Patterns of Inheritance
Chapter 12
Alleles
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A gene is a unit of heredity that encodes
information for the form of a particular
characteristic
The location of a gene on a chromosome
is called its locus
Genes for a characteristic found on
homologous chromosomes may not be
identical
Alternate versions or forms of genes found
at the same gene locus are called alleles
Alleles
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Each cell carries two alleles per characteristic,
one on each of the two homologous
chromosomes
If both homologous chromosomes carry the
same allele (gene form) at a given gene locus,
the organism is homozygous at that locus
If two homologous chromosomes carry different
alleles at a given locus, the organism is
heterozygous at that locus (a hybrid)
Early Ideas on Inheritance
• Original thought that inheritance was a
combination of both parents traits.
• Inheritance was thought to be blending of
the parents.
• One short parent, one tall parent =
medium height parent
Who Was Gregor Mendel?
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Mendel was a monk in a monastery in Brno
(now in Czech Republic) in late 1800s
Mendel studied botany and mathematics at
the university level before becoming a
monk
Experimentation with pea plant inheritance
took place in the monastery garden
Mendel’s background allowed him to see
patterns in the way plant characteristics
were inherited
The Secrets of Mendel’s
Success
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Important aspects of pea plants
– Pea flowers have male structures that
produce pollen (male gametes) by meiosis
– Pea flowers have female structures that
produce eggs (female gametes) by meiosis
– Pea flower petals enclose both male and
female flower parts and prevent entry of
pollen from another pea plant
The Secrets of Mendel’s
Success
• Pea plants can self pollinate
• Mendel could cross pollinate plants by
hand
• Pea plants have many contrasting traits
• Pea plants have a short generation turn
around time
The Language of a Genetic
Cross
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The parents used in a cross are part of
the parental generation (known as P)
The offspring of the P generation are
members of the first filial generation (F1)
Offspring of the F1 generation are
members of the F2 generation, etc.
Mendel’s Flower Color Experiments
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Mendel crossed a true-breeding purple
flower plant with a true-breeding whiteflower plant (P generation)
2. The F1 generation consisted of all purpleflowered plants
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What had happened to the white flower trait?
Mendel’s Flower Color
Experiments
3. Mendel allowed the F1 generation to self
fertilize
• The F2 were composed of ¾ purple flower
plants and ¼ white flower plants
Dominant and Recessive
Alleles
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Every cell in a pea plant carries two alleles
per characteristic (either the same or
different)
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The particular combination of the two alleles
carried by an individual is called the genotype
(PP, Pp, or pp)
The physical expression of the genotype is
known as the phenotype (e.g. purple or white
flowers)
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Mendel’s Law of Dominance
• If you cross parents that are pure
(homozygous) for contrasting traits, only
one form of the trait will be seen in the
offspring.
• The dominant allele masks the presence
of the recessive allele
• The recessive allele is only seen in
homozygous recessive individuals
Mendel’s Law of Segregation
• The two alleles for a characteristic
separate during gamete formation
(meiosis)
– Homologous chromosomes separate in
meiosis anaphase I
– Each gamete receives one of each pair of
homologous chromosomes and thus one of
the two alleles per characteristic
• The separation of alleles in meiosis is
known as Mendel’s Law of Segregation
Monohybrid Cross
• Cross between 2 hybrid (heterozygous)
individuals
• Offspring have a genotype ratio of 1:2:1
• Offspring have a phenotype ratio of 3:1
Practical Application: The Test
Cross
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A test cross is used to deduce the actual
genotype of an organism with a dominant
phenotype (i.e., is the organism PP or
Pp?)
– Cross the unknown dominant-phenotype
organism (P_) with a homozygous recessive
organism (pp)…
Practical Application: The Test
Cross
2. If the dominant-phenotype organism is
homozygous dominant (PP), only dominantphenotype offspring will be produced (Pp)
– If the dominant-phenotype organism is
heterozygous (Pp), approximately half of the
offspring will be of recessive phenotype (pp)
Rule of Multiplication
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Rule of multiplication is that the probability that independent events
will occur simultaneously is the product of their individual
probabilities. For example:
In a Mendelian cross between pea plants that are heterozygous for
flower color (Pp), what is the probability that the offspring will be
homozygous recessive?
