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Chapter 9 - Patterns of Inheritance
I. Genetics – the science of heredity
A. Selective breeding
1. pitbull/rottweiler vs. daschund
2. dogs from wolves – 14000 years ago
a) border collie – selected for by herders to control
flocks of animals
b) Labrador retriever – selected by hunters, good at
retrieving wounded prey – less aggressive – don’t
want dogs to eat the kill!
B. Genetics and the Environment – an intimate
collaboration
1. friendly rottweiler
2. shyness in humans has a genetic component – can be
amplified or reduced by environment.
a) ex. Tom Hanks
3. [READ] Avshalom Caspi and Terrie Moffitt
[interview with Moffitt here on npr] made
quite a splash in 2002 when they published the
paper “Role of Genotype in the Cycle of
Violence in Maltreated Children” in Science.
They reported that maltreated children would
differ in the development of antisocial
personality and violent behaviour depending
upon whether or not their genotype conferred
high or low levels of MAOA expression, a
neurotransmitter-metabolizing enzyme. Thus,
Caspi and Moffitt showed that a genetic
variation may moderate the influence of
environmental factors on behaviour in a rather
dramatic manner, fueling the growing suspicion
that the old nature/nurture dichotomy is much
too simplistic. Behaviour is most probably not
determined by either an innate genetic Bauplan
or the ever changing forces of our
surroundings. In Caspi and Moffitt’s study, at
least, children with a low-level MAOA genotype
only developed an antisocial personality if
maltreated (if you happen not to be
maltreated, a low-level MAOA polymorphism will
not cause you to develop an antisocial
personality); but, at the same time,
maltreatment doesn’t affect children with a
high-level MAOA polymorphism, so the
maltreatment is not a cause in itself either.
Genes and environmental factors interact to
produce behaviour, and the real question is
how they do so.
C. What we will exam in this chapter
1. rules that govern how inherited characteristics are
passed from parent to offspring
2. how to predict the ratio of offspring with particular
traits
3. how the behavior of chromosomes during gamete
formation (meiosis) and fertilization accounts for the
patterns of inheritance we observe.
II. History
A. Pangenesis - Hippocrates – ancient Greece 400BC
1. Pangenesis – particles (pangenes) travel from each
part of the organism’s body to the gametes, changes that
occur in the body during life are passed as well (work out
and get big muscles, you pass the muscles on…lol)
a) long been falsified, somatic cells have zero
influence over gametes
2. this idea of passing traits acquired over ones lifetime
persisted into the early 19th century!!
B. Blending hypothesis – early 19th century biologists used
ornamental plants
1. established that offspring inherit traits from both
parents
2. hereditary materials mix like mixing blue and yellow
paints to make green.
a) ex. pure black lab breeds with pure chocolate lab,
offspring will be somewhere b/w black and chocolate
and you can never get the original colors.
b) Falsified – what happens is you get all black and if
you mate the black, you get some black, but brown
can return!
Mendel’s Principles
III.Experimental genetics began in an abbey garden
A. Modern genetics began in the 1860’s (1856-63) with an
Augustinian monk by the name of Gregor Mendel.
B. Mendel was university trained in experimental
technique! – he was a physics teacher!
C. 3 keys to Mendel’s Success
1. Choice of organism:
a) He studied peas for their advantages:
(1) They grow easily and were readily available in
many varieties
(2) Have short life cycles (1 year) – annual plant
2. Experimental approach:
a) If you want to do hereditary experiments you need
to be able to control mating (you need to know who
the parents are). So how did Mendel control this?
b) Self fertilization vs. cross fertilization in plants
(1) Self-fertilization – sperm carrying pollen released
from stamens land on the tip of the egg containing
carpel of the same flower – naturally done by pea
plants (pedals enclose stamen and carpel)
(a) He controlled self-fertilization by simply covering
flower with a small bag.
(2) Cross-fertilization – fertilization of one plant by
pollen from a different plant.
(a) He prevented self-fertilization by cutting off the
stamens of immature flowers
(b) Dusted this female plant with pollen from
another plant
(c) Carpel will develop into a pod, containing seeds
(d) He planted these seeds and allowed them to grow
3. Selection of characteristics to study
a) Seven characteristics, each of which has two
distinct forms.
b) Each characteristic was defined by a single gene –
he didn’t know this, he was lucky
c) If you want to understand how something works,
you don’t’ pick the most complicated form like a
Ferrari. You pick the simplest form like a lawn
mower engine to learn the basic principles first.
