Chapter 14: Mendel and the Gene Idea
Overview: Drawing from the Deck of Genes
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Mendel had a theory that heredity came from genes and that’s what this chapter is about
14.1 Mendel used the scientific approach to identify two laws of
inheritance
Mendel’s Experimental, Quantitative Approach
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Two of Mendel’s main influences were physicist Christian Doppler and a botanist named Franzy
Unger
Mendel bred garden peas in the abbey garden to study inheritance
o Because they’re available in many varieties
Character—a heritable feature that varies among individuals
o Ex: a flower’s color
o Trait—each variant for a character
 Ex: purple and white color are variants for the character of color of a flower
Mendel also used peas because they have a short generation time, large number of offspring
from each mating, and he could control the mating between the plants
true breeding—a plant with purple flowers is true breeding of the seeds produced by selfpollination in successive generations all give rise to plants that also have purple flowers
Mendel cross-pollinated two contrasting, true-breeding pea varieties
o Ex: purple flowered plants and white flowered plants
o Hybridization—the mating, or crossing, of two true-breeding varieties
P generation—the true breeding parents (parental generation)
F₁ generation—the hybrid offspring of the P Generation
F₂ generation—when the F₁ generation self pollinates, it’s produced
o Mendel followed traits for these three generations
Through his tests, Mendel saw two fundamental principles of heredity: the law of segregation
and the law of independent assortment
o He put them to the test again and again
The Law of Segregation
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Results of first experiment:
o P generation: purple and white flowers
o F₁ generation: all purple flowers
o F₂ generation: 705 purple flowered plants, 224 white flowered plants—3 to 1 ratio
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There was confusion at first because the blending between white and purple
flowers didn’t make pale purple flowers, but it didn’t make all purple or all
white, so both of the genes had to be there
He named the purple flower color the dominant trait and the white flower color the recessive
trait
Mendel’s Model
 Mendel developed a model to explain the 3:1 inheritance pattern:
o 1. Alternative versions of genes account for variations in inherited characters
 Ex: gene for flower color in pea plants exists in two versions: one for purple and
other for white
 Alleles—the alternative versions of a gene
 We see this example today in how each gene is a sequence of
nucleotides at a specific place or locus along a certain chromosome, but
the DNA at that locus can vary slightly in its nucleotide sequence
o 2. For each character, an organism inherits two alleles, one from each parent
 Mendel didn’t even know about chromosomes when he made this rule
 Each organisms has two sts of chromosomes, and so a genetic locus occurs
twice in a cell—the two alleles might be the same at that locus (P generation) or
they may be different (F₁ hybrids)
o 3. If the two alleles at a locus differ, then one, the dominant allele, determines the
organism’s appearance; the other, the recessive allele, has no noticeable effect on the
organism’s appearance
o 4. Law of segregation—the two alleles for a heritable character segregate (separate)
during gamete formation and end up in different gametes
 Egg or sperm only gets one of the two alleles—this happens because
chromosomes segregate in meiosis—if the organism is true-breeding and has
identical alleles for that character then that allele is present in ALL gametes
 If different alleles are present (like in the F₁ hybrids) then 50% of the
gametes get the dominant and 50% get the recessive
 Punnett square—handy diagrammatic device for predicting he allele compostion of offspring
from a cross between individuals of known genetic makeup
o capital letter symbolizes dominant allele, lowercase symbolizes recessive
 Mendel’s model accounts for the 3:1 ratio of traits he observed in the F₂ generation
Useful Genetic Vocabulary
 Homozygous—and organism that has a pair of identical alleles for a character
o Ex PP or pp
o “breed true” because all of their gametes contain the same allele
o If we cross dominant homozygotes with recessive homozygotes every offspring will have
