Mendel and the Gene Idea
Early theories of Inheritance
I. EARLY THEORIES OF INHERITANCE
Many theories of inheritance have been proposed, and some data
back as far as ancient
- Aristotle proposed the theory of pangenesis which held that
particles (called pangenes) from
all parts of the body come together to form the eggs and sperm.
- Changes that occurred in the various body parts during an
organism's life could be passed on
to the next generation.
- Pangenesis was accepted by many (Lamarck and Darwin) and was the
prevailing theory into
the nineteenth century.
- Pangesesis is incorrect because reproductive cells are not
composed of contributions from
body cells and changes in body cells do not influence egg and
- In the seventeenth century, Aton van Leeuwenhoek "observed " the
homunculus, a miniature
human being, in human sperm cells. He and his followers
(spermists) believed that:
- The mother serves only as an incubator for the homunculus.
- All characteristics are inherited from the father.
- Also during the seventeenth century, Regnier de Graaf and his
followers (ovists) proposed
- The egg contains an entire human in miniature and that semen only
stimulates its growth.
- All characteristics are thus inherited from the mother.
deGraff was the first person to describe the ovarian follicle in
which human egg cells are
- Based upon their observations with ornamental plant breeding,
scientists in the nineteenth
century realized that both parents contribute to the
characteristics of offspring. The
"blending" theory then became the favored explanation of
inheritance. According to this
- Hereditary materials from male and female parents mix to form the
offspring, and once
blended, the hereditary material is inseparable.
- Since the hereditary material is inseparable, the population
should reach a uniform
appearance after many generations.
- This theory was inconsistent with the observations that:
- Populations do not reach a uniform appearance.
- Some traits are absent in one generation and present in the next.
- Modern genetics began in the 1860's with the experiments of an
Augustinian monk named
Gregor Mendel who discovered the fundamental principles of
Mendel and the Gene Idea
Mendel's Model of Inheritance
II. MENDEL'S MODEL OF INHERITANCE
pea life cycle
pea cross fertilisation
Mendel pea characters
Mendel's first experiment
geno- vs phenotypes
Chromosomes. Haploid vs diploid
Drosophila sex genes
ABO blood system
ABO blood test
- Mendel's work demonstrated that parents pass on to their
offspring discrete heritable factors
(genes) which retain their individuality generation after
generation (in contrast to the blending
- Although Mendel's work is the basis of modern genetics, it did
not influence the scientific
community until many years after his death.
- Mendel's Methods
- Unlike most nineteenth century biologists, Mendel adopted a
quantitative approach to his
- His experimental organisms were garden peas which proved to have
many advantages over
other possible organisms:
- They were easy to grow.
- They were available in many easily distinguishable varieties.
- Strict control over mating was possible (small bags over flowers
immature stamens could be removed to prevent self-pollination) to
insure the parentage of
- Mendel not only applied quantitative methods but also chose
traits which differed in a
relatively clear-cut manner.
- Mendel chose seven characteristics, each of which occurred in two
forms: seed shape (round
or wrinkled), seed color (yellow or green), seed coat color (gray
or white), pod shape
(inflated or constricted), pod color (green or yellow) flower
position (axial or terminal), stem
length (tall or dwarf).
- Mendel worked with his plants until he obtained true-breeding
plants (varieties which always
produced offspring with the same traits as the parents when the
parents were self-fertilizing).
He hybridized (cross-pollinated) these true-breeding varieties in
- Parental plants of such a cross are called the P generation.
- The hybrid offspring of the P generation are the F1 generation
- Mendel also allowed F1 generation plants to self-pollinate to
produce the next generation, the
- Mendel observed the transmission of selected traits for several
generations and arrived at two
principles of heredity which are now known as the law of
segregation and the law of
- Mendel's Law of Segregation
- When Mendel crossed true-breeding plants exhibiting different
forms of a trait, green-pod
and yellow-pod color, he bound that the traits did not blend, but
that the F1 progeny
(offspring) produced only green-colored pods.
- Mendel hypothesized that if the heritable factor for yellow pods
had been lost, then the F1
plants should only be capable of producing green-pod progeny.
- Mendel allowed the F1 plants to self-pollinate to produce the
next generation. In this F2
generation, there were 428 plants with green pods and 152 plants
with yellow pods which
gives a 3:1 ratio of green to yellow.
- From these types of experiments and observations, Mendel
concluded that the heritable factor
for yellow pods was not lost in the F1 plants, but was masked by
the presence of the green-pod factor.
- We now call these heritable factors genes.
- Mendel formulated his hypothesis of inheritance which can be
divided into four parts:
- There are alternative forms for genes, the units that determine
- The gene for pod color existed in two alternative forms, one for
green and one for yellow.
- Alternative forms of a gene are now called alleles.
- For each inherited characteristic, an organism has two alleles,
one inherited from each
- Mendel's experiment included on parental variety which had a pair
of alleles for green pod
color and one which had a pair of alleles for yellow pod color.
- The F1 hybrids inherited from the parental plants one allele for
green pod color and one
allele for yellow pod color.