D. Mendel needed a starting point. He needed true
breeding plants.
1. True breeds – plants that when self fertilized produced
offspring identical to the parent in the desired trait.
a) Ex. if you self fertilized a plant with yellow peas,
all the offspring would have yellow peas all the time.
E. Now Mendel was ready to ask what would happen if he
crossed the different varieties? (ex. Crossed purple flower
plants with white flower plants)
1. Hybrids – the offspring of two different varieties
(offspring of the purple-white flower cross)
2. Cross-fertilization = Cross or hybridization
3. P generation – the parental plants (P for parental)
4. F1 generation – the hybrid offspring of the P generation
(F for filial – Latin for “son”).
5. F2 generation – (filial 2) offspring of self-fertilization or
crossing of F1 generation.
F. Published his work in 1866 arguing for discrete,
heritable factors (genes) that retain their individuality when
transmitted from generation to generation.
IV. Mendel’s principle of segregation describes the
inheritance of a single characteristic (AIM: How can the
inheritance of a single characteristic be described?)
A. Monohybrid cross – parents differ in only one
characteristics (purple flower x white flower with all other
characteristics the same).
1. Results – Out of 929 F2 offspring, 705 (~3/4) were
purple and 224 (~1/4) were white.
2. The same was observed for all seven characteristics
3. There is no blending! Heritable factors (today called
genes) retain their individuality generation after
generation.
B. Four hypothesis were developed by Mendel:
1. There are alternative forms of genes called alleles
2. An organism has 2 genes for each inherited
characteristic, one from each parent – They may be the
same allele or different alleles
3. A sperm or egg carries only one allele for each
inherited trait, because allele pairs separate (segregate)
from each other during the production of gametes
4. When the two genes of a pair are different alleles and
one is fully expressed while the other has no noticeable
effect on the organism’s appearance, the alleles are called
the DOMINANT allele and the RECESSIVE allele,
respectively
a) Uppercase letters represent dominant alleles
b) Lowercase letters represent recessive alleles
P Plants
Gametes
c) Conventions for alleles: P, the dominant (purple)
allele, and p, the recessive (white) allele. P generation:
PPx pp; their gametes P and p; F1 generation – All Pp
PP
X
pp


All P
All p

F1 plants
(hybrids)
Gametes
All Pp


½P
½p



F2 Plants
*Punnet Square of PP X pp
Phenotypic ratio  3:1
Genotypic ration  1:2:1
d) Gentoype = genetic makeup
(1) Homozygous dominant - PP
(2) Homozygous recessive - pp
(3) Heterozygous - Pp
e) Phenotypes – what we see in the organism –
physical appearance – ex. purple flower or white
flower
f) Punnet Square – used to keep track of the gametes
(sides of the square) and offspring (cells within the
square) – shows possible combinations of gametes.
C. How can the disappearance of a trait in one generation,
then reappear the following generation be explained?
a) PRINCIPLE of SEGREGRATION  pairs of
genes segregate (separate) during gamete formation;
the fusion of gametes at fertilization pairs genes once
again
V. Homologous chromosomes bare the two alleles for
each characteristic
A. Alleles – alternative forms of genes – reside at the same
locus (location; loci is plural) on homologous
chromosomes
VI. The principle of independent assortment is
revealed by tracking two characteristics at once
A. Dihybrid cross – the mating of parents differing in two
characteristics
B. Question: Are traits passed as a package or
independently? Do white flowers always go with yellow
peas? – Go over experiment
1. Breed two strains true – ex. RRYY (round yellow) and
rryy (wrinkled green)
2. Hybridize the two strains resulting in F1: RrYy
3. Allow F1 to self-fertilize
a) Four possible gametes: RY, Ry, rY, ry if not linked
42 = 16 genotypic possibilities, only two if linked RY
and ry 22 = 4 genotypic possibilities. So what is
observed?
4. Results:
a) F1 exhibits only dominant phenotype (expected)
b) F2 has a phenotype ratio of 9:3:3:1 (round yellow:
round green: wrinkled yellow: wrinkled green)
c) Use Punnett square to analyze results
5. Conclusion – Independent Assortment (most often)
C. Principle of independent assortment – each pair of
alleles segregates independently during gamete formation.
VII. Geneticists use the testcross to determine unknown
genotypes
A. test cross – crossing an unknown genotype (expressing
the dominant phenotype – is it homo- or heterzygous) with
the recessive phenotype.
