Download Chapter 23: Patterns of Gene Inheritance

Chapter 23: Patterns of Gene
Inheritance
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Mendel’s Laws
Gregor Mendel was an Austrian monk
who in 1860 developed certain laws of
heredity after doing crosses between
garden pea plants.
Gregor Mendel investigated genetics at
the organismal level.
Examples of traits that can be observed
at the organismal level include facial
features (ex: big noses) that cause
generations to resemble each other.
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Gregor Mendel
Mendel’s law of segregation:
1.) Each individual has two factors (called
genes) for each trait (one from each parent).
2.) The genes segregate (separate) during
gamete formation (i.e., meiosis).
3.) Each gamete contains only one gene for
each trait (i.e., they are haploid).
4.) Fertilization gives the new individual two
genes for each trait (one from each parent,
restores diploid state).
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Diploid = Two copies of each type of chromosome
Loci = Physical position of a gene on a chromosome
Allele = Alternate
forms of a gene:
Homologous
Chromosomes
From Father
Genes
Alleles have the
same position
(locus) on a pair
of homologous
chromosomes
From Mother
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Alleles code for the same trait.
Examples of alleles:
-curly or straight (alleles), hair type (gene)
-attached or unattached (alleles), ear lobe type (gene)
Chromosomes segregate during the formation of the
gametes and each gamete has only one chromosome
from each pair.
Fertilization gives each new individual two
chromosomes again.
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The Inheritance and Expression
of a Single Trait
A capital letter indicates a dominant
allele, which is expressed when
present.
An example is W for widow’s peak.
A lowercase letter indicates a recessive
allele, which is only expressed only in
the absence of a dominant allele.
An example is w for a continuous or
straight hairline.
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Widow’s
peak
WIDOW’S PEAK
WW
Ww
STRAIGHT or CONTINUOUS HAIRLINE
ww
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Genotype and Phenotype
Genotype refers to the genes of an
individual which can be represented by
two letters or by a short descriptive
phrase.
Homozygous means that both alleles are
the same; for example, WW stands for
homozygous dominant and ww stands
for homozygous recessive.
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Heterozygous means that the members
of the allelic pair are different—for
example, Ww.
Phenotype refers to the physical or
observable characteristics of the
individual – widow’s peak or straight
hairline.
Both WW and Ww result in widow’s peak,
two genotypes with the same
phenotype.
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Gamete Formation
Because homologous pairs separate during
meiosis, a gamete has only one allele from
each pair of alleles (for a specific gene).
If the allelic pair is Ww, the resulting gametes
would contain either a W or a w, but not both
– (gametes are haploid).
Ww represents the genotype of an individual.
Gametes that could be produced by this
individual are W or w.
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Genotype
Gametes
AA
A
A
Aa
A
a
AABb
AB
Ab
AaBb
AB
Ab
aB
ab
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One-Trait Crosses
In one-trait crosses, only one trait (such
as type of hairline) is being considered.
When performing crosses, the original
parents are called the parental
generation, or the P generation.
All of their children are the filial
generation, or F generation.
Children are monohybrids when they are
heterozygous for one pair of alleles.
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Male
Female
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If you know the genotype of the parents, it
is possible to determine the gametes and
use a Punnett square to determine the
phenotypic ratio among the offspring.
W
w
W
WW
Ww
w
Ww
ww
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Monohybrid cross
Genotypes of parents are known
(both are heterozygous Ww)
Genotypic Ratio
1 WW homozygous dominant
2 Ww heterzygous
1 ww homozygous recessive
Phenotypic Ratio
3 widow’s peak
1 straight hairline
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The One-Trait Testcross
It is not always possible to discern a
homozygous dominant from a heterozygous
individual by inspection of phenotype (they
have the same phenotype – both will have
widow’s peak).
A testcross crosses the dominant phenotype
with the recessive phenotype.
If a homozygous recessive phenotype is
among the offspring, the parent must be
heterozygous.
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One-trait testcross
?
