Observing Patterns
in Inherited Traits
Chapter 11
Hsueh-Fen Juan
Oct. 16, 2012
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
Video: Genetics of skin color
11.1 Mendel, Pea Plants,
and Inheritance Patterns
 Recurring inheritance patterns are observable
outcomes of sexual reproduction
 Before the discovery of genes, it was thought
that inherited traits resulted from a blend of
parental characters
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
Garden Pea
Plant:
Self Fertilization
and CrossFertilization
carpel
anther
A Garden pea flower, cut in half. Sperm form in
pollen grains, which originate in male floral parts
(anthers). Eggs develop, fertilization takes place,
and seeds mature in female floral parts (carpels).
B Pollen from a plant that breeds true for purple flowers is
brushed onto a floral bud of a plant that breeds true for white
flowers. The white flower had its anthers snipped off. Artificial
pollination is one way to ensure that a plant will not self-fertilize.
C Later, seeds develop inside pods of the cross-fertilized
plant. An embryo in each seed develops into a mature pea plant.
D Each new plant’s flower color is indirect but
observable evidence that hereditary material
has been transmitted from the parent plants.
Fig. 11-3, p. 170
Animation: Crossing garden pea plants
Terms Used in Modern Genetics
 Genes
• Heritable units of information 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
• May cause a trait to change
• Alleles (等位基因) 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
 An individual with nonidentical alleles of a gene
is heterozygous for that gene (異型合子)
 An individual with identical alleles of a gene is
homozygous for that gene (同型合子)
Terms Used in Modern Genetics
 An allele is dominant (顯性) if its effect masks
the effect of a recessive (隱性) 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 (基因表現)
• The process by which information in a gene is
converted to a structural or functional part of a
cell or body
• Expressed genes determine traits
Terms Used in Modern Genetics
 Genotype (基因型)
• The particular alleles an individual carries
 Phenotype (表型)
• 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
Testcrosses
 Testcross (試交)
• A method of determining if an individual is
heterozygous or homozygous dominant
• An individual with unknown genotype is crossed
with one that is homozygous recessive (AA x aa)
or (Aa x aa)
Monohybrid Experiments
 Monohybrid experiments (單性雜交)
• Testcrosses that check for a dominance
relationship between two alleles at a single
locus (某基因位上，等位基因僅兩種才能用)
• May be crosses between true breeding
(homozygous) individuals (AA x aa), or between
identical heterozygotes (Aa x Aa)
Mendel’s Monohybrid Experiments
 Mendel used monohybrid experiments to find
dominance relationships among pea plant traits
• When he crossed plants that bred true for white
flowers with plants that bred true for purple
flowers, all F1 plants had purple flowers
• When he crossed two F1 plants, ¾ of the F2
plants had purple flowers, ¼ had white flowers
Segregation of Alleles at a Gene Locus
Mendel’s Monohybrid Experiments
Calculating Probabilities
 Probability
• A measure of the chance that a particular
outcome will occur
 Punnett square (旁氏表)
• 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
Mendel’s Law of Segregation (分離律)
 重點：單性雜交實驗驗證了分離律
 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
in modern terms (當時尚未知染色體的存在)
1.Diploid cells have pairs of genes on pairs of
homologous chromosomes
2.The two genes of each pair separate during
meiosis, and end up in different gametes
11.2 Key Concepts
Insights from Monohybrid Experiments
 Some experiments yielded evidence of gene
segregation: When one chromosome separates
from its homologous partner during meiosis, the
alleles on those chromosomes also separate
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
Dihybrid Experiments
 Dihybrid experiments (兩性雜交)
• Tests for dominance relationships between
alleles at two loci
• Individuals that breed true for two different traits
are crossed (AABB x aabb)
• F2 phenotype ratio is 9:3:3:1 (four phenotypes)
• Individually, each dominant trait has an F2 ratio of
3:1 – inheritance of one trait does not affect
inheritance of the other
Independent Assortment at Meiosis
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
aB
B
Mendel’s Dihybrid Experiments
P
generation
A Meiosis in homozygous
individuals results in one
kind of gamete.
parent plant parent plant
homozygous homozygous
for purple
for white
flowers
flowers
and long
and short
stems
stems
aabb
AABB
B A cross between plants
AB
homozygous for two different traits
yields one possible combination of gametes:
x
ab
Fig. 11-9a, p. 175
F1
generation
All F1 offspring are AaBb,
with purple flowers and tall stems.
