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GENETICS:
Observing Patterns
in
Inherited Traits
Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a
chromosome
Alleles
• Different molecular forms of a gene
found on homologous chromosomes
• Arise by mutation
• Dominant allele masks a recessive
allele that is paired with it
Allele Combinations
• Homozygous
– having two identical alleles
– Homozygous dominant, AA
– Homozygous recessive, aa
• Heterozygous
– having two different alleles
– Aa
Genotype & Phenotype
• Genotype refers to particular genes an
individual carries (RR or Rr or rr)
• Phenotype refers to an individual’s
observable traits (flower color, seed
shape, etc)
Other Definitions
•
Dominant allele – in a heterozygous individual,
a trait that is fully expressed in the phenotype
•
Recessive allele – in a heterozygous individual,
a trait that is completely masked by the
expression of the dominant allele
•
Pure (true) breeding – a population with only
one type of allele for a given trait
•
Self cross – when individuals of a generation
fertilize themselves (e.g., self-fertilized flower).
Chromosomes
A pair of homologous
chromosomes,
each in the unduplicated state
(most often, one from a male
parent and its partner from a
female parent)
A gene locus (plural, loci), the
location for a specific gene on
a specific type of chromosome
A pair of alleles (each being a
certain molecular form of a gene)
at corresponding loci on a pair of
homologous chromosomes
Three pairs of genes (at three loci
on this pair of homologous
chromosomes); same thing as
three pairs of alleles
Fig. 8-1, p.113
Gregor Mendel
1822-1884
• Father of Genetics
• Austrian Monk
• Strong background in
mathematics
• observed evidence of
how parents transmit
genes to offspring
• Unaware of cells,
chromosomes or
genes
Fig. 10-2, p.152
Mendel studied the Garden Pea
• Mendel began by examining varieties of
peas suitable for study
– Character- an observable feature, such
as flower color
– Trait – actual flower color, such as purple
or white
– Heritable trait – is this character passed
on to progeny ?
• Experimentally cross-pollinated
Mendel’s methods
Mendel’s
Monohybrid
Cross Results
Mendel crossed
round x wrinkle seeded plants
• P (parental generation)
 round x wrinkled
• F1 (1st filial generation
offspring)
 round
• F2 (2nd filial generation
offspring)
 round & wrinkled
round x wrinkled
Credit: © Wally Eberhart
Dominant / Recessive Traits
• Mendel observed
each parent carried
two “units” for a given
trait
• We know these “units”
are genes on
chromosomes
• Dominant traits –
show up each
generation
• Recessive traits –
may be masked by
dominant traits
F1 Results of One Monohybrid Cross
True-breeding
homozygous recessive
parent plant
F1
PHENOTYPES
aa
True-breeding
homozygous dominant
parent plant
a
a
A
Aa
Aa
A
Aa
Aa
Aa
Aa
Aa
Aa
AA
Fig. 10-7b1, p.155
Monohybrid Cross
Experimental intercross between
two F1 heterozygotes
AA X aa
Aa (F1 monohybrids)
Aa X Aa
?
