Transcript Unit 5 genetics powerpoint part 1

Human Genetics
Concepts and Applications
Ninth Edition
RICKI LEWIS
6
Matters of Sex
PowerPoint® Lecture Outlines
Prepared by Johnny El-Rady, University of South Florida
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
SEX Determination
Maleness or femaleness is determined at
conception
Humans are monoecious: meaning that each
person can make one type of gamete either
eggs or sperm
Dioecious organisms can make both types of
gametes in the same body
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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), while females are
heterogametic (ZW)
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In grasshoppers, the female contains 24 chromosomes and is
XX while the male contains 23 chromosomes and is XO
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X and Y Chromosomes
X chromosome
- Contains > 1,500 genes
- Larger than the Y chromosome
- Acts as a homolog to Y in
males
Y chromosome
- Contains 231 genes
- Many DNA segments are
palindromes and may
destabilize DNA
Figure 6.2
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Anatomy of the Y Chromosome
Pseudoautosomal regions
(PAR1 and PAR2)
- 5% of the chromosome
- Contains genes shared with
X chromosome
Male specific region (MSY)
- 95% of the chromosome
- Contains majority of genes
including SRY and AZF
(needed for sperm production)
Figure 6.3
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SRY Gene
Encodes a transcription factor protein
Controls the expression of other genes
Stimulates male development
Developing testes secrete anti-Mullerian
hormone and destroy female structures
Testosterone and dihydrotesterone (DHT)
are secreted and stimulate male
structures
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Abnormalities in Sexual Development
Pseudohermaphroditism = Presence of
male and female structures but at
different stages of life
- Androgen insensitivity syndrome = Lack
of androgen receptors
- 5-alpha reductase deficiency = Absence
of DHT
- Congenital adrenal hyperplasia = High
levels of androgens
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Abnormalities in Sexual Development
The degree to which pseudohermaphroditism disturbs the
individual depends as much on society as it does on
genetics.
In the Dominican Republic in the 1970s, 22 young girls
reached the age of puberty and began to transform into
boys.
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Abnormalities in Sexual Development
They had a form of 5-alpha reductase deficiency that was
fairly common due to consanguinity ( relatives having
children with relatives).
The parents were happy that they had had sons after all, and
so these special adolescents were given their own gender
nave at age 12.
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Figure 6.4
Figure 6.4
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Table 6.1
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Sex Ratios
The proportion of males to females in a
human population
Calculated by # of males / # of females
multiplied by 1,000
Primary sex ratio – At conception
Secondary sex ratio – At birth
Tertiary sex ratio – At maturity
Sex ratios can change markedly with age
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Sex Ratios
Sex ratios can be altered intentionally by
a society
- Example: China’s
one-child policy has
led to a scarcity of
females
Figure 6.5
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In China by the year 2020, 20 million men will find themselves without
female partners as a result of the nations “ one-child” policy
The average number of births per woman fell from 5.4 in 1971 to 1.8 in
2001.
There is about 117 boys born for every 100 girls.
The Chinese government is now promoting a “Care for Girls” program
which awards housing subsidies and scholarships to families with
children
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At the other end of the human life cycle, sex ratio favors
females in most populations.
For people over the age of 65 in the United States there is a
ratio of 75 men for every 100 women
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Sex Determination in Humans
Figure 6.6
Figure 6.6
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Inheritance related to sex
Genes located exclusively on the x chromosome are called sex-linked
Genes that occur only on the Y chromosome can produce their effects
only in males and are called Holandric genes
The mechanisms where a given trait is limited to one sex is called sexlimited and fi dominance of a given allele depends on the sex of the
bearer is called sex-influenced.
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Y-linked Traits
Genes on the Y chromosome are said to
be Y-linked
Y-linked traits are very rare
Transmitted from male to male
No affected females
Currently, identified Y-linked traits involve
infertility and are not transmitted
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X-linked Traits ( we will do some
examples on the board
Possible genotypes
X+X+  Homozyogus wild-type female
X+Xm  Heterozygous female carrier
XmXm  Homozygous mutant female
X+Y  Hemizygous wild-type male
XmY Hemizygous mutant male
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X-linked Recessive Inheritance
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X-linked Recessive Traits
Examples:
- Ichthyosis = Deficiency of an enzyme
that removes cholesterol from skin
- Color-blindness = Inability to see red
and green colors ( only about 0.4 % are
girls)
- Hemophilia = Disorder of blood-clotting
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Figure 6.7
Figure 6.7
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Figure 6.8
Figure 6.8
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X-linked Dominant Inheritance
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X-linked Dominant Traits
Incontinentia
pigmenti
A newborn girl with IP has
yellow, pus filled
vesicles on her limbs,
the lesions become
warty and eventually
give way to brown
splotches that remain for
lifeFigure 6.9
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X-linked Dominant Traits
Congenital
generalized
hypertrichosis
Produces many extra hair
follicles, and more denser
more abundant upper body
hair. An affected man
would pass the trait to all
his daughter, but non of his
sons. Why?
Figure 6.10
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Solving Genetic Problems
Steps to follow:
1) Look at the inheritance pattern
2) Draw a pedigree
3) List genotypes and phenotypes and their
probabilities
4) Assign genotypes and phenotypes
5) Determine how alleles separate into gametes
6) Use Punnett square to determine ratios
7) Repeat for next generation
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Sex-Limited Traits
Traits that affect a structure or function
occurring only in one sex
The gene may be autosomal or X-linked
Examples:
- Beard growth
- Milk production
- Preeclampsia in pregnancy
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Sex-limited characteristics do some examples on
board
Genotype
HH
Hh
hh
male chickens
female
Hen feathered
Hen feathered
Hen feathered
Hen feathered
cock-feathered Hen feathered
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Sex-Influenced Traits
Traits in which the phenotype expressed
by a heterozygote is influenced by sex
Allele is dominant in one sex but recessive
in the other
Example:
- Pattern baldness in humans
- A heterozygous male is bald, but a
heterozygous female is not
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Sex influenced traits
Baldness: do some problems on the board
Genotype
Men
women
BB
bald
Bald
Bb
bald
not
bb
not
not
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X Inactivation
Females have two alleles for X chromosome
genes but males have only one
In mammals, X inactivation balances this
inequality and one X chromosome is
randomly inactivated in each cell
The inactivated X chromosome is called a
Barr body
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X Inactivation
X inactivation occurs early in prenatal
development
It is an example of an epigenetic change
- An inherited change that does not alter
the DNA base sequence
The XIST gene encodes an RNA that binds
to and inactivates the X chromosome
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Barr Bodies
Were discovered in
1940s in female cats but
not male
Barr body is the inactive
X chromosome in a
female somatic cell,
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Figure 6.11
Figure 6.12
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X Inactivation Animation
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Figure 2.3
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X Inactivation
A female that expresses the phenotype
corresponding to an X-linked gene is a
manifesting heterozygote
X inactivation is
obvious in
calico cats
LOOK AT KEY ON HOW TO
SOLVE THESE PROBLEMS
Figure 6.12
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X inactivation


