Transcript Human Genetics - Madison Public Schools

Human Genetics
Chapter 12: Inheritance Patterns
and Human Genetics
Pedigrees
Geneticists can study human genetic
traits and trace genetic diseases from
one generation to the next by studying
the phenotypes of family members in a
pedigree.
 A pedigree is a diagram that shows how
a trait is inherited over several
generations.

Pedigrees
Patterns of Inheritance

Biologists learn about genetic diseases by
analyzing patterns of inheritance.
 For Example:

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If a trait is autosomal, it will appear in both sexes
equally. If a trait is sex-linked it is usually only seen
in males. Most sex-linked traits are recessive.
If a trait is autosomal dominant, every individual
with the trait will have a parent with the trait. If the
trait is recessive, an individual with the trait can
have one, two or neither parent exhibit the trait.
Pedigrees
Pedigrees can be analyzed to determine
whether a trait is autosomal or sexlinked.
 Pedigrees can be analyzed to determine
whether a trait is dominant or recessive.
 Individuals on a pedigree that have the
allele but do not express the trait are
known as carriers.

Genetic Traits and Disorders
Genes controlling human traits show
many patterns of inheritance.
 Some of these genes cause genetic
disorders.
 GENETIC DISORDERS are diseases or
disabling conditions that have a genetic
basis

Polygenic Inheritance
Most human characteristics (traits) are
polygenic.
 Polygenic characters (traits) are
influenced by several genes.
 Polygenic traits include: height, nose
length, size of foot, hair color, skin color,
and eye color.

Complex Characters
Many human conditions are complex
characters.
 Complex characters (traits) are
influenced strongly by both the
environment and by genes.
 Skin color and height are both polygenic
and complex. Explain why.
 Heart disease, diabetes, cancer are also
thought to be complex.

Complex Characters
 Biologists
hope that by
identifying environmental
components that contribute to a
disease, they can educate
people in ways that minimize
their risk of developing the
disease.
Multiple Alleles
 Many
genes have more than two
alleles. Genes with three or
more alleles are said to have
multiple alleles.
 The ABO blood groups are
governed by multiple alleles.
Codominance
Alleles that are codominant are
expressed equally.
 The IA and IB alleles in the ABO blood
group gene are both codominant since
they both produce a corresponding
antigen (protein) on the surface of
erythrocytes (red blood cells).

Incomplete Dominance
Sometimes an individual displays a trait
that is intermediate between two parents,
a condition known as incomplete
dominance.
 Example: In Caucasians, the child of a
straight-haired parent and a curly-haired
parent would have wavy hair.

X-linked Traits
X-linked traits are carried on the X
chromosome of the 23rd pair. This means
that X-linked traits are carried on a sex
chromosome.
 Colorblindness, Hemophilia are both Xlinked traits.

Sex-Influenced Traits

Sex-influenced traits are involved in other complex
characters.
 Males and females can show different phenotypes
even when they share the same genotype.
 Sex-influenced traits are usually autosomal.
 For example, an allele that is dominant in males but
recessive in females controls pattern baldness, the
type of baldness usually found in men. The difference
is due to higher levels of the hormone testosterone in
men, which interacts with the genotype to produce
pattern baldness.
Detecting Genetic Disease
Many people with a family history of
genetic disease seek genetic screening
before having children.
 Genetic Screening is an examination of a
person’s genetic makeup. It may involve
karyotypes, blood tests for certain
proteins, or direct test of DNA.
 Doctors can now also detect more than
200 genetic disorders in the fetus.

Amniocentesis
Performed between the 14th and 16th
week of pregnancy.
 Allows a physician to remove some
amniotic fluid from the amnion, the sac
that surrounds the fetus.
 Geneticists can analyze fetal cells for
genetic disease by examining
chromosomes and proteins in the fluid.

Amniocentesis
Chorionic Villi Sampling (CVS)
Performed between the 8th and 10th
week of pregnancy.
 The physician takes a sample of the
chorionic villi derived from the zygote
that grow between the mother’s uterus
and the placenta.
 Technicians analyze the cells,
chromosomes and proteins to detect
genetic disease.

Chorionic Villi Sampling (CVS)
Genetic Counseling
Many people with a family history of a
genetic disease also undergo genetic
counseling.
 Genetic counseling is the process of
informing a person or couple about their
genetic makeup.
 Genetic counseling is a form of medical
guidance that informs individuals about
problems that might affect their offspringl

Treating Genetic Disease
Physicians treat genetic diseases in
several ways.
 For some diseases, physicians can
implement symptom-prevention
measures. Example: insulin injections
for diabetes.
 Physicians can even do some types of
surgery to correct genetic defects in a
fetus before birth.

Gene Therapy

Gene Therapy is a technique that replaces a
defective gene with a healthy copy of the
gene.
 A virus is used to inject the gene into the cells.
The gene functions until the cells die. Gene
Therapy needs to be repeated.
 Gene therapy, in which only body cells are
altered, is called somatic cell gene therapy.
 Germ-cell gene therapy is the attempt to alter
sperm or eggs. This form of gene therapy is
highly controversial.
Critical Thinking Review
A woman with cystic fibrosis marries a
man who is heterozygous for cystic
fibrosis. What is the likelihood
(probability) that their children will have
cystic fibrosis?
 (Cystic fibrosis is autosomal recessive)
 (Give yourself a key, determine the
genotypes of parents, perform the cross)

Critical Thinking Review

Why is colorblindness less common
among females?
ABO Blood Group

A man with blood type B marries a
woman with blood type A. Their first
child is blood type O. What is the
probability their next child will be blood
type AB? Blood Type B?
Analyzing a Pedigree
Analyzing a Pedigree