Transcript Human Heredity and Sex

IV. Human Heredity & Sex-linked Disorders
A. Human Chromosomes
-Humans have 46 chromosomes
in their cells.
-Cell biologists analyze
chromosomes using karyotypes.
Karyotype – a picture of
chromosomes arranged/grouped
in order in pairs.
Sex-chromosomes vs. Autosomes
*Two of our 46 chromosomes are known as
sex chromosomes, because they determine a person’s sex.
-Females have two copies of a large X chromosome (XX)
& males have one X & one Y chromosome (XY).
-Females can only pass on an X to a child & males pass
on either X or Y, determining the sex of a baby.
-All human egg cells contain an X chromosome, while ½
of the sperm cells carry X’s & ½ carry Y’s.
-The remaining 44 chromosomes (non-sex chromosomes)
are known as autosomal chromosomes or autosomes.
-The total # of chromosomes in human cells (autosomes
& sex chromosomes) is represented as : 46,XX for females
& 46,XY for males.
Chromosomes 21 & 22
-The smallest of human
autosomes, chromosomes 21 & 22,
were the first to be studied.
-Genetic disorders on chromosome
22 are responsible for one form of
leukemia (cancer of the blood) &
neurofibromatosis, a tumor
causing disease of the nervous
system.
-Genetic disorders on chromosome
21 include amyotrophic lateral
sclerosis (ALS) or Lou Gehrig’s
disease.
B. Sex-linked Genes & Disorders
Sex-linked gene – gene
located on either the X or Y
chromosome.
-Special patterns occur on each
sex chromosome.
-Because the X chromosome is
larger, it carries more sex-linked
genes & disorders.
Types of Sex-linked Disorders
1. Colorblindness – sex-linked disorder in
which an individual can’t perceive
certain colors. It is passed to offspring
on the X chromosome.
-If an X carries the recessive allele for
colorblindness it may or may not be
expressed in a female but will be
expressed in a male.
-Males receive one X chromosome, so all
X-linked alleles are expressed even if they
are recessive.
-Sex-linked genes move from fathers to
daughters and from those daughters to
their sons.
Types of Sex-linked Disorders
2. Hemophilia – sexlinked disorder in
which the blood is
unable to clot because
it lacks a certain
protein.
-It is caused by a point
mutation & is carried on
the X chromosome, so it
usually affects males.
Types of Sex-linked Disorders
3. Duchenne Muscular Dystrophy –
sex-linked disorder that results in
the progressive weakening & loss
of skeletal muscle.
-Affected muscle tissue starts to
break down during childhood.
-Is carried on the X chromosome
& is caused by a defective
version of a gene that codes for a
muscle protein.
C. Dominant, Recessive, & Chromosomal Disorders
*All genetic info, including human disorders, is in
the Human Genome – our complete set of genetic
info.
Recessive Alleles and Disorders (pg. 345)
Recall : Recessive alleles aren’t expressed in
heterozygous individuals.
*Disorders caused by recessive alleles don’t affect a
carrier’s health but may be passed on to offspring.
Types of Recessive disorders :
1. Phenylketonuria (PKU) –
genetic disorder in which the
body lacks the enzymes to
break down the amino acid
phenylalanine (in milk & other
foods). Can be tested &
treated.
Result : Phenylalanine builds up
in the tissues causing severe
nerve damage & possible mental
retardation.
Types of Recessive disorders :
2. Tay-Sachs disease – a fatal
genetic disorder that causes
lipids to accumulate in the
brain when the body fails to
break them down.
*Appears frequently in Jewish
families of central/eastern
European ancestry.
Result : Nervous system
breakdown including : blindness,
brain damage, and eventually
death in the first few years of life.
Has no cure.
Types of Recessive disorders :
3. Cystic fibrosis (CF) – common
genetic disorder in which
the body produces excessive
secretions of thick mucus.
*Appears frequently in people
whose ancestors came from
Northern Europe.
Result : Malfunction of tissues in
the body & mucus accumulation
in the digestive tract (digestive
difficulty) & in the lungs
(difficulty breathing).
Types of Recessive disorders :
4. Albinism – genetic
disorder in which
individuals have no skin,
hair, or eye pigment.
*Occurs in ALL races.
Types of Recessive disorders :
5. Galactosemia – genetic
disorder in which
galactose (a sugar)
accumulates in tissues.
Result : Mental retardation,
eye and liver damage.
Dominant Alleles and Disorders
Recall : Dominant alleles are
always expressed in a person’s
phenotype, ex : freckles,
widow’s peak, farsightedness,
broad lips,& polydactyly
(extra/long fingers/toes).
-If passed on, dominant
disorders usually kill the
offspring before he/she is
capable of reproduction.
*For this reason, dominant
disorders are very rare.
Types of Dominant Disorders :
1. Huntington’s Disease – a
fatal genetic disorder in which
the nervous system gradually
deteriorates, especially the
brain.
Results : Progressive loss of
muscle control and mental
function until death occurs.
*Signs of the disease show
up later in life (late 30’s or 40’s).
Types of Dominant Disorders :
2. Achondroplasia – a form
of dwarfism.
3. Hypercholesterolemia –
genetic disease in which a
person has excess
cholesterol in the blood.
Result : Heart disease
Codominant Alleles and Disorders
*Codominant alleles are both expressed
at the same time.
Types of Codominant Disorders :
1. Sickle Cell Disease – genetic disease
in which one allele causes a change in
the shape of normal red blood cells.
-Affected blood cells are moon-shaped or
“sickle-shaped” instead of round.
Result : Poor blood-flow during which
the cells clump together blocking &
damaging parts of the circulatory
system. Can cause weakness, anemia,
brain damage, spleen damage, heart
damage, & even death.
What causes Sickle-cell disease?
*Sickle-cell disease results from
Pleiotropy – when a single gene
affects more than one trait.
*Is commonly found in African
Americans whose ancestors are
from west-central Africa.
*People who are heterozygous
for sickle-cell disease are
generally healthy & are
resistant to malaria.
Chromosomal Disorders
*Most chromosomal disorders
are caused by mutations.
*The most common form of
mutation that results in
chromosomal disorders is
Nondisjunction – the addition
or loss of a whole chromosome
when the chromosomes are
supposed to separate in meiosis.
Types of Chromosomal Disorders :
1. Down Syndrome
chromosomal disorder that
results from an extra copy of
chromosome number 21.
An affected person has three
number 21 chromosomes in each
cell.
Result : Mental retardation &
Physical abnormalities.
*Down Syndrome is also called
Trisomy 21, Trisomy meaning
“three bodies” or three copies.
Types of Chromosomal Disorders :
2. Turner’s syndrome – sex chromosome disorder in which females
have underdeveloped sexual characteristics.
*It is caused by the presence of only 1 sex chromosome (Monosomy),
an X, in the cells.
Allele pair = XO due to a mutation in sex cells (germ mutation).
Result : Affected females are sterile and their sex organs DON’T
develop at puberty.
Types of Chromosomal Disorders :
3. Klinefelter’s syndrome – sex chromosome disorder in which males
have underdeveloped sex organs.
*It is caused by having two X chromosomes and one Y chromosome
in each cell. Allele pair = XXY.
Result : The extra X chromosome interferes with Meiosis & usually
keeps them from reproducing.