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Impacts, Issues:
The Color of Skin
 Like most human traits, skin color has a genetic
basis; more than 100 gene products affect the
synthesis and deposition of melanins
Mendel’s Experimental Approach
 Mendel was a monk with training in plant
breeding and mathematics
 He studied the garden pea (Pisum sativum),
which breeds true for a number of traits
Terms Used in Modern Genetics
 Genes
• Heritable units of info about traits
• Parents transmit genes to offspring
• Each gene has a specific locus on a
chromosome
 Diploid cells (chromosome number 2n) have
pairs of genes on homologous chromosomes
Terms Used in Modern Genetics
 A mutation is a permanent change in a gene
• Alleles are different molecular forms of a gene
 A hybrid has nonidentical alleles for a trait
• Offspring of a cross between two individuals that
breed true for different forms of a trait are hybrids
Terms Used in Modern Genetics
 Nonidentical alleles of a gene is heterozygous
 Identical alleles of a gene is homozygous
Terms Used in Modern Genetics
 An allele is dominant if its effect masks the
effect of a recessive allele paired with it
• Capital letters (A) signify dominant alleles;
lowercase letters (a) signify recessive alleles
• Homozygous dominant (AA)
• Homozygous recessive (aa)
• Heterozygous (Aa)
Terms Used in Modern Genetics
 Gene expression
• Process by which info in a gene is converted to a
structural or functional part
• Expressed genes determine traits
Terms Used in Modern Genetics
 Genotype
• The particular alleles an individual carries
 Phenotype
• An individual’s observable traits
Terms Used in Modern Genetics
 P stands for parents, F for filial (offspring)
 F1: First generation offspring of parents
 F2: Second generation offspring of parents
11.1 Key Concepts
Where Modern Genetics Started
 Gregor Mendel gathered the first experimental
evidence of the genetic basis of inheritance
 His meticulous work gave him clues that
heritable traits are specified in units
 The units, which are distributed into gametes in
predictable patterns, were later identified as
genes
11.2 Mendel’s Law of Segregation
 Garden pea plants inherit two “units” of
information for a trait, one from each parent
homozygous
dominant parent
homozygous
recessive parent
(chromosomes
duplicated before
meiosis)
meiosis I
meiosis II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Fig. 11-5, p. 172
Calculating Probabilities
 Probability
• A measure of the chance that a particular
outcome will occur
 Punnett square
• A grid used to calculate the probability of
genotypes and phenotypes in offspring
Construction of a Punnett Square
Phenotype Ratios
in a Monohybrid Experiment
Phenotype Ratios
in a Monohybrid Experiment
F1 offspring
aa
True-breeding homozygous
recessive parent plant
a
a
A
Aa
Aa
A
Aa
Aa
Aa
Aa
AA
True-breeding homozygous
dominant parent plant
Aa
Aa
B A cross between two plants that breed true for different forms
of a trait produces F1 offspring that are identically heterozygous.
Fig. 11-7b, p. 173
Mendel’s Law of Segregation
 Mendel observed a phenotype ratio of 3:1 in the
F2 offspring of his monohybrid crosses
• Consistent with the probability of the aa genotype
in the offspring of a heterozygous cross (Aa x Aa)
 This is the basis of Mendel’s law of segregation
• Diploid cells have pairs of genes on pairs of
homologous chromosomes
• The two genes of each pair separate during
meiosis, and end up in different gametes
11.3 Mendel’s Law
of Independent Assortment
 Mendel’s law of independent assortment
• Many genes are sorted into gametes
independently of other genes
One of two possible alignments
a Chromosome
alignments at
metaphase I:
The only other possible alignment
A
Aa
a
A
Aa
a
B
Bb
b
b
bB
B
b The resulting
alignments at
metaphase II:
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
c Possible
B
combinations
of alleles in
gametes:
A
A
a
a
A
A
a
a
AB
B
b
ab
b
b
Ab
b
B
B
aB
Fig. 11-8, p. 174
• Dihybrid Cross:
• In humans, there is a gene that controls
formation (or lack thereof) of muscles in the
tongue that allow people with those muscles
to roll their tongues, while people who lack
those muscles cannot roll their tongues. The
ability to roll one’s tongue is dominant over
non-rolling. The ability to taste certain
substances is also genetically controlled.