Theoretically, X inactivation evens out the sexes for
expression of X-linked genes.
In actuality, a female may not be equivalent, in gene
expression, to a male because she has two cell populations,
where the male has only one
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X inactivation
A situation in which
being heterozygote for
an X-linked gene is
harmful is
craniofrontonasal
syndrome. Males and
homzygous females
have asymmetrical facial
features
Heterozygous females have
much more sever phenotypes
with very abnormal faces
resulting from abnormal fusing
of skull bones
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Rett Syndrome: Curious inheritance pattern
Nearly always affects females
One symptom is that the girls
do strange things with their
hands__ uncontrollably taping
objects, clapping , repeatedly
put hands to mouth and wring
their hands
This is dominant and in 99% of
cases is not passed from parent
to child but arises anew

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Genomic Imprinting
The phenotype of an individual differs
depending on the gene’s parental origin
Genes are imprinted by an epigenetic event:
DNA methylation
- Methyl (CH3) groups bind to DNA and
suppress gene expression in a pattern
determined by the individual’s sex
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Imprints are erased
during meiosis
- Then reinstituted
according to the
sex of the
individual
Figure 6.13
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Importance of Genomic Imprinting
Function of imprinting isn’t well understood,
but it may play a role in development
Research suggests that it takes two opposite
sex parents to produce a healthy embryo
- Male genome controls placenta development
- Female genome controls embryo development
Genomic imprinting may also explain
incomplete penetrance
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Imprinting and Human Disease
Two distinct syndromes result from a small
deletion in chromosome 15
- Prader-Willi syndrome
- Deletion inherited from father
- Angelman syndrome
- Deletion inherited from mother
The two syndromes may also result from
uniparental disomy
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Imprinting and Human Disease
Deletion on chromosome 15 reveals imprinting
Figure 6.16
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Turner Syndrome
Called the XO syndrome
1 in 2,500 female births
99% of affected fetuses die in utero
Features include short stature, webbing at
back of neck, incomplete sexual
development (infertile), impaired hearing
Individuals who are mosaics may have
children
No bar bodies in cells
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XYY Syndrome
Also known as Jacobs syndrome
1 in 1,000 male births
96% are phenotypically normal
Modest phenotypes may include great
height, acne, speech and reading
disabilities
Studies suggesting increase in aggressive
behaviors are not supported
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Triplo-X
Called the XXX syndrome
1 in 1,000 female births
Few modest effects on phenotype include
tallness, menstrual irregularities, and
slight impact on intelligence
X-inactivation of two X chromosomes
occurs and cells have two Barr bodies
May compensate for presence of extra X
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Klinefelter Syndrome
Called the XXY syndrome
1 in 500 male births
Phenotypes include:
- Incomplete sexual development
- Rudimentary testes and prostate
- Long limbs, large hands and feet
- Some breast tissue development
Most common cause of male infertility
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XXYY Syndrome
Likely arises due to unusual oocyte and
sperm
Associated with more severe behavioral
problems than Klinefelter syndrome
- AAD, obsessive compulsive disorder,
learning disabilities
Individuals are infertile
Treated with testosterone
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XYY Syndrome
Also known as Jacobs syndrome
1 in 1,000 male births
96% are phenotypically normal
Modest phenotypes may include great
height, acne, speech and reading
disabilities
Studies suggesting increase in aggressive
behaviors are not supported
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