Transcript You Light Up My Life

Get Ready for A & P!
Genetics, Genetic Abnormalities
& Bioethics
Early Ideas about Heredity
 People
knew that sperm and eggs
transmitted information about traits
 Blending theory - traits blended
 Problem:


Would expect variation to disappear
However, variation in traits persists
Gregor Mendel
 The
founder of modern genetics
Fig. 11-2, p.170
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
 Each
from parents to offspring
has a specific location (locus) on a
chromosome
Alleles
 Different
 Arise
molecular forms of a gene
by mutation
 Dominant
allele masks a recessive
allele that is paired with it
Allele Combinations
 Homozygous


having two identical alleles at a locus
AA or aa
 Heterozygous


having two different alleles at a locus
Aa
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 chromosome. Alleles are at corresponding
loci on a pair of homologous chromosomes
A pair of alleles may be identical or nonidentical.
They are represented in the text by letters such as D
or d
Three pairs of genes (at three loci on this pair of
homologous chromosomes); same thing as three
pairs of alleles
Fig. 11-4, p.171
Genotype & Phenotype
 Genotype
refers to particular genes an
individual carries
 Phenotype refers to an individual’s
observable traits
 Cannot always determine genotype by
observing phenotype
 3 major genotypes :
homozygous dominant (AA)
homozygous recessive (aa)
heterozygous (Aa)
Tracking Generations
 Parental
generation
mates to produce
 First-generation
offspring
mate to produce
 Second-generation
offspring
P
F1
F2
Monohybrid Crosses
Experimental intercross between
two F1 heterozygotes
AA X aa
Aa (F1 monohybrids)
Aa X Aa
?
Mendel’s
Monohybrid
Cross Results
F2 plants showed
dominant-torecessive ratio that
averaged 3:1
5,474 round
1,850 wrinkled
6,022 yellow
2,001 green
882 inflated
299 wrinkled
428 green
152 yellow
705 purple
224 white
651 long stem
207 at tip
787 tall
277 dwarf
Fig. 11-6, p. 172
Trait Studied
Dominant
Form
Recessive
Form
F2 Dominant-toRecessive Ratio
SEED SHAPE
5,474 round
1,850 wrinkled
2.96:1
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
787 tall
277 dwarf
2.84:1
FLOWER POSITION
STEM LENGTH
Fig. 11-6, p.172
Probability
The chance that each outcome of a given
event will occur is proportional to the
number of ways that event can be reached
Monohybrid
Cross
Illustrated
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
Figure 11.7
Page 173
a
Aa
aa
Test Cross
 Individual
that shows dominant phenotype
is crossed with individual with recessive
phenotype
 Examining
offspring allows you to
determine the genotype of the dominant
individual
Punnett Squares of Test Crosses
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. 11-7b1, p.173
Punnett Squares of Test Crosses
An F1 plant self-fertilizes
and produces gametes:
F2
PHENOTYPES
Aa
A
a
A
AA
Aa
a
Aa
aa
AA
Aa
Aa
aa
Fig. 11-7b2, p.173
Dihybrid Cross
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
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
Dominance Relations
Complete dominance
Incomplete dominance
Codominance
Incomplete
Dominance
X
Incomplete
Homozygous
Homozygous
parent
parent
Dominance
All F1 are
heterozygous
X
F2 shows three phenotypes in 1:2:1 ratio
Incomplete
Dominance
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. 11-11, p.176
Codominance: ABO Blood Types
 Gene
that controls ABO type codes for
enzyme that dictates structure of a
glycolipid on blood cells
alleles (IA and IB) are codominant
when paired
 Two
 Third
allele (i) is recessive to others
ABO Blood Type
Range of genotypes:
IAIA
IBIB
or
or
IAi
Blood
Types:
A
IAIB
AB
IBi
ii
B
O
Fig. 11-10a, p.176
ABO and Transfusions
 Recipient’s
immune system will attack
blood cells that have an unfamiliar
glycolipid on surface
 Type
O is universal donor because it has
neither type A nor type B glycolipid
Temperature Effects
on Phenotype
 Rabbit
is homozygous for
an allele that specifies a
heat-sensitive version of an
enzyme in melaninproducing pathway
 Melanin is produced in
cooler areas of body
Figure 11.16
Page 179
Autosomal Recessive
Inheritance Patterns
 If
parents are
both
heterozygous,
child will have a
25% chance of
being affected
Fig. 12-10b, p. 191
Fig. 11-21, p.183
Autosomal
Dominant Inheritance
Trait typically
appears in
every
generation
Fig. 12-10a, p. 190
Huntington Disorder