Answer:
Probability that an egg from the F1 (Pp) will receive a p allele = 1/2.
Probability that a sperm from the F1 will receive a p allele = 1/2.
The overall probability that two recessive alleles will unite, one from
the egg and one from the sperm, simultaneously, at fertilization is:
1/2 X 1/2 = 1/4.
Rule of Addition
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Rule of addition is that the probability of an event that can occur in two or
more independent ways is the sum of the separate probabilities of the
different ways. For example:
In a Mendelian cross between pea plants that are heterozygous for flower
color (Pp), what is the probability of the offspring being a heterozygote?
There are two ways in which a heterozygote may be produced: the dominant
allele (P) may be in the egg and the recessive allele (p) in the sperm, or the
dominant allele may be in the sperm and the recessive in the egg.
Consequently, the probability that the offspring will be heterozygous is the
sum of the probabilities of those two possible ways:
Probability that the dominant allele will be in the egg with the recessive in
the sperm is 1/2 X 1/2 = 1/4.
Probability that the dominant allele will be in the sperm and the recessive in
the egg is 1/2 X 1/2 = 1/4.
Therefore, the probability that a heterozygous offspring will be produced is
1/4 + 1/4 = 1/2.
Law of Independent Assortment
• Seed color (yellow vs. green peas) and seed
shape (smooth vs. wrinkled peas) were the
characteristics studied
• The allele symbols were assigned:
– Y = yellow (dominant), y = green (recessive)
– S = smooth (dominant), s = wrinkled (recessive)
– Genes of pea color and pea shape (S, s and Y, y)
separate independently during meiosis (Mendel’s
Law of Independent Assortment)
• Possible gametes of parent SSYY are SY, SY, SY, and SY
(each S can combine with each Y)
• Possible gametes of parent ssyy are sy, sy, sy, and sy (each
s and combine with each y)
Traits Are Inherited
Independently
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Punnett Square from SSYY x ssyy cross
Gametes
¼sy
¼SY
¼SY
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16
SsYy
1
16
SsYy
1
16
¼SY SsYy
¼SY
1
16
SsYy
¼sy
¼sy
¼sy
1
16
1
16
1
16
1
16
1
16
1
16
1
16
SsYy SsYy
SsYy SsYy
SsYy
1
16
SsYy
SsYy
1
16
SsYy
1
16
SsYy SsYy
1
16
1
16
SsYy SsYy
F1: All SsYy
Smooth yellow peas
Traits Are Inherited
Independently
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A dihybrid cross yields:
– 9/16 smooth yellow peas
– 3/16 smooth green peas
– 3/16 wrinkled yellow peas
– 1/16 wrinkled green peas
A dihybrid cross yields a 9:3:3:1 phenotype ratio
Gene Linkage
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Mendel’s Law of Independent Assortment
only works for genes whose loci are on
different chromosomes
Different gene loci located on the same
chromosome tend to be inherited together
Characteristics whose genes tend to
assort together are said to be linked
Recombination
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Genes on the same chromosome do not
always sort together
Crossing over in Prophase I of meiosis
creates new gene combinations
Crossing over involves the exchange of
DNA between chromatids of paired
homologous chromosomes in synapsis
Incomplete Dominance
• Dominance of one allele over another
breaks down in incompletely dominant
characteristics
• When the heterozygous phenotype is
intermediate between the two homozygous
phenotypes, the pattern of inheritance is
called incomplete dominance
• Example: Four o’clock flowers. White and
red flowers. Heterozygous are pink
Incomplete Dominance
• Human hair texture is influenced by a gene
with two incompletely dominant alleles, C1
and C2
– A person with two copies of the C1 allele has
curly hair
– Someone with two copies of the C2 allele has
straight hair
– Heterozygotes (C1C2 genotype) have wavy hair
Incomplete Dominance
• If two wavy-haired people marry, their
children could have any of the three hair
types: curly (C1C1), wavy (C1C2), or
