsons through
his daughters, but never to his sons.
• X-linked recessive characteristics may appear to
alternate between the sexes across generations.
Go through the Worked Problem on p 89 [p 81] of
Pierce.
In ZZ-ZW system:
Males are homogametic sex (ZZ), so can be
homozygous or heterozygous for Z-linked genes.
Females are heterogametic sex (ZW), so are
hemizygous and possess only single allele for any
Z-linked gene.
Inheritance of Z-linked characteristics is the same
as for X-linked characteristics, only the pattern in
males and females is reversed.
Z-linked plumage colour in peafowls
ZCa+ allele (blue) > Zca allele (cameo)
♂ ZCa+ZCa+
♀ ZCa+W blue
blue
Ca+
ca
Z Z
ZcaW cameo
ca
ca
Z Z
cameo
Example:
In chickens, the allele for black feathers (B) is
dominant over the allele for brown feathers (b) and
the B-locus is carried on the Z chromosome.
A black hen is crossed with a heterozygous black
rooster. Give the expected genotypic and
phenotypic ratios of the F1 progeny.
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Y-Linked Characteristics
Recognising Sex-Linked Inheritance
• Y-linked traits present only in males.
• All male offspring display a Y-linked trait present
in the father.
• X and Y chromosomes probably evolved from a
pair of autosomes.
• Modern human Y chromosome is small and
carries few genes.
• Y-linked genetic markers are passed from father
to son and can be used to study male ancestry.
• NB: Many autosomal characteristics also differ
between males and females, as their
expression is influenced by sex hormones
(Chapter 5).
• Y-linked traits are always inherited only from
father to son.
• X-linked traits exhibit a distinctive pattern of
inheritance, and is one possible explanation for
when reciprocal crosses give different results.
Males always inherit X-linked traits only from
their mother.
4.3 Dosage Compensation Equalises the Amount
of Protein Produced by X-Linked Genes in
Males and Females, p 92 [p 84]
♀ XX : 2 copies of every X-linked gene
♂ XY : 1 copy of every X-linked gene.
Females would produce twice as much gene products
(proteins) for X-linked genes as males → would be
highly detrimental → prevented by mechanism of
dosage compensation.
Fruit flies: Activity of X-linked genes doubled in males.
Humans: One X chromosome inactivated in females.
Lyon Hypothesis
Barr body observed in nuclei of female cells.
Mary Lyon hypothesised this represents an inactive X
chromosome.
All but one of the X chromosomes in a cell are
inactivated.
Random process in early embryonic development.
Inactive X remains inactivated in progeny cells.
Females are therefore functionally hemizygous for Xlinked genes at cellular level.
For females heterozygous at an X-linked locus, a
mosaic pattern of expression is observed for this gene,
unique in every individual.
Approximately 50% of cells will express one allele and
50% the other allele.
E.g. black and orange fur patterns in female calico and
tortoiseshell cats.
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Class work
Mechanism of Random X Inactivation
X chromosome counting.
Xist gene activated on one X chromosome → Xist RNA
transcript coats chromosome → alters chromatin
structure → inactivated.
Xist gene on other X chromosome repressed → X
remains active.
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