Transcript What are Sex-Linked Traits?

Human Genetics
Review – What is a GENE?
• A gene is the unit
that controls traits
• Genes are passed
from parents to
offspring
• Genes are located
on our
chromosomes
KARYOTYPE: A SET OF PHOTOGRAPHS OF
CHROMOSOMES GROUPED IN ORDER OF PAIRS.
Karyotypes are used to detect chromosomal disorders
and to determine the sex of an unborn child
The first 44
chromosomes
(22 pairs) are
called
autosomes.
The 23rd pair of chromosomes are the sex
chromosomes which determine an individual’s sex.
XX – Female or XY - Male
At least 1 (X)
chromosome MUST
be present for
survival
The presence OR absence
of the (Y) chromosome
determines sex of
offspring
Normal Female Karyotype
This karyotype
would be labeled
as: 46,XX
Normal Male Karyotype
This karyotype
would be
labeled as:
46,XY
Chromosome Number Disorders
Nondisjunction - Failure of chromosomes to
separate during meiosis.
– Abnormal offspring
– Monosomy: one copy of a chromosome
– Trisomy: three copies of a chromosome
– Polyploidy: multiple copies of chromosomes
Nondisjunction in Anaphase I
Trisomy 21 (Down Syndrome)
Turner’s Syndrome: 45XO
Klinefelter’s Syndrome: 47XXY
CHROMOSOMAL MUTATIONS
Deletion- piece breaks off and is lost.
Duplication - piece breaks off and
attaches to a homologous
chromosome
Translocation - piece breaks off
and attaches to non-homologous
chromosome.
Inversion - piece breaks off and
reattaches upside-down.
What are Sex-Linked Traits? Trait determined by a gene
on a sex chromosome (usually X chromosome).
• Females must receive 2
affected X’s in order to
have the trait.
• Females with only 1
affected X are carriers.
• Males will always show a
recessive trait located on
the X chromosome
because he only has 1 X.
Examples:
Colorblindness, Hemophilia,
Duchenne’s Muscular Dystrophy
Sex-Linked Disorders
• Genotypes for sex-linked traits are written using
the X and Y chromosomes to show path of
inheritance.
C = normal vision c = colorblindness
Colorblind male = XcY
Colorblind female = XcXc
Carrier female = XCXc
**Remember males CANNOT be carriers**
Colorblindness
Sex-linked traits: Color blind father x carrier mother
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Sex-Linked Punnett Squares
• In sex-linked traits, probabilities for male and
female offspring must be calculated separately
because traits are inherited differently.
Sex-Linked Punnett Square Practice
1.
A colorblind female marries a man with normal vision.
What is the probability of them having colorblind
children?
Key: ________________________________________
Cross: _______________________________________
Probability of having a colorblind daughter = ___________
Probability of having a colorblind son = ____________
Sex-Linked Punnett Square Practice
2. A genetics counselor interviews a couple with a family history of
hemophilia to evaluate the possibility of having offspring with the
disorder. The wife does not have hemophilia, but states that her father
had the disorder. The husband is normal.
Key: ________________________________________
Cross: _______________________________________
Probability of having a daughter with hemophilia = ___________
Probability of having a son with hemophilia = ____________
Predictions and People
• What is a Pedigree - chart that shows how
a trait and the genes that control it are
inherited within a family.
How to Read a Pedigree
Circle = female
Square = male
Horizontal lines connect
siblings or spouses
Shaded figure = Individuals
who are affected by the trait
Non-shaded = individuals who
are unaffected
***Carriers are half shaded***
Vertical lines connect
parents and children
**When illustrated in a pedigree, children are placed in
birth order from left to right (Oldest to youngest).
• What is a Carrier? - an individual who
carries a recessive trait (heterozygous) that
is not expressed and can pass on genetic
disorders to offspring.
• Has normal phenotype
Practice
The following pedigree shows the
inheritance of a recessive trait.
1. How many generations are
shown?
2. How many children did
Parents of Generation 1
have?
How many were boys?
How many were girls?
3. How many children did
Parents of Generation II
have?
How many were boys?
How many were girls?
4. Use the key to determine
the genotypes of each
person in this pedigree.
(Hint: label all homozygous
Key: R = Non-affected; r = affected
recessives first)
I Generation
1
2
II Generation
3
5
4
6
III Generation
7
8
9
10
Widow’s Peak is dominant over no widow’s peak
W = dominant
w = recessive
Ww
ww
Many genetic disorders caused by
AUTOSOMAL RECESSIVE GENES.
Examples:
PKU (an inability to break down the amino acid
Phenylalanine (can cause mental retardation)
Tay-Sachs (results in a nervous system
breakdown and death)
ALBINISM - a lack of pigment in hair, skin and eyes.
CYSTIC FIBROSIS - caused by a recessive allele on
chromosome #7 - causes excess mucous in lungs
and digestive tract, increased susceptibility to
infections.
Tay-Sachs Recessive Trait Disorder
T = normal
t = tay-sachs
tt
Tt
Tt
Tt
Tt
Tt
TT
Autosomal Dominant Disorders
• Dominant allele disorders are less common
(typically a homozygous person (HH) dies
therefore the trait can't carry on to offspring).
• Ex. Huntington Disease
Huntington’s Disease - dominance (H)
hh
Hh
Hh
hh
Affected
Normal
a. No symptoms until 30’s or 40’s
b. Nerve damage: loss of muscle
control & mental function until death
Huntington’s Disease (dominant trait disorder)
H = affected w/disease
h = normal
Hh
hh
Hh
hh
Hh
Hh
HH