Download Chromosomal

Chapter 12
Inheritance Patterns and Human
Genetics
12-1
Chromosomes and Inheritance
Remember…
• DNA in
chromosomes has
instructions for
protein synthesis
• Chromosomes are
transmitted from
one generation to
the next
Sex Determination
• 1900s: Thomas Hunt Morgan bred Drosophila
melanogaster (fruit flies)
• Determined:
– Females: XX
– Males: XY
• “Y” chromosome is
smaller and
hook-shaped
• Autosome: all
other chromosomes
• Sex chromosomes segregate (form pairs)
during meiosis
• SO! Each sex cell either has an “X” or a “Y”
• Male gametes: can
contain “X” OR “Y”
• Female gametes: ONLY “X”
Why????
Sex Linkage
• Morgan also hypothesized that more genes
are carried by X chromosome than the Y
• X-linked: genes carried on
the X chromosome
• Y-linked: genes found on the
Y chromosome
• Sex linkage: presence of gene
on sex chromosome
Sex Linked in Drosophila
• Normal: Red eyes
• Mutation: WHITE eyes
• P1 x P1  F1
– Red eye female x White
eye male ALL RED
EYES
• F1 x F1  F2
– Red female x red male
 3(red):1(white)
– HOWEVER! All white
eyes on MALES ONLY
No White Eyed Females
• Trait for eye color is carried on X
• Red eye female: XR XR
• White eye male: Xr Y
Linkage Group
• Thousands more genes than chromosomes 
many genes on same chromosome
• Linkage group: multiple genes on same
chromosome
• Linked genes tend to
be inherited together
Why?
Linked Groups in Drosophila
• Morgan looked at fly body color and wing size
– Gray body, Long wing x Black body, Short wing
– BBLL x bbll
• If on separate
chromosomes:
– 9:3:3:1
• If on the same
chromosome:
– 3:1
So, what actually happened?
• Results:
– several Gray, long (Ggll)
– Several black, short (bbLl)
• HOW????
Crossing Over
• Exchange of pieces of
DNA between
homologous
chromosomes
• Accounts for
unexpected
phenotypes in the F2
generation of
Morgan’s experiment
Chromosome Mapping
• Likelihood of genes crossing over depends on
distance from each other on chromosome
• Farther apart the genes, the more likely that
they will cross
• If more offspring show
the new combination
of traits, the farther
the genes are on a
chromosome
• Scientist conduct breeding
experiments to determine
how frequently genes
separate from one another
• THUS! They measure
distance of genes on
chromosomes
• THUS! They prepare
chromosome maps:
diagram used to show
linear sequence of gene on
chromosome
Alfred H. Sturtevant
• Morgan’s student
• Used crossing over data to construct
chromosome map of Drosophila
• Genes that are separated
by crossing over 1% of
the time are 1 map unit
apart
Mutation
• MANY Different types:
– Chromosomal: Whole chromosome
– Genetic: Single nucleotide
• Germ-cell mutations: in gametes
– Do NOT affect organism
– Affect organism’s offspring
• Somatic mutations: in body
cells
– affect organism (skin cancer,
leukemia)
– NOT passed on to offspring
Chromosome Mutations
• Deletion: loss of a piece of
chromosome due to chromosomal
breakage
– All info of that chromosome lost
• Inversion: piece of chromosome
broken off and reattached upsidedown
• Translocation: piece of
chromosome breaks off and
attaches to another,
nonhomologous chromosome
• Nondisjunction: failure of chromosome to
separate from homologue during meiosis
– Result: one gamete receives EXTRA COPY of a
chromosome
Gene Mutations
• Point mutation: single based mutated
– Substitution, deletion, insertion
• Substitutions: single nucleotide replaced with a
different nucleotide ( ATC TTC)
* REMEMBER: DNA mRNA  amino acid
• Results:
1- base codes for same A.A.
