Chapter 11, Complex
Genetic Patterns
Recessive Genetic Disorders
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In chapter 10 we discussed that in order for a
recessive trait to be expressed, an individual must
be homozygous recessive for that trait.
Recessive genetic disorders are disorders that are
caused by recessive alleles when a person is
homozygous recessive for that trait.
A person who is heterozygous for a trait does not
express the trait but can pass the trait to their
offspring. They are called carriers.
Recessive Genetic Disorders
Recessive Genetic Disorders
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Cystic Fibrosis – Causes a thick, sticky mucus to build
up in the lungs and digestive tract.
Albinism – No color in the skin, eyes, and hair.
Galactosemia – Inability of the body to break
down the simple sugar galactose.
Tay-Sachs disease – Deadly disease of the nervous
system that is most common in people of Jewish
descent.
Albinism
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Mother of five, with
three albino children
Dominant Genetic Disorders
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Dominant genetic
disorders.
Caused by dominant
alleles (genes).
If only one parent
has one allele
(heterozygous), 50%
of the children will
inherit the disease.
Dominant Genetic Disorders
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Huntington's disease – Causes nerve cells in the
brain to degenerate, causing a gradual loss of
brain function. Occurs most commonly in people
ages 30 – 50 years old.
Achondroplasia – A disorder that affects the growth
of bones and causes dwarfism.
Polydactyly – A condition resulting in an extra
number of fingers and toes.
Polydactyly
Pedigrees
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Geneticists use diagrams that trace inheritance of
particular traits through several generations called
pedigrees.
A pedigree uses symbols to illustrate different meanings.
Males are represented by a square.
Females are represented by a circle.
Usually someone who expresses a trait is dark.
Usually someone without the trait is light.
A carrier is half dark and half light.
Pedigrees
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Horizontal lines
between symbols
represent parents of
the offspring in the
lines below them.
Offspring are ordered
from first to last.
Roman numerals
represent generations.
Albinism and Polydactyly Pedigrees
Incomplete Dominance
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Remember from chapter 10 we learned that if an
organism's genotype is homozygous dominant (TT)
or heterozygous (Tt) the dominant allele is
expressed.
Only when an individual organism's genotype is
homozygous recessive (tt) is the recessive allele
expressed.
In incomplete dominance, when an organism's
genotype is heterozygous (Tt), an intermediate
phenotype is expressed.
Incomplete Dominance
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When Red (RR) and White (rr) snapdragons
are crossed, the heterzygous offspring are all
pink (Rr).
Codominance
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Codominance is when two alleles can be dominant at
the same time.
In coat color for cows, you can have red cows (RR),
white cows (WW), and roan cows (RW). Roan cows
have both red and white hairs.
In chickens, you can have chickens with black feathers
(BB), chickens with white feathers (WW), and chickens
with black and white feathers (BW), but not gray
feathers.
Codominance (Roan Cattle)
Multiple Alleles
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Some genes are controlled by more than just two
different alleles (dominant and recessive).
Blood types in humans is determined by multiple
alleles. Humans can have either A blood, B blood,
AB blood (codominant), or O blood (neither A or B
blood).
Multiple alleles can also express a hierarchy of
dominance. For example in rabbit coat color, Full
coat color is dominant to Chinchilla, Chinchilla is
dominant to Himalayan, and Himalayan is dominant
to Albino.
Multiple Alleles
and Dominance Hierarchy
Epistasis
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Some genes can be altered or modified by the
effects of one or more other genes.
This effect is called epistasis.
A good example of this is coat color in
Labrador retrievers. In this case, the dominant
allele (E) determines whether or not the fur is
dark.
A second gene (B) determines how dark the
coat color will be.
Sex Determination
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Humans have 46 chromosomes.
2 of those 46 chromosomes are called sex
chromosomes.
The other 44 chromosomes are called autosomes.
Most of your chromosomes are autosomes are
chromosomes that determine your traits.
Sex chromosomes can determine traits as well, but
they also determine gender or sex.
Sex Determination in Chromosomes
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Sex chromosomes can
either be X or Y.
If an individual's sex
chromosomes are both X
chromosomes, that person
will be a female.
