Complex Inheritance &
Human Heredity
Basic Patterns of Human Inheritance
 A recessive trait is expressed when an individual holds
TWO recessive alleles (homozygous recessive)
 Example s recessive genetic disorders include:
 Cystic fibrosis
 Albinism
 Tay-Sachs disease
 Galactosemia
 Individuals heterozygous for a recessive disorder will
express the dominant trait (no disorder) but will be a
carrier of the disorder
 Some disorders are caused by dominant genes
 Those people without the disorder are homozygous
recessive
 Examples of Dominant Genetic Disorders:
 Huntington’s Disease
 Achondroplasia
 Huntington’s Disease -
http://www.youtube.com/watch?v=65xf1olEpQM
 A diagram that traces the inheritance of a particular
trait through several generations
 Read about Pedigrees on pp. 299-301
 Complete the Mini-Lab on p. 300
Chapter 11.2
Sex Determination, Sex-Linked Traits, &
Sex Inf luenced Traits
RED Flower x WHITE Flower ---> PINK Flower
With incomplete
dominance, a cross
between organisms with
two different phenotypes
produces offspring with a
third phenotype that is a
blending of the parental
traits.
 There are foxes that roam the forests in northern
Baltimore County. They come in three colors, blue,
gold, and green. This trait is controlled by a single gene
with incomplete dominance.
 A homozygous (BB) individual is blue, a homozygous
(bb) individual is gold, and a heterozygous (Bb)
individual is green.
 What would be the genotypes and phenotypes of the
offspring if a blue fox were crossed with a gold one?
CODOMINANCE
red x white ---> red & white spotted
With codominance, a
cross between organisms
with two different
phenotypes produces
offspring with a third
phenotype in which both
of the parental traits
appear together.
 Longhorns may be white (CWCW), red (CRCR) or roan
(CRCW). Roan longhorns have a mixture of both white
hairs and red hairs due to a codominant gene.
 "A single copy can be expressed by just a few white
hairs on the face or extremities, unevenly roaned
patches, or an even mix of white and colored hairs all
over the body. Two copies produce an almost white
animal, with some pigment around the ears.”
 What would the phenotypes and genotypes of the
offspring if a red male were crossed with a white
female?
 Sickle cell disease is caused by the




allele that controls the formation of
the protein hemoglobin.
Hemoglobin is the part of the red
blood cell that carries oxygen.
The allele for normal hemoglobin (A)
results in red blood cells that are discshaped.
The sickle cell allele (S) changes the
hemoglobin and results in red blood
cells that are sickle -shaped.
People who are heterozygous for these
two alleles (AS) have both normal and
sickle-shaped red blood cells.
PROBLEM:
What is the risk that two people
heterozygous for the sickle-shaped allele
will have a child with sickle cell disease?
MULTIPLE ALLELES
 A single gene that has more than two possible alleles.
ALLELE
IA
IB
i
CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood
Notice that that the allele for "O" (i) is recessive
to the alleles for "A" & "B".
With three alleles we have a higher number of possible
combinations in creating a genotype.
GENOTYPES
IAIA
IAi
IBIB
IBi
IAIB
ii
RESULTING PHENOTYPES
Type A
Type A
Type B
Type B
Type AB
Type O
 A woman with Type O blood (ii) and a man who is
Type AB (IAIB) have are expecting a child.
 What are the possible blood types of the child?
 One allele hides the effects of another allele
Primula flowers produce a blue
pigment with the dominant Gene K
However, the presence of a dominant Gene D will
mask effect of Gene K
(KKDD, KKDd, KkDD, KkDd will all appear another color)
 Gender is determined by a pair of
chromosomes called the sex
chromosomes.
 two types: X and Y
 females have two X chromosomes
 males have one X and one Y
chromosome
 In humans, the remaining 22 pairs
of chromosomes are homologous
and are referred to as autosomes.
 X chromosome is larger than Y
 ‘X’ contains 900-1400 genes
 ‘Y’ contains fewer than 100 genes –
mostly related to “maleness”
 In females, one of the X
chromosomes is inactivated.
 X-inactivation occurs randomly in each
body cell.
 Form of dosage compensation.
 Inactivated X-chromosome condenses
into a darkly staining region called a
Barr body.
 Example:
 Calico color in cats
Calico cat
 Traits controlled by genes on the sex chromosomes are
called sex-linked traits.
 Mostly X-linked traits; very few Y-linked traits.
 Males disproportionately affected by recessive alleles on the X
chromosome.
 Examples:
 Red-green color blindness
 Hemophilia
 A recessive X-linked trait
 Affects 8-12% of males in the United States; less than 0.5% of females.
 Results when the color-detecting cones in the retina of the eyes
function ‘poorly’ in discriminating between red and green colors.
Normal vision
Red-green colorblind vision
PROBLEM:
 A man with normal vision and a
woman who is heterozygous for the
colorblind allele want to have a
child.
 ‘B’ is the allele for normal vision
 ‘b’ is the allele for color blindness
 What is the probability that their
child will be colorblind?
