genetics midterm lesson 1 Flashcards
Prevalent alleles in a
natural population.
* Example: Elderflower orchid,
Dactylorhiza sambucina
Wild-type alleles
more than one
wild-type allele may occur
Genetic polymorphism
tend to promote the
reproductive success of organisms in
their native environments.
Wild-type alleles
random mutations occur
in populations and alter preexisting
alleles. Called as such to distinguish
them from the more common wild-type
alleles
Mutant alleles
usually cause a substantial decrease in the expression of a functional protein supported by the analysis of many
human genetic diseases.
Recessive mutant alleles
is usually caused by a mutant
allele.
genetic disease
much less common than recessive mutant alleles.
Dominant mutant allele
- Gain-of-function mutation
- Dominant-negative mutation
- Haploinsufficiency.
Three explanations account for most dominant mutant
alleles :
*Change the gene or the protein encoded by a gene
so it gains a new or abnormal function. For
example, a mutant gene may be overexpressed or it
may be expressed in the wrong cell type.
Gain-of-function mutations
*Change a protein such that the mutant protein acts
antagonistically to the normal protein. In a
heterozygote, the mutant protein counteracts the
effects of the normal protein, thereby altering the
phenotype.
Dominant-negative mutations
- The dominant mutant allele is a loss-of-function
allele.
Haploinsufficiency
used to describe patterns of
inheritance in which a heterozygote (with one
functional allele and one inactive allele) exhibits an
abnormal or disease phenotype.
Haploinsufficiency
An example in humans is polydactyly.
Haploinsufficiency
allele that is expected to cause
a particular phenotype does not.
INCOMPLETE PENETRANCE
The measure of penetrance is described at the
population level. For example, if 60% of the
heterozygotes carrying a dominant allele exhibit the
trait, we say that this trait is 60% penetrant.
INCOMPLETE PENETRANCE
At the individual level, the trait is either present or not.
INCOMPLETE PENETRANCE
Human pedigree for a dominant trait
known as
polydactyly
This trait causes the affected individual to
have additional fingers or toes (or both)
polydactyly
the allele is found in a
gene located on an autosome (not a sex
chromosome)
autosomal dominant allele
a single copy of this
allele is sufficient to cause this
condition.
autosomal dominant allele
term used to describe the outcome of traits is the degree to which the trait is expressed
Expressivity
goes through two color
phases. During the cold winter, it is
primarily white, but in the warmer
summer, the fox is mostly brown
arctic fox (Alopex lagopus)
The
phenotypic effects are dependent on
the temperature
Temperature-sensitive allele
is caused by a defect in
a gene that encodes the enzyme
phenylalanine hydroxylase.
Phenylketonuria (PKU)- autosomal
recessive disease
manifest a variety of
detrimental traits including mental
impairment, underdeveloped teeth, and
foul-smelling urine.
Phenylketonuria
When given a
standard diet containing phenylalanine,
which is found in most protein-rich
foods,
Phenylketonuria
restricted diet free of
phenylalanine, they develop properly
Phenylketonuria
first
observed this phenomenon in the color of the
flowers of the four-o’clock plant (Mirabilis
jalapa)
Carl Correns
lack of a functional protein required for pigmentation.
INCOMPLETE DOMINANCE
heterozygotes may produce only 50% of the normal protein, but this amount is not sufficient to produce the same phenotype
haploinsufficiency
50% of the functional protein cannot accomplish the same level of pigment synthesis that 100% of the protein can.
50% of the functional protein cannot accomplish the same level of pigment synthesis that 100% of the protein can.
heterozygote has greater reproductive succes
OVERDOMINANCE
aka:Heterozygote Advantage
an autosomal recessive disorder in which the affected individual produces an altered form of the protein hemoglobin, which carries oxygen within red blood cells.
Sickle cell disease
Individuals affected with sickle cell disease are
homozygous for the HbS allele
This causes their red blood cells to deform into a
sickle shape under
This causes their red blood cells to deform into a sickle shape under conditions of low oxygen concentration
homozygous for the HbS allele
high frequency among human populations that are exposed to malaria.
OVERDOMINANCE
have better resistance to malaria than do HbA HbA homozygotes, while not incurring the ill effects of sickle cell disease.
heterozygous
How can we explain the observation that two protein variants in the HbA HbS heterozygote produce a more favorable phenotype?
- Disease Resistance
- Subunit Composition of Proteins
- Differences in Protein Function
two
alleles are both expressed
in the heterozygous
individual
CODOMINANCE
when a gene is located on the X
chromosome but not on the Y chromosome.
X-linked inheritance-
to indicate that males have a single copy of an X-linked gene
Hemizygous
males are more likely to be
affected by rare,
recessive X-linked disorders.
- X-linked recessive, the allele causing
the disease is recessive and located on the X chromosome
- Duchenne muscular dystrophy
refers to a gene that is found on one of the two types
of sex chromosomes but not on both.
Sex-linked gene
relatively few genes are located only on the
Y chromosome.
Holandric genes
found in mammals.
SRY gene
Its expression is necessary for proper
male development.
SRY gene
The gene is transmitted only from fathers to sons
y linked genes
phenomenon in which an allele is dominant
in one sex but recessive in the opposite sex.
Sex-influenced inheritance
Therefore, sex influence is a
phenomenon of
heterozygotes
controlled by sex hormones or by the pathway
that leads to male and female development
Sex-limited inheritance-
The genes that affect sex-limited traits maybe
autosomal or X-linked.
are the presence of ovaries in females and the presence of testes in males.
sex-limited traits
in which members of the
opposite sex have different morphological features.
Sexual dimorphism
An allele that has the potential to cause the death of an
organism.
LETHAL ALLELES
one that must be present for survival
Essential gene
not absolutely required for survival, although they
are likely to be beneficial to the organism.
Nonessential genes
some lethal alleles may kill an organism only when certain environmental conditions prevail
Conditional lethal alleles
conditional lethal alleles cause an organism to die only in a particular temperature range.
Temperature-sensitive (ts) lethal alleles
may also be identified when an individual is exposed to a particular agent in the environment.
Conditional lethal alleles
People with a defect in the gene that encodes the e_
_____ have a negative reaction to the
ingestion of ____.
enzyme glucose-6- phosphate dehydrogenase (G-6-PD),
fava beans
certain lethal alleles act only in some individuals.
Semilethal alleles-
environmental conditions
and the actions of other genes within the organism may help to
prevent the detrimental effects of certain semilethal alleles.
environmental conditions
and the actions of other genes within the organism may help to
prevent the detrimental effects of certain semilethal alleles.
The time when a lethal allele exerts its effect can vary.
The time when a lethal allele exerts its effect can vary.
multiple effects of a single gene
PLEIOTROPY
all traits are affected by
the contributions of many genes.
GENE INTERACTIONS
