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Genetics Chapter 4
Terms in this set (85)
describes inheritance patterns that obey two laws..
1. Law of segregation
2. Law of independent assortment
Table 4.1 describes several different patterns of Mendelian inheritance. These patterns are examined with two goals in mind...
1. Predict the outcome of crosses
2. Understand the relationship between the molecular expression of a gene and the trait itself
Simple Mendelian inheritance involves...
-A single gene with two different alleles
-Alleles display a simple dominant/recessive relationship
heterozygote phenotype is between corresponding homozygotes
dominant phenotype is not expressed even though an individual carries a dominant allele
heterozygote trait is more beneficial than either homozygote
heterozygote expresses both alleles simultaneously (NOT forming an intermediate phenotype, shows BOTH)
inheritance of genes located on x-chromosome
-effect of sex on the phenotype of the individual; some alleles are recessive in one sex and dominant in the other.
this refers to traits that occur in only one of the two sexes
an allele that has the potential of causing the death of the organism
-prevalent alleles in a population
-typically encode proteins that function normally/are made in the proper amounts
a phenomenon in large populations where more than one wild-type allele may occur
-Alleles that have been altered by mutation
-often defective in their ability to express a functional protein
-tend to be rare in natural populations
-often inherited in a recessive fasion
Genetic diseases are usually caused by ________
In many human genetic diseases , the _____allele contains a mutation, which....??
-prevents the allele from producing a fully functional protein (In other words, a single copy of the dominant allele is sufficient to mask the effects of the recessive allele)
Are dominant mutants more or less common than recessive?
Dominant Mutants are much less common than recessive
Three explanations for most dominant mutant alleles:
-Protein encoded by the mutant gene is changed so it gains a new or abnormal function
-lead to increased activity of a certain protein in tissues which normally express it (i.e. higher gene dosage causing Downs Syndrome)
-Example: a mutant gene may be over-expressed or expressed in the wrong cell type hemoglobin Kempsey, a mutant hemoglobin with such high oxygen affinity that it does not release oxygen to tissues
-Protein encoded by the mutant gene acts antagonistically to the normal protein
-In a heterozygote, the mutant protein counteracts the effects of the normal protein, thereby altering the phenotype
-Dominant mutant allele is a loss-of-function allele
-Heterozygote exhibits abnormal phenotype (does not make enough product to give the wild type phenotype)
-allele doesn't cause the phenotype we'd expect
-trait is either PRESENT or NOT
-The term indicates that a dominant allele does not always "penetrate" into the phenotype of the individual
-ie the dominant allele does not influence the outcome of a trait in a heterozygote individual
INCOMPLETE PENETRANCE EXAMPLE.
-Autosomal dominant trait
-Affected individuals have additional fingers and/or toes
-A single copy of the polydactyly allele is usually sufficient to cause this condition
-In some cases, however, individuals carry the dominant allele but do not exhibit the trait
The measure of penetrance is described at the____level
If 60% of heterozygotes carrying a dominant allele exhibit the trait allele, the trait is ____ penetrant
the degree to which a trait is expressed
In the case of polydactyly, the number of digits can vary.
A person with several extra digits has _______ of this trait. A person with a single extra digit has_______.
high expressivity; low expressivity
How do we explain incomplete penetrance and variable expressivity?
-The molecular explanation of expressivity and incomplete penetrance may not always be understood
-In most cases, the range of phenotypes is thought to be due to influences of the ENVIRONMENT and/or OTHER 'MODIFIER' GENES
How might the environment impact phenotype?
Environmental conditions may have a great impact on the phenotype of the individual
-temperature-sensitive allele, diet
phenotypic effects are dependent on the temperature
Temperature-sensitive allele example (Arctic Fox)
-Arctic fox changes coat color
-grayish brown in summer, white in winter
Temperature-sensitive allele example (Siamese Cat)
-The allele produces a temperature-sensitive tyrosinase gene that is inactive at the cat's core body temperature, leaving a light brown background.
-However, at the tips of the extremities, which are much cooler, the enzyme is active and produces normal amounts of pigment, creating the characteristic dark points of the Siamese.
-Indeed, indoor Siamese cats living in warm homes tend to be lighter than their outdoor compatriots, which can become quite dark during cold winters.
-The cb allele is also temperature sensitive, but less so than the cs allele, and therefore produces a darker coat.
-Humans affected by phenylketonuria (PKU) are unable to metabolize phenylalanine (which is found in most protein rich foods)
-symptoms include mental retardation, underdeveloped teeth, foul smelling urine
-When detected early, individuals can be fed a restricted diet essentially free of phenylalanine and remain symptom free
Geneticists often examine a range of conditions when studying the effect of environment on phenotype, which allows them to see...
the "norm of reaction" of the environmental influence on phenotypic range
To evaluate the "norm of reaction", researchers begin with _______ strains that have ______ genotypes and subject them to ________.
different environmental conditions
In the case of polydactyly, the number of digits can vary.
