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PATTERNS OF INHERITANCE
Ch. 10
SUDDEN DEATH ON THE COURT
Flo Hyman, 6ft, 5in tall, graceful and athletic, was
probably the best woman volleyball player of her time.
Captain of the American women’s volleyball team that
won a silver medal in the 1984 Olympics, Hyman later
joined a professional Japanese squad. In 1986, at the age
of 31, she was taken out of a game for a short breather,
and died while sitting quietly on the bench. How could
this happen to someone so young and fit?
Hyman had a genetic disorder called Marfan syndrome, which affects
about 1 in 5,000 to 10,000 ppl, probably including classical musicians and
composers Sergei Rachmaninoff and Nicolo Paganini, and possibly
American president Abe Lincoln. Ppl with M.S. are typically tall and
slender, with long, flexible limbs and large hands and feet. For some ppl
with M.S., these characteristics lead to fame and fortune. Unfortunately,
M.S. can also be deadly.
Hyman died from a ruptured aorta, the massive artery that carries blood
from the heart to most of the body. Why did Hyman’s aorta burst? What
does a weak aorta have in common with tallness and large hands?
M.S. is caused by a mutation in the gene that encodes a protein called
fibrillin, an essential component of connective tissue. Many parts of
your body contain connective tissue, including the tendons that attach
muscles to bones, the ligaments that fasten bones to other bones in
joints, and the walls of arteries. Fibrillin forms long fibers that give
strength and elasticity to connective tissue. Normal fibrillin also traps
certain growth factors, preventing them from stimulating excessive
cell division in, for example, bone-forming cells. Defective fibrillin
cannot trap these growth factors, with the result that the arms, legs,
hands, and feet of ppl with M.S. tend to become unusually long. The
combination of defective fibrillin and high concentrations of growth
factors also weakens bone, cartilage, and artery walls.
Diploid organisms, including ppl, generally have 2 copies of each
gene, one on each homologous chromosome. One defective copy of
the fibrillin gene is enough to cause M.S. What does this tell us about
the inheritance of M.S.? Are all inherited diseases caused by a single
defective copy of a gene?
10.1 What Is The Physical Basis of Inheritance
Inheritance: process by which traits of organisms are
passed to their offspring.
Genes: encode the information
needed to produce proteins, cells,
and entire organisms
Locus: a gene’s physical location
on a chromosome
Alleles: different versions of a gene
at a given locus
Homozygous – 2 alleles are the same (AA or aa)
Heterozygous – 2 alleles are different (Aa)
Genotype – alleles of an individual (AA, Aa, aa)
Phenotype – physical appearance of a trait (purple, white)
10.2: How Were the Principles of Inheritance Discovered?
Gregor Mendel  Father of Genetics
genetics  branch of biology;
focuses on the passing of traits from
parents to offspring (heredity)
Why Mendel used peas?
 sharp contrasting traits
 controlled mating (self-fertilization or
cross-pollination
 easy to grow, mature
quickly
 produce many offspring
Mendel’s Experiments:
1. self-pollinated pea plants to produce a truebreeding parent generation
purple flower X purple flower  purple flowers (P1)
white flower X white flower  white flowers (P1)
2. cross-pollinated 2 P1 plants to produce the
F1 generation
white flower (P1) X purple flower (P1) purple flowers (F1)
3. cross-pollinated 2 F1 plants to produce the
F2 generation
purple flower (F1) X purple flower (F1)
purple & white flowers (F2)
Sample Problems
In humans, freckles is a dominant trait, while not
having freckles is a recessive trait (Use F and f)
•
•
•
•
•
•
•
•
Dominant allele______
Recessive allele _____
Homozygous for freckles ______
Homozygous for no freckles _____
Heterozygous for freckles _____
Heterozygous for no freckles _____
Possible Genotypes ________
Possible Phenotypes________
Principle of Dominance  organisms with at least 1
dominant trait will exhibit that trait
B = black
b =brown
BB  black
Bb  black
Law of Independent Assortment:
alleles on different
genes separate
independently of each
other during egg/sperm
(gamete) formation
Law of Segregation: two alleles (for the same trait)
separate when gametes are formed
 inheritance of one trait DOES NOT influence the
inheritance of any other trait
How can we use probability to predict traits?
Probability  likelihood that a particular event will
occur; expressed as %, decimal, or fraction
Probability = # of one kind of possibility
total # of possible outcomes
 What is the probability of flipping a penny and it
landing on heads?
= ½ = 50%
 Probability of flipping a penny and nickel and both
landing on heads
=½*½=¼
Punnett square  diagram used to predict outcomes
of genetic crosses
 considers all possible combinations of gametes
1. monohybrid cross 
involves 1 pair of traits
(color; D – dark / d-light)
Monohybrid Cross
Problem: Cross a pea plant
homozygous for yellow (Y) seed color
and a pea plant that is homozygous for
green (y) seed color.
Y
Y
y
y
Results:
• 4 Yy
•100% of the offspring are expected to
be heterzygous (Yy)
• genotype of offspring: 4 Yy
•Phenotype of offspring; 4 yellow
2. dihybrid cross  involves 2 pairs of traits
Ex. Eye color  B-black / b – brown
Fur texture  S – soft / s – rough
BbSs
BS
Bs
bS
x
bs
Ex. Cross a fox that
is heterozygous for
brown eyes and
heterozygous for
soft fur.