two different alleles
 Pp
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Heterozygous—an organism that has two different alleles for a gene
because of different effects of dominant and recessive alleles, an organism’s traits don’t always
reveal its genetic composition
o phenotype—an organism’s appearance or observable traits
 note that this can mean physiological traits like a pea that lacks the ability to self
pollinate vs. one that can pollinate as well as actual physical looks like color
o genotype—genetic makeup
 ex: plants can have same phenotype of purple, but have different genotypes of
PP or Pp
The Testcross
 testcross—breeding an organism of unknown genotype with a recessive homozygote—it can
reveal the genotype of that organism
o ex: if you have a purple flower and you want to know whether it’s PP or Pp, you
testcross it and if it was PP all offspring will turn out purple because they’ll all be Pp, but
if it was originally Pp, there will be some white ones because there will be at least 1 pp
in the mix
The Law of Independent Assortment
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monohybrids—heterozygous for one character
o Mendel derived his law of segregation from experiments with monohybrids
Mendel identified second law of inheritance by following two characters at the same time
o Ex: seed color and shape
If you cross two true breeding pea varieties that differ in both characters (ex: yellow round
seeds that are YYRR and green wrinkled peas that are yyrr) you will get dihybrids—individuals
heterozygous for two characters (YyRr)
We know that genes have to independently assort instead of staying together as YR and yr..this
means you can get YR, yR, Yr, and yr—we know this because when we had F₁ self reproduce, the
F₂ generation got a 9:3:3:1 ratio of results
o Alleles of one gene are sorted into gametes independently of the alleles of other genes
law of independent assortment—each pair of alleles segregates independently of each other
pair of alleles during gamete formation
o this law only applies to genes located on different chromosomes (not homologous)
 genes located near each other on the same chromosome tend to be inherited
together and have more complex inheritance patterns than predicted by the law
of independent assortment
14.2 The law of probability govern Mendelian inheritance
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two basic rules of probability can help us predict the outcome of the fusion of gametes in simple
monohybrid crosses and more complicated crosses
The Multiplication and Addition Rules Applied to Monohybrid Crosses
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multiplication rule says that in order to determine that probability that two or more
independent events will occur together in some specific combination, we multiply the
probability of one event by the probability of another
o ex: probability that two coin tosses will end up both heads—1/2 (probability that one
toss will end up heads) * ½ (probability that the other toss will end up heads) = ¼
(probability that the two tosses will both end up heads)
o ex for genes: if two Rr plants reproduce, the probability that the offspring will end up rr
is ¼ because there is a ½ chance the egg will have an r and there’s a ½ chance the sperm
will have an r and ½ * ½ = 1/4
 same for RR—this is the rule we use to figure out that probability of an F₂ plant
from a monohybrid cross being homozygous
addition rule—the probability that any one of two or more mutually exclusive events will occur
is calculated by adding their individual probabilities
o this is how we find the probability of a F₂ plant being heterozygous
 ex: ½ (probability that mother will give r) * ½ (probability that father will give R)
= ¼ (probability that there will be Rr)
 you can also get heterozygous if father gives r and mother gives R—1/2
(probability that mother will give R) * ½ (probability that father will give r) = ¼
 ¼+¼=½
Solving Complex Genetics Problems with the Rules of Probability
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We can use the multiplication rule to find probabilities:
o Ex: YyRr— ¼ chance it will be YY, ½ chance it will be Yy, ¼ chance it will be yy; ¼ change
it will be RR, ¼ chance it will be rr, ½ chance it will be Rr
 This means probability of YYRR will be ¼ * ¼ = 1/16; probability of YyRr is 1/2 *
½=¼
o For more examples see pg 271 at the top!