A person with several extra digits has _______ of this trait
A person with a single extra digit has low expressivity
the heterozygote exhibits a phenotype that is intermediate between the corresponding homozygotes
Incomplete Dominance example: Flower color in the four o'clock plant
CR = wild-type allele for red flower color
CW = allele for white flower color
-50% of the CR protein is not sufficient to produce the red phenotype, produces pink
-result: 1:2:1 phenotypic ratio NOT the 3:1 ratio observed in simple Mendelian inheritance
Whether a trait is dominant or incompletely dominant may depend on...
how closely the trait is examined
-Traits may appear dominant at the macroscopic level but may actually be incomplete dominant at the microscopic level
--AKA heterozygote advantage
-the phenomenon in which a heterozygote is more vigorous than both of the corresponding homozygotes
-when heterozygotes have superior traits
Overdominance example: Sickle-cell anemia
-Autosomal recessive disorder
-Affected individuals produce abnormal form of hemoglobin
HbA Encodes the normal hemoglobin, hemoglobin A
HbS Encodes the abnormal hemoglobin, hemoglobin S
The sickle cell allele has been found at a fairly high frequency in parts of Africa where malaria is found
-Malaria is caused by a protozoan, Plasmodium. This parasite undergoes its life cycle in two main parts (one inside the Anopheles mosquito, the other inside red blood cells)
-Red blood cells of heterozygotes, are likely to rupture when infected by Plasmodium sp., which prevents the propagation of the parasite
-Therefore, HbAHbS individuals have an "advantage" over
HbSHbS (because they do not suffer from sickle cell anemia) AND HbAHbA (because they are more resistant to malaria)
Overdominance at the molecular level is due to...
two alleles that produce slightly different proteins
Three possible explanations for overdominance at the molecular/cellular level
1. Disease resistance
2. Homodimer formation
3. Variation in functional activity
-A microorganism will infect a cell if certain cellular proteins function optimally
-Heterozygotes have one altered copy of the gene. Therefore, they have slightly reduced protein function
-This reduced function is not enough to cause serious side effects, but it is enough to prevent infections
Disease Resistance examples:
-Sickle-cell anemia and malaria
-Tay-Sachs disease and tuberculosis
-PKU and fungal toxins
-a protein composed of two different subunits is called a dimer
-When both subunits are encoded by the same gene, the protein is called a homodimer
Homodimer formation example with A1 and A2
-A1A1 homozygotes make only A1A1 homodimers; A2A2 homozygotes make only A2A2 homodimers
-A1A2 heterozygotes Make A1A1 and A2A2 homodimers AND A1A2 homodimers
-For some proteins, the A1A2 homodimer may have better functional activity, giving the heterozygote superior characteristics
Variation in functional activity
-A gene, E, encodes a metabolic enzyme
-Allele E1 encodes an enzyme that functions better at lower temperatures; Allele E2 encodes an enzyme that functions better at higher temperatures
-E1E2 heterozygotes produce both enzymes, therefore, they have an advantage in that they function over a wider temperature range than either E1E1 or E2E2 homozygotes
-Typically three or more different alleles
-Many genes have multiple alleles
-Multiple alleles are commonly found within natural populations
Multiple alleles example
see powerpoint. ABO blood slides.
Many species have males and females that differ in their sex chromosome composition. Certain traits are governed by genes on the ______. A pedigree for an X-linked disease shows that it is mostly _____ that are affected with their ______ as carriers. ____are more likely to get an X-linked disease because...??
-hey only have one X chromosome and therefore, only one copy of the gene
-refers to the very few genes found on both X and Y chromosomes
-Found in homologous regions needed for chromosome pairing
-The X and Y chromosomes contain short regions of homology where the X and Y chromosomes carry the same genes
-These regions promote the necessary pairing of the X and Y chromosomes that occur during Meiosis I
-Inheritance patterns resembles that of a gene located on an autosome.
refers to chromosomes that differ between males and females
gene is located on x-chromosome, but not y-chromosome
x-linked recessive pattern
the allele causing the disease is recessive and located on the x-chromosome
a second cross in which the sexes and phenotypes are reversed
the few genes that are located only on the y-chromosome
-Traits where an allele is dominant in one sex but recessive in the opposite sex, thus, sex influence is a phenomenon of heterozygotes
-Sex-influenced does NOT mean sex-linked
-Sex-influenced traits are autosomal
-Scurs (hornlike growth) in cattle; This trait is characterized by small hornlike growths on the frontal bone.
