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Chapter 12
Inheritance Patterns and
Human Genetics
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I. Sex Determination
A. Autosomes: are matching chromosomes, in humans
there are 22 pairs.
B. The 23rd pair are the sex chromosomes.
Females are XX
Males are XY
II. Effects of Gene Location:
A. Sex – linked: Traits that are carried on the X
chromosome, not the Y.
Colorblindness and Hemophilia are two examples.
–Because males have only one X chromosome, a male
who carries a recessive allele on the X chromosome
will exhibit the sex-linked trait.
The Red X represents one carrying a defect
such as hemophilia
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B. Linked genes
– Pairs of genes that tend to be inherited together
are called linked genes.
C. Chromosome Mapping
– The farther apart two genes are located on a
chromosome, the more likely a cross-over will
occur.
– Researchers use recombinant percentages to
construct chromosome maps showing relative
gene positions.
III. Mutations
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Germ-cell mutations occur in gametes and
can be passed on to offspring.
Somatic-cell mutations occur in body
cells and affect only the individual organism.
Chromosome Mutations
– Chromosome mutations are changes in the
structure of a chromosome or the loss or gain of
an entire chromosome.
Gene Mutations
Gene mutations are changes in one or more of the
nucleotides in a gene.
IV. Inheritance of
Traits

Pedigrees
– Geneticists use pedigrees to trace
diseases or traits through families.
– Pedigrees are diagrams that reveal
inheritance patterns of genes.
Pedigree is a chart that shows the
inheritance of a trait.
Question of the day:
Color blindness is a sex-linked trait
that is carried on the X chromosome. If a
boy is born color-blind, what would
have to be true?
A. His father had normal vision.
B. His grandmother was colorblind.
C. His mother carried at least one
gene for color blindness.
D. His grandfather passed on the
color-blind trait to his father
V. Genetic Disorders
Huntington’s Disease: Rare
dominant gene. Results in brain and
nervous system deterioration. Onset
between the ages of 30 – 50.
 Cystic Fibrosis: Recessive gene.
Build up of mucous in the lungs and
digestive tract. Pneumonia, poor
digestion. Onset at birth

Typical Pedigree of Huntington’s
Disease
I
II
III
Tay-Sachs disease: Recessive gene, Lipid
Accumulation in the brain, seizures, blindness,
Onset at birth.
PKU: Recessive gene, Inability to digest
Phenalanine, mental retardation, Onset birth.
Can be controlled with diet.
Sickle-Cell Anemia: Co-dominant genes.
Deformed hemoglobin with the inability to
properly carry oxygen. Onset birth. If one gene
is normal and one sickle cell – ½ the blood
will be normal. Has been treated with stem
cells.
Color blindness: Sex-linked gene. Inability to
distinguish hues.
Hemophilia: Sex-linked. Missing factor 9 the
protein need for blood to clot.
Down’s Syndrome: Nondisjunction of the 21st
chromosome. Trisomy 21. Developmentally
disabled, some medical problems at birth.
Klinefelter’s Syndrome: Nondisjunction of the
sex chromosomes. XXY, males that are
developmentally disabled.
Turner’s Syndrome: Nondisjunction of the
sex chromosomes. Single X. Female, failure to
develop sexually without medication.
Cri-du-chat: Large section of chromosome 5
missing. Extremely small head, rarely survive
more than 1 to 2 years.
Most chromosomal mutations are lethal
Karyotype is a picture of one’s chromosomes.
Amniocentesis can be done during pregnancy
and the a karyotype made to see any
Chromosomal abnormalities.
VI. When Heredity
Follows Different Rules
A. Incomplete Dominance: The traits blend, neither
trait is expressed fully.
B. Co dominance: Both alleles are expressed equally.
Cross a black chicken with a white rooster and you
get a black and white chicken
Blood type is also co dominant. AB have equal effect,
O is a recessive gene.
C. Multiple Alleles: When there are
more than two possible alleles for a
trait. Blood type again is an example,
three possible alleles A, B or O.
 D. Polygenic Traits: Takes more than
one gene pair to code for a trait.
*Height requires 3 gene pairs
*Hair, eye color and skin tone require 7
gene pairs.

E. Sex-Influenced traits:
Traits that are controlled by
the hormones of the body.
Examples: lion’s mane, a
man’s beard, woman’s
breast development, horn
size on animals, feather
color in birds.
Baldness in males
F. Environmental
Controls
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Genes can fail in their
expression if the
environmental
conditions are not
present.
Temperature, nutrition,
light, chemicals,
infectious agents.
Negative genes can be
expressed if conditions
are also right. Genes that
cause alcoholism.
You do not need to
write this!!!
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The sex of alligators is determined by nest
temperature early in the incubation period
(from 7th to 21st day). If the nest
temperature is below 86 degrees, all
hatchlings are female. Above 93 degrees,
all hatchlings are males. At temperatures in
between there are both males and females,
usually with a greater percentage of
females.
http://www.centralfloridazoo.org/animals/Am
erican_alligator.htm
Complex Inheritance of
Human Traits
Surface molecule B
A. Blood Typing is an example of Codominance and multiple Alleles.
B. A, B are dominant to O
Genotype Surface Protein Phenotype
IAIA, IAi
A
A
IBIB, IB i
IAIB
ii
B
AB
NONE
B
AB
O
Rh factor: + is dominant, negative is recessive.
Person that is positive could be ++ or +Person that is negative would be –
AB = universal recipient, O = universal Donor
Surface molecule A

Gene Therapy
– In gene therapy, a defective gene is
replaced with a copy of a healthy gene.
– Somatic cell gene therapy alters only
body cells.
– Germ cell gene therapy attempts to alter
eggs or sperm.