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Mindarie Senior College
2A/2B Human Biological Science
Genetic Variation
Mendelian Patterns of Inheritance
Gregor Mendel
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Born in the Czech Republic in 1822.
Died in 1884 with his work still unnoticed.
Worked in an Austrian monastery as a
monk/teacher/scientist/gardener.
Experimented with purebred tall and short
peas.
Discovered some of the basic laws of
heredity.
Studied seven pure bred traits in peas.
Called the stronger hereditary factor
dominant.
Called the weaker hereditary factor
recessive.
Presentation to the Science Society
in 1866 went unnoticed. ie, no comtemporary
recognition
Work and findings rediscovered in 1900 by
Hugo de Vries and Carl Correns
Known as the “Father of Genetics”.
Moral: you too can be famous!
Mendel’s Observations
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He noticed that peas are easy to breed
for pure traits and he called the pure
strains purebreds.
He developed pure strains of peas for
seven different traits (i.e. tall or short,
round or wrinkled, yellow or green, etc.)
He crossed these pure strains to
produce hybrids.
He crossed thousands of plants and kept
careful records for eight years.
Mendel’s Peas
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In peas many traits appear in two forms (i.e. tall or
short, round or wrinkled, yellow or green.)
The flower is the reproductive organ and the male and
female are both in the same flower.
He crossed pure strains by putting the pollen (male
gamete) from one purebred pea plant on the pistil
(female sex organ) of another purebred pea plant to
form a hybrid or crossbred plant.
Mendel’s Results
Mendel crossed purebred tall plants with purebred
short plants and the first generation plants were all
tall.
When these tall offspring were crossed the result
was a ratio of 3 tall to 1 short.
Mendel’s Peas
Mendel’s Experiments
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He experimentally
crosses different
strains to develop
hybrids.
He then crossed
the hybrids and
analyzed the
results.
Dominant Traits RULE
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Strong Hereditary
traits cover weak
traits.
Mendal called
stronger traits
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DOMINANT
Mendal called
weaker traits
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recessive
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Dominant traits
are represented
by capital letters
(T) while recessive
traits are
represented by
lower case letters
(t).
Important terms used in
genetics
Term
Meaning
Monohybrid
cross
The inheritance of a genetic feature
controlled by a single pair of genes.
(monohybrid = an organism that is hybrid
for one genetic feature)
The different forms of a gene.
Most
genes have only two alleles (e.g. the
height of the pea plants in the above
example was controlled by a gene, which
had a tall allele and a short allele).
Although some genes have more than two
alleles a hybrid individual can only
possess two of them.
Allele
Homozygous
Where
both
alleles
determining
a
genetic
feature are the same (e.g.
TT)
Heterozygous
Where
the
two
alleles
determining
a
genetic
feature are different (e.g.
Tt)
Genotype
The combination of genes
that determine a character,
whether or not they are
expressed (e.g. Tt)
Phenotype
Autosome
Sex chromosome
(X-chromosome)
The physical expression or
appearance of a particular
genotype (e.g. the physical
expression or phenotype of
a pea plant with the
genotype Tt will be a plant
that is tall).
A non-sex chromosome
The
23rd
pair
of
chromosomes
that
determine the sex of an
individual
Mendel’s first law - The Law of
Segregation
This law states that:
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allele pairs separate or segregate during
gamete formation, and randomly unite at
fertilization
The underlying assumptions
The Law of Segregation is based on the assumption
that:
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There are alternative forms for genes (known as
alleles)
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For each characteristic, or trait, organisms inherit
two alternative forms of that gene - one from each
parent
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When gametes (sex cells) are produced, allele pairs
separate leaving the gametes with a single allele for
each trait
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When the two alleles of a pair are different, one is
dominant and the other is recessive
Mendel’s hypothesis - 1
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Mendel predicted that if he crosspollinated true-breeding long-stemmed
plants (homozygous dominant) with truebreeding short-stemmed plants
(homozygous recessive), all of the
resulting offspring (the F1 generation)
would be long-stemmed.
Dominant allele
(long stem)
Recessive allele
(short stem)
The
gametes or
sex cells
Tall
TT
t
T
Tt
Tt
Tall
Short
tt
Tt
Tall
Tt
Tall
Tall
Calculating autosomal dominantrecessive patterns of inheritance
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Let the allele for long stems be represented by T
Let the allele for short stems be represented by t
Parental phenotype Tall x Short
Parental genotype
TT x tt
Possible gametes
T x t only
F1 genotype
All Tt
F1 phenotype
All tall
Punnett Squares
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Predicting the result of a genetic cross
is made easier by using a Punnett
square.
Genotype of
second parent
T
t
T
TT
Tt
t
Tt
tt
Genotype of
first parent
Predicted
genotypes
of offspring
* Statistical predictions
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The result of genetic crosses only
shows the statistical probability of an
offspring having a particular genotype
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Example:
The chance of a baby being a particular
sex is ½ male: ½ female (or 1:1).
This means that EVERY baby has an
equal chance of being a boy or a girl – if
the first baby is a boy the second baby
has a 50% chance of also being a boy.
The predicted ratio can be seen best in
large populations, such as Mendel’s pea
experiments, which involved thousands
of plants.
Some Mendelian traits in Humans
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Ability to taste PTC
Albinism
Blood type
Brachydactyly (Shortness of fingers and toes)
Cheek dimples
Cleft chin (cleft=dominant; smooth=recessive)
Free or attached earlobes
Wet or dry earwax
Face freckles
Hitch hiker’s thumb
Polydactyly (six fingers)
Widow’s peak (Widow's peak=dominant, straight
hairline=recessive)
Tongue rolling