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SIMPLE PATTERNS OF
INHERITANCE
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Gregor Johann Mendel
1822–1884
Entered monastery
and became a priest
Historic studies on
pea plants
Paper ignored at the
time
Independently
rediscovered years
later
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Garden Pea, Pisum sativum
Several advantageous properties
Many
readily available characters or traits
Normally self-fertilizing
True-breeding lines exhibit the same traits
Ease
of making crosses with large flowers
Cross-fertilization or hybridization
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Single-factor cross
Experimenter
follows the variants
of only 1 trait
P generation
True-breeding
parents
F1 generation
Offspring of P cross
Monohybrids – if
parents differ in 1
trait
F2 generation
F1 self-fertilizes
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3 important ideas
1.
Dominant and recessive traits
Dominant is displayed trait
Recessive trait masked by dominant trait
2.
Genes and alleles
Mechanism of inheritance
“Unit factors” are genes
Every individual has 2 genes for a trait
Gene has two variant forms or alleles
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3 important ideas
3.
Segregation of alleles
Approximately 3:1 ratio
Two copies of a gene carried by an F1 plant
segregate (separate) from each other, so
that each sperm or egg carries only one
allele
Mendel’s Law of Segregation
2 copies of a gene segregate from each other
during the transmission from parent to offspring
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Genotype and phenotype
Genotype
Genetic
composition of individual
TT or tt – homozygous
Tt – heterozygous
Phenotype
Characteristics
that are the result of gene
expression
TT and Tt are tall
tt is dwarf
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Punnett square
Step 1. Write down the genotypes of both
parents
Male
parent: Tt
Female parent: Tt
Step 2. Write down the possible gametes
that each parent can make
Male
gametes: T or t
Female gametes: T or t
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Step 3. Create an empty Punnett square.
Male gametes
♂
T
t
♀
Female gametes
T
t
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Step 4. Fill in the possible gentoypes.
Male gametes
♂
T
t
TT
Tt
Tt
tt
♀
Female gametes
T
t
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Step 5. Determine relative proportions of
genotypes and phenotypes.
Male gametes
♂
T
t
TT
Tt
Genotype ratio
TT:Tt:tt
1:2:1
tt
Phenotype ratio
Tall: dwarf
3:1
Female gametes
♀
T
t
Tt
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Testcross
A dwarf pea plant must be tt
A tall pea plant could be either TT or Tt
Cross unknown individual to a homozygous
recessive individual
If some offspring are dwarf, unknown individual
must have been Tt
If all offspring are tall, the unknown individual
was TT
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Two-factor cross
Follow inheritance of 2 different traits
Possible patterns
2
genes linked so that variants found together
in parents are always inherited as a unit
2 genes are independent and randomly
distributed
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Linkage Hypothesis
Independent
Assortment
Hypothesis
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Dihybrid offspring- offspring are hybrids
with respect to both traits
Data for F2 hybrids consistent only with
independent assortment
Law of Independent Assortment
Alleles
of different genes assort independently
of each other during gamete formation
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Chromosome theory of inheritance
1.
Chromosomes contain the genetic material, which is transmitted from parent to offspring and
from cell to cell. Genes are found in the chromosomes.
2.
Chromosomes are replicated and passed from parent to offspring. They are also passed from
cell to cell during the multicellular development of an organism. Each type of chromosome
retains its individuality during cell division and gamete formation.
3.
The nucleus of a diploid cell contains two sets of chromosomes, which are found in homologous
pairs. One member of each pair is inherited from the mother and the other from the father. The
maternal and paternal sets of homologous chromosomes are functionally equivalent; each set
carries a full complement of genes.
4.
At meiosis, one member of each chromosome pair segregates into one daughter nucleus and its
homologue segregates into the other daughter nucleus. Each of the resulting haploid cells
contains only one set of chromosomes. During the formation of haploid cells, the members of
different chromosome pairs segregate independently of each other.
5.
Gametes are haploid cells that combine to form a diploid cell during fertilization, with each
gamete transmitting one set of chromosomes to the offspring. In animals, one set comes from
the mother and the other set comes from the father.
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Chromosomes and segregation
Mendel’s law of segregation can be
explained by the pairing and segregation
of homologous chromosomes during
meiosis
Locus – physical location of a gene on a
chromosome
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Chromosomes and Independent
Assortment
Law of independent assortment can also
be explained by the behavior of
chromosomes during meiosis
Random alignment of chromosome pairs
during meiosis I leads to the independent
assortment of alleles found on different
chromosomes
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Probability
Chance that an event will have a particular
outcome
For a single coin toss
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Self-fertilization of a pea plant that was
heterozygous for the height gene (Tt)
Punnett square predicted that one-fourth
of the offspring would be dwarf
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Sample size
Accuracy of prediction depends on the
number of events observed or sample size
Random sampling error – deviation
between observed and expected outcome
Larger samples have smaller sampling
errors
Humans have small families and observed
data may be very different from expected
outcome
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Product rule
Probability that two or more independent events
will occur is equal to the product of their
individual probabilities
If we toss a coin twice, what is the probability
that we will get heads both times?
The product rule says that it is equal to the
probability of getting heads on the first toss (1/2)
times the probability of getting heads on the
second toss (1/2), or one in four
½x½=¼
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Sum rule
Probability that one of two or more mutually
exclusive outcomes will occur is the sum of
the probabilities of the possible outcomes
In a cross between two heterozygous (Tt)
pea plants, we may want to know the
probability of a particular offspring being a
homozygote
¼+¼=½
Half of the offspring will be homozygotes
(TT or tt)
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Mendelian inheritance
Inheritance pattern of genes that
segregate and assort independently
Simple Mendelian inheritance – one trait is
completely dominant over the other
X-linked inheritance – pairs of dominant
and recessive alleles found on the X
chromosome
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Mendelian inheritance
Wild-type allele
Prevalent
allele in a population
Encodes a protein made in the proper amount
and functioning normally
Mutant alleles
Altered
by mutation
Tend to be rare in natural populations
Defective in its ability to express a functional
protein
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Mendelian inheritance
In simple dominance, the recessive allele
does not affect the phenotype of the
heterozygote
A single copy of the dominant allele is
sufficient to mask the recessive allele
Purple pigment, P
One
P allele makes enough functional protein
to provide a normal phenotype
In other cases, the heterozygote may make
more than 50% of the normal amount of
protein – up-regulated
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Incomplete dominance
Heterozygote has
intermediate phenotype
Neither allele is dominant
Pink four-o’clocks
50% of normal protein
not enough to give red
color
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Multiple alleles
3
or more variants in a population
Phenotype depends on which 2 alleles are inherited
ABO blood types in humans
Type AB is codominance- expressing both alleles equally
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Sex-influenced inheritance
Allele
is dominant in one sex but recessive in the other
Pattern baldness
Baldness allele dominant in men but not women
Only a woman homozygous for baldness allele would be bald
Not
X-linked
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Role of environment
Norm of reaction – effects of
environmental variation on a phenotype
Genetically identical plants grow to
different heights in different temperatures
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