straight (C2C2)
Multiple Alleles
• A species may have more than two alleles
for a given characteristic
– Each individual still carries two alleles for this
characteristic
Multiple Alleles
• Examples of multiple allelism
– Thousands of alleles for eye color in fruit flies,
producing white, yellow, orange, pink, brown,
or red eyes
– Human blood group genes producing blood
types A, B, AB, and O
• Three alleles in this system: A, B, and O
Codominance
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Sometimes both alleles are expressed
fully
Example: Human blood group alleles
– Alleles A and B are codominant
– Type AB blood is seen where individual has
the genotype AB
Polygenic Inheritance
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Some characteristics show a range of
continuous phenotypes instead of discrete,
defined phenotypes
– Examples include human height, skin color, and
body build, and grain color in wheat
Polygenic Inheritance
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Phenotypes produced by polygenic
inheritance are governed by the interaction
of more than two genes at multiple loci
Human skin color is controlled by at least 3
genes, each with pairs of incompletely
dominant alleles
Pleiotropy
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Some alleles of a characteristic may create
multiple phenotypic effects (pleiotropy)
– Mendel’s rules specify only one phenotype
possible for any allele
– Example: The SRY gene in male humans
– SRY gene stimulates development of gonads
into testes, which in turn stimulate
development of the prostate, seminal vesicles,
penis, and scrotum
Epistasis
• Epistasis is the interaction between genes.
• Epistasis takes place when the effects of one
gene are modified by one or several other genes
• Example: Coat color in Labrador retrievers
• The genes that are involved in a specific
epistatic interaction have phenotypic ratios may
appear to deviate from those expected with
independent assortment.
Labrador Retrievers
• Fur color in Labrador Retrievers is controlled by
two separate genes.
– Fur color is a polygenic trait!
Gene 1: Represented by B
: Controls color
Gene 2: Represented by E
: Controls expression of B
Labrador Retrievers
• If a Labrador retriever has
a dominant B allele, they
will have black fur.
• If they have two recessive
alleles (bb) they will have
brown fur.
Labrador Retrievers
• If a retriever receives at least one
dominant “E” allele, they will remain the
color that the “B” allele coded for.
– Either black of brown
• However, if a dog receives a pair of
homozygous recessive “e” alleles, they
will be yellow regardless of their “B”
alleles!
Labrador Retrievers
• BBEE and BbEe --> Black retrievers
• bbEE and bbEe --> Brown retrievers
• BBee, Bbee, or bbee --> Yellow retrievers
Environmental Influence
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The environment can module how genes
are expressed
Example: Himalayan rabbit
– Himalayan rabbits have the genotype for black
fur all over the body
– Black pigment is only produced in colder
areas of the body: the nose, ears, and paws
Environmental Influence
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Both heredity and environment play major
roles in the development of some
characteristics
– Identical twin studies in humans reveal
different IQ scores between twins
Pedigree Analysis
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Pedigree analysis is often combined with
molecular genetics technology to elucidate
gene action and expression
Recessive Genetic Disorders
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Heterozygous individuals are carriers of a
recessive genetic trait (but otherwise have
a normal phenotype)
Recessive genes are more likely to occur
in a homozygous combination (expressing
the defective phenotype) when related
individuals have children
Lethal Genes
• Can be recessive or dominant
• Lethal genes that are recessive stay in a
population.
