Download Chapter-13

Observing Patterns in
Inherited Traits
Chapter 13
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
Cystic Fibrosis
 Most common fatal genetic disorder in the US
 Gene  CFTR (encodes protein that moves chloride ions
out of the epithelial cells)
• Water follows chloride ions  osmosis
• Goal  thin film of water on surface of epithelial sheets
that allow mucus to slide easily over the wet sheets of
cells
• Mutation in CFTR gene  deletion of three base pairs
• Disrupts membrane trafficking of CFTR
• Protein is made by never released from the cell
• Prevents binding with bacteria at the cell surface 
decrease immune response
Menacing Mucus
• Outcome  transport of chloride ions and water out of
the epithelial cells are interrupted
• Thick globs of mucus accumulate and clog passageways
• Breathing becomes difficult due to mucus obstruction
• Treatment 
• posture change, thumps on chest and back to clear the
lungs
• Antibiotics to control infections
• Lifespan  30 yrs
 Genetics  must have 2 copies of the inherited
mutated gene (one from each parent)
• 1:3,300 births
Mendel,
Pea Plants, and Inheritance Patterns
 By experimenting with pea plants, Mendel was
the first to gather evidence of patterns by which
parents transmit genes to offspring
Mendel’s
Experiments
carpel
stamen
a Garden pea flower, cut in half. Sperm form in
pollen grains, which originate in male floral parts
(stamens). 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
stamens snipped off. This is one way to assure
cross-fertilization of plants.
c Later, seeds develop inside pods of the crossfertilized plant. An embryo within 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. 10.3, p.154
Producing Hybrids
 Hybrids  Heterozygous individual
• Offspring of a cross between two individuals that
breed true for different forms of a trait
 Each inherits nonidentical alleles for a trait being
studied
Mendel’s Experimental Approach
 Control over reproduction
• Removing a flower’s pollen-bearing anthers
 “Bred true” all offspring have the same form of
the traits as the parent(s), generation after
generation, mutations aside
 “Cross-fertilize”  transfer pollen among
individuals that have different traits
• Mendel discovered  traits of the offspring from
cross-fertilized pea plants appear in predictable
patterns
• Outcome  Hereditary information is passed from
one generation to the next in discrete units
Producing Hybrid Offspring
Homozygous
dominant parent
Homozygous
recessive parent
(chromosomes
duplicated
before meiosis)
meiosis
I
meiosis II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Fig. 10.5, p.156
Heritable Units of Information
 Genes
• Heritable units of information about traits
• Each has its own locus on the chromosome
 Alleles
• Different molecular forms of the same gene
 Locus  location of a gene on a chromosome
• Figure 13.3 and 13.4 in text
 Mutation
• Permanent change in a gene’s information
Heritable Units of Information
a A pair of homologous chromosomes,
both unduplicated. In most species, one
is inherited from a female parent and its
partner from a male parent.
b A gene locus (plural, loci), the location
for a specific gene on a chromosome.
Alleles are at corresponding loci on a
pair of homologous chromosomes
c A pair of alleles may be identical
or not. Alleles are represented in
the text by letters such as D or d.
d Three pairs of genes (at three
loci on this pair of homologous
chromosomes); same thing as
three pairs of alleles.
Fig. 10.4, p.155
Modern Genetic Terms
 Genotype
• An individual’s alleles at any or all gene loci
• A set of alleles carried by an individual
 Phenotype
• An individual’s observable traits
 Dominant allele may mask effect of a recessive allele
paired with it
 Recessive allele is masked by a dominant allele
Modern Genetic Terms
 Homozygous  having identical alleles of a gene
 Heterozygous  having 2 different alleles of a gene
 Homozygous dominant
• Has two dominant alleles for a trait (AA)
 Homozygous recessive
• Has two recessive alleles (aa)
 Heterozygote
• Has two nonidentical alleles (Aa)
Key Concepts:
MODERN GENETICS
 Gregor Mendel gathered the first indirect,
experimental evidence of the genetic basis of
inheritance
 His meticulous work tracking traits in many