If an individual's sex
chromosomes are X and Y,
that person will be a male.
Sex-Linked Traits
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Traits controlled by genes on the sex
chromosomes are called sex-linked traits.
Because males have only one X chromosome,
they are affected by recessive traits found on
the X chromosome more than females.
Females are less likely to express a recessive
trait on an X chromosome, because the other X
chromosome may have a dominant allele.
Are you Color Blind?
How are Sex-linked traits inherited?
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This would be the result
from a cross between a
female carrier (XBXb)
for color blindness and
a normal male (XBY).
½ of the children
would be normal
½ would either be
color blind or be
carriers
Sex-linked traits
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Hemophilia is another sex-linked trait that causes delayed
blood clotting.
Most sex-linked traits like color blindness and hemophilia
affect males more than females.
Calico cats are always females because the gene for coat
color is on the X chromosome. If a female is heterozygous,
she will be a calico cat.
Male pattern baldness is technically an autosomal trait, but
it acts like a sex-linked trait.
Eye color in Drosophila Fruit flies is a sex-linked trait.
Calico Cats
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Males can either be
black or orange.
Females can be black,
orange, or calico.
This is caused by one
of the X chromosomes
becoming inactive in
female cats.
Queen Victoria's Pedigree (Hemophilia)
Male Pattern Baldness
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The gene for baldness
is recessive in females,
but dominant in males.
Males can either be
heterozygous or
homozygous dominant.
Females must be
homozygous recessive
to be bald.
Drosophila Fruit Flies
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Red-eyes are dominant to
white-eyes in fruit flies.
The gene for eye color in
fruit flies is found on the
X chromosome.
Fruit flies make good
specimens for studying
sex-linked traits.
Polygenic Traits
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Genes that are controlled by multiple genes at the same
time are called polygenic traits
One characteristic of polygenic traits is that there tends to
be a higher frequency of intermediate forms of a trait.
When graphed, the frequency of phenotypes for
polygenic traits appears as a bell-shaped curve.
Some examples of polygenic traits are skin color, eye
color, and height.
Polygenic Traits (Height)
Polygenic Traits (Skin Color)
Polygenic Traits (Eye Color)
What are Karyotypes?
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Some genetic disorders can be caused by the
presence or absence of chromosomes.
A Karyotype is a photograph of a person's
chromosomes arranged in order.
They are arranged from largest to smallest.
Homologous pairs are arranged together.
Once arranged, karyotypes can reveal genetic
disorders by looking for patterns.
Normal Karyotype
Nondisjunction
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If chromosomes fail to separate properly during meiosis, the
gametes (sex cells) will not end up with the right number of
chromosomes.
Later on during fertilization if the egg or sperm contains one
of these abnormal gametes, a nondisjunction can occur.
If a zygote ends up with one extra chromosome, this results in
a trisomy (2n + 1)
If a zygote ends up with one less chromosome, this results in a
monosomy (2n - 1)
Nondisjunction
Disorders caused by Nondisjunction
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Down syndrome – result of an extra chromosome number 21
(often called trisomy 21).
Symptoms include short stature, heart defects, and mental
disability, increased risk of infection.
About 1 in 800 children born in the U.S. have Down syndrome.
The frequency of children being born with Down syndrome
increase with the age of the mother.
Mothers over 45 have about a 6% chance of giving birth to a
child with Down syndrome.
Down Syndrome Karyotype
Other Disorders caused by Nondisjunction
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Turner's syndrome – (XO) Women with only one x chromosome.
Symptoms include short height, webbed neck, lack of underarm and
pubic hair, and underdeveloped ovaries.
Klinefelter's syndrome – (XXY) Symptoms in males include tall height,
low IQ scores, speech and language difficulty, sterility.
Patau syndrome – (trisomy 13) Heart defects, abnormalities of the
eyes, ears, brain and spinal cord, cleft palate and/or lip, small head,
low IQ scores, additional fingers and toes, 95% mortality in the first
year of life, children rarely live past a few months.
Edward’s Syndrome – (trisomy 18) Slow growth before birth, low
birth weight, abnormal organ growth, heart defects, small head, 9095% mortality rate during the first year.