Transcript You Light Up My Life

Observing Patterns
in
Inherited Traits
Chapter 8
Cystic Fibrosis
• Recessive genetic disorder;
thick mucus in lungs,
digestive tract shortens life
span
• Carriers of CF may not
know they have mutant
gene
• Potential parents can be
tested for gene
Impacts, Issues Video
One Bad Transporter and
Cystic Fibrosis
Gregor Mendel
• Strong background
in plant breeding
and mathematics
• Using pea plants,
found indirect but
observable
evidence of how
parents transmit
genes to offspring
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
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 correspinding 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
Genetic terms
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 guarantee a plant will not self-fertilize.
c Later, seeds develop inside pods of the cross-fertilized 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. 8-3, p.114
Gregor Mendel
Crossing garden pea plants
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
Mendel’s Monohybrid
Cross Results
Monohybrid cross
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
Mendel’s Monohybrid
Cross Results
F2 ratios interaction
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
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
Monohybrid Cross
Illustrated
Testcross
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
Dihybrid Cross
AB X ab
Experimental cross between
individuals that are heterozygous
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
Dihybrid Cross: F2 Results
Dihybrid cross
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
Codominance
Incomplete dominance
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
Fig. 8-9, p.118
ABO Blood Type:
A Multiple Allele System
Range of genotypes:
Blood
types:
IA IA
IB IB
or
or
IA i
IA I B
IB i
ii
A
AB
B
O
ABO Blood Type
Codominance: ABO blood types
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
Incomplete dominance
Interactions among Gene Pairs
• Common among genes for hair
color in mammals
BLACK LABRADOR
YELLOW LABRADOR
CHOCOLATE LABRADOR
Pleiotropy
• Alleles at a single locus may affect two
or more traits
– Marfan syndrome
– Cystic fibrosis
Fig. 8-12, p.119
Pleiotropy
Pleiotropic effects of Marfan syndrome
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
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
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
Coat Color
Coat color in the Himalayan rabbit
Environmental Effects
on Phenotype
Homologous Chromosomes
• Homologous autosomes:
– identical in length, size, shape, and gene
sequence
• Sex chromosomes:
– nonidentical but still homologous
• Homologous chromosomes interact,
then segregate from one another during
meiosis
Karyotype Diagram
1
2
3
4
13
14
15
16
5
17
6
7
8
9
18
19
20
21
10
22
11
12
XX (or XY)
Karyotype preparation
Alleles
• Different molecular forms of a gene
• Arise through mutation
• Diploid cell has a pair of alleles at each
locus
• Alleles on homologous chromosomes
may be same or different
Sex
Determination
female
(XX)
male
(XY)
eggs
sperm
X
x
Y
X
x
X
X
X
X
XX
XX
Y
XY
XY
Sex Determination
Human sex determination
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
Crossing Over
Crossover review
Pedigree
Symbols
male
female
marriage/mating
offspring in order of
birth, from left to right
Individual showing
trait being studied
sex not
specified
I, II, III, IV...
generation
Fig. 8-22a, p.126
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
Pedigree for Polydactyly
Pedigree diagrams
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 dominant inheritance
Autosomal Recessive Inheritance
Patterns
• If parents are
both
heterozygous,
child will have a
25% chance of
being affected
Autosomal Recessive
Inheritance Patterns
Autosomal recessive inheritance
X-Linked Recessive Inheritance
• Males show
disorder more
than females
• Son cannot inherit
disorder from his
father
X-Linked Recessive Inheritance
X-linked inheritance
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
Cri-du-chat
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
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
Testing for Genetic Disorders
• Carrier screening
• Prenatal diagnosis
– Amniocentesis
– Chorionic villi sampling
– Fetoscopy
• Preimplantation diagnosis
Amniocentesis
Amniocentesis
Fetoscopy
• For
prenatal
diagnosis