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Biology Chapter 12
Patterns of Heredity &
Human Genetics
12.1 Pedigrees
• Pedigree- a graphic representation
of genetic inheritance
– There are some
common symbols
used in pedigrees
as shown
Known
heterozygotes
(carrier)
12.1 Analyzing a Pedigree
• From a pedigree, you can
determine:
– whether the trait is dominant or
recessive
– The phenotypes of individuals
– the genotypes of homozygous
recessive individuals
• You can only predict genotypes of
those that exhibit the dominant
phenotype based on the phenotypes of
their offspring and parents
12.1 Recessive Heredity
• Most genetic disorders result from a
recessive allele combination
• Examples:
– Cystic fibrosis: common among white
Americans, results in the formation of thick
mucus in the lungs and digestive tract
– Tay-Sachs: results in the absence of an
enzyme that breaks down fats, so it gets
stored in the cells
– Phenylketonuria (PKU): results in the
absence of an enzyme that converts
phenylalanine to tyrosine and damages
the central nervous system
12.1 Dominant heredity
• Some traits are inherited because of
having 1 or 2 dominant alleles
– Examples include: cleft chin, widows peak,
earlobe attachment, eye shape, etc.
– Genetic disorders can also be dominant,
such as Huntington’s disease, where
certain areas of the brain (CNS) break
down
• Huntington’s does not appear until the 30’s and
40’s so it is often passed on to offspring before
the individual is aware that they have it
12.2 Complex Patterns of Inheritance
• Mendel’s studies on heredity use only
simple inheritance-having dominant
and recessive traits
• There are other inheritance patterns:
–
–
–
–
–
Incomplete dominance
Co-dominance
Multiple alleles
Sex linked inheritance
Polygenic inheritance
12.2 Incomplete Dominance
• In incomplete dominance, the
phenotype of the offspring is
intermediate between that of the
parents
– For example: a red snapdragon crossed
with a white snapdragon results in pink
snapdragons
– The alleles for incomplete dominance are
both capital, with one having a prime
symbol
• For example R (red), R‘ (white) and RR’ (pink)
12.2 Co-dominance
• In co-dominance, the phenotype of the
offspring includes both of the parental
phenotypes
– For example: a black rooster crossed with
a white hen results in black and white
offspring, called checkered
– The alleles for co-dominance are two
different capital letters
• For example: B (black) and W (white) and BW
(checkered)
12.2Multiple Alleles
• Multiple alleles are traits
controlled by more than 2 alleles
– Each organisms will only have 2 of
these alleles, but there are more
than 2 possibilities
• For example pigeons can be:
– BA, which is red ash and is the most
dominant
– B, which is blue and is recessive to BA but
dominant to b
– b, which is chocolate and recessive to both
BA and B
12.2 Sex Determination
• The gender of offspring is determined
by the sex chromosomes
– In humans there should be 2 sex
chromosomes
• XX results in a female
• XY results in a male
– Because only the male can contribute different
alleles, the father’s sex chromosome determines
the sex of the baby
• The other chromosomes are identical
and called autosomes
– Humans have 22 pairs (or 44) of these
chromosomes
12.2 Sex Linked Inheritance
• Some traits are controlled by genes
located on sex chromosomes
– The alleles for sex linked traits are written
as superscripts on the sex chromosomes
• For example: XR or Xr
• X linked traits can be passed on to male or
female offspring
• Y linked traits can only be passed on from
father to son
12.2 Polygenic Inheritance
• Polygenic inheritance occurs
when multiple genes (with 2
alleles each) can control for a trait
– This is present when traits can
occur over a wide range, such as
height and skin color
– These genes can be on the same or
different chromosomes
12.2 Environmental Influence
• The environment of an organism (both
internal and external) can also
determine the phenotype that is
expressed
– Examples of external factors include light,
temperature, infectious agents and
nutrition
• For example the artic fox has brown fur when it
is warm and white fur when it is cold to help
them blend in with their surroundings
– Examples of internal agents include
hormones associated with gender
• For example, mountain sheep males have
bigger horns than mountain sheep females
12.3 Co-dominance in Humans
• Sickle Cell disease is an example
of co-dominance in humans
– The sickle shaped cells result from a
change in the protein hemoglobin
• Normal red blood cells are disc shaped
• Homozygous sickle cell individuals
have red blood cells that are all sickle
shaped (half-moon shaped)
• Heterozygous individuals have some
disc shaped and some sickle shaped
red blood cells
12.3 Multiple Alleles in Humans
• Blood Type is an Genotype Phenotype
example of
AIA or IAi
I
A
multiple alleles in
humans
IBIB or IBi
B
• The following
genotypes and
I AI B
AB
phenotypes can
occur:
ii
O
12.3 Sex Linked Traits in Humans
• Sex linked traits can be inherited on
the X chromosome or the Y
chromosome
– If a male receives a recessive allele on the
X chromosome, the trait will be expressed
– A female must receive a recessive allele
on both X chromosomes to express the
trait
– If the recessive allele is on the Y
chromosome, it can only be passed from
father to son
• Females cannot exhibit sex linked traits on the
Y chromosome
12.3 X-linked human traits
• Red-Green colorblindness
– Recessive
– Individuals cannot tell the difference
between red and green
• Hemophilia
– Recessive
– Individuals do not have enough clotting
factor in their blood, so it does not clot
12.3 Polygenic Traits in Humans
• Skin Color
– Intermediate skin colors exist in the
first generation, with the original skin
colors as well as the intermediates
appearing in the 2nd generation
• Eye Color
– Eyes can be a range of colors,
including solids and mixtures of
colors
12.3 Changes in Chromosome Numbers
• Karyotypes are diagrams that
show the number and types of
chromosomes
– Individuals can have an extra
autosome, that results in diseases
such as in Down’s syndrome
– Individuals can have an extra sex
chromosome that results in
diseases such as Kleinfelter’s