1. If homozygous dominant – phenotype ratio in F1 is all
dominant
2. If heterozygous – F1 phenotype ratio is half dominant
and half recessive
VIII. Mendel’s principles reflect the rules of probability
A. Mendel had a strong background in Math
B. Probability scale ranges from 0 to 1
1. 0 representing the chance an event will NOT occur
2. 1 presenting that an event WILL occur
3. All probabilities must add up to 1
a) Ex. flip a coin – chance of heads is .5, and chance
of tails is .5 = 1
b) Drawing card. Chance of drawing an Ace of
spades? Chance of drawing any other card?
C. Independent events – an event that does not influence
the outcome of a later event
1. coin flipping
a) 1st flip does not influence 2nd – still ½ chance of
getting heads.
D. Flip a coin twice (compound event). What is the
probability of getting heads both times?
1. Rule of multiplication – probability of 2 events
occurring together is the product of the probabilities of
the 2 events occurring apart
2. Toss two coins at the same time – chance of getting
two heads is ½ * ½ , same as if they were separated by
time.
3. Getting a kina of hearts and then a queen of spades?
1/52 x 1/52
E. Rule of multiplication in genetics
1. Bb x Bb
2. What is the probability of getting a bb genotype?
a) What is probability of getting sperm with b gene?
b) What is probability of getting egg with b gene?
c) Multiply the two independent events and you get ¼
F. Rule of addition - the probability that an event can occur
in two or more ways is the sum of the probability of each
event
1. probability of getting a jack of hearts or a queen of
spades? 1/52 + 1/52
2. What is the probability of getting a Bb genotype?
a) Probability of getting a Bb is ¼ and probability of
getting a bB is ¼. Probability of getting either is ¼ +
¼ = ½.
G. Applying these rules allows us to predict probabilities
for very complex crosses (like trihybrid) that would require
too complex a Punnet Square
1. Trihybrid cross
a) AaBbCc x AaBbCc
b) What is the probability of getting aabbcc
(1) Look at them each independently
(2) odd of getting aa = ¼, bb = ¼, cc = ¼ and ¼ x ¼ x
¼ = 1/64
c) We could also make a 64-section Punnett square
2. Example 1:
a) Probability of a recessive phenotype occurring in a
monohybrid cross (PP x pp) is ¼.
b) The probability of two recessives occurring
together in a dihybrid cross (RRYY x rryy and
getting rryy) is ¼ * ¼ = 1/16.
c) What about a trihybrid? ¼ * ¼ * ¼ = 1/64.
IX. Genetic traits in humans can be tracked through
family pedigrees
A. Mendel’s principles apply to many human traits
1. Fig. 9.8A shows some simple dominant-recessive traits
at one gene locus
2. A dominant trait does NOT mean that it is normal or
more common than a recessive one.
a) wild-type traits (trait prevailing in nature) are not
always dominant traits.
b) recessive is often more common than dominant
B. How do we know how particular human traits are
inherited?
1. We can’t cross humans like peas and dogs! Can’t
control mating.
2. Must analyze the results of natural mating
a) collect family history of trait
b) assemble info into a family tree or pedigree (a
visual family history of a trait).
(1) squares are male and circles are female
(2) colored symbols indicate an affected individual
C. Carriers – people with one copy of the allele for a
recessive disorder, symptomless
X. Many inherited disorders in humans are controlled by a single
gene
A. >1000 known human genetic disorders inherited as
dominant or recessive traits controlled by a single gene
locus
1. Recessive disorders
a) cystic fibrosis
(1) autosomal recessive – must have two copies of
recessive allele
(2) carried by 1 in 25 Caucasians
(3) 1 in 1800 Caucasians affected
(4) excessive secretion of thick mucus from lungs,
pancreas and other organs
(a) interferes with breathing, digestion, liver function
and renders person vulnerable to pneumonia and
other infections.
(5) untreated, most die by 5 years old
(6) diet, antibiotics, frequent pounding of chest and
back to clear lungs can prolong life to adulthood.
b) Taboos or laws forbidding marriages b/w close
relatives
2. dominant disorders
a) achondroplasia
(1) 1 in 25,000
(2) homozygous dominant causes death of the embryo
(3) Thus 99.99% of population are homozygous
recessive
b) What about lethal dominant alleles? Would you
expect them to be common?
c) Huntington’s disease
(1) a lethal dominant that escapes elimination – does
not cause death until beyond reproductive age.
