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By the late nineteenth century…Charles Darwin’s theory of natural selection
suggested that a population could evolve if members show variation in heritable traitsVariations that improved an individual’s
chances of surviving would be more common in each generation
Over time, a population____________________Prevailing view:“____________ _______________”
Hereditary material must be fluidFluids from both parents blend at
fertilization, causing ___________________________
What’s Wrong with This Theory?Blending inheritance doesn’t explain the
observable variations in a population that allows for evolutionWould produce uniform populationsMany observations did not support blending
Example:
The theory of natural selection does not fit with this view of inheritance
Gregor MendelCalled:
Used experiments in plant breeding and his knowledge of mathematics to form his own hypotheses about inheritance
_________ _____Pisum sativum: the garden pea plantThis plant can fertilize itself
Its flowers produce both male and female gametes, which fertilize and give rise to new plants
Mendel could use true-breeding varietiesSuccessive generations will be ____________
in a certain traitOffspring grown from seeds of white-flowered
parent plants also have white flowersPeas can also be cross-fertilized by human
manipulation of pollen
Mendel’s Experimental Approach
Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits
For example, white flowers vs. purple flowers
Figure 10.3
Producing HybridsTrue-breeding lineage occurs when
offspring inherit __________ _________ in each generation
___________ are the offspring of a cross between two individuals that breed true for different forms of a trait
Each inherits ______________ ___________ for a trait being studied
Terms Used in Modern Genetics_______ : heritable units of information
about traitsEach is located at its own particular ________ on the chromosome
_________ : different molecular forms of the same gene
____________ : permanent change in a gene’s information that alters a gene’s molecular structure
AllelesWhen both alleles are the same, the
condition is the _______________If the alleles differ, then it is the
______________ conditionAn allele is ___________ when its effect on a
trait masks the effect of any __________ allele paired with itDominant alleles are signified by
capitalized lettersLowercase letters signify recessive ones
AllelesA homozygous dominant individual
has a pair of dominant alleles: ______A heterozygous individual has a pair
of nonidentical alleles: ______ Homozygous recessive: pair of
recessive alleles _______
Gene ExpressionDominant allele may mask effect of recessive
allele on the homologous chromosome Gene expression is the process in which the
gene’s information is converted to its function.Expressed genes _________________________.
_____________ : an individual’s alleles at any or all gene loci (their genetic make up)
______________ : an individual’s observable traits; how their genes are expressed
Mendel’s ExperimentsMendel suspected that every plant inherits two
“units” of information for a trait, one from each parent
Mendel’s first experiments were _____________ __________
Monohybrid crosses have two parents that are true-breeding for contrasting forms of a trait For example, pea plants with white flowers and
pea plants with purple flowers____ = parent generation____ = first-generation offspring____ = second-generation offspring
Monohybrid CrossesIn the first-generation offspring (F1):
One form of the trait (white flowers) disappears
All offspring have purple flowersWhen Mendel crossed those offspring (F1 x
F1), the white flowers reappeared!What is going on?
The parent pea plants must have been:
__________________________________________________
Monohybrid CrossThe parent pea plants must have been:
F1: The offspring, therefore, must ALL be:
F2: The offspring of the second cross (F1 x F1) will:
Monohybrid CrossWe now know that all members of the F1
offspring are heterozygous (Aa) because one parent could produce only an A gamete and the other could produce only an a gamete
Mendel assumed that each sperm has an equal probability of fertilizing an egg
Monohybrid CrossThe numerical ratios of crosses suggested
that genes do not blendFor example, the F2 offspring showed a
3:1 phenotypic ratio of purple to whiteThus, each new plant has three chances
in four of having at least one dominant allele
Testcross___________ : method of determining genotype
One individual of unknown genotype is crossed with another that is homozygous recessive
The results will show if the individual is homozygous or heterozygous for a dominant trait
To support his concept of segregation, Mendel crossed F1 plants with homozygous recessive individuals
A ratio of ______ of recessive and dominant phenotypes supported his hypothesis
Theory of SegregationMendel’s Theory of Segregation: diploid
organisms have pairs of genes, on pairs of homologous chromosomes
States that diploid organisms inherit two genes per trait on pairs of chromosomes. Each gene segregates from the other during meiosis such that each gamete will receive only one gene per trait.
Mendel’s Dihybrid Experiments_______________ are the offspring of parents
that breed true for different versions of two traits
In addition to his monohybrid crosses, Mendel also performed experiments involving two traits: a ___________ _______________
True breeding parents: AABB x aabb
F1 hybrid offspring:
Gametes
Dihybrid CrossesMendel correctly predicted that all F1
plants would show both of the dominant allelesExample:
He wondered if the genes for flower color and plant height would always travel together when two F1 plants were crossed
Dihybrid CrossThe F2 results:
tall and purple-flowered:dwarf and white-flowered:
dwarf and purple-flowered:tall and white-flowered:
Mendel’s Theory of ______________ ______________
The Mendelian theory of independent assortment states that as meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another, independently of gene pairs of other chromosomes
Requires qualification, because gene pairs do not always assort independently
CodominanceIn _______________ , nonidentical alleles for a gene are
fully expressed in heterozygotesMay occur in ___________ _________ __________, in which
three or more alleles of a gene persist among individuals
Example: ______________________________Blood type is determined by markers produced
by three genes (a multiple allele system)Red blood cells have membrane glycolipids
which helps the body identify its own blood cellsThe ABO gene encodes an enzyme which
determines the form of the glycolipidIA and IB are each dominant to i, but are
codominant to each other
ABO Blood TypesBlood type is determined by the alleles
carried for the ABO gene: _________________A and B are codominant when paired; this
means that some people can express both genes and have _____ blood
The O allele is __________when paired with A or B
Figure 10.10
Incomplete DominanceIn ___________ __________, one dominant allele
cannot completely mask the expression of anotherOne allele of a pair is not fully dominant over its
partnerExample: ______________________________
A true-breeding red-flowered snapdragon crossed with a white-flowered snapdragon will produce _________ ____________
This pink color is because there is not enough red pigment (produced by the dominant) allele to completely mask the effects of the white allele
EpistasisTraits are expressed through ____________ :
interactions among products of two or more gene pairs
Two alleles can mask the expression of another gene’s alleles because of these interactions
Examples: variations in chicken combs and Labrador retriever coat colors
Single Genes with a Wide ReachSometimes the expression of alleles at one
location can have effects on two or more traits, which is called ______________
Marfan syndrome: genetic disorder which arises by mutations in the fibrillin geneFibrillin protein allows elasticity in many tissues,
including the heart, skin, blood vessels, skeleton, and tendons
Characterized by these effects: lanky skeleton, leaky heart valves, weakened blood vessels, deformed air sacs in lungs, pain, and lens displacement in the eyes