Let’s let R represent tongue-rolling, r
represent a non-roller, T represent ability to
taste PTC, and t represent non-tasting.
Suppose a woman who is both a homozygous tongueroller and a non-PTC-taster marries a man who is a
heterozygous tongue-roller and is a PTC taster, and
they have children Draw the Punnett square that
predicts what their children will be.
If the man is both Rr and Tt (How do we know that?),
he would be RrTt and so could produce gametes with
either R or r and either T or t (one allele for each
gene). There are two choices for the first trait (R or r).
No matter which of those go into a given sperm, there
are still two choices for the second trait (T or t).
Woman RRtt
Man RrTt
RT
Rt
Rt
Rt
Rt
Rt
rT
rt
RRTt RRtt RrTt Rrtt
Mendel’s Law of Independent Assortment
 Mendel’s dihybrid experiments showed that
“units” specifying one trait segregated into
gametes separately from “units” for other traits
 Exception: Genes that have loci very close to
one another on a chromosome tend to stay
together during meiosis
Codominance in ABO Blood Types
 Codominance
• Two nonidentical alleles of a gene are both fully
expressed in heterozygotes, so neither is
dominant or recessive
• May occur in multiple allele systems
 Multiple allele systems
• Genes with three or more alleles in a population
• Example: ABO blood types
Genotypes:
Phenotypes
(Blood type):
AA
BB
or
or
AO
AB
BO
OO
A
AB
B
O
Fig. 11-10, p. 176
Incomplete Dominance
 Incomplete dominance
• One allele is not fully dominant over its partner
• The heterozygote’s phenotype is somewhere
between the two homozygotes, resulting in a
1:2:1 phenotype ratio in F2 offspring
 Example: Snapdragon color
• RR is red
• Rr is pink
• rr is white
homozygous
homozygous
x parent (rr)
parent (RR)
heterozygous
F1 offspring (Rr)
A Cross a red-flowered with a white-flowered plant,
and all of the F1 offspring will be pink.
Fig. 11-11a, p. 176
B Cross two F1 plants,
and the three phenotypes
of the F2 offspring will
occur in a 1:2 :1 ratio:
R
r
RR
Rr
Rr
rr
R
r
Fig. 11-11b, p. 176
Epistasis
 Epistasis
• Two or more gene products influence a trait
• Typically, one gene product suppresses the effect
of another
 Example: Coat color in dogs
• Alleles B and b designate colors (black or brown)
• Two recessive alleles ee suppress color
Epistasis in Coat Colors
Pleiotropy
 Pleiotropy
• One gene product
influences two or
more traits
• Example: Some tall,
thin athletes have
Marfan syndrome, a
potentially fatal
genetic disorder
The Distance Between Genes
 The probability that a crossover event will
separate alleles of two genes is proportional to
the distance between those genes
11.6 Genes and the Environment
 Expression of some genes is affected by
environmental factors such as temperature,
altitude, or chemical exposure
 The result may be variation in traits
Effects of Temperature
on Gene Expression
 Enzyme tyrosinase, works at low temperatures
11.7 Complex
Variations in Traits
 Individuals vary in
some of their shared
traits
 Many traits (such as
eye color) show a
continuous range of
variation
Continuous Variation
 Continuous variation
• Traits with a range of small differences
• The more factors that influence a trait, the more
continuous the distribution of phenotype
 Bell curve
• When continuous phenotypes are divided into
measurable categories and plotted as a bar chart,
they form a bell-shaped curve
Continuous Variation and the Bell Curve
Regarding the Unexpected Phenotype
 Phenotype results from complex interactions
among gene products and the environment
• Enzymes and other gene products control steps
of most metabolic pathways
• Mutations, interactions among genes, and
environmental conditions may affect one or more
steps
11.4-11.7 Key Concepts
Variations on Mendel’s Theme
 Not all traits appear in Mendelian inheritance
patterns
• An allele may be partly dominant over a
nonidentical partner, or codominant with it
• Multiple genes may influence a trait; some genes
influence many traits
• The environments also influences gene
expression