AaBb
AaBb
AaBb
C Meiosis in AaBb dihybrid plants
results in four kinds of gametes:
AB Ab aB
F2
generation
ab
These gametes can meet up in one of 16
possible wayswhen the dihybrids are
crossed (AaBb X AaBb):
Fig. 11-9b, p. 175
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that
are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall;
and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9:3:3:1.
Fig. 11-9c, p. 175
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
(亦即一對等位基因之分離，與另一對等位基因之
分離毫不相干)
 重點：兩性雜交實驗驗證了自由配合律
 Exception: Genes that have loci very close to
one another on a chromosome tend to stay
together during meiosis (基因連鎖則不成立)
11.3 Key Concepts
Insights from Dihybrid Experiments
 Some experiments yielded evidence of
independent assortment: Genes are typically
distributed into gametes independently of other
genes
11.4 Beyond Simple Dominance
 Mendel focused on traits based on clearly
dominant and recessive alleles; however, the
expression patterns of genes for some traits are
not as straightforward
Codominance in ABO Blood Types
 Codominance (等顯性)
• 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 (複對偶基因)
• Genes with three or more alleles in a population
• Example: ABO blood types
Codominance in ABO Blood Types
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
Incomplete Dominance in Snapdragons
Fig. 11-11a, p. 176
Fig. 11-11b, p. 176
Epistasis
 Epistasis (上位基因)
• Two or more gene products influence a trait
• Typically, one gene product suppresses the
effect of another, so the resulting phenotype is
somewhat unexpected.
• 通常會有一個基因在眾基因的交互作用中佔優勢，
稱此基因為上位基因(例：藍眼瑪莉花色基因)
 Example: Coat color in dogs
• Alleles B and b designate colors (black or brown)
• Two recessive alleles ee suppress color
• Allele E promotes the deposition of melanin in fur,
but two recessive alleles (ee) reduce it.
Epistasis in Chicken Combs
Dramatic variations in their combs
Animation: Comb shape in chickens
Epistasis in Coat Colors
EE/Ee/ee為上位基因，決定是否有黑色素，因此只
要出現基因型ee，不論是BB, Bb或bb，皆因無黑色
素而呈表型yellow
EB
Eb
eB
eb
EB
EEBB
black
EEBb
black
EeBB
black
EeBb
black
Eb
EEBb
black
EEbb
chocolate
EeBb
black
Eebb
chocolate
eB
EeBB
black
EeBb
black
eeBB
yellow
eeBb
yellow
eb
EeBb
black
Eebb
chocolate
eeBb
yellow
eebb
yellow
Allele B (black) is dominant to b (brown).
Allele E promotes the deposition of melanin in fur, but two
recessive alleles (ee) reduce it.
Fig. 11-13a, p. 177
Pleiotropy
 Pleiotropy (基因多效性)
• One gene product
influences two or more
traits
• Example: Some tall, thin
athletes have Marfan
syndrome, a potentially
fatal genetic disorder
• 單基因(產物)多表型
FBN1, which encodes a connective protein called fibrillin-1
Long fibers of fibrillin impart elasticity to the tissues of the heart,
skin, blood vessels, tendons and other body parts.
11.5 Linkage Groups
 The farther apart two genes are on a
chromosome, the more often crossing over (互
換) occurs between them
 Linkage group (基因連鎖群)
• All genes on one chromosome are called a
linkage group
• Linked genes are very close together; crossing
over rarely occurs between them
Linkage and Crossing Over
Animation: Crossover review
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 (酪胺酸酶), works at low
temperatures
Animation: Coat color in the Himalayan
rabbit
Effects of Altitude
on Gene Expression
Effects of Predation
on Gene Expression
 Predators of daphnias emit chemicals that
trigger a different phenotype
11.7 Complex
Variations in Traits
 Individuals of most
species 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