- A sperm or egg carries only one allele for each inherited
characteristic, because allele pairs
separate (segregate) from each other during the production of
gametes. At fertilization, the
sperm and egg unite with both contributing their alleles. This
restores the gene to the paired
- In Mendel's experiment, each gamete of a parental plant carried
one allele for pod color,
specifying either green or yellow.
- Cross-pollination to produce the F1 resulted in the combination
found in this generation.
- When the two alleles of a pair are different, one is fully
expressed and the other is
completely masked. These are called the dominant allele and
recessive allele, respectively.
- Mendel's F1 hybrid had green pods, which was recessive.
- Mendel's hypothesis explains the 3:1 ratio of progeny plant types
he observed in the F2
- It predicts that the F1 hybrids (Gg) will produce two classes of
gametes when the pairs
separate during gamete formation.
- Half will receive a green-pod (G) allele and the other half the
yellow-pod allele (g).
- During self-pollination, these two classes of alleles unite in a
- Eggs containing green-pod alleles have equal chances of being
fertilized by sperm carrying
preen-pod alleles or sperm carrying yellow-pod alleles.
- Since the same is true for eggs containing yellow-pod alleles,
there are four equally likely
combinations of sperm and eggs.
- The combinations resulting from a genetic cross may be predicted
by using a device called a
GG x gg
Gg x Gg
- The F2 progeny would include:
- One-fourth of the plants with two alleles for green pod color.
- One-half of the plants with one allele for green pods and one
allele for yellow pods. Since
the green pod allele is dominant, these plants have green pods.
- One-fourth of the plants with two alleles for yellow pod color
which will have yellow pods
since no dominant allele is present.
- The pattern of inheritance for all seven of the characteristics
studied by Mendel was the
same: one parental trait disappeared in the F1 generation and
reappeared in one-fourth of the
- The mechanisms producing this pattern of inheritance is stated by
Mendel's Law of
Segregation: allele pairs segregate (separate) during gamete
formation, and the paired
condition is restored by the random fusion of gametes at
- Research since Mendel's time has established that the law of
segregation applies to all
sexually reproducing organisms.
- Some Useful Genetic Vocabulary:
- Dominant alleles are indicated by a capital letter: G=green pod
- Recessive alleles are indicated by a lowercase letter: g=yellow
- An organism with a pair of identical alleles for a characteristic
(GG or gg) is homozygous
and all gametes carry that allele.
- Homozygotes are true-breeding.
- An organism that has two different alleles for a trait (Gg) is
said to be heterozygous and one-
half of the gametes carry one allele (G) with the other half
carrying the other allele (g).
- Heterozygotes are not true-breeding.
- Phenotype = An organism's expressed traits (green or yellow).
In Mendel's experiment, the F2 generation had a 3:1 phenotypic
ratio of plants with green
pods to plants with yellow pods.
Genotype = An organism's genetic makeup (GG, Gg or gg).
The genotypic ratio of the F2 generation was 1:2:1 (1GG:2Gg:1gg).
- The Testcross
- Because some alleles are dominant over others, the phenotype of
an organism does not
always reflect its genotype.
- A recessive phenotype (yellow) is only expressed with the
organism is homozygous recessive
- A pea plant with green pods may be either homozygous dominant
(GG) or heterozygous
- To determine whether an organism with a dominant phenotype (e.g.
green pod color) is
homozygous dominant or heterozygous, you use a testcross.
- Testcross = The breeding of an organism of unknown genotype with
- If all the progeny of the testcross have green pods, then the
green pod parent was probably
homozygous dominant since a GG x gg cross produces Gg progeny.
- If the progeny of the testcross contains both green and yellow
phenotypes, then the green pod
parent was heterozygous since a Gg x gg cross produces Gg and gg
progeny in a 1:1 ratio.
- The testcross was devised by Mendel and is still an important
tool in genetic studies.
- Inheritance as a Game of Chance
- Segregation of allele pairs during gamete formation and
reformation of pairs at fertilization
obey the rules of probability.
- The probability scale ranges from 0 to 1 with an event that is
certain to occur having a
probability of 1, and an event that is certain NOT to occur
having a probability of 0.
- The same rules of probability apply to tossing a coin or rolling
- The probability of tossing a head with a two-headed coin is 1.
The probability of tossing a
tail with the two-headed coin is 0.
- The probability of tossing a head with a normal coin is 1/2 with
the probability of tossing a
tail also being 1/2.
- The probability of rolling a 3 on a six-sided die is 1/6.
- The probability of rolling a number other than 3 is 5/6.
- The probabilities of all possible outcomes for an event must add
up to 1.
- For every toss of a normal coin, the probability of heads is 1/2.
The outcome of any particular toss is unaffected by what has
happened on previous attempts.
- This phenomena is referred to as independent events.
- It is possible the five successive tosses of a normal coin will
produce five heads; however,
the probability of heads on the sixth toss is still 1/2.
- Two basic rules of probability govern the possibilities: the rule
of multiplication and the rule
- Rule of Multiplication
- If two coins are tossed simultaneously, the outcome of each coin
is an independent event,
unaffected by the other coin.