 NO CHANGE
2- base codes for different
A.A.  Protein change
3- base code for STOP
codon  Protein change
Sickle Cell Anemia
• Point substitution mutation
• Substitutes adenine for
thymine
• Protein changed: hemoglobin
• Result: irregular shaped red
blood cells
– Circulatory problems: heart,
brain, lungs and many other
organs and tissues damaged
– Widespread in African
Americans ( 1 of 500 in US)
• In US 1 of every 10
African Americans are
heterozygous for sickle
cell anemia
• Heterozygous: produced
both normal and
mutated forms of
hemoglobin
– Usually healthy
Frame Shift Mutations
• Deletion: one or more nucleotide deleted
from sequence
• Insertion: one or more nucleotides inserted
• Frame shift mutation: one or more nucleotide
deleted or inserted
• MUCH more serious effects
• Entire rest of sequence
mutated
• More DNA mutated if
deletion/insertion occurs
closer to beginning of
sequence
– More amino acids
inaccurately coded for
12-2 Human Genetics
Quick Look:
• 23 pairs of
chromosomes
• 100,000 genes
• Scientists focus on
disease causing
genes
– Easily traced from
one generation to
the next
Pedigree Analysis
• Pedigree: a family
record that shows
how a trait is inherited
over several
generations
• Study phenotypes of
family to see how
traits are inherited
Patterns of Inheritance
• Certain phenotypes usually repeat in
predictable patterns
• Carriers: individuals who
have one recessive
autosomal allele
Genetic Disorders
• Diseases or condition that has a genetic basis
Single-allele Traits
• Controlled by
single allele of
a gene
• 200+
Huntington’s Disease (HD)
*“Autosomal-dominant pattern of inheritance”
– Caused by dominant allele on autosome
• Symptoms: mild forgetfulness; irritability
• Results: loss of muscle control; uncontrollable
physical spasms;
severe mental illness;
death
• On-set: 40’s or later
*Usually unknown until
after individuals have children
*THUS! Usually passed
on without knowing
Genetic Marker
• Short section of DNA that is known to have
close association with particular gene
• If marker is present, USUALLY gene is present
• Sampling can show presence of gene
(disorder)
– Marker present:
96% chance of HD
Homozygous Recessive Single-allele
Traits
•
•
•
•
Expressed ONLY in homozygous state (aa)
Show autosomal-recessive patt. of inherit.
250+
Ex: Cystic fibrosis (CF)
and sickle cell anemia
Multiple-allele Traits
• Controlled by three or more alleles of the
same gene that code for a single trait
• Ex: ABO blood groups
– Three alleles: IA, IB, i
– Codominant: IA, IB
Polygenic Traits
• Traits controlled by two or more
genes
• Show many degrees of variation
• Skin color; 3-6 genes
• Eye color;
– Light blue: very little melanin
– Dark brown: a lot of melanin
• Many influenced by
environment
– Height: also influenced by
nutrition and disease
X-Linked Traits
• Not all are diseases
• Ex: Colorblindness, Hemophilia
Sex-Influenced Traits
• When presence of female or male hormones
influence expression of certain human traits
• Ex: Pattern Baldness
– Male and female: BB  lose hair
– Male: BB’  lose hair due to high testosterone
– Female: B’  does NOT lose hair
• Genes that code for trait
on autosomes NOT gametes!
Due to Nondisjunction
• Lack a chromosome OR have an extra
• Monosomy: only one copy of a chromosome
– 45 instead of 46
• Trisomy: an extra copy of a chromosome
– 47 instead of 46
Down Syndrome
• AKA Trisomy-21
• Extra chromo. 21
• Severe mental
retardation,
characteristic facial
features, muscle
weakness, heart
defects, short
stature
Nondisjunction in Sex Chromosome
• Klinefelters syndrome: XXY
– Female appearance; mental retardation; infertile
• Turner’s syndrome: XO
– Female; do not mature sexually; infertile
• Just Y CANNOT
survive
• X contains info
vital to survival
Detecting Human Genetic Disorders
• Family history of genetic disorders  Genetic
screening before becoming a parent
– Examination of genetic makeup
• Make karyotype
• Test blood for certain proteins
• Genetic counseling:
medical guidance to inform
parents about problems
that could affect offspring
Amniocentesis
• Physician removes small amount of amniotic
fluid from amnion (sac that surrounds fetus)
– Fetal cells and proteins analyzed
– Karyotype prepared
Chronic villi Sampling
• Physician takes sample of chronic villi (tissue
that grow between mother’s uterus and
placenta)
– Produce karyotype
• Has same genetic make
up as fetus
Sample Taken After Birth
• Immediately after birth genetic testing done
of fetus
• Ex: PKU (phenylketonuria)
– Disorder when infant cannot metabolize the
amino acid phenylalanine
– Excess
phenylalanine can
cause brain damage
– Infants put on diet
without
phenylalanine