-Caused by an autosomal gene
-Allele B is dominant in males, but recessive in females
a trait occurs in only one of the two sexes
-one that has the potential to cause the death of an organism
-These alleles are typically the result of mutations in essential genes
-They are usually inherited in a recessive manner
-Can kill an organism at a very early age (during development) or later in life.
-those that are absolutely required for survival
-The absence of their protein product leads to a lethal phenotype
-It is estimated that about 1/3 of all genes are essential for survival
-Loss of function mutations in essential genes cause death
-those not absolutely required for survival
-Loss of function mutations in nonessential genes will notcause death
Conditional lethal alleles
-may kill an organism only when certain environmental conditions prevail
-Temperature-sensitive (ts) lethals
A developing Drosophila larva may be killed at 30°C
But it will survive if grown at 22°C (permissive
-Typically caused by mutations that alter structure of the protein at the nonpermissive temperature
-Kill some individuals in a population, not all of them
-Environmental factors and other genes may help prevent the detrimental effects of semilethal genes
A lethal allele may produce ratios that ______ Mendelian ratios. The allele causes death at ____ age and the _____ individual is not apparent in the population.
-seemingly deviate from
Lethal Allele example:
-Manx cat carries a dominant mutation that affects the spine
-This mutation shortens the tail
-This allele is lethal as a homozygote
When will the lethal allele affect the individual?
-Many lethal alleles prevent cell division; these will kill an organism at an early age
-Some lethal alleles exert their effect later in life
Example of a lethal allele that exerts effect later in life
-characterized by progressive degeneration of the nervous system, dementia and early death
-The age of onset of the disease is usually between 30 and 50
-when a single gene affects the expression of more than one phenotype
what can cause Pleiotropy?
-The gene product can affect cell function in more than one way
-The gene may be expressed in different cell types
-The gene may be expressed at different stages of development
Pleiotropy example: Cystic fibrosis
-Normal allele encodes the cystic fibrosis transmembrane conductance regulator (CFTR), which regulates ionic balance by transporting Cl- ions
-Mutant does not transport Chloride effectively. In lungs, this causes very thick mucus on the skin, causes salty sweat, males are often sterile because Cl- transport is needed for proper development of the vas deferens
-Defect in CFTR can have multiple effects
Pleiotropy example: Albinism
-A mutation that results in a non-functional enzyme anywhere along the pathway will cause a failure to produce one or both forms of melanin.
-The result affects the expression of multiple phenotypic characters (eye color, skin color, hair color, sensitivity to sunlight, vision-melanin is necessary for normal eye development)
-occur when two or more different genes influence the outcome of a single trait
-epistasis, complementation, gene redundancy
A Cross Involving a Two-Gene Interaction Can Still Produce ____ distinct phenotypes
Gene Interaction example: Inheritance of comb morphology in chicken
-Comb types come in four different morphologies
-Thus, the F2 generation consisted of chickens with four types of combs
9 walnut : 3 rose : 3 pea : 1 single
-conclusion: comb morphology is determined by two different genes
R (rose comb) is dominant to r
P (pea comb) is dominant to p
R and P are codominant (walnut comb)
rrpp produces single comb
Gene Interaction example: flower color in the sweet pea
-Lathyrus odoratus normally has purple flowers
-carried out the following cross
P: True-breeding purple X true-breeding white
F1: Purple flowered plants
F2: 9 purple : 7 white
-conclusion: flower color is determined by two different genes
C (one purple-prod) allele is dominant to c (white)
P (another purple-prod) allele is dominant to p (white)
cc or pp masks P or C alleles, producing white color
-Thus, a plant that is homozygous for either recessive white allele would develop a white flower, regardless whether of not the other gene contains a purple-producing allele
-describes the situation in which a gene can mask the phenotypic effects of another gene
-often arise because two (or more) different proteins participate in a common cellular function
Epistasis example: Enzymatic pathway
-If an individual is homozygous for either recessive allele it will not make any functional enzyme C or enzyme P
-Therefore, the flowers remain white
-both homozygotes of the mutant gene have the same phenotypic effect
A Cross Involving a Two-Gene Interaction Can Produce ____distinct phenotypes due to Epistasis
Epistasis example: coat color in rodents
-Geneticists have developed techniques to directly generate loss-of-function alleles
-Allows scientists to understand the affects of the gene on structure or function of the organism
existence of several genes in the genome of an organism that perform the same role to some extent
This set is often in folders with...
Genetics: Chapter 4
Gene & Gene Mutations
Genetics Extensions of Mendelian Inheritance
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