A
Monohybrid
Cross
True-breeding
homozygous recessive
parent plant
F1
PHENOTYPES
aa
True-breeding
homozygous dominant
a
parent plant
Aa
Aa
Aa
Aa
a
A
Aa
Aa
A
Aa
Aa
AA
An F1 plant
self-fertilizes
and produces
gametes:
F2
PHENOTYPES
Aa
A
AA
Aa
Aa
aa
a
A AA Aa
a
Aa
aa
F2 Results of Monohybrid Cross
An F1 plant self-fertilizes
and produces gametes:
F2
PHENOTYPES
Aa
A
a
A
AA
Aa
a
Aa
aa
AA
Aa
Aa
aa
Fig. 10-7b2, p.155
Dominant Form
Recessive Form
FLOWER
COLOR
705 purple
224 white
3.15:1
FLOWER
POSITION
651 along stem
207 at tip
3.14:1
STEM
LENGTH
787 tall
227 dwarf
Average F2 dominant-to-recessive
ratio for all of the traits studied:
2.84:1
3:1
Fig. 8-5, p.115
Probability and the Punnett Square
male gametes
female 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
Fig. 8-6a, p.115
POSSIBLE EVENT
sperm A
sperm A
sperm a
sperm a
meets egg A
meets egg a
meets egg A
meets egg a
PROBABLE OUTCOME
1/4 AA offspring
1/4 Aa
1/4 Aa
1/4 aa
p.115
Mendel’s Theory
of Segregation
• Individual inherits a unit of information
(allele) for a trait from each parent
• During gamete formation, the alleles
segregate from each other
homozygous
dominant parent
homozygous
recessive parent
(chromosomes
duplicated
before meiosis)
meiosis
I
meiosis
II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Fig. 8-4, p.114
Dihybrid Cross
AB X ab
Experimental cross between
individuals that are homozygous for
different versions of two traits
Dihybrid Cross: F1 Results
purple
flowers,
tall
TRUEBREEDING
PARENTS:
AABB
GAMETES:
AB
x
white
flowers,
dwarf
aabb
AB
ab
ab
AaBb
F1 HYBRID
OFFSPRING:
all purple-flowered, tall
1
AABB
purpleflowered,
tall parent
(homozygous
dominant)
AB
X
ab
2
aabb
whiteflowered,
dwarf parent
(homozygous
recessive)
3 F1 OUTCOME: All of the F1 plants are purple-flowered, tall
(AaBb heterozygotes)
Fig. 8-7, p.116
AaBb
meiosis,
gamete formation
AaBb
meiosis,
gamete formation
Fig. 8-7, p.116
Dihybrid Cross: F2 Results
AaBb X
AaBb
1/4 AB 1/4 Ab 1/4 aB
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
1/4 ab
1/16
AABB
1/16
AABb
1/16
AaBB
1/16
AaBb
1/16
AABb
1/16
AAbb
1/16
AaBb
1/16
Aabb
1/16
AaBB
1/16
AaBb
1/16
aaBB
1/16
aaBb
1/16
AaBb
1/16
Aabb
1/16
aaBb
1/16
aabb
9/16 purple-flowered, tall
3/16 purple-flowered, dwarf
3/16 white-flowered, tall
1/16 white-flowered, dwarf
Independent Assortment
• “Units” for one trait were assorted into
gametes independently of the “units” for
the other trait
• Members of each pair of homologous
chromosomes are randomly sorted into
gametes during meiosis
Independent Assortment
Metaphase I:
A
A a
a
B
B b
b
OR
A
A a
a
b
b B
B
Metaphase II:
Gametes:
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
B
A
B
A
1/4 AB
b
a
b
a
1/4 ab
b
A
b
A
1/4 Ab
B
a
B
a
1/4 aB
Tremendous Variation
Number of genotypes possible in offspring as
a result of independent assortment and
hybrid crossing is
3n
(n is the number of gene loci
at which the parents differ)
Dominance Relations
Complete dominance
Incomplete dominance
Codominance
Codominance: ABO Blood Types
• Gene that controls ABO type
codes for enzyme that
determines structure of a
glycolipid on blood cells
• Two alleles (IA and IB) are
codominant when paired
• Third allele (i) is recessive to
others
ABO Blood Type:
A Multiple Allele System
Range of genotypes:
Blood
types:
IA IA
IB IB
or
or
IA i
I A IB
IB i
ii
A
AB
B
O
Fig. 14.10
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
ABO and Transfusions
• Type O is universal donor – neither type
A nor type B antigens produced
• Type AB is universal receiver – no
immune response to A or B antigens
Incomplete
Dominance
X
Incomplete
homozygous
homozygous
parent
parent
Dominance
All F1 are
heterozygous
F2 shows three
phenotypes in
1:2:1 ratio
X
homozygous parent X homozygous parent
All F1 offspring
heterozygous for
flower color:
Cross two of the F1
plants and the F2
offspring will show
three phenotypes in
a 1:2:1 ratio:
Fig. 8-10, p.118
Dominance Relations
Complete dominance
Incomplete dominance
Codominance
Pleiotropy
• Alleles at a single locus may affect two or
more traits
– Marfan syndrome
• Protein: fibrillin-1
– Cystic fibrosis
• Protein: cystic fibrosis transmembrane
conductance regulator (CFTR)
– Color and crossed eyes in Siamese
cats
– Sickle Cell Anemia
Gene interactions and phenotypic
expression
• Genes may interact with each other:
one gene influences phenotypic
expression of others
• Complex variations: phenotype
influenced by gene interactions and/or
environmental conditions
Interactions among Gene Pairs
