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Powerpoint Lecture Outline
Human Genetics
Concepts and Applications
Seventh Edition
Ricki Lewis
Prepared by
Mary King Kananen
Penn State Altoona
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Chapter 6
Matters of Sex
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Sexual
Development
•In early embryos
unspecialized gonads
and two sets of
reproductive ducts
exist until week 6
•An embryo develops
as a male or female
using information from
the Y chromosome
Figure 6.1
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Male or Female?
•Gender is ultimately a genetic phenomenon
•It also has psychological and sociological
components
•Males have 22 pairs of autosomes and X
and Y chromosomes
•Females have 22 pairs of autosomes and
Two X chromosomes
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Sex Chromosomes Determine Gender
•Human males are the heterogametic sex with
different sex chromosomes, (XY)
•Human females are the homogametic sex (XX)
•In other species sex can be determined in many
ways
For example, in birds and snakes
• males are homogametic
ZZ
• female are heterogametic
ZW
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X and Y Chromosomes
• X chromosome
– contains more than 1,500 genes
– Larger than the Y chromosome
– Acts as a homolog to Y
chromosome in males
• Y chromosome
– 95% harbor male specific genes
– Many DNA segments are
Figure 6.2
palindromes and may
destabilize DNA
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Genes on the Y Chromosome
Genes shared with X
chromosome define
the pseudoautosomal
regions (PAR1 and
PAR2)
•Male specific (MSY)
including SRY gene
•SRY gene is
important in
determining sex
Figure 6.3
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SRY Gene
•Encodes for a transcription factor protein
•Controls the expression of other genes
•Stimulates male development
•In response, developing testes secrete antiMullerian hormone and destroy female
structures
•Testosterone and DHT are secreted and
stimulate male structures
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Mutations that Disrupt Normal Sexual
Development
Figure 6.4
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Table 6.1
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Homosexuality
•Homosexuality has been seen in all cultures for
thousands of years
•Evidence may suggest a genetic component
•Research in this area is controversial
•Twin studies
•Identifying possible markers
•Altered gene expression in male Drosophila
•White gene causing decreased serotonin levels
and homosexual behavior
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Figure 6.6
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Y-linked Traits
•
•
•
•
•
Genes on the Y chromosome
Very rare
Transmitted male to male
No affected females
Currently, identified Y-linked traits
involve infertility and are not transmitted
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X-linked Traits
Possible genotypes
X+Y Hemizygous wild type male
XmY Hemizygous mutant male
X+X+ Homozyogus wild female
X+Xm Heterozygous female carrier
XmXm Homozygous mutant female
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X-linked Recessive Traits
•Always expressed in hemizygous males
•Female homozygotes show the trait but
female heterozygotes do not
•Affected males: Inherited from affected or
heterozygous mother
•Affected females : affected fathers and
affected or heterozygous mothers
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X-linked Recessive Inheritance
Figure 6.7
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Figure 6.8
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X-linked Dominant Inheritance
• Expressed with one copy
• Males are often more severely affected
• Typically associated with miscarriage or
lethality in males
• Passed from father to all his daughters
but none of his sons
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X-linked Dominant Inheritance:
Congenital Generalized Hypertrichosis
Figure 6.10
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Genetics Problems
• Examples
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X Inactivation
•Females have two alleles for every gene on the X
chromosome but males have only one
•In mammals, X inactivation balances this
inequality and one X chromosome is randomly
inactivated in each cell
•The XIST gene inactivates the chromosome and
encodes for RNA
•It alters phenotype but not genotype
•Inactivated X chromosomes form a Barr body
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Figure 6.11
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Inactive X Chromosome Is Visible
as Barr Body
Figure 6.12
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Cats Heterozygous for the Coat
Color Gene
Figure 6.13
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Sex-limited Traits
• Traits that affect a structure or function
occurring only in one gender
• May be autosomal or X linked
Examples:
• Beard growth
• Milk production
• Pregnancy phenotypes
• Sperm production levels
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Sex-influenced Traits
•Traits are those in which the phenotype expressed by a
heterozygote is influenced by gender.
•Allele appears dominant in one gender and recessive in
the other
Example:
•Pattern baldness is a sex-influenced trait:
men
women
m/m
bald
bald
m/+
bald
unaffected
+/+
unaffected
unaffected
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Figure 6.14
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Genomic Imprinting
• Occurs when the expression of a gene differs
when the allele is transmitted maternally
versus paternally
• Function unknown, may play a role in
development
• Genes silenced by an epigenetic event
• Information not encoded by DNA
• Imprints maintained in mitotic divisions but lost
in meiosis
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Genomic Imprinting
Figure 6.15
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Importance of Genomic Imprinting
• Experiments suggest that it takes two opposite
sex parents to produce a healthy embryo
• Genes from female parent direct different
activities from the genes of the male parent
• May explain why cloning mammals is so difficult
and the offspring are almost always unhealthy
• Certain assisted reproductive technologies may
produce imprinting disorders
• May explain incomplete penetrance
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Figure 6.16
Two male pronuclei form a hydatidiform mole
Two female pronuclei form a teratoma
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Imprinting and Human Disease
Deletion on chromosome 15 reveals imprinting
Figure 6.17
• Inherited paternally
• Prader-Willi syndrome
• Inherited maternally
• Angelman syndrome
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Callipyge (“beautiful buttock”) Sheep
Is Caused by Genomic Imprinting
• Over-muscled hindquarters
• Autosomal dominant
• Trait only passed if it came from the father
and the female may not carry the trait
• Seven other genes are overexpessed on
chromosome 18
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Figure 6.18
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