Download 1. Basic Genetic Concepts The Nature of Inheritance (Genetics) Chapter 9:

Chapter 9:
Patterns of Inheritance
1. Genetic Concepts
2. Single Gene Inheritance
3. Multiple Gene Inheritance
4. Sex-linked Inheritance
1. Basic Genetic Concepts
The Nature of Inheritance
(Genetics)
Genetics is the study of how genes are
inherited AND how they influence the
physical characteristics of each individual.
Genetics relates to 2 basic processes:
1) the distribution of genes into haploid gametes
• i.e., by meiosis
2) how the interaction of gene pairs in a diploid
individual influences physical characteristics
1
haploid
diploid
Genes, Alleles & Chromosomes
gene loci
genotype:
dominant
allele
P
a
B
P
a
b
aa
Bb
homozygous
for the
recessive allele
heterozygous
PP
homozygous
for the
dominant allele
recessive
allele
• all genes have a specific chromosomal location or
locus & can have different versions or alleles
Important Genetic Terms
Trait (or character)
• physical characteristic (e.g., eye color)
Phenotype
• individual’s observable traits (e.g., brown eyes)
Allele
• different forms or versions of the same gene
Genotype
• the alleles an individual has for a given gene
(e.g., AA or Aa or aa)
2
Homozygous
• the 2 alleles for a gene are identical (AA or aa)
Heterozygous
• the 2 alleles for a gene are different (Aa)
Dominant allele (designated by capital letter: A)
• 1 copy of the allele determines phenotype (A-)
Recessive allele (designated by lower-case letter: a)
• affects phenotype only when homozygous (aa)
Homozygous dominant = AA
Homozygous recessive = aa
Heterozygous = Aa
2. Single Gene Inheritance
TRAIT
purple
white
FLOWER
POSITION
axial
terminal
SEED COLOR
yellow
green
SEED SHAPE
round
wrinkled
POD SHAPE
inflated
FLOWER
COLOR
constricted
POD COLOR
green
yellow
STEM LENGTH
tall
dwarf
Gregor Mendel
Deduced the basics of
inheritance in the 19th
century by analyzing pea
plant crosses:
• examined
several traits
exhibiting
dominant &
recessive
phenotypes
3
Crossing
Pea Plants
White
1
Removed
stamens from
purple flower
Stamens
Carpel
2
Transferred pollen from
Purple
stamens of white flower
to carpel of purple flower
Parents
(P)
3
Pollinated carpel
matured into pod
4
Planted seeds
from pod
Offspring
(F1)
The Inheritance of Flower Color
P generation
(true-breeding
parents)
Based on results
such as this for
various traits,
Mendel correctly
deduced that:
×
Purple flowers
White flowers
All plants have
purple flowers
F1 generation
• offspring inherit
2 “alleles”,
1 from ea parent
Fertilization
among F1 plants
(F1 × F1)
• one “allele” is
dominant over
the other
(e.g., purple
over white)
F2 generation
¾ of plants
¼ of plants
have purple flowers have white flowers
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
1
–
2
Gametes
1
–
2
P
p
Basis of
Mendel’s
results
In Mendel’s F1
crosses ½ of the
gametes from each
parent contained P
& ½ contained p:
Sperm
P
p
F2 plants
Phenotypic ratio
3 purple : 1 white
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp
Fertilization results
in 4 possible
combinations,
3 of which result in
purple flowers
4
The Punnett
Square
1) write out the cross
(genotypes of each
parent)
F1 genotypes
Bb x Bb
Bb female
Formation of eggs
1
–
2
2) indicate all possible
gametes for each
parent on axes of a
Punnet square
1
–
2
B
p
1
–
2
B
b
b
b
1
–
4
B
b
b
1
–
4
1
–
4
F2 genotypes
P
p
PP
Pp
Pp
B
b
B
Pp x
P
1
–
2
B
1
–
4
3) fill in all possible
combinations
(i.e., fertilizations)
4) determine expected
ratios of phenotypes
Bb male
Formation of sperm
Pp
Expected
Genotypic Ratio:
1 PP : 2 Pp : 1 pp
Expected
Phenotypic Ratio:
pp
3 purple : 1 white
Pedigrees & Human Genetics
First generation
(grandparents)