All offspring have dominant
phenotype. Therefore the
dominant parent (genotype
we are tying to figure out) must
be homozygous dominant.
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?
Offspring have dominant and
recessive phenotypes.
Therefore the dominant parent
(genotype we are tying to
figure out) must be
heterozygous dominant.
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1.) Both a man and woman are heterozygous for
tongue rolling. Tongue rolling is dominant over
non-tongue rolling. What is the chance that their
child will be a tongue roller?
Male
Female
T
GENOTYPE
Tt
Tt
T
t
T
t
T
t
TT
Tt
Tt
tt
MALE
t
GAMETES
FEMALE
Offspring Phenotypes
3 Rollers
1 Non-Roller
3 of 4 chances for roller
child (75% chance).23-19
2) Both you and your sibling are non-rollers and
your parents are rollers. Tongue rolling is dominant
over non-tongue rolling. What are the genotypes of your
parents?
OFFSPRING
tt
PARENTS
T
t
T
T
t
t
TT
Tt
Tt
tt
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The Inheritance of Many Traits
Independent Assortment
The law of independent assortment states that
each pair of alleles segregates independently
of the other pairs and all possible
combinations of alleles can occur in the
gametes.
This law is dependent on the random
arrangement of homologous pairs at
metaphase.
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Segregation and independent
assortment
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Two-Trait Crosses
In two-trait crosses, genotypes of the
parents require four letters because
there two alleles for each trait.
Gametes will contain one letter for each
trait.
When a dihybrid (heterozygous for both
traits) reproduces with another dihybrid
the phenotypic results are 9 : 3 : 3 : 1.
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Widow’s Peak is dominant over Straight Hairline
W
w
Short Fingers are dominant over Long Fingers
S
s
Phenotype
Widow’s Peak / Short Fingers
Genotypes
WWSS WWSs WwSS WsSs
Widow’s Peak / Long Fingers
WWss Wwss
Straight HL / Short Fingers
wwSS wwSs
Straight HL / Long Fingers
wwss
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Dihybrid cross (two traits)
Widow’s Peak
Short Fingers
Straight Hairline
Long Fingers
homozygous dominant
homozygous recessive
Widow’s Peak
Short Fingers
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WwSs
WwSs
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The Two-Trait Testcross
A testcross is done to determine
genotype of individual that has
dominant phenotypes (for both traits).
(Homozygous dominant or heterozygous
for the two traits under consideration).
Cross heterozygote for both traits with
homozygous recessive for both traits results in 1 : 1 : 1 : 1 ratio.
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Two-trait testcross
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SELECTED TRAITS IN HUMAN HEREDITY
Dominant
Recessive
normal skin pigmentation
freckles
broad lips
tongue roller
PTC taster
large eyes
migraine headaches
normal foot arch
albinism
no freckles
thin lips
non-tongue roller
PTC non-taster
small eyes
no migraine headaches
flat feet
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If a man that is homozygous recessive for eye size
(i.e., has small eyes) and is homozygous dominant for
freckles (i.e., has freckles) has children with a woman
that is homozygous dominant for eye size (i.e., has large
eyes) and is homozygous recessive for freckles
(i.e., does not have freckles), what are the potential
phenotypes and genotypes of their children?
Man
llFF  IF only for gametes
Woman LLff  Lf only for gametes
GENOTYPE
IF
IF
Lf
ILFf
ILFf
Lf
ILFf
ILFf
PHENOTYPE
All are heterozygous for
both traits and show large
eyes with freckles
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If one of the children reproduces with another person
that has the same genotype, what are the chances that
they will have a child with large eyes and freckles?
LlFf
LF
X
Lf
LF LLFF LLFf
Lf
LLFf LLff
lF
LlFf
lf
LlFF LlFf
LlFf
Llff
large eyes/freckles
9/16 or  56 %
Large eyes/no freckles
3/16
Small eyes/freckles
lF
LlFF LlFf
lf
LlFf
Llff
llFF
llFf
llFf
llff
3/16
Small eyes/no freckles
1/16
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Genetic Disorders
Patterns of Inheritance
When studying human disorders, biologists
often construct pedigree charts to show the
pattern of inheritance of a characteristic
within a family.
