Download Chapter 14 - Speedway High School

Chapter
p
14
Mendel and the Gene Idea
Concept 14.1: Mendel used the scientific approach
to identify two laws of inheritance
• Mendel discovered the basic principles of heredity
by breeding garden peas in carefully planned
experiments
LE 14-2
Removed stamens
from purple flower
Transferred sperm
spermbearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
Carpel
Stamens
Pollinated carpel
matured into pod
Planted seeds
from pod
First
generation
offspring
(F1)
Examined
offspring:
all purple
flowers
• Mendel chose to track only those characters that
varied in an “either-or”
either-or manner
• He also used varieties that were “true-breeding”
( l t th
(plants
thatt produce
d
offspring
ff i off the
th same variety
i t
when they self-pollinate)
The Law of Segregation
• When any individual produces gametes, the
copies of a gene separate, so that each gamete
receives only one copy.
PP x pp
Pp x Pp
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
F2 Generation
Purple
White
flowers flowers
All p
plants had
purple flowers
• Mendel called the
purple flower color
a dominant trait
and white flower
color a recessive
trait
• “heritable factor” is
what we now call a
gene
Mendel’s Model
• Mendel developed four related concepts to explain the
3:1 inheritance pattern he observed in F2 offspring.
• First Concept – alternate versions of genes, alleles,
exist and explain for inherited variations.
Allele for purple flowers
Locus for flower-color g
gene
Allele for white flowers
Homologous
pair of
chromosomes
• Second concept - for each character an
organism inherits two alleles
alleles, one from each
parent
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
F2 Generation
Purple
White
flowers flowers
All p
plants had
purple flowers
• Third concept - if the two alleles at a locus differ,
then the dominant allele determines the
organism’s
i ’ appearance, and
d th
the recessive
i allele
ll l
has no noticeable effect on appearance
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
F2 Generation
Purple White
flowers flowers
All plants
l t had
h d
purple flowers
• Fourth concept – The Law of Segregation
states that the two alleles for a heritable character
separate (segregate) during gamete formation and
end up in different gametes.
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
F2 Generation
Purple White
flowers flowers
All plants
l t had
h d
purple flowers
LE 14-5_2
P Generation
Appearance:
Genetic makeup:
Purple
flowers
PP
White
flowers
pp
P
p
Gametes
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1
Gametes:
2
1
P
p
2
F1 sperm
P
p
PP
Pp
Pp
pp
F2 Generation
P
F1 eggs
p
3
:1
Useful Genetic Vocabulary
• Homozygous - An organism with two identical
alleles for a character
character.
• Heterozygous - An organism that has two
diff
different
t alleles
ll l ffor a gene.
– Unlike homozygotes,
yg
, heterozygotes
yg
are not
true-breeding
LE 14-6
3
Phenotype
Genotype
Purple
PP
(homozygous
Purple
Pp
(heterozygous
1
2
1
Purple
Pp
p
(heterozygous
White
pp
(homozygous
Ratio 3:1
Ratio 1:2:1
1
LE 14-7
Dominant phenotype,
unknown genotype:
PP or Pp?
Test Cross
Recessive phenotype,
known genotype:
pp
If Pp,
then 2 offspring purple
and 1 2 offspring white:
If PP,
then all offspring
purple:
p
1
p
P
p
p
Pp
Pp
pp
pp
P
Pp
Pp
P
P
Pp
Pp
The Law of Independent Assortment
• Monohybrids - individuals that are heterozygous
for one character
• A cross between such heterozygotes is called a
monohybrid
h b id cross
Yy x Yy
• Dihybrids - heterozygous for two characters
• A dihybrid cross, a cross between F1 dihybrids,
can determine whether two characters are
t
transmitted
itt d to
t offspring
ff i as a package
k
or
independently
YyRr x YyRr
?
LE 14-8
P Generation
YYRR
yyrr
Gametes YR
yr
YyRr
F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm
1
Sperm
1
2
YR
1
2
yr
1
1
2
2
1
4
Yr
1
4
yR
1
4
yr
YR
4
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
1
YR
Eggs
Eggs
F2 Generation
(predicted
offspring)
4
YyRr
1
Yr
4
yr
YyRr
3
4
yyrr
1
1
yR
y
4
4
1
Phenotypic ratio 3:1
yr
4
9
16
3
16
3
16
3
16
Phenotypic ratio 9:3:3:1
• Law of Independent Assortment - each pair of
alleles segregates independently of other pairs of
alleles during gamete formation
– This law applies only to genes on different,
non-homologous chromosomes
Concept 14.2: The laws of probability govern
Mendelian inheritance
• Mendel’s laws of segregation and independent
assortment reflect the rules of probability
• The alleles of one gene segregate into gametes
independently of another gene’s
gene s alleles
The Multiplication and Addition Rules Applied to
Monohybrid Crosses
• Multiplication rule - the probability that two or
more independent events will occur together is
the product of their individual probabilities
• Rr x Rr
– Probability of gamete having:
• RR?
• rr?
LE 14-9
Rr
Rr
Segregation of
gg
alleles into eggs
Segregation of
alleles into sperm
Sperm
1
R
2
R
1
2
1
r
2
R
R
R
1
r
1
4
4
Eggs
r
r
1
2
R
r
1
4
r
1
4
• Rule of addition - the probability that any one of
two or more exclusive events will occur is
calculated by adding together their individual
p
probabilities
• Rr x Rr
– Probability of gamete having:
• Rr?
Solving Complex Genetics Problems with the Rules
of Probability
• A dihybrid cross is equivalent to two or
more independent monohybrid crosses
occurring simultaneously
• Cross: YyRr x YyRr
• What is the probability of YYRR?