generations of pea plants gave him clues that
heritable traits are specified in units
 The units, distributed into gametes in predictable
patterns, were later identified as genes
Mendel’s Theory of Segregation
 Mendel’s Theory of Segregation:
• Diploid organisms have pairs of genes, on pairs
of homologous chromosomes
• Based on monohybrid experiments
• Law of Segregation  During meiosis
• Genes of each pair separate
• Each gamete gets one or the other gene
Producing Hybrid Offspring
 Crossing two true-breeding parents of different
genotypes yields hybrid offspring
 All F1 offspring are heterozygous for a gene, and can
be used in monohybrid experiments
• All F1 offspring of parental cross AA x aa are Aa
• Monohybrid cross
• Breeding experiment Aa x Aa (heterozygote's) are
crossed
• Frequency of traits among the offspring offers information
about the dominance relationship between the alleles
 Punnett square  predict the genetic and phenotypic
outcome of a cross
A Monohybrid Cross
 Crosses between F1 monohybrids resulted in
these allelic combinations among F2 offspring
• Phenotype ratio 3:1
• Evidence of dominant and recessive traits
F2 Offspring:
Dominant and Recessive Traits
Trait
Studied
Dominant
Form
Recessive
Form
F2 Dominantto-Recessive
Ratio
Seed
shape
5,474 round
1,850 wrinkled
2.98:1
Seed
color
6,022 yellow
2,001 green
3.01:1
Pod
shape
882 inflated
299 wrinkled
2.95:1
Pod
color
428 green
152 yellow
2.82:1
Flower
color
705 purple
224 white
3.15:1
651 long stem
207 at tip
3.14:1
Flower
position
Stem
length
787 tall
277 dwarf
2.84:1
Fig. 10.6, p.156
Predicting Probability: Punnett Squares
female gametes
male gametes
A
a
A
a
A
a
A
aa
a
A
A
Aa
aa
a
Aa
a
A
a
Aa
A
AA
Aa
aa
a
Aa
aa
Stepped Art
Fig. 10-7a, p.157
Predicting F1 Offspring
Predicting F2 Offspring (3:1)
TESTCROSS
 Testcross 
• Method of determining genotype in which an
individual of unknown genotype is crossed with
one that is known to be homozygous recessive
• Ex. Offspring is homozygous recessive for a trait
• One parent is homozygous dominant for that trait,
what is the genotype for the other parent???
• SOLVE and WRITE here.
Key Concepts:
MONOHYBRID EXPERIMENTS
 Some experiments yielded evidence of gene
segregation
 When one chromosome separates from its
homologous partner during meiosis, the pairs of
alleles on those chromosomes also separate and
end up in different gametes
 Law of segregation
• Diploid cells carry pairs of genes, on pairs of
homologous chromosomes
• The two genes of each pair are separated from each
other during meiosis, so they end up in different
gametes
Mendel’s Theory of
Independent Assortment
 Mendel’s Theory of Independent Assortment:
• Meiosis assorts gene pairs of homologous
chromosomes independently of gene pairs on all
other chromosomes
• Based on dihybrid experiments
 Pairs of homologous chromosomes align
randomly at metaphase I
Independent Assortment in Meiosis I
One of two possible alignments
a Chromosome
alignments at
metaphase I:
b The resulting
alignments at
metaphase II:
B
c Possible
combinations
of alleles in
gametes:
The only other possible alignment
A
a
a
A
Aa
a
B
Bb
b
b
bB
B
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
A
A
a
a
A
A
a
a
AB
B b
ab
b
b
Ab
b
B
B
aB
Fig. 10.8, p.158
Dihybrid Experiments
 Start with a cross between true-breeding
heterozygous parents that differ for alleles of two
genes (AABB x aabb)
 All F1 offspring are heterozygous for both genes
(AaBb)
 Frequency of traits among the offspring provides
information about the dominance relationship
between the paired alleles
Mendel’s Dihybrid Experiments
 AaBb x AaBb
 Phenotypes of the F2 offspring of F1 hybrids
were close to a 9:3:3:1 ratio
•
•
•
•
9 dominant for both traits
3 dominant for A, recessive for b
3 dominant for B, recessive for a
1 recessive for both traits
Results of
Mendel’s Dihybrid Experiments
Meiosis, gamete formation
in true-breeding parent
plants
parent homozygous dominant
for purple flowers, tall stems
parent homozygous recessive
Gametes at fertilization for white flowers, short stems
meiosis,
gamete
formation
Possible genotypes resulting from a cross between two F1 plants:
meiosis,
gamete
formation
All F1 plants are AaBb
heterozygotes with purple
flowers and tall stems.