- The probability that both coins will come up heads (a compound
event) is equal to the
product of the separate probabilities of the independent single
events: 1/2 x 1/2 = 1/4.
- A Mendelian F1 cross with one trait which has two alleles (pea
pod color) is analogous to
the coin toss. With an F1 genotype of Gg, the probability that an
F2 plant will have yellow
pods is 1/4.
- The probability that an egg from the F1 will receive a g allele
- The probability that a sperm from the F1 will receive a g allele
is also 1/2.
- Thus the probability that two g alleles will unite at
fertilization is 1/2 x 1/2 = 1/4.
- Rule of Addition
The Statistical Nature of Inheritance
- The probability that an F2 plant will be heterozygous is the sum
of the two single events.
- Summation is used since there are two ways in which a
heterozygous F2 may be produced:
the dominant allele (G) may be in the egg and the recessive
allele (g) in the sperm or the
dominant allele may be in the sperm and the recessive in the egg.
- The probability that the dominant allele will be in the egg with
the recessive in the sperm is
1/2 x 1/2 = 1/4.
- The probability that the dominant allele will be in the sperm and
the recessive in the egg is
1/2 x 1/2 = 1/4.
- Using this rule, the probability that a heterozygous F2 will be
produced is 1/4 + 1/4 = 1/2.
- If a seed is planted from the F2 generation, we cannot predict
with absolute certainty that the
plant will grow up to produce yellow pods.
- It can be said that there is exactly a 1/4 chance that the plant
will have yellow pods.
- Stated in statistical terms: among a large sample of F2 plants,
25% (one-fourth) will have
- The larger the sample size, the closer the results will conform
- Mendel's Law of Independent Assortment
- Mendel deduced the law of segregation from experiments with
monohybrid crosses, breeding
experiments that employ parental varieties differing in a single
- A dihybrid cross is a mating between parents that differ in two
- Mendel performed dihybrid crosses by mating two individuals which
differed in seed color
(yellow and green) and seed shape (round and wrinkled).
- Mendel knew from monohybrid crosses that the allele for round
seeds was dominant to the
allele for wrinkled and that yellow was dominant to green.
- To determine if these traits, seed shape and seed color, are
inherited together or
independently, Mendel performed dihybrid crosses using homozygous
- Plants homozygous for round yellow seeds (RRYY) were crossed with
for wrinkled green seeds (rryy).
- All the F1 progeny were heterozygous for both traits (RrYy) and
had round yellow seeds (the
- The F1 generation provided no proof as to whether the traits are
inherited independently or
not, proof could only be obtained from an F2 generation.
- If segregation in the F1 plants resulted in only two classes of
gametes (RY and ry) as were
provided from the P generation, then the F2 generation would have
a 3:1 phenotypic ratio
(three-fourths round yellow seeds and one-fourth wrinkled green
- If segregation in the F1 plant resulted in four classes of
gametes from each plant because the
genes segregated independently (RY, Ry, rY, and ry), then the F2
generation would have a
9:3:3:1 phenotypic ratio (nine round yellow seeds to three round
green seeds to three
wrinkled yellow seeds to one wrinkled green seed).
- Mendel allowed self-pollination of F1 plants (RrYy x RrYy) to
produce an F2 generation.
- When he categorized peas from the F2 generation he found a ratio
of 315:108:101:32 which approximates a 9:3:3:1 phenotypic ratio.
- The phenotypic ratio for each trait singly was still 3:1 (round
vs wrinkled and yellow vs green).
- The experimental results supported the hypothesis that each
allele pair segregates independently during gamete formation.
- Mendel tried all seven of his traits in various combinations in
dihybrid crosses and found the same 9:3:3:1 in each case.
- This behavior of genes during gamete formation is called
independent assortment and the principle is referred to as
Mendel's Law of Independent Assortment.
- The rules of probability applied to segregation and independent
assortment can solve complex genetics problems. For example,
Mendel crossed pea varieties that differed in three traits
- A trihybrid cross between two organisms with the genotypes AaBbCc
and AaBbCc will result in a 1/64 probability of producing an
offspring with the genotype aabbcc.
Aa x Aa: probability for aa offspring = 1/4
Bb x Bb: probability for bb offspring = 1/4
Cc x Cc: probability for cc offspring = 1/4
- Because segregation of each allele pair is an independent event,
the rule of multiplication is used to calculate the overall
probability that the offspring will be aabbcc:
1/4 aa x 1/4 bb x 1/4 cc = 1/64
- Mendel's two laws explain inheritance in terms of discrete
factors (genes) which as passed from generation to generation
according to simple rules of chance.
- These principles apply to all sexually reproducing organisms for
simple patterns of inheritance.
- Experiments with many organisms indicate that more complicated
patterns of inheritance exist.
- The more complicated patterns of inheritance include situations
where one allele is not completely dominant over another allele,
there are more than two alleles for a trait, or the genotype does
not always dictate the phenotype in a rigid manner.