(2) causes uncontrollable movements in all parts of
body, brain cell loss leading to loss of memory and
judgement…depression. Loss of motor skills prevents
swallowing and speaking.
XI. Fetal testing can spot many inherited disorders
early in pregnancy
A. Amniocentesis – extract ~10ml of amniotic fluid from
preganant woman between 14 and 20 weeks (fetus is 6
inches long)
1. the fluid part is immediately tested for certain telltale
chemicals
2. the cells are cultured, allowing them to undergo cell
division to a sufficient number to do
a) biochemical tests like DNA testing or appearance
of specfic proteins, etc…
b) karyotyping (need cell division)
3. 1% complication rate (maternal bleeding, miscarriage,
premature birth)
B. Chronic villus sampling (CVS) – narrow tube inserted
through vagina and cervix used to suction off a small
amount of fetal tissue (chorionic villi) from the placenta –
1. cells are undergoing rapid cell divisiosn – perfect for
karyotyping
2. advantages over amnio
a) done earlier - 10-12 weeks
b) fast, only takes a few hours to get results
3. can’t test for everything an amnio tests for
4. 2% complication rate
C. Ultrasound imaging – use sound waves to look for
anatomical deformities.
1. can be used in combo with CVS or amnio to determine
position of fetus and where to insert needle.
D. Maternal blood tests
1. help identify fetuses at risk for further testing like
amnio.
2. look for alpha-fetoprotein (AFP) – protein produced by
fetus
a) high levels may indicate Down syndrome or neural
tube defects
3. Triple screen test
a) measures AFP, estriol, and human chorionic
gonadotropin (hCG) - hormones produced by
placenta
b) abnormal levels also point towards Down
syndrome
E. Summary – family history, blood tests, genetic
counseling and fetal testing
Variations on Mendel’s Principles
XII. The relationship of genotype to phenotype is rarely
simple
A. Mendel’s principles work for some traits, but most are
inherited in ways that follow more complex patterns –
these complex patterns are extensions of Mendel’s rules,
not exceptions.
XIII. Incomplete dominance results in intermediate
phenotypes
A. Incomplete dominance – F1 hybrids have a phenotype in
between that of the parental varieties
B. One allele is not completely dominant over the other
XIV. Many genes have more than two alleles in the
population
A. Example : ABO blood groups in humans
B. Individuals have only two alleles per trait, but there are
three possibilities here.
C. These alleles code for the presence of two different
carbohydrates (A or B) or no carbohydrate (O) on the
surface of RBC’s.
Phenotype
Genotypes
O
Ii
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
D. Codominance – both alleles are expressed in
heterzygotes (AB blood)
E. Relevance in transfusions
1. Type O = universal donor
2. Type AB = universal acceptor
F. Blood type can be quick tool to disprove or suggest
parentage in paternity suits (DNA is MUCH better!)
XV. A single gene may affect many phenotypic
characteristics
A. Pleiotropy – impact of a single gene on more than one
characteristic
1. Sickle cell anemia
a) (strikes 1 in 500 African American children born
in US annually) –
b) 100,000 a year die from it in world
2. homozygous for sickle cell allele  results in
abnormal hemoglobin  results in sickle-shaped red
blood cells  breakdown of RBC’s, clogging of small
blood vessels, accumulation of sickle cells in spleen,
etc… (Fig. 9.14)
3. results in physical weakness, anemia, pain and fever,
heartfailure, brain damage, spleen damage, damage to
other organs, kidney failure, etc…
XVI. Genetic testing can detect disease causing alleles
A. CARRIER TESTING  used to determine if a person
carries a harmful allele
B. DIAGNOSTIC TESTING  can confirm or rule out an
existing disorder
C. PRENATAL TESTING  checks for disorders in unborn
babies
D. NEWBORN SCREENING  can catch disorders right
after birth; allowing infants to receive medical care
E. PREDICTIVE TESTING  used to determine a person’s
risk for developing on specific disorder on the future
XVII. A single characteristic may be influenced by many
genes
A. Known as polygenetic inheritance – additive effect of
two or more genes on a single phenotypic characteristic
B. Ex. Skin Color, Height
The chromosomal basis of inheritance
XVIII.
Chromosome behavior accounts for Mendel’s
principles
A. chromosome theory of inheritance – genes are on
chromosomes and thus the behavior of chromosomes
during meiosis and fertilization accounts for observed
inheritance patterns.