Fig. 10.9, p.159
Linkage Groups
 All genes on the same chromosome are part of
one linkage group
 Crossing over between homologous
chromosomes disrupts gene linkages
Crossover
 Distance between gene
• Genes far a parts on a chromosome 
high frequency of crossing over occurs
between them
• Assort into gametes independently, just as if
they were on different chromosomes
• Genes close together on a chromosome 
DO NOT assort independently and low
frequency of crossing over
• Tightly linked genes stay together during
meiosis more frequently
Linkage and Crossing Over
Key Concepts:
DIHYBRID EXPERIMENTS
 Some experiments yielded evidence of
independent assortment
 During meiosis, the members of a pair of
homologous chromosomes are distributed into
gametes independently of all other pairs
Beyond Simple Dominance
 Other types of gene expression
•
•
•
•
Codominant alleles
Incomplete dominance
Epistasis
Pleiotropy
Codominant Alleles
 Both alleles are expressed at the same time in
heterozygotes
• Example: Multiple allele system (gene for which
three or more alleles persist in a population)
• ABO blood typing
Genotypes:
Phenotypes
(Blood type):
AA
or
AO
A
AB
BB
or
BO
OO
AB
B
O
Fig. 10.10, p.160
ABO Gene
 O is recessive when paired with A or B
 Incorrect blood transfusion  Dangerous
• Result: Red blood cells can clump or burst
 O type = universal donor
 AB type = universal recipient
Incomplete Dominance
 An allele is not fully dominant over its partner on
a homologous chromosome
• Both are expressed
• Produces a phenotype between the two
homozygous conditions
Incomplete Dominance
homozygous
homozygous
parent (RR) x parent (rr)
Cross two of the
F1 plants, and
the F2 offspring
will show three
phenotypes in
a 1:2:1 ratio:
RR
Rr
heterozygous
F1 offspring (Rr)
Rr
rr
Fig. 10.11, p.160
Epistasis
 Interacting products of one or more genes affect
the same trait
 Trait is influenced by the products of multiple
genes
RRpp (rose comb)
F1 of spring:
F2 offspring:
RRPP, RRPp,
RrPP, or RrPp
9/16 walnut
RrPp
RRpp or Rrpp
3/16 rose
X
rrPP (pea comb)
RrPp (all walnut comb)
X
RrPp
rrPP or rrPp
3/16 pea
rrpp
1/16 single comb
Fig. 10.12, p.161
More Epistasis – Skin color and Fur color
 Color
•
•
•
•
Dominant B = black
Recessive b = brown
Dominant E = melanin to deposit in fur
Recessive e = reduces melanin deposition
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. 10.13, p.161
Pleiotropy
 A single gene may affect two or more traits
• Example: sickle-cell anemia, cystic fibrosis, and
Marfan syndrome
 Marfan syndrome
• Gene encodes fibrillin  give elasticity to the
heart, skin, blood vessels, etc
• Mutation makes vessels thin and leaky
• Inflamed, thinned, and weakened  aorta can
rupture during exercise
Complex Variations in Traits
 Polygenic Inheritance
• When products of many
genes influence a trait,
individuals of a population
show a range of continuous
variation for the trait
• Continuous variation  a
range of small differences in
a shared trait in a population
• Ex. Eye Color
Continuous Variation
# of individuals fall into each category.
This gives the relative frequencies of
Phenotypes across a range of measurable
Values. Plot data on bar chart
If bell curve 
trait varies continuously
Number of individuals
with a measurable
value for the trait
This red graph line of the
range of variation for a trait
in a population plots out as
a bell-shaped curve. Such
curves indicate continuous
variation in a population.
Range of values for the trait
Fig. 10.19a, p.164
Environmental Effect on Phenotype
 Environmental factors may affect gene
expression in individuals  affect phenotype
• Example: Temperature and fur color
• Length of day affect production of melanin that
affect skin and fun color
Elevation and Plant Height
Height (centimeters)
60
Height (centimeters)
60
Height (centimeters)
60
0
0
0
a Mature cutting
at high elevation
(3,060 meters
above sea level)
b Mature cutting
at mid-elevation
(1,400 meters
above sea level)
c Mature cutting
at low elevation
(30 meters above
sea level)
Fig. 10.17, p.163
Predation and Body Form
Few
Many
Predators. Predators.
Round Head Pointy head
Variations in Gene Expression
 Gene interactions and environmental factors
affect most phenotypes
• Gene products control metabolic pathways
• Mutations may alter or block pathways
Key Concepts:
VARIATIONS ON MENDEL’S THEME
 Not all traits have clearly dominant or recessive
forms
 One allele of a pair may be fully or partially
dominant over its nonidentical partner, or
codominant with it
 Two or more gene pairs often influence the
same trait, and some single genes influence
many traits
 The environment also influences variation in
gene expression
Animation: Coat color in Labrador
retrievers
Animation: Coat color in the Himalayan
rabbit
Animation: Codominance: ABO blood
types
Animation: Comb shape in chickens
Animation: Continuous variation in
height
CLICK HERE TO PLAY
Animation: Crossing garden pea plants
Animation: Crossover review
Animation: Dihybrid cross
Animation: F2 ratios interaction
Animation: Genetic terms
Animation: Incomplete dominance
Animation: Monohybrid cross
Animation: Pleiotropic effects of Marfan
syndrome
Animation: Testcross