Transcript Patterns of Heredity and Human Genetics

PATTERNS OF HEREDITY
AND HUMAN GENETICS
Chapter 12
Objectives
 Interpret a pedigree
 Determine human genetic disorders that are
caused by inheritance of recessive alleles
 Predict how a human trait can be determined my a
simple dominant allele
 SCS:
Mendelian Inheritance of
Human Traits
How can we trace traits in
our family?
Geneticists use a tool called
a pedigree (graphic
representation of genetic
inheritance)
Male
Female
Marriage
Heterozygous or carrier
I, II, III – represent
generations
pedigrees are used by genetic
counselors to help trace
genetic disorders and other
traits
Simple Recessive Heredity
most genetic disorders are
caused by recessive alleles
(rare) – you have to receive 2
alleles, 1 from mom and 1
from dad in order to inherit
the disorder
Examples
Cystic Fibrosis (CF)
1/2000 are affected by CF,
1/200 are carriers
characterized by a defective
protein that causes mucas
to build up in the lungs
Tay Sachs Disease
 common in Ashkenazi Jews
 characterized by the absence
of an enzyme that breaks
down fat in the brain
 most children die by the age
of 5
Simple Dominant Heredity
only need a single dominant
allele for the trait to be
expressed
 examples of
dominant traits
in humans:
unattached ear
lobes, tongue
rolling,
hitchhikers
thumb, mid
digit hair
Examples of dominant genetic
disorders
Huntington’s Disease
doesn’t affect individuals
until their 30-50 years of age
characterized by the
breakdown of areas of the
brain
12-2 Whey Heredity follows different rules
 Distinguish between incompletely dominant and
codominant alleles
 Compare multiple allelic and polygenic
inheritance
 Analyze the pattern of sex-linked inheritance
 Summarize how internal and external
environments affect gene expression
 SCS:
Complex Patterns of Heredity
most traits observed
are not a result of
simple dominant/
recessive inheritance
Types
Incomplete dominance-
heterozygous condition
results an intermediate
phenotype or “mixing”;
neither allele is completely
dominant over the other
 Example: Snap
dragons.
Homozygous
Red and white
flowers are
crossed, and
they produce all
pink offspring
 Codominance- both alleles are
expressed equally
Example: Cows. Homozygous Red
and white cows are crossed (mated),
and they produce offspring that
appear pink from a distance because
they both red and white hairs
Multiple Alleles- traits
controlled by two or more
alleles
Sex linked inheritancetraits controlled by genes
located on the sex
chromosomes
 Thomas Morgan was the
first to discover
Worked with fruit flies
Problems involving sex linkage
In fruit flies, red eyes are
dominant over white
example of a cross:
XRXr x XrY
Polygenic Inheritancetraits that are controlled
by more than 1 gene
Genes might be on the
same or different
chromosomes
Other factors influence the
expression of genes
 Environmental Influences
 External Environmental (temp.
nutrition, light, chemicals,
infectious agents)
 Internal Environment (hormones,
age)
 genetic makeup determines the
potential to develop and function
12-3 Complex Inheritance of Human Traits
 Compare codominance, multiple alleleic, sex-
linked, and polygenic patterns of inheritance in
humans
 Distinguish among conditions in which extra
autosomal or sex chromosomes exist
 SCS:
Complex Inheritance
of Human Traits
Codominance Sickle Cell
Anemia
most
common in
black
Americans
Characterized by the
sickling of red blood
cells, which hampers
circulation and affects
oxygen levels of the
blood
In malaria prone areas
people with this gene
are protected against
malaria
Homozygous conditions
results in full blown SC,
and medication has to be
taken
Heterozygous condition
results in a mild case, may
not need medication
Multiple Alleles
Blood types
There are 4 blood types in
humans (A, B, AB, and 0)
the type of marker (protein
on your red blood cells
determine your blood type)
Phenotypes
Type A
Type B
Type AB
Type O
Genotypes
IAIA or IA i
IBIB or IBi
IAIB
ii
knowing your blood type
is important:
blood transfusions,
child birth, blood
donations
 Type Blood
Can Receive
A
A, O
B
B, O
AB
AB, O, A, B
Universal recipient: AB
Universal Donor: O
Sex Linkage
 Some human traits are
determined by genes found on
the sex chromosomes (mostly
X)
Examples of sex linked traits:
 Red/Green Color Blindnesscan’t differentiate red from
green
 recessive disorder
 More males have it because they
only need one allele
Hemophilia
 bleeders disease; blood does
not clot naturally so a person
can bleed to death
 1/10,000 males are afflicted,
only 1/ 100 million females are
affeced
Polygenic Traits
example include skin
color, eye color,
Changes in chromosome
number
Normally humans have 46
chromosomes, but
sometimes during
replication, chromosomes
do not separate correctly
Geneticist use a tool called
a karyotype to determine if
an extra chromosome is
present
Take a sample of a person’s
DNA and line up the
metaphase chromosomes
Examples of Disorders
Downs Syndrome (Trisomy
21)
a result of an extra
chromosome in the 21st pair
some degree of mental
retardation
Turners Syndrome
 XO, female, infertile
Kleinfelter’s
 XXY, Male, infertile