Autosomal dominant allele

Causes involuntary movements, nervous system
deterioration, death

Symptoms don’t usually show up until person is
past age 30

People often pass allele on before they know
they have it
Achondroplasia
 Autosomal
dominant allele
 In
homozygous form usually leads to
stillbirth
 Heterozygotes
 Have
display a type of dwarfism
short arms and legs relative to other
body parts
Autosomal
Dominant
Inheritance
Fig. 12-5, p.190
Sex Chromosomes
 Discovered
 Mammals,

 In
in late 1800s
fruit flies
XX is female, XY is male
other groups XX is male, XY female
 Human
X and Y chromosomes function as
homologues during meiosis
The X Chromosome
 Carries
 Most
more than 2,300 genes
genes deal with nonsexual traits
 Genes
on X chromosome can be
expressed in both males and females
The Y Chromosome
 Fewer
than two dozen genes identified
 One
is the master gene for male sex
determination

SRY gene (sex-determining region of Y)
 SRY
present, testes form
 SRY
absent, ovaries form
Crossing Over
•Each chromosome
becomes zippered to its
homologue
•All four chromatids are
closely aligned
•Nonsister chromosomes
exchange segments
Effect of Crossing Over
 After
crossing over, each chromosome
contains both maternal and paternal
segments
 Creates
new allele combinations in
offspring
Crossover Frequency
Proportional to the distance that
separates genes
A
B
C
D
Crossing over will disrupt linkage between
A and B more often than C and D
In-text figure
Page 178
X-Linked Recessive Inheritance
 Males
show
disorder more
than females
 Son cannot inherit
disorder from his
father
Fig. 12-10, p.194
Examples of X-Linked Traits
 Color

blindness
Inability to distinguish among some of all
colors
 Hemophilia

Blood-clotting disorder

1/7,000 males has allele for hemophilia A

Was common in European royal families
Color Blindness
Fig. 12-12, p.195
male
Pedigree
Symbols
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. 12-19a, p.200
Duplication
 Gene
sequence that is repeated several to
hundreds of times
 Duplications
occur in normal
chromosomes
 May

have adaptive advantage
Useful mutations may occur in copy
Duplication
normal chromosome
one segment
repeated
three repeats
Deletion
 Loss
of some segment of a chromosome
 Most are lethal or cause serious disorder
Deletion
 Cri-du-chat
Fig. 12-13, p.196
Inversion
A linear stretch of DNA is reversed
within the chromosome
segments
G, H, I
become
inverted
In-text figure
Page 196
Translocation
A
piece of one chromosome becomes
attached to another nonhomologous
chromosome
 Most are reciprocal
 Philadelphia chromosome arose from a
reciprocal translocation between
chromosomes 9 and 22
Translocation
one chromosome
a nonhomologous
chromosome
nonreciprocal translocation
In-text figure
Page 206
In-text
figure
Aneuploidy
 Individuals
have one extra or 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
 Lethal
in flowering plants
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
Figure 12.16
Page 198
Nondisjunction
Fig. 12-16a, p.198
Down Syndrome
 Trisomy
of chromosome 21
 Mental
impairment and a variety of
additional defects
 Can
 Risk
be detected before birth
of Down syndrome increases
dramatically in mothers over age 35
Down Syndrome
Fig. 12-17, p.199
Turner Syndrome
 Inheritance
 98%
of only one X (XO)
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
 Most
otherwise phenotypically normal
 Some
 Once
than average males
mentally impaired
thought to be predisposed to
criminal behavior, but studies now
discredit
Genetic Abnormality
A
rare, uncommon version of a trait
 Polydactyly

Unusual number of toes or fingers

Does not cause any health problems

View of trait as disfiguring is subjective
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
Genetic Disorders and Genetic Abnormalities
Gene Mutations
Base-Pair Substitutions
Insertions
Deletions
Base-Pair Substitution
a base substitution
within the triplet (red)
original base triplet
in a DNA strand
During replication, proofreading
enzymes make a substitution
possible outcomes:
or
original, unmutated
sequence
a gene mutation
Frameshift Mutations
 Insertion

Extra base added into gene region
 Deletion

Base removed from gene region
 Both
shift the reading frame
 Result
in many wrong amino acids
Frameshift Mutation
part of DNA template
mRNA transcribed from DNA
THREONINE
PROLINE
GLUTAMATE
GLUTAMATE
LYSINE
resulting amino acid sequence
base substitution in DNA
altered mRNA
THREONINE
PROLINE
VALINE
GLUTAMATE
LYSINE
altered amino acid sequence
deletion in DNA
altered mRNA
THREONINE
PROLINE
GLYCINE
ARGININE
altered amino acid sequence
Fig. 14-10, p.226
Mutation Rates
 Each
gene has a characteristic mutation
rate
 Average rate for eukaryotes is between
10-4 and 10-6 per gene per generation
 Only mutations that arise in germ cells can
be passed on to next generation
Mutagens
 Ionizing
radiation (X rays)
 Nonionizing
 Natural
radiation (UV)
and synthetic chemicals