• Common among genes for hair
color in mammals
BLACK LABRADOR
YELLOW LABRADOR
CHOCOLATE LABRADOR
Genetics of Coat Color in
Labrador Retrievers
Epistasis: Phenotypic expression of one
gene is governed by another
• Two genes involved
- One gene influences melanin production
• Two alleles - B (black) is dominant over b
(brown)
- Other gene influences melanin deposition
• Two alleles - E promotes pigment deposition
and is dominant over e
• Black color – dominant B & E must be present
• Yellow color – recessive e & either B or b
• Chocolate color – dominant E & recessive b
Continuous Variation
• A continuous range of
small differences in a
given trait among
individuals
• The greater the number
of genes and
environmental factors
that affect a trait, the
more continuous the
variation in that trait
Controlled by
more than one
gene
• Two genes
A or a
and
B or b
dark brown
4 dominants
medium brown
3 dominants
light brown or
hazel
2 dominants
dark blue, grey
or green
1 dominants
light blue
0 dominants
Fig. 10-14, p.160
X
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
dark brown
med. brown
med. brown
hazel or lt br.
AABb
AAbb
AaBb
Aabb
med. brown
hazel or lt br.
hazel or lt br.
gray, grn, dk. bl.
AaBB
AaBb
aaBB
aaBb
med. brown
hazel or lt br.
hazel or lt br.
gray, grn, dk. bl.
AaBb
Aabb
aaBb
aabb
hazel or lt br.
gray, grn, dk. bl.
gray, grn, dk. bl.
pale blue
Ab
aB
ab
Number of individuals with
some value of the trait
Plotting Variation
The line of a
bell-shaped
curve reveals
continuous
variation in the
population
Range of values for the trait
Fig. 8-14a, p.120
Range of values
for the trait
Fig. 8-14b, p.120
Number of individuals with
some value of the trait
Fig. 8-15, p.121
Continuous Variation
• Skin Color in humans:
three genes
with multiple alleles
Fig. 14.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Environmental Effects on
Phenotype
• Genotype and environment can interact
to affect phenotype
– Himalayan rabbit ice pack experiment
– Transplantation of plant cuttings to different
elevations
– Human depression
Environmental Effects
on Phenotype
Hydrangeas and Soil
Soil pH or trace elements?
Phenotypic Plasticity
• Phenotype change
in response to the
environment.
• Examples:
– Humans tan in
response to sun
exposure; increased
melanin protects
cells from harmful
solar radiation
Phenotypic Plasticity
• Phenotype change
in response to the
environment.
• Examples:
– Mussels exposed to
seastar “scents”
develop stronger
adductor muscles
– Mussels exposed to
dog whelk “scent”
develop thicker
shells
Phenotypic Plasticity
• Phenotype change
in response to the
environment.
• Predator mediated
Human Genetics and Linkages
• Autosome Linkages
• Sex chromosome linkages
• Linkage group; all of the genes along
the length of a chromosome
• Full linkages stay together after crossover
• Incomplete linkages separate at
crossover
Sex
Determination
female
(XX)
male
(XY)
eggs
sperm
X
x
Y
X
x
X
X
X
X
XX
XX
Y
XY
XY
Embryonic
Development
At seven weeks, appearance
of “uncommitted”
duct system of embryo
At seven weeks, appearance of
structures that will give rise to
external genitalia
Y chromosome Y chromosome Y chromosome Y chromosome
present
absent
present
absent
testes
ovaries
10 weeks
ovary
penis
testis
penis
10 weeks
vaginal opening
uterus
vagina
birth approaching
The Y Chromosome
• Small, with few genes
• Master gene for male sex determination
– SRY gene (sex-determining region of Y)
• SRY present, testes form
• SRY absent, ovaries form
The X Chromosome
• Carries more than 2,000 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be
expressed in both males and females
Crossover Frequency
Proportional to the distance between
genes
A
B
C
D
Crossing over will disrupt linkage between
A and B more often than C and D
Full Linkage
Parents:
AB
ab
B
A
B
b
x
A
a
b
F1 offspring:
a
All AaBb
meiosis, gamete formation
Equal
ratios of
two types
of gametes:
50%
AB
B
A
b
a
50%
ab
Fig. 8-20a, p.125
Incomplete Linkage
AC
A
Parents:
ac
a
C
A
x
c
a
c
C
All AaCc
F1 offspring:
meiosis, gamete formation
A
Unequal
ratios of
four types
of gametes:
C
a
c
Most gametes have
parental genotypes
A
c
a
C
A smaller number have
recombinant genotypes
Fig. 8-20b, p.125
Genetic Abnormality
• A rare, uncommon version of a trait
• Polydactyly
– Unusual number of toes or fingers
– Does not cause health problems
– View of trait as disfiguring is subjective
Pedigree for Polydactyly
Genetic Disorder
• Inherited conditions that cause mild to
severe medical problems
• Why don’t they disappear?
