Transcript Chapters 6 & 7 Genetics

Gregor Mendel’s Discoveries
•
Pre-Mendel  Blending Theory of
Heredity
– Hereditary material from each
parent mixes in the offspring
•
•
•
2 problems
Individuals of a population should
reach a uniform appearance after
many generations
Once traits are blended, they can’t
be separated
• Gregor Mendel 
Particulate Theory of
Heredity
– Traits are inherited as
separate factors
• Mendel used quantitative
approach
• Studied peas for 3 reasons:
– Many varieties
– Self pollinating/cross pollinating
– Each variety had 2 alternative
forms
• Used true breeding varieties
• Used large sample sizes and
accurate observations
• Used math to develop
probabilities and perform
statistical analyses
• Used terms to define
generations as: P, F , F
1
2
• Developed terms such as:
–
–
–
–
–
Alleles (factor)
Dominant/Recessive
Homozygous/ Heterozygous
Phenotype/Genotype
Testcross
• Derived 2 principles:
– Law of segregation – two alleles for a character
separate when gametes are formed
Law of Independent assortment – each pair of alleles segregate into gametes
independently
Degrees of Dominance
• Complete dominance occurs when
phenotypes of the heterozygote and
dominant homozygote are identical
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-9

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
R
1/
2
r
R
R
Eggs
4
r
2
r
2
R
1/
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Fig. 14-16
Parents
Normal
Aa

Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
Fig. 14-17
Parents
Dwarf
Dd

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
d
Dd
Dwarf
Eggs
Normal
dd
Normal
• Dominant condition
• Fatal
• Only one Huntington’s
allele needed
• Produces abnormal
protein that clumps up
in cell nuclei –
especially nerve cells in
the brain
Concept Quiz
If an allele for tall plants (T) is dominant to
short plants (t), what offspring would you
expect from a TT x Tt cross?
A. ½ tall; ½ short
B. ¾ tall; ¼ short
C. All tall
Identify vocab term:
• This allele gets masked in the
phenotype
• Genetic Makup
• Alternate forms of a gene
• The allele that gets fully expressed
• Two identical alleles for a trait
• Physical appearance
If two heterozygotes are crossed
for dimpled chin. What will be
the expected genotypic ratio?
Phenotypic ratio?
• 1:2:1
• 3:1
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single
gene may deviate from simple Mendelian
patterns in the following situations:
– When alleles are on the sex chromosomes
– When alleles are not completely dominant or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• In codominance, phenotypes of both
alleles are exhibited in the heterozygote
• Affects 1 in 2500 individuals
in European populations
Fig. 14-10-1
P Generation
Red
CRCR
Gametes
White
CWCW
CR
CW
Fig. 14-10-2
P Generation
Red
CRCR
Gametes
White
CWCW
CR
CW
Pink
CRCW
F1 Generation
In incomplete
dominance, the
phenotype of F1 hybrids
is somewhere between
the phenotypes of the
two parental varieties
Gametes 1/2 CR
1/
2
CW
Fig. 14-10-3
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
Incomplete Dominance
Concept Quiz
A red carnation and a white carnation
produce offspring that are all pink. The type
of inheritance pattern occurring is:
A. Complete dominance
B. Incomplete dominance
C. Codominance
Red is dominant to white in flower petal
color. If a homozygous dominant is crossed
with a homozygous recessive and this
inheritance is incomplete dominance. What
will be the phenotypic ratio of this cross?
• 1:2:1
Concept Quiz
Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FWFW cross?
A. 3 white : 1 brown
B. 3 white : 1 cream
C. 2 white : 2 cream
Multiple Alleles
• Most genes exist in populations in more than
two allelic forms
• For example, the four phenotypes of the ABO
blood group in humans are determined by
three alleles for the enzyme (I) that attaches A
or B carbohydrates to red blood cells: IA, IB,
and i.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-11
Allele
IA
IB
Carbohydrate
A
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
Who is the universal recipient?
Who is the universal donor?
What blood types would the offspring have if one parent
Is type O and one parent is AB? Both parents AB?
Pleiotropy
The ability of a gene to affect
an organism in many ways
Fig. 14-12
BbCc

BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
Eggs
1/
1/
1/
1/
4 BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bC
4 Bc
4 bc
9
: 3
: 4
A gene at one locus
Alters a gene at another
locus
Epistasis
•
•
•
•
B = Black
b = Brown
C = Pigment
c = nonpig
Polygenic Inheritance
• Additive effect of 2
or more genes on
a single
phenotypic
character
• Eye color, skin
color
Identify each type of inheritance?
• Both alleles are expressed in
heterozygote
• More than two alleles are possible for a
trait
• The dominant allele masks the
recessive allele
• Additive effect of two or more genes
• Intermediate phenotype in
heterozygotes
• A gene at one locus controls a gene at
another locus
Quantitative Genetics
• The environment plays a
role – traits such as
height, weight, musical
ability, susceptibility to
cancer,and intelligence
• Quantitative traits show
continuous variation;
we can see a large
range of phenotypes in
the population
• The amount of variation
in a population is called
variance
Genetically the same but
Phenotypically different
Nutrition, exercise, and exposure
to sun can cause differences in
phenotypes
Diversity in Offspring
• Mutation, independent assortment, crossing
over, and random fertilization result in unique
combinations of alleles
• These processes produce the diversity of
individuals found in humans and all other
sexually reproducing biological populations
• You are one out of 64 trillion genetically
different children that your parents could
produce
• Fraternal (nonidentical)
– dizygotic: two
separate fertilized
eggs
– not genetically the
same
• Identical
– monozygotic:
one single
fertilized egg
that separates
– genetically the
same
Sex Determination and
Sex Linkage
• Some genes are on
the X chromosome
and are inherited in
a specific manner
• In humans, sex
determination
involves the X and Y
chromosomes
Sex Linkage
• The genes on the X
or Y chromosomes
are called sexlinked genes
• Genes on X are
called “X-linked,”
while those on Y are
called “Y-linked”
• The X chromosome
is much larger and
carries far more
genetic information
X-Linked Genes
• Since males only have
one X chromosome,
they are more likely to
suffer from X-linked
diseases
– Hemophilia
– red-green color
blindness
– muscular dystrophy
• Since females get one
X chromosome from
each parent, and have
two copies, they are
less likely to suffer from
X-linked diseases
• X inactivation
allows some female
organisms to shut
off their X
chromosomes
7.3 Pedigrees
• A pedigree is a
chart showing
inheritance patterns
in a family
• Pedigrees can be
used to identify
different types of
inheritance patterns