CH 14 NOTES, part 2 – Extending Mendelian Genetics
The Spectrum of Dominance
● Complete dominance occurs when the phenotypes of the heterozygote and dominant homozygote are identical
CODOMINANCE:
● In codominance
affect the phenotype in separate, distinguishable ways
● Example:
● inheritance characterized by
in the heterozygote
Example: in chickens, BB =
, Bb = “
, bb =
” (both black and
white feathers)
INCOMPLETE DOMINANCE:
● inheritance where one allele is not completely dominant over the other, so the heterozygote has a
between the phenotype of the 2 homozygotes
Ex: RR =
, rr =
, Rr =
The Relationship Between Dominance and Phenotype
● Dominant and recessive alleles do not really “interact”
● they lead to
that produce a phenotype
Frequency of Dominant Alleles
● Dominant alleles are not necessarily more common in populations than recessive alleles
Multiple Alleles
● Most genes exist in populations in more than two allelic forms
● The
is determined by multiple alleles
Polygenic Inheritance
● Many human characters vary in the population along a continuum and are called quantitative characters
● mode of inheritance in which the
determines a single
phenotypic character
Examples:
(4 gene pairs)
(2 gene pairs)
● Quantitative variation usually indicates polygenic inheritance (An additive effect of 2 or more genes on a single
phenotype)
PLEIOTROPY:
● the ability of a
Examples:
sickle cell anemia
Siamese cats & tigers
EPISTASIS:
● interaction between 2 nonallelic genes in which
Example: gene for pigment deposition is epistatic to gene for melanin production in mice
Nature and Nurture: The Environmental Impact on Phenotype
● Another departure from simple Mendelian genetics arises when the phenotype for a character depends on
environment as well as on genotype
● The norm of reaction is the phenotypic range of a particular genotype that is influenced by the environment
MULTIFACTORIAL INHERITANCE:
● A trait depends on many factors; a variety of genotypes as well as environmental influences (such as
,
, or
)
Examples of disorders that may be a result of multifactorial inheritance: diabetes, heart disease, neural tube
defects, autism, Alzheimer disease, ALS, and many cancer syndromes
● An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
● the phenotype reflects its overall genotype and unique environmental history
Inheritance Patterns for Genetic Diseases in Humans
Pedigree Analysis
● A pedigree is a
that describes the interrelationships of parents and children
across generations
● Inheritance patterns of particular traits can be traced and described using pedigrees
1) Autosomal Recessive:
● recessive alleles that cause human disorders are usually
● defective alleles code for either a malfunctioning protein or no protein at all
● heterozygotes can be phenotypically normal, if 1 copy of the normal allele is all that is needed to produce
sufficient quantities of the “good” protein
Examples: cystic fibrosis, Tay-Sachs, sickle cell anemia
● “Carriers” are
who carry the recessive allele but are
Cystic Fibrosis
● Symptoms of cystic fibrosis include:
-
in the some internal organs
-Abnormal absorption of nutrients in the small intestine
Sickle-Cell Disease
● Sickle-cell disease:
-Affects one out of 400 African-Americans
-Is caused by the substitution of a single amino acid in the
● Symptoms include:
-Physical weakness, pain, organ damage, and even paralysis
2) Autosomal Dominant:
● only 1 dominant allele is needed in order to produce the effects of these diseases (heterozygous)
●
condition results in spontaneous abortion of fetus
● homozygous recessives are of normal phenotype
Examples: achondroplasia, Huntington’s Disease
● ACHONDROPLASIA: a
that is lethal when homozygous for the dominant allele
● HUNTINGTON’S DISEASE: a
-it has no obvious phenotypic effects until about 35 to 40 years of age
Genetic Testing and Counseling
● Genetic counselors can provide information to prospective parents concerned about a family history for a specific
disease
● Using family histories genetic counselors help couples determine the odds that their children will have genetic
disorders
Tests for Identifying Carriers
● For a growing number of diseases tests are available that identify carriers and help define the odds more
accurately
Fetal Testing
● AMNIOCENTESIS: the
● CHORIONIC VILLUS SAMPLING (CVS): a
tested
is removed and tested
is removed and