 Could they have a colorblind
daughter?
XB
Xb
XB
Y
XBXB XBY
XBXb XbY
SOLUTION:
 25%
 No, they cannot have a colorblind daughter. The only child that could be
affected would be a boy.
Other
Colorblindness
Tests
Hemophilia
 A recessive X-linked disorder
 Results in a delayed clotting of
blood
 Will it be more prevalent in
males or females? Why?
 A man with hemophilia has
children with a woman who is
a carrier for hemophilia. What
is the chance that their next
child will have hemophilia?
Their next son? Their next
daughter?
 Traits located on the autosomes can sometimes be
affected by the proportion of sex hormones produced by
the body.
 Ultimately sex hormone production is prescribed by the sex
chromosomes.
 These are called sex-influenced traits
 Example:
 Male pattern baldness – gene expressed in the presence of testosterone
 Traits affected by more
than one gene
 Examples:
 Skin color
 Eye color
 Height
ADAM Inc., Male Pattern Baldness. 14 Apr 2008. U.S. National Library of Medicine 16 Apr 2008
<http://www.nlm.nih.gov/medlineplus/ency/imagepages/17083.htm>
ADAM Inc., Various Tests for Color Blindness. 2007. New York Times Company. 16 Apr 2008
<http://www.nytimes.com/imagepages/2007/08/01/health/adam/9962Colorblindnesstests.html>.
Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.
Color in Computer Graphics. 25 Feb 1998. Cornell University Program in Computer Graphics. 16 Apr 2008
<www.graphics.cornell.edu/online/tutorial/color/>.
Waggoner, Terrace L.. "About Color Blindness (Color Vision Deficiency)." Colors for the Color Blind. U.S. Naval Hospital,
Pensacola, FL. 16 Apr 2008 <http://www.toledo-bend.com/colorblind/aboutCB.html>.
"X chromosome." Genetics Home Reference: Your Guide to Understanding Genetic Conditions. 14 Apr 2008. U.S. National
Library of Medicine. 16 Apr 2008 <http://ghr.nlm.nih.gov/chromosome=X>.
“X chromosome.” Photo Researchers, Inc. 16 Apr 2008.
Chapter 11.3
Karyotypes and Nondisjunction
 Some inherited traits can be identified at the
chromosome level.
 Geneticists use karyotypes.
 Chromosomes are stained.
 A photomicrograph is taken of a cell’s chromosomes
during metaphase.
 Chromosomes pairs are arranged in order of decreasing
size.
Amniocentesis
procedure
Test
Benefit
Risk
Amniocentesis
• Diagnosis of chromosome abnormalities
• Diagnosis of other defects
• Discomfort for expectant
mother
• Slight risk of infection
• Risk of miscarriage
Chorionic villus
sampling
• Diagnosis of chromosome abnormality
• Diagnosis of certain genetic defects
•Risk of miscarriage
•Risk of infection
•Risk of newborn limb defects
Fetal blood sampling
• Diagnosis of genetic or chromosome
abnormality
• Checks for fetal blood problems and oxygen
levels
• Medications can be given to the fetus before
birth
• Risk of bleeding from sample
site
• Risk of infection
• Amniotic fluid might leak
• Risk of fetal death
 Cell division in which either the homologous pairs or
sister chromatids do not separate correctly, resulting in
gametes with an abnormal number of chromosomes.
 Monosomy – having only one of a particular type of
chromosome.
 Trisomy – having a set of three chromosomes of one kind
 e.g. Down syndrome (Trisomy 21)
 Results in distinctive facial features, short stature, heart defects,
and mental disability
 Females can survive with only one X chromosome or with three. Males
can survive with either an extra X or an extra Y. Males cannot survive
with only a Y chromosome.
Genotype
XO
XXX
XXY
XYY
OY
Phenotype
Female with
Turner’s
syndrome
(sterile)
Nearly normal
female
Male with
Klinefelter’s
syndrome
(sterile)
Normal or nearly
normal male
Results in death
Biggs, Haggins, Holliday. Biology (Glencoe Science)(Teacher Wraparound Edition). New York: GLENCOE MCGRAW HILL,
2007.
Farabee, M. J. Human Genetics. 2001. 01 Apr. 2009
<http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookhumgen.html>.
Downschild. Digital image. About Downs Syndrome. 2008. The Coventry and Rugby Down's Syndrome Support Group. 01
Apr. 2009 <http://www.downssupport.org.uk/aboutdowns.html>.
"Genetics and pregnancy loss." Miscarriage Management™ -. 2009. Sydney IVF Limited. 01 Apr. 2009
<http://www.miscarriage.com.au/basepage.cfm?id=16>.
Karyotype. Digital image. AP Biology. 01 Apr. 2009 <http://www.nkellogg.com/apbiology.htm>.
Nondisjunction. Digital image. 01 Apr. 2009 <http://porpax.bio.miami.edu/~cmallery/150/mendel/heredity.htm>.