Genetic counselors construct pedigree charts
to determine the mode (dominant or
recessive) of inheritance of a condition.
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Pedigree Analysis: determine how a genetic disorder
is inherited, chances of offspring having a genetic disorder.
Unaffected male
Unaffected female
Affected Male
Affected female
“UNION”
OFFSPRING
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Genetic Disorders: medical conditions caused by alleles
inherited from parents, hereditary disorder.
Autosomal Genetic Disorders: genetic disorders caused by
Alleles on autosomal chromosomes (non-sex
Chromosomes – similar to somatic).
Autosomal Disorders can be:
1) Autosomal Dominant
2) Autosomal Recessive
Autosomal Dominant AA or Aa have disorder (phenotype)
Aa
Aa
aa
Aa
aa
aa
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Autosomal Recessive: aa have disorder (phenotype)
AA or Aa
aa
aa
aa
aa
Aa
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CARRIER –
Has allele but
is unaffected
*
* HOW DO YOU KNOW INDIVIDUAL
IS HETEROZYGOUS?
Autosomal recessive pedigree chart (Taysachs disease, Cystic fibrosis, PKU)
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* HOW DO YOU KNOW INDIVIDUAL
IS HETEROZYGOUS?
Autosomal dominant pedigree chart
(Neurofibromatosis, Huntington disease)
*
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Polygenic Inheritance
Polygenic traits are governed by more
than one gene pair (e.g., several pairs
of genes may be involved in
determining the phenotype).
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Polygenic inheritance
Such traits produce a
continuous variation
representing a bellshaped curve (Ex:
height in humans).
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Skin Color
The inheritance of skin color, determined
by an unknown number of gene pairs,
is a classic example of polygenic
inheritance.
A range of phenotypes exist from very
dark to very light.
The distribution of these phenotypes also
follows a bell-shaped curve.
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Polygenic Disorders
Many human traits, like allergies,
schizophrenia, hypertension, diabetes,
cancers, and cleft lip, appear to be due
to the combined action of many genes
plus environmental influences.
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Multiple Allelic Traits
Inheritance by multiple alleles occurs
when more than two alternative
alleles exist for a particular gene locus.
A person’s blood type is an example of a
trait determined by multiple alleles (A,
B, and O).
***Each individual inherits only two
alleles for these genes.
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ABO Blood Types
A person can have an allele for an A
antigen (blood type A) or a B antigen
(blood type B), both A and B antigens
(blood type AB), or no antigen (blood
type O) on the red blood cells.
Human blood types can be type A (IAIA or
IA i), type B (IBIB or IBi), type AB (IAIB), or
type 0 (ii).
Alleles: A, B, O
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Inheritance of blood type….
(Who’s your daddy?)
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Incompletely Dominant Traits
Codominance means that both alleles are
equally expressed in a heterozygote.
(Ex: sickle cell anemia)
Incomplete dominance is exhibited when
the heterozygote doesn’t show the
dominant trait but shows an
intermediate phenotype, representing a
blending of traits. (Ex: curly, wavy, or
straight hair)
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Incomplete dominance
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Sickle-Cell Disease
Sickle-cell disease is an example of a
human disorder controlled by
incompletely dominant alleles.
Sickle cell disease involves irregular,
sickle shaped red blood cells caused by
abnormal hemoglobin.
HbA represents normal hemoglobin; and
HbS represents the sickled condition.
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HbAHbA individuals are normal; HbSHbS
individuals have sickle-cell disease and
HbAHbS individuals have the
intermediate condition called sickle-cell
trait.
Heterozygotes have an advantage in
malaria-infested Africa because the
pathogen for malaria cannot exist in
their blood cells.
This evolutionary selection accounts for
the prevalence of the allele among
African Americans.
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