Download Observing Patterns in Inherited Traits

Observing Patterns
in Inherited Traits
Chapter 11
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
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 Cross-Fertilization
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 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
 An individual with identical alleles of a gene is
homozygous for that gene
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
• 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
• 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
Testcrosses
 Testcross
• 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
• Testcrosses that check for a dominance
relationship between two alleles at a single locus
• 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
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
homozygous
dominant parent
homozygous
recessive parent
(chromosomes
duplicated before
meiosis)
meiosis I
meiosis II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Stepped Art
Fig. 11-5, p. 172
Mendel’s Monohybrid Experiments
Trait
Studied
Dominant
Form
Recessive
Form
F2 Dominant-toRecessive Ratio
Seed
shape
5,474 round 1,850 wrinkled
2.98 to 1
Seed
color
6,022 yellow 2,001 green
3.01 to 1
Pod
shape
882 inflated 299 wrinkled
2.95 to 1
Pod
color
428 green
152 yellow
2.82 to 1
Flower
color
705 purple
224 white
3.15 to 1
Flower
position 651 along stem 207 at tip
3.14 to 1
Stem
length
2.84 to 1
787 tall
277 dwarf
Fig. 11-6, 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
Fig. 11-7, p. 173
male gametes
female gametes
A
a
A
a
A
a
A
A
aa
a
A
Aa
aa
a
Aa
a
A
a
Aa
A
AA
Aa
aa
a
Aa
aa
A From left to right, step-by-step construction of a Punnett square. Circles
signify gametes, and letters signify alleles: A is dominant; a is recessive.
The genotypes of the resulting offspring are inside the squares.
Fig. 11-7a, p. 173
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
F2 offspring
Aa
Heterozygous
F1 offspring
A
a
A
AA
Aa
a
Aa
aa
AA
Aa
Aa
aa
Aa
Heterozygous
F1 offspring
C A cross between the F1 offspring is the monohybrid experiment. The
phenotype ratio of F2 offspring in this example is 3:1 (3 purple to 1 white).
Fig. 11-7c, 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
11.2 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
• 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
B
aB
Fig. 11-8, p. 174
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
Stepped Art
Fig. 11-8, p. 174
Mendel’s Dihybrid Experiments
Fig. 11-9a, p. 175
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
Fig. 11-9b, 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
Fig. 11-9c, 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
Animation: Dihybrid cross
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
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
• 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
Codominance in ABO Blood Types
Genotypes:
Phenotypes
(Blood type):
AA
BB
or
or
AO
AB
BO
OO
A
AB
B
O
Fig. 11-10, p. 176
Animation: Codominance: 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
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
Fig. 11-11b, 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
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
Fig. 11-13a, p. 177
Epistasis in Chicken Combs
Pleiotropy
 Pleiotropy
• One gene product
influences two or
more traits
• Example: Some tall,
thin athletes have
Marfan syndrome, a
potentially fatal
genetic disorder
11.5 Linkage Groups
 The farther apart two genes are on a
chromosome, the more often crossing over
occurs between them
 Linkage group
• All genes on one chromosome
• Linked genes are very close together; crossing
over rarely occurs between them
Linkage and Crossing Over
Parental
generation
AC
ac
X
F1 offspring
All AaCc
meiosis, gamete formation
Gametes
Most gametes have
parental genotypes
A smaller number have
recombinant genotypes
Fig. 11-15, p. 178
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, works at low temperatures
Animation: Coat color in the Himalayan
rabbit
Effects of Altitude
on Gene Expression
Height (centimeters) Height (centimeters) Height (centimeters)
60
a Mature
cutting at high
elevation (3,060
meters above sea
level)
0
60
b Mature
cutting at midelevation (1,400
meters above
sea level)
0
60
c Mature
cutting at low
elevation (30
meters above
sea level)
0
Fig. 11-17, p. 179
Effects of Predation
on Gene Expression
 Predators of daphnias emit chemicals that
trigger a different phenotype
Fig. 11-18a, p. 179
Fig. 11-18b, p. 179
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
Fig. 11-19a, p. 180
Fig. 11-19b, p. 180
Fig. 11-19c, p. 180
Animation: Continuous variation in
height
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
Animation: Testcross
Animation: Coat color in Labrador
retrievers
Animation: Comb shape in chickens
Animation: F2 ratios interaction
Animation: Genetic terms
Animation: Incomplete dominance
Animation: Monohybrid cross
Animation: Pleiotropic effects of Marfan
syndrome
Video: Genetics of skin color