– Mutation introduces new rare alleles
– In heterozygotes, harmful allele is masked,
so it can still be passed on to offspring
Autosomal
Dominant Inheritance
Trait typically
appears in
every
generation
Achondroplasia
• Autosomal dominant
inheritance
• Homozygous form
usually leads
to stillbirth
• Heterozygotes display a
type of dwarfism
Autosomal Recessive Inheritance
Patterns
• If parents are
both
heterozygous,
child will have a
25% chance of
being affected
Autosomal Recessive Galactosemia
X-Linked Recessive Inheritance
• Males show
disorder more
than females
• Son cannot inherit
disorder from his
father
Examples of X-Linked Traits
• Color blindness
– Inability to distinguish among some
or all colors
• Hemophilia
– Blood-clotting disorder
– 1/7,000 males has allele for hemophilia A
– Was common in European royal families
Color Blindness
Fig. 8-27, p.128
Hemophilia
Structural Changes in Chromosomes
• Duplication
• Deletion
• Inversion
• Translocation
Duplication
normal chromosome
one segment
repeated
three repeats
Deletion
• Loss of some segment of a chromosome
• Most are lethal or cause serious disorder
Inversion
A linear stretch of DNA is reversed
within the chromosome
segments
G, H, I
become
inverted
Translocation
one chromosome
a nonhomologous
chromosome
nonreciprocal translocation
In-text figure
Page 206
Changes in Chromosome Number
• Aneuploidy
• Polyploidy
• Most changes in chromosome
number are due to nondisjuction
Aneuploidy
• Individuals have one extra or one less
chromosome (2n + 1 or 2n - 1)
• Major cause of human
reproductive failure
• Most human miscarriages are
aneuploids
Polyploidy
• Individuals have three or more of each
type of chromosome (3n, 4n)
• Common in flowering plants
• Lethal for humans
– 99% die before birth
– Newborns die soon after birth
Nondisjunction
n+1
n+1
n-1
chromosome
alignments at
metaphase I
n-1
nondisjunction alignments at
at anaphase I metaphase II
anaphase II
Down Syndrome
• Trisomy of chromosome 21
• Mental impairment and a variety of
additional defects
• Can be detected before birth
• Risk of Down syndrome increases
dramatically when mothers are over age 35
Down Syndrome
• Trisomy of
chromosome 21
Down Syndrome
Turner Syndrome
• Inheritance of only one X (XO)
• 98% spontaneously aborted
• Survivors are short, infertile
females
– No functional ovaries
– Secondary sexual traits reduced
– May be treated with hormones,
surgery
Klinefelter Syndrome
• XXY condition
• Results mainly from nondisjunction in
mother (67%)
• Phenotype is tall males
– Sterile or nearly so
– Feminized traits (sparse facial hair,
somewhat enlarged breasts)
– Treated with testosterone injections
XYY Condition
• Taller than average males
• Most otherwise phenotypically normal
• Some mentally impaired
• Once mistakenly associated with
criminal behavior
X Chromosome
Inactivation
• One X inactivated in
each cell of female
• Creates a “mosaic”
for X chromosomes
• Dosage compensation
Mosaic Expression