• Heterozygous (carriers) allow the alleles to
stay “hidden”
• Dominant lethal genes much more rare
• Homozygous dominant & heterozygous
individuals will both show the trait
Sex Chromosomes and Autosomes
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Mammals and many insect species have a
set of sex chromosomes that dictate
gender
– Females have two X chromosomes
– Males have an X chromosome and a Y
chromosome
– Sex chromosomes segregate during meiosis
Sex-Linked Genes Are on the X or the Y
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Genes carried on one sex chromosome
are sex-linked
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X chromosome is much larger than the Y and carries
over 1000 genes
Y chromosome is smaller and carries only 78 genes
Females (XX) can be homozygous or heterozygous
for a characteristic
Males (XY) have only one copy of the genes on the X
or the Y
How Sex-Linkage Affects Inheritance
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Patterns of sex-linked inheritance were first
discovered in fruit flies (Drosophila) in early
1900s
Eye color genes were found to be carried by
the X chromosome
– R = red eyes (dominant)
– r = white eyes (recessive)
How Sex-Linkage Affects Inheritance
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Sex-linked (specifically X-linked) recessive
alleles displayed their phenotype more often
in males
– Males showed recessive white-eyed phenotype
more often than females in an
XRXr x XrY cross
How Sex-Linkage Affects Inheritance
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Males do not have a second X-linked gene
(as do females) which can mask a recessive
gene if dominant
Sex-Linked Genetic Disorders
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Several defective alleles for characteristics
encoded on the X chromosome are known
Sex-linked disorders appear more frequently in
males and often skip generations
Examples of sex-linked (X-linked) disorders
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Red-green color blindness
Hemophilia (deficiency in blood clotting protein)
• Hemophilia gene in Queen Victoria of England was
passed among the royal families of Europe
Albinism
• Melanin is the dark pigment that colors
skin cells
• Melanin is produced by the enzyme
tyrosinase
• An allele known as TYR (for tyrosinase)
encodes a defective tyrosinase protein in
skin cells, producing no melanin
Albinism
• Humans and other mammals who are
homozygous for TYR have no skin, fur, or
eye coloring (skin and hair appear white,
eyes are pink)
Sickle-Cell Anemia
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Autosomal recessive disorder
Mutation occurs on the HBB gene
HBB gene locus: 11p15.5
A mutant hemoglobin gene causes hemoglobin
molecules in blood cells to clump together
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Red blood cells take on a sickle (crescent) shape and
easily break
Blood clots can form, leading to oxygen starvation of
tissues and paralysis
Condition is known as sickle-cell anemia
Sickle-Cell Anemia
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About 8% of the African population is
heterozygous for sickle-cell anemia
– Heterozygous individuals have some
resistance to malaria
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The presence of the mutant allele can be
detected by a blood test
Results of blood testing can help couples
understand odds of giving birth to a child
with sickle-cell anemia
Cystic Fibrosis
• Lethal gene mutation that cause the
production of abnormally thick mucus
• Autosomal recessive mutation
• Mutation occurs on the CFTR gene
• CFTR gene locus: 7q31.2
• Have found over 1000 mutations
Tay-Sachs
• Lethal gene mutation that causes
premature death of brain cells
• Autosomal recessive mutation
• Mutation occurs on the HEXA gene
• HEXA gene locus: 15q23-24
• Normally only found in populations of Eastern
European Jews
Non-Disjunction
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Incorrect separation of chromosomes or
chromatids in meiosis known as nondisjunction
Most embryos arising from gametes with
abnormal chromosome numbers abort
spontaneously (are miscarried)
Some combinations of abnormal
chromosome number survive to birth or
beyond
Abnormal Sex Chromosome
Number
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Non-disjunction of sex chromosomes in
males or females produce abnormal
numbers of X and Y chromosomes
– Turner Syndrome (XO): an underdeveloped,
infertile woman with only one X chromosome
– Trisomy X (XXX): a fertile, “normal” woman
with an extra X chromosome
Abnormal Sex Chromosome
Number
– Kleinfelter Syndrome (XXY): an infertile man
with an extra X chromosome, having partial
breast development and small testes
– XYY Male: a tall man with an extra Y that
produces high levels of testosterone and may
score lower on IQ tests
Abnormal Autosome Number
• Trisomy 21 (Down Syndrome) is an
example of an abnormal autosomal
number
– Down syndrome individuals have three copies
of chromosome 21
– Down syndrome characterized by distinctively
shaped eyelids, among other physical
features