Transcript Human Genetics - Castle High School

Human Genetics
Chapter 14
Quick Chromosome Review
• What are chromosomes made of?
– DNA & PROTEINS
• When are chromosomes visible?
– CELL DIVISION
• What does it mean to be diploid?
– Have two sets of chromosomes
– One from mom & one from dad!
• Which cells are not diploid? Why?
– Gametes: haploid sperm (23) haploid egg (23)
– They must combine to form a diploid
zygote (46)!
Sex Chromosomes
• Determine an individual’s gender
• Two chromosomes within the genome
• XX – Female
– All human eggs carry an X chromosome (23, X)
– X chromosome has > 1200 genes
• XY – Male
– About half of sperm cells carry an X, the other
half carry a Y (23, X or 23, Y)
– Y chromosome is much smaller and has only about
140 genes
– mostly associated with male sex
determination and sperm development
Autosomal Chromosomes
• Remaining 44 human
chromosomes
• Humans have 46 total
chromosomes
– 44 autosomal chromosomes (22
pairs)
– 2 sex chromosomes
• To correctly label a karyotype
or summarize the number of
chromosomes in a human cell:
– 46, XX (females); 46, XY (males)
Human Pedigrees
• Chart that shows relationships within a family
• Shows presence or absence of a trait and how
it is passed on through a family
• used for any species
• used to infer genotypes of family members
• can determine if allele is dominant,
recessive, autosomal, or sex-linked
• used to determine who in a family is
at risk for genetic conditions by
genetic researchers or counselors
Human Pedigree
Children are placed from oldest  youngest
Key explains what trait is!
ff
Steps:
•Identify all people who have the
trait.
Ff
•For the purpose of this class all
traits will be given to you. In
other instances, you would have
to determine whether or not the
trait is autosomal dominant,
autosomal recessive, or sexlinked.
•In this example, all those who
have the trait are homozygous
recessive.
•Can you correctly identify all
genotypes of this family?
•F- Normal
•f- cystic fibrosis
Key:
ff
ff
Ff
Ff
affected male
affected female
unaffected male
unaffected female
Pp
Pp
• PKU
– P- Unaffected
– p- phenylketonuria
PP or Pp
Pp
pp
pp
Key:
pp
Pp
affected male
unaffected male
Pp
affected female
unaffected female
hh
hh
Key:
Hh
Hh
• H-huntington’s
disease
• h-Unaffected
hh
Hh
affected male
unaffected male
Hh
hh
hh
affected female
unaffected female
Sex-Linked Inheritance
• Colorblindness
XCY
XcY
Key:
XcXc
XCXc
XCXc
affected male
unaffected male
XcY
affected female
unaffected female
XcY
Karyotypes
• To look at the human genome, biologists
photograph human chromosomes during
mitosis (usually metaphase)
• Cut them out and arrange them into a
picture called a karyotype
• Shows the complete diploid set of
chromosomes grouped together
in pairs
• Arranged in order of decreasing
size
Normal Human Male (46, XY)
Normal Human Female (46, XX)
X Chromosome Inactivation
• Females have an extra X
chromosome
• Most of genes in one of
X chromosomes turned
off
• Forms dense region in
nucleus called a Barr
body
– Same process in other
mammals
– i.e. calico cats
Human Genetic Disorders
• Review: what is a mutation?
– Change in DNA sequence
– Changes proteins by altering amino acid
sequence!
• Can directly affect the phenotype
expressed!
• Many genetic disorders are caused
by changes in an individual gene!
Sickle Cell Disease
• Caused by a defective allele for a protein in
hemoglobin
• Causes hemoglobin molecules to stick
together and form sickle shape
• More rigid and get stuck in capillaries
• Results in loss of blood flow and damage to
cells, tissues, and organs
Cystic Fibrosis
• Known as CF
• Results from deletion of just three bases in gene for a
protein called CFTR
• Protein is destroyed and doesn’t work as a transport
protein, so proper ions aren’t transported across cell
membrane
• Recessive trait – need two copies of defective allele
for CF
• Serious digestive problems and
thick, heavy mucus clogs lungs and
breathing passageways
Huntington’s Disease
• Caused by dominant
allele for a protein
found in brain cells
• Mental deterioration,
uncontrollable
movements, appears
in middle age
Genetic Advantages
• How do fatal alleles remain in the gene
pool?
• Heterozygous individuals often show
genetic advantages in certain situations
• Those heterozygous for sickle-cell are
highly resistant to malaria
• Those heterozygous for CF had an
advantage in medieval times and
were resistant to Typhoid Fever
Chromosomal Disorders
• Errors can occur in meiosis
• Homologous chromosomes can fail to separate
during Anaphase I
– Nondisjunction
• Results in gametes with an abnormal number of
chromosomes
– Trisomy = 3 copies of a chromosome
– Down syndrome = Trisomy 21
• Mild to severe mental retardation
• High frequency of birth defects
Male: 47, XY, +21
Female: 47, XX, +21
Trisomy 21
Sex Chromosome
Disorders
Klinefelter’s
Syndrome,
47 XXY
• Turner’s Syndrome
– Nondisjunction of X chromosome
– Female inherits only one X chromosome
– Results in sterility and reproductive organs do not
develop properly
• Klinefelter’s syndrome
– Males inherit extra X chromosome
– Interferes with meiosis and
usually causes sterility
• No cases of babies born without X!
– Necessary for survival of embryo
XXY
Human Genome Project
• Launched in 1990
• Main goal: sequence 3 billion base pairs of
human DNA and identify all human genes
• Completed in 2003
• Completed genomes of many other organisms
while working through human genome
• 40% of our proteins have strong similarity to
proteins in many of those organisms (fruit fly,
worms, yeast)
Human Genome Project
• Only about 2% of genome encodes instructions
for synthesis of proteins
– Large areas of chromosomes with very few genes
• Pinpointed genes and associated particular
sequences in those genes with diseases and
disorders
• Identified about three million locations where
single-base DNA differences occur in humans
• Help us find sequences associated with diabetes,
cancer, and other health problems
HGP: What now?
• Ethical, legal, social issues
– Who owns and controls genetic information?
– How does genetic privacy relate to medical privacy?
• May 2008: Genetic Information
Nondiscrimination Act
– Prohibits US insurance companies and employers from
discriminating on basis of information derived from
genetic tests
• Still don’t understand as many as 50% of the
human genes thus far discovered