Transcript Human Genetics - Northwest Allen County Schools

Human
Genetics
Chapter 14
Human Chromosomes
How many kinds of chromosomes do humans
have?
23
How many chromosomes do humans have?
46 (23 from each parent)
What are the sex chromosomes?
The two chromosomes that determine your
sex (X & Y; females = XX; males = XY) The X
is much larger (X has 1200 genes; Y has
about 140).
Human Chromosomes
What are autosomes (autosomal chromosomes)?
The other 22 chromosomes that are not sex
chromosomes. (they are named chromosomes 1,
2, 3 . . . 22)
The sex chromosomes are the 23rd pair.
Human Chromosomes
What is a karyotype?
1. An arrangement of pictures of
chromosomes taken during metaphase.
This picture is called a metaphase spread.
2. Chromosomes are arranged based on size,
centromere location, and banding
patterns. (The banding was the inspiration
for bar codes.)
3. Karyotypes are used to diagnose genetic
disorders based on chromosomal number
and for sex determination.
Human Chromosomes
Metaphase Spread
Karyotype of a normal human male.
46,XY
Human Pedigree
What is a pedigree?
It is a chart that traces a Mendelian trait through a
family.
Examples of Autosomal Dominant
Traits:
1. Dwarfism
2. Progeria
3. Polydactyly and Syndactyly
Examples of Autosomal
Recessive Traits:
1. Albinism
2. Cystic Fibrosis
3. Phenylketoneuria (PKU)
Human Blood Types
ABO blood types are traits inherited by
multiple alleles that show both
codominance & complete dominance.
Rh blood group determines the (+) or (–) part
of the blood type. (+) is completely
dominant over the (–).
Human Autosomal Disorders
Cystic Fibrosis
1.
2.
3.
Caused by a recessive allele on chromosome 7 that
affects a chloride ion channel in the plasma/cell
membrane. The CFTR protein folds improperly.
Causes the production of a thick mucus that
interferes with digestion & breathing.
Allele is found in populations originating from
Northern Europe. The heterozygous people during the
Middle Ages had an evolutionary advantage against
typhoid fever.
Sickle Cell Disease
1.
2.
3.
4.
5.
Found in people of Sub-Saharan African
descent.
The allele is the result of a point mutation that
changes just one amino acid in hemoglobin.
The affected hemoglobin causes the red
blood cells to be misshapen when O2 levels
drop.
The allele shows codominance (sickle betaglobin gene along with the normal beta-globin
gene). Homozygous recessive people are
most affected, heterozygous are partially
affected, and homozygous dominant are not
affected.
The allele is relatively common because the
heterozygous people have resistance to
malaria.
Sickle Cell Disease
Human Chromosomes
1.
2.
3.
Each diploid cell has 6 billion base pairs.
Only 2% actually codes for proteins.
The average gene is 3000 base pairs; the longest is
2 million base pairs.
PS-1, PS-2, APOE, & APP
are all genes involved in
Alzheimer’s Disease
Sex-Linked Traits
These are traits whose genes are found on one of the
sex chromosomes.
1.
Most of these genes are found on the X
chromosome because it is larger.
2.
X-linked traits have a tendency to show up more
in males because they only have one X
chromosome.
Sex-Linked Traits
1.
2.
3.
Red/Green
Colorblindness:
X-linked recessive
allele
Can’t see the colors
red and green very
well.
Most common in
males.
Sex-Linked Traits
Hemophilia:
1. X-linked
recessive
2. One of the
proteins that is
involved in the
clotting process
is defective.
3. Once known as
the “royal
disease”
because it was
common in the
royal families of
Europe.
XH
Xh
XH
XHXH
XHXh
Y
XHY
XhY
Sex-Linked Traits
Duchenne Muscular
Dystrophy:
1.
X-linked recessive
2.
Causes progressive
weakening & loss of
skeletal muscle.
3.
Affects 1 out of 3000
American males
X-Chromosome Inactivation
1.
2.
3.
4.
In females, one of the X
chromosomes is randomly
inactivated.
This process is called
Lyonization after Mary
Lyon, the scientist who
discovered it.
The inactive X
chromosome forms a
dense region in the
nucleus called a Barr
body.
This phenomenon can
cause interesting traits like
the calico color pattern in
cats. The genes for the
black and orange color
are on the X chromosome.
Nondisjunction
1.
2.
3.
4.
Sometimes during meiosis, the homologous
chromosomes fail to separate.
This causes the gametes to have more or less than
23 chromosomes.
Down Syndrome is caused by nondisjunction of
chromosome 21.
One gamete had two copies of chromosome 21.
The resulting child has 3 copies of chromosome 21
(trisomy 21)
Nondisjunction
Nondisjunction can also occur with the sex
chromosomes.
A. Turner’s Syndrome (monosomy X)
- 45,X (1 in 2500 females)
- Mentally normal, typically short, webbed
neck, sterile, underdeveloped sex organs &
2o sex characteristics, heart problems.
B. Klinefelter’s Syndrome (47, XXY)
- 1 in 1000 males
- Some cognitive problems, tall & slim, some
female 2o sex characteristics, sterile.