Chapter 12
Section 3-Complex Inheritance
of Human Traits
I) Codominance in Humans
• **Note: phenotypes of both
homozygotes are produced in
heterozygotes  ex/sickle cell anemia
B) Sickle Cell Anemia (SCA)
i) Basics of SCA
• Major health problems in US & Africa
• Most common among in black Americans w/
origins in Africa & white Americans with origins
in the countries that surround the Med. Sea
• 1/12 African American is heterozygote for SCA
I) Codominance in Humans
i) Basics of SCA
• Hemoglobin is a protein that carries O2 in
red blood cells (RBC)
• Individuals who are homozygotes for SCA
have defective hemoglobins which form
crystal-like structures that cause shape
changes in RBC
Normal RBC
Disc Shaped
Sickle Cell RBC
Half-moon or sickle shaped
I) Codominance in Humans
ii)How sickle cell works?
1. Gene for hemoglobin produces protein that differs from
normal hemoglobin by 1 AA
2. SCA hemoglobin crystallizes when O2 levels are low after
hemoglobin delivers O2 to cells in narrow capillaries
3. Causes change in RBC from disc shaped  sickle shape (see
Fig.12.16)
4. Sickle shaped RBC slow blood flow and block small vessels
– Can result in:
◊ Pain
◊ Tissue damage
◊ Short life span of RBC
I) Codominance in Humans
iv) Codominance of SCA
– Individuals who are heterozygotes for SCA means
that they produce both:
• normal hemoglobin
• SCA hemoglobin (ex/ of codominance) and they have the
SCA trait
– These individuals produce enough normal
hemoglobins to prevent serious health problems
and can live relatively normal lives
– These individuals can show signs of SCA related to
disorders if O2 availability reduced
II) Multiple Alleles govern
blood type
– ABO blood type grp classic example of
single gene with multiple alleles in humans
i) Human Blood Types
– Human blood types determined by presence
or absence of certain molecules on surface
of RBC
– Blood Type Gene: I
– (3) alleles that can be present on I gene:
• IA= surface molecule A
• IB= surface molecule B
• i = no surface molecule
GENOTYPES Type A:_____ Type B:____ Type AB:_______ Type O:___
Possibilities Type A:_____ Type B:_____
II) Multiple Alleles govern
blood type
B) Importance of Blood typing (B.T.)
– Blood typing important b/c:
• Incompatible blood type given through blood
transfusion can cause other blood types to
clump causing death
• Can be used to solve parentage:
• Ex/ Child has blood type AB, Mother has blood
type A, what must the father’s blood type be?
II) Sex-linked traits in
Humans
– Many human traits are determined by genes
carried on sex chromosomes
– Most genes carried on the X chromosome
A) How sex chromosomes are passed on?
– Males
X
Passed on to
Daughters only
Females
Y
Passed on to
Sons only
X
X
Can be passed to either
sons or daughters
II) Sex-linked traits in
Humans
• If son receives allele on X chromosomes it will be
expressed b/c males have only 1 chromosome
• 2 traits that are X-linked disorders are:
1. Red-green blindness
2. Hemophilia
B) Red-green colorblindness
– Individual can’t distinguish between red & green
– Caused by recessive allele found at 1 of (2) gene
sites on X chromosome
– Both genes affect red & green receptors in eye
cells
– Can cause problems w/ driving  traffic lights
Color Blindness Tests
II) Sex-linked traits in
Humans
C) Hemophilia: An X-linked Disorder
– Clotting ability necessary human adaptation
– If clotting ability not present, could result
in:
• Excess blood loss due to minor injury
• Serious blood loss due to internal injury
• Major blood loss due to bruising
– Hemophilia A
• X-linked disorder causing a blood clotting
problem
• 1/10,000 males have hemophilia
• 1/100 million female have hemophilia
II) Sex-linked traits in
Humans
C) Hemophilia: An X-linked Trait
– Hemophilia A
• Males receive hemophilia allele from carrier
mothers on X chromosome
1. 1 recessive allele = hemophilia in males
2. 2 recessive alleles = hemophilia in females
– Treatment of hemophilia:
1. Blood transfusions
2. Injections of Factor VIII (this is a blood clotting
enzyme that is absent in hemophilic patients
III) Polygenic Inheritance
in Humans
•
Many human traits are determined by
polygenic inheritance
–
–
Ex/ variations in eye color (humans)
Ex/variations in skin color (humans)
i) Skin Color: A polygenic trait
•
•
•
1900s-skin color data used to test polygenic
inheritance theory
Light skinned individual x Dark Skinned individual
Intermediate skin color
III) Polygenic Inheritance
in Humans
– F2 generation:
•
results in range of skin colors from light skinned dark skinned like P1
generation w/ most having intermediate skin color
– Variation led to skin color being polygenic & has 3-4 genes
involved
III) Changes in
Chromosome Numbers
• Remember abnormal chromosomes #
usually happens b/c of meiosis accidents
(Process Called _________________)
• Human chromosome:
– Pairs of chromosomes = 23
– Total chromosomes = 46
A) Abnormal numbers of Autosomes
– Trisomic Humans-those individuals who
have either partial/whole extra
chromosome
– Trisomy usually caused by nondisjunction
III) Changes in
Chromosome Numbers
i) How abnormal chromosomes are ID’ed?
1.
2.
•
•
3.
Sample of cells obtained from fetus or individual
Metaphase chromosomes photographed & images are
enlarged & arranged by length & location of Centromere
Chart of chromosomes is called a karyotype
Karyotype-def-pg329-chart of metaphase
chromosome pairs arranged according to length &
location of the Centromere; used to pinpoint
unusual chromosome #s in cells
Karyotypes valuable in ID’ing unusual chromosome
#s in cells
III) Changes in
Chromosome Numbers
B) Down Syndrome: Trisomy 21
–
Most embryo w/abnormal chromosome # result in death
of the embryo
Down syndrome only autosomal trisomy that results in
embryo surviving to birth and adulthood.
Down syndrome happens in 1/700 live births
–
–
ii) Symptoms of Down Syndrome:
1.
2.
3.
4.
5.
6.
–
Some degree of mental retardation
Short stature
Weak muscles through out body
Short, wide neck with excess skin
Short/stocky arms & legs
Facial features: sm. Low set ears/nasal bridge pushed in/
irregular mouth & tongue
Incident of Down Syndrome births is er in older
mothers (especially over 40)
III) Changes in
Chromosome Numbers
C) Abnormal numbers of sex chromosomes
–
–
–
Many abnormalities in # of sex chromosomes
Any individuals w/ at least one Y chromosome is
male
Most individuals w/abnormal # of sex
chromosomes lead normal lives, but some may
result in:
1. Klinefelter syndrome (XXY)-sm testes, usually sterile
2. XYY-may be taller than normal
3. Turner’s syndrome (XO)- monosomy X-have sex organs
which don’t mature/women sterile/short/heart defects
common