14.3 Inheritance patterns are often more complex than predicted by
simple Mendelian genetics
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Mendel went off the assumption that there were two alleles, one completely dominant and the
other completely recessive, but it’s even more complex than that
Extending Mendelian Genetics for a Single Gene
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Deviation from Mendelian patterns occurs when alleles aren’t completely dominant or
recessive, when a particular gene has more than two alleles, or when a single gene produces
multiple phenotypes
Degrees of Dominance
 Complete dominance—when the phenotype of the heterozygote and the dominant
homozygote are indistinguishable
 Incomplete dominance—when neither allele is completely dominant and so F₁ hybrids have a
phenotype somewhere between those of the two parental varieties
o Ex: red plus white snapdragons makes pink snap dragons—you know however that it’s
not blending because they still produce 1 all red and 1 all white along with the 2 pink
 Codominance—the two alleles both affect the phenotype in separate, distinguishable ways
o Ex: M and N on blood cells—at a single gene locus two variation are possible
 Individuals can either be homozygous MM, homozygous NN, or heterozygous
MN (where red blood cells have both M and N)
 It’s not intermediate between M and N phenotypes (like complete
dominance is) but rather both M and N phenotypes are exhibited since
both molecules are present
The Relationship between Dominance and Phenotype
 Dominant alleles don’t subdue a recessive allele—in fact they don’t interact at all, and it is only
in the pathway from genotype to phenotype that dominance and recessiveness come in to play
 Illustration of relationship between dominance and phenotype:
o Dominant allele for round pea produces an enzyme that makes starch so when the seed
dries, it’s not wrinkled while the recessive allele codes for a defective form of this
enzyme
 One dominant allele results in enough of the enzyme to make enough of the
starch, which is why homozygous dominant and heterozygous have the same
phenotype
 For any character, the observed dominant/recessive relationship of alleles depends on the level
at which we examine the phenotype
o Ex: Tay-Sachs disease—an inherited disorder in humans in which the brain cells of a
child cannot metabolize certain lipids because a crucial enzyme doesn’t work right and
as the lipids accumulate in brain cells, the child begins to suffer seizures, blindness,
degeneration of motor skills and eventual death
 At organismal level it is recessive because only children who inherit two copies
of the alleles have it
 At biochemical level it is characteristic of incomplete dominance because the
activity level of the lipid metabolizing enzyme is intermediate between that in
people with homozygous for the normal allele and those with Tay-Sachs disease
 At the molecular level the normal allele and the Tay-Sachs are codominant
because the heterozygous individuals produce equal number of normal and
dysfunctional enzyme molecules (the number of normal produces is sufficient so
that a heterozygote doesn’t have Tay-Sachs disease
Frequency of Dominant Alleles
 Dominant allele is not always the most popular—oftentimes most people are recessive
Multiple Alleles
 Most genes exist in more than two allelic forms
o Ex: ABO blood groups in humans—blood can be A, B, AB, or O
Pleiotropy
 Pleiotropy—when genes have multiple phenotypic effects
o Most genes are this
 Pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary
diseases
Extending Mendelian Genetics for Two or More Genes
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Dominance relationships, multiple alleles, and pleiotropy all have to do with the effects of the
alleles of a single gene, but now we look at situations in which two or more genes are involved
in determining the phenotype
Epistasis
 Epistasis—when a gene at one locus alters the phenotypic expression of a gene at a second
locus
o Ex: in mice black fur (BB) is dominant to brown, but C decides if there will be color, so if
mice are cc then they have a white coat and it doesn’t matter whether it’s BB or bb at
the other allele
 The gene for pigment (C or c) is epistatic to the gene that codes for black or
brown (B or b)
Polygenic Inheritance
 Quantitative characters—characters that vary in the population along a continuum
o Ex: human skin color and height
 Polygenic inheritance—an additive effect of two or more genes on a single phenotypic
character (opposite of pleiotropy where one gene is affecting several phenotypic characters)
o Although it’s more complicated, even if there were just three genes that controlled skin
color (ABC) there could be tons of variation from very dark (AABBCC) to very light
(aabbcc)
 These genes have a cumulative effect and each makes the same genetic
contribution (three units) to skin darkness
Nature and Nurture: The Environmental Impact on Phenotype
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Another departure from Mendelian genetics is when the phenotype for a character depends on
environment as well as genotype
o Ex: wind and sun exposure does something to trees, exercise alters builds of humans,
etc.
