Download Chapter 9: Patterns of Inheritance

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
haploid
diploid
gene 1
gene 2
gene 3
gene 4
Genes, Alleles &
Chromosomes
• diploid organisms (e.g.,
humans) have 2 copies of ea
chromosome, hence 2 copies
of each gene
• genes can come in different
versions called alleles
• all genes have a
characteristic chromosomal
location or locus
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)
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
Gregor Mendel
Deduced the basics
of inheritance by
analyzing pea plant
crosses:
• examined a number
of traits exhibiting
dominant & recessive
phenotypes
The Inheritance of Flower Color
P generation
(true-breeding
parents)
×
Purple flowers
White flowers
All plants have
purple flowers
F1 generation
Fertilization
among F1 plants
(F1 × F1)
F2 generation
3
4
1
of plants
of plants
4
have purple flowers have white flowers
Based on results
such as this for
various traits,
Mendel correctly
deduced that:
• offspring inherit
2 “alleles”,
1 from ea parent
• one “allele” is
dominant over
the other
(e.g., purple
over white)
Basis of
Mendel’s results
In Mendel’s F1 crosses
½ of the gametes from
each parent contained
“P”, ½ contained “p”:
Fertilization results in 4
possible combinations,
3 of which result in
purple flowers
The Punnett Square
…is useful for determining
expected results of a cross:
1) write out the cross (i.e., the
genotypes of each parent)
2) indicate all possible gametes
for each parent on either axis
of a Punnet square
3) fill in all possible fertilizations
(i.e., combinations of gametes)
4) determine the expected
proportions of ea phenotype
Pp x
P
p
P
p
PP
Pp
Pp
pp
Pp
Expected
Genotypic Ratio:
1 PP : 2 Pp : 1 pp
Expected
Phenotypic Ratio:
3 purple : 1 white
Pedigrees & Human Genetics
Pedigrees are extremely useful for deducing
patterns of human
genetic inheritance…
D?
Dd
D?
Dd
Joshua
Lambert
John
Eddy
Abigail
Linnell
Parents
Hepzibah
Daggett
Sperm
D
dd
Jonathan
Lambert
D?
Abigail
Lambert
Dd
Elizabeth
Eddy
D
Offspring
Dd
Dd
dd
Dd
**Inheritance pattern
shows this form of
deafness to be recessive**
Dd
Dd
DD
Normal
d
Dd
Normal
(carrier)
Eggs
dd
Female Male
Deaf
Hearing
d
Dd
Normal
(carrier)
dd
Deaf
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
Dominant allele is expressed to lesser degree
when only 1 allele is present (heterozygous):
• heterozygotes have an intermediate phenotype
Codominance & Blood Type
• “A” & “B” alleles are codominant (both alleles expressed)
• “O” allele is recessive
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
A
B
AB
IAi x
IB
IA
i
IAI B
I Bi
IB i
i
IAi
ii
Expected
Phenotypic Ratio:
¼ type AB
¼ type A
¼ type B
¼ type O
3. Multiple Gene Inheritance
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 ab
AABb AaBB AaBb
AABb AAbb AaBb Aabb
Expected Ratios:
9 A-B3 A-bb
aB
ab
AaBB AaBb aaBB aaBb
3 aaB1 aabb
AaBb Aabb
aaBb
aabb
One of Mendel’s
“Dihybrid” Crosses
• 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
(dihybrid
cross)
Gamete Production by Meiosis
• all possible gametes will be produced in equal
proportions if each gene is on a different chromosome
F1 generation
All round yellow seeds
(RrYy)
R
r
y
Y
R
Y
R
r
Y
y
Metaphase I
of meiosis
(alternative arrangements)
r
R
Y
y
r
Anaphase I
of meiosis
y
r
R
r
R
Y
y
r
R
Y
y
Metaphase II
of meiosis
y
Y
Y
y
Y
Gametes
R
R
1
y
r
r
1
RY
4
Y
r
r
1
ry
4
y
y
Y
R
R
1
rY
4
Ry
4
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
X-linked Genes
Genes on the X
chromosome have a
unique inheritance
pattern in males:
• only 1 allele, so no
masking of
recessive alleles
H
h
X X
x
H
X
XH
H
X Y
Y
H
H
X X
H
X Y
H
h
X X
h
X Y
• X-linked alleles are
always inherited
from the mother (carrier)
e.g., Hemophilia is
caused by a recessive
X-linked allele (h):
Xh
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