BbSs
Test Cross
 determines if an organism with a dominant
phenotype is heterozygous or homozygous for a trait
 cross individual with the unknown genotype with
a homozygous recessive individual
Sample Problem:
A plant with yellow seeds but an unknown
genotype (Y_) is test crossed with a plant with
green seeds (yy). What is the unknown genotype if
all of the offspring have yellow seeds? 50%
yellow seeds and 50% green seeds?
Y
y Yy
y
Y
Y
y
y Yy yy
y
• If 100% of offspring have yellow
seeds, the unknown genotype must be
YY.
• If 50% have yellow seeds and 50%
have green seeds, the unknown genotype
must be Yy.
Incomplete dominance:
 individual displays a trait that is intermediate
between 2 parents
 neither trait is completely dominant over the
other
Ex. White 4 O’clock (WW) x Red 4 O’Clock (RR) = Pink
4 O’clock flowers (RW)
Straight (SS) x Curly (CC) = Wavy hair (SC)
Codominance
 when two dominant alleles are expressed at the same
time
Ex. ABO blood group: Type AB (AB)
Ex. Roan color in horses: red and white strands (RW)
Ex. Sickle Cell Anemia: the carrier has no disease but
has a few sickle shaped (NS)
blood cells
Multiple Alleles
 genes with 3 or more alleles
Ex. ABO blood group - A, B, o
“A” and “B” refer to carbohydrates located on
the surface of red blood cells
 o means that neither carb is present
 “A” and “B” are both dominant over o, but
neither are dominant over each other
Multiple Alleles: Blood Types
Type
A-type blood
B-type blood
Homozygous Heterozygous
AA
BB
Ao
Bo
AB – type
blood
-----------
AB
O - blood
oo
--------
Polygenic Inheritance:
 trait that is influenced by two or more genes and often
also influenced by environment
 the more genes that contribute to a single trait, the
greater the number of possible phenotypes
 genes may be
scattered on the same
chromosome or on
different chromosomes
Ex. Eye color, height (180 genes),
weight, skin color (3 genes)
Pleiotrophy
 when single genes have multiple phenotypic effects
 Ex. mutation in a single gene in a lab mouse in 1962
 Hairless
 Lack a thymus gland
 Have virtually no immune response
 Females don’t develop functional mammary glands
Traits Influenced by Environment:
 phenotype influenced by the environment
Ex. Arctic fox is reddish brown in summer and white in
winter
Ex. Skin color is influenced by exposure to sun.
Ex. Fur color in Siamese cats changes with temperature
10.6 How Are Genes Located on the Same Chromosome Inherited
Gene linkage – genes on the same chromosome tend to be inherited
together (unless crossing over occurs)
Genetic recombination – new combinations of alleles of genes that
are located on same chromosome
10.7 How Are Sex and Sex-Linked Traits Inherited
Autosomal traits
appear in both sexes equally
Ex. Albinism: body unable to produce melanin
Sex-linked traits:
 genes located only on the sex chromosomes
(X-1,000 genes or Y-100 genes)
 more common for males to have sex-linked traits;
 most sex-linked traits are recessive
Ex. Hemophilia and colorblindness
Normal Female: XX
Female w/ hemophilia: XhXh
Female carrier of hemophilia: XXh
Male w/ hemophilia: XhY
Normal Male: XY
Some Human Genetic Disorders Are Controlled by Single Genes

For many genes, a normal allele (functional protein) is dominant
to a mutant allele (nonfunctional protein)

carrier: person who is heterozygous, with 1 normal, dominant
allele and one defective, recessive allele
 phenotypically healthy but can pass on defective allele to offspring
Disorder
Dominant/
Recessive
Symptoms
Sickle Cell
Anemia
Recessive
Poor blood circulation
Red-Green
colorblindness
Sex-linked
recessive
Recessive
Inability to see red and
green colors.
Huntington’s
Disease
Dominant
Gradual deterioration
of brain; fatal
Hemophilia
Sex-linked
recessive
Failure of blood to clot
Cystic
Fibrosis
Mucus clogs lungs,
liver, pancreas; fatal
Some Genetic Disorders are Caused by…..
 non-disjunction
 autosomes (trisomy 21
 sex chromosomes (XO, XYY, XXY, XXX)
genetic counseling – informs parents of problems
that could affect their child
 gene therapy– replacing defective genes with healthy
gene
Pedigree
 diagram that shows how a trait is inherited over
several generations
 depicts if who carries the gene and possibility of
having affected offspring.
CSI: Sudden Death on the Court
Medical examiners revealed that Flo Hyman’s father and sister
have M.S., but her mother and brother do not. A single defective
fibrillin allele is enough to cause M.S. What can we conclude
about the inheritance of M.S. from these data?
First, if even one defective fibrillin allele produces M.S., then
Hyman’s mother must carry two normal alleles, because she does
not have M.S. Second, because Hyman’s father has M.S. it is very
likely that Hyman inherited a defective fibrillin allele from him.
The fact that her sister also had M.S. makes this virtually certain.
Third, is M.S. inherited as a dominant or recessive condition? If
one defective allele is enough to cause normal allele must be
recessive. Fourth there seems to be no gender effect in Hyman’s
family. M.S. is not sex-linked because the fibrillin gene is on an
autosome. Finally, if Hyman had born children, could they have
inherited M.S. from her? For a dominant disorder, any children who
inherited her defective allele would develop M.S. Therefore, her
children would have had a 50% chance of inheriting M.S