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Norm of reaction—the phenotypic range that occurs due to all the possibilities of
environmental influences
o Sometimes norms of reaction have no breadth (ex ABO blood group system—you can’t
change that) and some have very wide breadth (count of red blood cells varies a lot
depending on altitude, physical fitness, and infectious agents
o They’re generally broadest for polygenic characters because environment contributes to
the quantitative nature of these characters
 Multifactorial—when many factors, both genetic and environmental,
collectively influence phenotype
Integrating a Mendelian View of Heredity and Variation
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Phenotype can refer to specific characters as well as to an organism in its entirety
Genotype can refer to alleles for a single genetic locus or it can refer to the organism’s entire
genetic makeup
14.4 Many humans traits follow Mendelian patterns of inheritance
Pedigree Analysis
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Pedigree—a collection of information about a family’s history for a particular trait and the
assemblage of this information into a family tree describing the traits of parents and children
across the generations
see figure 14.15 to see how to trace through a pedigree
helps calculate the probability that a child will have a particular genotype and phenotype
Recessively Inherited Disorders
The Behavior of Recessive Alleles
 an allele that causes a genetic disorder codes either for a malfunctioning protein or for no
protein at all
o if a disorder is recessive, the heterozygotes are normal in phenotype because one copy
of the normal allele produces a sufficient amount of the specific protein—person can
only get it if they inherit two recessive alleles
 carriers—heterozygotes who may transmit the recessive allele to their offspring
 most people with recessive disorders have parents who are carriers of the disorder but have a
normal phenotype themselves
 genetic disorders aren’t even distributed among all groups of people
o in olden times people reproduced within a small area so the gene was more likely to be
passed on, so for example Ashkenazic Jews have the Tay-Sachs disease occurring 100
times more than non Jews or Mediterranean Jews
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when disease causing recessive allele is rare, it’s mostly unlikely that two people will reproduce
who both carry that allele, but if you mate with someone in your family or someone with the
same ancestor as you not too long ago, the chances increase exponentially
o so rules society has about marriage between close relatives might have evolved aout of
observation that stillbirths and birth defects are more common when the parents are
closely related
Cystic Fibrosis
 cystic fibrosis—genetic disease that strikes one out of every 2,500 people of European descent
o 1 in 25 are carriers
 Normal allele for gene codes for membrane protein that functions in transport of chloride ions
between cells and extracellular fluid—when you have two recessive alleles you have cystic
fibrosis and the protein doesn’t exist so mucus around lungs becomes stickier than usual and it’s
way hard for your body to fight infections
o If left untreated people with it die before their 5th birthdays, but there’s medicine now
that helps people survive longer
Sickle-Cell Disease
 Sickle-cell disease—most common inherited disorder among people of African descent—affects
one out of every 4 African Americans
 cause by substitution of one amino acid in hemoglobin protein; when oxygen content is low in
blood, the sickle cell hemoglobin molecules aggregate into long rods that deform the red cells
into a sickle shape
o these cells may clump and clog small blood vessels which leads to physical weakness,
pain, organ damage, and even paralysis
 regular blood transfusions prevent brain damage and medicine helps prevent or
treat other problems, but there’s no cure to sickle-cell
 two sickle-cell alleles are necessary to have full blown disease, but heterozygotes can
experience some symptoms too during long periods of reduced blood oxygen
o this is an example of how at the organismal level, the normal allele is incompletely
dominant to the sickle cell allele, but at the molecular level the two alleles are
codominant because both normal and abnormal hemoglobins are made
Dominantly Inherited Disorders
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they’re much less common because all lethal alleles arise by mutations in cells that produce
sperm or eggs, but if a lethal dominant allele causes death of offspring the allele won’t be
passed on, whereas if it’s recessive the parent doesn’t show signs of it so they mature and
produce and pass it on
Huntington’s Disease
 sometimes lethal dominant alleles don’t become apparent until after the person who has it is
old, and they’ve already had kids and may have given the allele to their child
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ex: Huntington’s disease—a degenerative disease of the nervous sytem that’s cause by
a lethal dominant allele and has no obvious phenotypic effect until the individual is
about 35 to 45 years old
 once deterioration begins it’s irreversible and fatal
 affects one in 10,000 people
Multifactorial Disorders
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many diseases—heart disease, diabetes, cancer, alcoholism, etc.—are multifactorial, which
means there are genetic components and a significant environmental influence
Genetic Testing and Counseling
Counseling Based on Mendelian Genetics and Probability Rules
 we can use Mendel’s laws to predict possible outcomes of mating and see if people will have an
afflicted baby
Tests for Identifying Carriers
 key is finding out whether parents are carriers but there are ethical dilemmas
Fetal Testing
 amniocentesis—procedure to determine beginning at the 14th to 16th week of pregnancy
whether or not the fetus has the disease
o done by using amniotic fluid
 chorionic villus sampling (CVS)—does the same thing, but sample is taken from the placenta
o this is much faster because these cells are derived from the fetus and have the same
genotype and they’re proliferating rapidoly enough to allow karyotyping to be carried
out immediately
 as opposed to amniocentesis which has to be cultured for several weeks
Newborn Screening
 some genetic disorders can be detected at birth
o ex: PKU—kids can’t metabolize phenylalanine and it accumulates to toxic levels in the
blood causing mental retardation—if it’s caught at birth though precautions can be
taken to allow normal development