Ff
Second generation
(parents, aunts,
and uncles)
FF
or
Ff
Third generation
(two sisters)
Female Male
Affected
Unaffected
ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
5
Test Cross
Test crosses are used
to reveal an unknown
genotype
• cross the unknown with
a homozygous recessive:
P? x pp
• if all offspring display
dominant phenotype:
homozygous dominant (PP)
• if ½ recessive, ½
dominant phenotype:
heterozygous (Pp)
Incomplete
Dominance
P generation
Red
RR
White
rr
Dominant allele is
expressed to lesser
degree when only 1
copy is present
(heterozygous):
r
R
Gametes
F1 generation
Pink
Rr
1
–
2
R
1
–
2
R
Gametes
• heterozygotes have
an intermediate
phenotype (e.g., pink F2 generation
vs red or white)
1
–
2
r
Sperm
1
–
2
r
1
–
2
R
RR
rR
1
–
2
r
Rr
rr
Eggs
Codominance & Blood Type
Blood
Group
(Phenotype) Genotypes
O
Red Blood Cells
ii
A
IAIA
or
IAi
Carbohydrate A
B
IBIB
or
IBi
Carbohydrate B
AB
• “A” & “B”
alleles are
codominant
(both alleles
expressed)
• “O” allele is
recessive
IAIB
6
More on ABO Blood Type
Blood
Group
(Phenotype) Genotypes
Antibodies
Present in
Blood
O
ii
Anti-A
Anti-B
A
IAIA
or
IAi
Anti-B
B
IBIB
or
IBi
Anti-A
AB
IAIB
—
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
O
IAi x
IA
i
IB
i
IAIB
IAi
IB i
ii
A
B
AB
IB i
Expected
Phenotypic Ratio:
¼ type AB
¼ type A
¼ type B
¼ type O
3. Multiple Gene Inheritance
7
Inheritance of 2 Traits
The results of a cross involving 2 genes can
also be predicted using a Punnet square.
First the possible gametes produced by each
parent with regard to both genes must be
determined:
e.g.
Aa Bb x Aa Bb
• the # of possible gametes is the product of the
# of different alleles for each gene
2 “A” alleles (A & a) x 2 “B” alleles (B & b) = 4
The 4 possible gametes for each parent in
this cross (Aa Bb x Aa Bb) are:
Aa Bb
AB Ab
AB AABB
aB ab
AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
AaBB AaBb
aaBB aaBb
ab
AaBb Aabb
aaBb
Expected Ratios:
9 A-B3 A-bb
3 aaB1 aabb
aabb
One of Mendel’s
“Dihybrid” Crosses
(dihybrid
cross)
• crossed plants that breed
true for 2 different traits,
then crossed F1 plants:
e.g., plants with smooth,
yellow seeds (SSYY) X
wrinkled, green seeds (ssyy)
SSYY x ssyy = all SsYy
SsYy x SsYy = 9:3:3:1
8
Gamete Production by Meiosis
F1 generation
all possible
gametes will be
produced in
equal
proportions if
each gene is on a
different
chromosome
R
r
Y
R
r
Y
y
R
r
Y
y
r
Y
y
r
R
r
R
Y
y
Metaphase II
of meiosis
y
Y
r
r
R
y
r
R
Y
y
r
y
y
Y
R
R
1
–
4
1
– rY
4
1
– ry
4
1
– RY
4
r
Y
Gametes
y
Y
R
Metaphase I
of meiosis
(alternative
arrangements)
Anaphase I
of meiosis
R
Y
All round yellow seeds
(RrYy)
y
Ry
Fertilization among the F1 plants
F2 generation
9
:3
:3
:1
4. Sex-linked Inheritance
Human Sex
Determination
XX = female
XY = male
Sex-linked
Inheritance
Sex-linked inheritance
involves genes on the
X (or Y) chromosome:
• human females have 2
X chromosomes, hence
2 alleles for each
X-linked gene
• human males have 1
X chromosome, hence
1 allele for each
X-linked gene
9
X-linked Genes
Genes on the X
chromosome have a
unique inheritance
pattern in males:
• only 1 allele, so no
masking of
recessive alleles
XHXh x
XHY
XH
Y
XHXH
XHY
H h
Xh X X
XhY
XH
• X-linked alleles are
always inherited
from the mother (carrier)
e.g., Hemophilia is
caused by a recessive
X-linked allele (h):
An “X-linked” Pedigree
The inheritance pattern
shows that color-blindness
is recessive & X-linked
Key Terms for Chapter 9
• trait, phenotype, genotype, allele
• homozygous, heterozygous
• dominant, recessive, codominant
• true-breeding, monohybrid, dihybrid
• test cross, Punnet square
• sex-linked, X-linked
• pedigree
Relevant Review Questions:
1-7, 10-12, 15-17
10