B. Mendel knew NOTHING about genes and chromosomes
C. Principle of Segregation – homologous pairs of
chromosomes accounts for this
D. Principle of independent assortment – accounted for by
the fact that there are several sets of homologous
chromosomes – Mendel’s 7 garden pea traits all separated
independently because they were all on different
chromosomes.
E. What if Mendel’s traits were on the same chromosome?
XIX. Genes on the same chromosome tend to be
inherited together
A. Linked genes – genes located on the same chromosome
B. Inheritance of these traits does not follow principles of
independent assortment – they are normally inherited
together
C. So how might nature unlink linked genes?
XX. Crossing over produces new combinations of
alleles
A. In the case of many linked genes, there are some
offspring that appear to unlink the genes and follow
independent assortment
B. These situation are accounted for by crossing over,
which recombines linked genes into assortments of alleles
not found in parents
C. Recombination frequency – the percentage of offspring
that are recombinants (having a genotype not found in
either parent)
D. Early recombination experiments were demonstrated in
fruit flies by embryologist T. H. Morgan and colleagues in
the early 1900’s.
XXI. Geneticists use crossover data to map genes
A. Drosophila melanogaster – the fruit fly – model
organism that has greatly aided in our understanding of
genetics (the modern day pea plant).
1. Many characteristics, short life cycle, easily raised and
bred, only 4 chromosomes, chromosomes can easily be
visualized in nondividing cells in the salivary glands.
B. A. H. Sturtevant (colleague of Morgan) developed a
technique for using crossover data to map the location of
genes on chromosomes on which they are linked
C. The greater distance between two genes on the same
chromosome, the more likely it is that a crossover event
will occur between those genes
D. Thus, one can use recombination frequency to
ESTIMATE the location of genes on a chromosome relative
to other genes.
Chromosomes and Sex-linked genes
XXII. Chromosomes determine sex in many species
A. Sex chromosomes – many animals (humans, fruit flies,
others) have a pair of chromosomes that determine their
gender – designated X and Y – The X-Y system
1. Eggs are all X, sperm (X or Y) determines sex
2. XY – male
3. XX – female
B. Other sex determination systems exist:
1. X-O system – grasshoppers, crickets, roaches:
a) females – XX and
b) males – XO (O = absence of sex chromosome)
(1) egg is always X, sperm can be X or O
2. Z-W system (opposite of XY system) – certain fish,
butterflies, and birds –
a) males ZZ
b) females are ZW
(1) egg has either Z or W, sperm are all Z
3. Chromosome number – most ants and bees –
a) females develop from fertilized eggs
(1) and are thus diploid.
b) Males develop from non-fertilized eggs (no fathers)
(1) and are thus haploid.
4. Many plants have separate sexes – male and female
flowers on different organisms
a) spinach, marijuana, and others – X-Y system
b) wild strawberries – Z-W system
C. Not all organisms have separate sexes
1. Most plant and some animal species have individuals
that produce both sperm and eggs.
a) Monoecious - plants of this type (pea plants!)
b) Hermaphroditic – animals of this type –
earthworms and garden snails
D. Many other exotic sex determination systems exist
1. Temperature-dependent sex determination – some
species of reptiles – alligators, turtles – sex determined
by temperature of the egg!! – so not ALWAYS genetic.
XXIII.
Sex-linked genes exhibit a unique pattern of
inheritance
A. sex-linked genes – any gene found on the sex
chromosomes
B. Sex chromosomes contain genes specifying sex as well
as genes having nothing to do with sex.
1. Those not related to sex are most often found on
which gene, X or Y? Explain… (X of course)
C. Example using fruit fly eye color
1. Red is dominant to white
2. Y chromosome does not have a locus for eye color
a) male phenotype based solely on X
b) First go over the possible combinations for male
and female.
c) XrY have white eyes, XRY have red
XXIV.
males?
Why do sex-linked disorders affect mostly
A. Males only have one X-chromosome, so we only need to
inherit one copy of the compromised allele gene. Females
need to inherit two compromised alleles – less likely.
B. RED-GREEN COLOR BLINDNESS  a common sexlinked disorder characterized by a malfunction of light
sensitive cells in the (eyes?)
1. involves several genes
a) normal vision – we see 150 colors
b) color blind – see fewer than 25
C. HEMOPHILIA  a sex-linked recessive trait
characterized by excessive bleeding due to a defective
gene involved in blood clotting
D. DUCHENNE MUSCULAR DYSTROPHY  a sex-linked
recessive disorder characterized by a progressive
weakening and loss of muscle tissue