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Mendelian Inheritance: Principles of Genetics

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BEYOND MENDEL
Chapter 14
Mendelian Inheritance in
Humans
Incomplete Dominance
• Heterozygotes have intermediate
phenotype
– i.e. Snapdragon flower color
– Notation varies widely, so on the AP test use
the words “incompletely dominant”
Figure 14.9 Incomplete dominance in snapdragon color
Figure 14.9x Incomplete dominance in carnations
Codominance
• Two alleles affect phenotype in separate,
distinguishable ways
– Both show up if both are present
– i.e. checkered chicken feathers, ABO blood types
• Blood type
– Alleles IA and IB specify different enzymes that
attach different sugars
– IAIB - both enzymes, both sugars = codominance
– ii – no sugars, no enzymes
Multiple Alleles
• Most genes have more than two allele
forms
– i.e. ABO blood types is also an example of a
trait with 3 alleles for 4 different phenotypes
• Alleles: IA, IB, i
• Phenotypes: A, B, AB, or O blood types
Figure 14.10 Multiple alleles for the ABO blood groups
Figure 14.10x ABO blood types
Principles of Dominance and
Recessiveness Relationships
•
•
Ranges from complete dominance and various
degrees of incomplete and codominance
Alleles do not involve the ability of one allele to
supress another at the level of the DNA
–
i.e. round vs. wrinkled pea shape
•
•
•
Dominant allele codes for synthesis of an enzyme that coverts
sugar to starch.
Recessive wrinkled pea, starch is not made, the sugar causes
osmotic uptake of water and when it dries it wrinkles
Dominance does not mean “common” in a
population
–
i.e. polydactyly is dominant
Polydactyly is Dominant!!
Pleiotropy
• One gene affects an organism in many ways
– i.e Sickle-cell disease and Marfan Syndrome
• Marfan Syndrome is a mutation in the gene for
fibrillin, a protein that occurs in connective
tissue. It affects 1 in every 10,000 people
– Skeleton
– Heart and blood vessels
– Lungs
– Nervous System
– Eyes
– Skin
Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
Marfan Syndrome
Epistasis
• Gene at one locus alters the phenotypic
expression of a different gene at a second locus
– i.e. Hair color in mammals: mice, labradors; comb
shape in poultry
• In mice, black (B) coat is dominant to brown
coat color (b)
– A second gene determines whether or not pigment
will be deposited in the hair…dominant (C) for color
results in deposition
– If the mouse is cc, it will be albino regardless of
the genotype at the black/brown locus.
Figure 14.11 An example of epistasis
Polygenes
• Quantitative characteristics vary along a
continuum in a population
• Additive effect of two or more genes on a single
phenotype.
• This is the converse of pleiotropy!
– i.e. Skin pigmentation is controlled by at least 3
separately inherited genes (A, B, C) that add
together to determine the darkness of skin color
• AaBbCc would have an intermediate skin color
Figure 14.12 A simplified model for polygenic inheritance of skin color
• What would the genotype of a
person with light skin color be?
____________
• What would the genotype of a
person with very dark skin
color be? _____________
Environmental Effects
• Nature vs. Nurture
• genetic and environmental factors affect phenotypes
– i.e. hydrangea color – affected by soil (pH, water, temp, etc.)
Figure 14.14 Pedigree analysis
• Reveals the patterns in human inheritance
through generations in families
Recessively Inherited
Genetic Disorders
• Allele that causes a genetic disorder codes
either for a malfunctioned protein or for no
protein at all
• Heterozygotes produce a sufficient amount of
the specific protein
– Carriers because they can transmit the recessive
allele to their offspring
– They do not have the disease
• To have the disorder, one must be
homozygous recessive
Recessively Inherited
Genetic Disorders
•
•
•
•
•
•
•
Albinism
Cystic Fibrosis
Tay-Sachs
ALS (Lou Gehrig’s)
Sickle Cell Anemia
Phenylketonuria (PKU)
Alkaptonuria
Amyotropic Lateral Sclerosis
(Lou Gerhig’s Disease)
• Neurodegenerative –
degradation of motor
nerves that control
voluntary
– Leads to twitching,
denervation, atrophy – loss of
all movement
• Mutation on chromosome
21
• 1-2 of 100,000 people/year
Tay-Sachs Disease
Heterozygotes resistant to tuberculosis
Sickle Cell Anemia
Sickle cell
patients
resistant to
malaria
Cystic Fibrosis
Heterozygotes resistant to cholera
Dominantly Inherited
Genetic Disorders
• Offspring have a 50%
chance of receiving the
dominant allele.
• Most lethal dominant
alleles have survived
in the population b/c
late onset
–
–
–
–
Achondroplasia
Huntington’s disease
Progeria
Marfan syndrome
Multifactorial Disorders
• Have a genetic component and a
significant environmental influence
– i.e.
•
•
•
•
•
•
Heart disease
Cancer
Diabetes
Alcoholism
Schizophrenia
Manic-depressive disorder
Genetic Testing and
Counseling
• Amniocentesis
– 14th -16th week of pregnancy, needle inserted into
uterus and extracts 10ml of amniotic fluid, fetal cells
are cultured for several weeks then karyotyped
• Chorionic Villus Sampling (CVS)
– 8-10 weeks of pregnancy, narrow tube inserted
vaginally that suctions out sample of fetal tissue
from placenta; immediate karyotyping
• Newborn Screening
– used for many diseases such as PKU
PKU: Phenylketonuria
Figure 14.17 Testing a fetus for genetic disorders
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