– of Yyrr?
Genetics Problems
• PPYyRrTt x PpyyRrtt
• What is the probability of PpyyrrTt?
Genetics Problems
• PpYyRr x Ppyyrr
•
What fraction of the offspring from this cross would exhibit the
recessive phenotypes for at least two of the three characteristics?
Concept 14.3: Inheritance patterns are often more
complex than predicted by simple Mendelian genetics
• Inheritance of characters by a single gene may
deviate from simple Mendelian patterns in the
following situations:
– Wh
When alleles
ll l are nott completely
l t l d
dominant
i
t or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
The Spectrum of Dominance
• Complete dominance
– PP=Purple Pp=Purple
• Codominance
– BB=Black Fur WW=White Fur BW=Black & White Fur
• Incomplete dominance
– RR=Red WW=White RW=Pink
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes
1
1
F2 Generation
2
CR
2
CR
1
2
1
CW
Sperm
2
CW
Eggs
1
1
2
2
CR
CRCR
CRCW
CRCW
CWCW
CW
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common in populations than recessive alleles
• For example, one baby out of 400 in the United
St t is
States
i born
b
with
ith extra
t fi
fingers or ttoes
– this unusual trait is dominant
• Known as polydactyly
• Natural Selection at work!
Multiple Alleles
• Most genes exist in populations in more than
two allelic forms
• Four phenotypes of the ABO blood group
– three alleles for the enzyme (I): IA, IB, and i
Pleiotropy
• Most genes have multiple phenotypic effects, a
property called pleiotropy
• For example, the multiple symptoms of
– cystic fibrosis and sickle-cell disease
Epistasis
• Epistasis - a gene at one locus alters the
phenotypic expression of a gene at a second
locus
• IIn mice
i and
d many other
th mammals,
l coatt color
l
depends on two genes
– One gene determines the pigment color
– The other gene determines whether the pigment will
be deposited in the hair
LE 14-11
BbCc
BbCc
Sperm
1
1
1
1
1
4
BC
1
4
bC
1
4
1
Bc
4
bc
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
4
Bc
BBCc
BbCc
BBcc
Bbcc
4
bc
BbCc
bbCc
Bbcc
bbcc
9
16
3
16
4
16
Polygenic Inheritance
• Quantitative
characters are those
th t vary in
that
i th
the
population along a
continuum
• Skin color in humans
is an example of
polygenic inheritance
AaBbCc
aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCcAABBCC
20/64
15/64
Fraction
n of progeny
• usually indicates
polygenic
l
i inheritance
i h it
AaBbCc
6/64
1/64
Nature and Nurture: The Environmental Impact
on Phenotype
• Norm of reaction - the phenotypic range of a
genotype influenced by the environment
• Hydrangea flowers of the same genotype
range from blue-violet to pink, depending on
soil acidity
Concept 14.4: Many human traits follow
Mendelian patterns of inheritance
• Pedigree - a family tree that describes the
interrelationships of parents and children across
generations
• Inheritance patterns of particular traits can be
traced and described using pedigrees
LE 14-14a
Ww
ww
ww
Ww ww ww Ww
WW
or
Ww
Ww
Ww
ww
Dominant trait (widow’s peak)
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters)
ww
Widow’s peak
First generation
(grandparents)
No widow’s peak
LE 14-14b
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff ff
Third
Thi
d
generation
(two sisters)
Attached earlobe
Recessive trait (attached earlobe)
Ff
ff
ff
Ff
Ff
ff
FF
or
Ff
Ff
ff
Free earlobe
• Pedigrees can also be used to make
predictions about future offspring
Dominantly Inheritance
Recessively Inherited Disorders
• Recessively inherited disorders show up only in
individuals homozygous for the allele
– Carriers are heterozygous individuals who
carryy the recessive allele but appear normal
Recessively Inheritance
X-Linked Recessive Inheritance
X-Linked Dominant Inheritance
Mitochondrial Inheritance
Cystic Fibrosis
• Cystic fibrosis is the most common lethal genetic
disease in the United States
States, striking one out of
every 2,500 people of European descent
– Common
C
symptom
t
- abnormal
b
l mucus b
buildup
ild
Sickle-Cell Disease
• Sickle-cell disease
affects one out of
400 AfricanAmericans
• Symptoms include
physical
p
y
weakness,,
pain, organ
damage, and even
paralysis
l i
Dominantly Inherited Disorders
• Some human disorders are due to dominant
alleles
• One example is achondroplasia, a form of
d
dwarfism
fi
th
thatt iis llethal
th l when
h h
homozygous ffor th
the
dominant allele
Fetal Testing
• In amniocentesis, the liquid that bathes the fetus
is removed and tested
• In chorionic villus sampling (CVS), a sample of
th placenta
the
l
t is
i removed
d and
d ttested
t d
Video: Ultrasound of Human Fetus I
LE 14-17a
Amniocentesis
Amniotic
fluid
withdrawn
Fetus
A sample of
amniotic fluid can
b taken
be
t k starting
t ti
att
the 14th to 16th
week of pregnancy.
C t if
Centrifugation
ti
Placenta
Uterus
Cervix
Fluid
Fetal
cells
Biochemical tests can be
performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Biochemical
tests
Several
weeks
k
Karyotyping
LE 14-17b
Chorionic villus sampling (CVS)
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
Fetus
Suction tube
inserted through
cervix
Placenta Chorionic villi
Fetal
cells
Biochemical
tests
Several
h
hours
Karyotyping
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.