Chapter 14 Mendel and the Gene Idea
Overview: Drawing from the Deck of Genes
• Eor blue eyes) among individuals in a population.s brown, green,
• These traits are transmitted from parents to offspring.
• One possible explanation for heredity is a “blending” hypothesis.
This hypothesis proposes that genetic material contributed by
each parent mixes in a manner analogous to the way blue and
yellow paints blend to make green.
With blending inheritance, a freely mating population will
eventually give rise to a uniform population of individuals.
Etell us that heritable traits do not blend to become uniform.
• Aparents pass on discrete heritable units, genes, that retain their
separate identities in offspring.
Genes can be sorted and passed on, generation after
generation, in undiluted form.
• Modern genetics began in an abbey garden, where a monk named
Gregor Mendel documented a particulate mechanism of
Concept 14.1 Mendel used the scientific approach to identify two
laws of inheritance
• Mendel discovered the basic principles of heredity by breeding
garden peas in carefully planned experiments.
• Mendel grew up on a small farm in what is today the Czech
• In 1843, Mendel entered an Augustinian monastery.
• Hhe was influenced by a physicist who encouraged experimentation
and the application of mathematics to science and by a botanist
who stimulated Mendel’s interest in the causes of variation in
• These influences came together in Mendel’s experiments.
• After university, Mendel taught at the Brunn Modern School and
lived in the local monastery.
• The monks at this monastery had a long tradition of interest in the
breeding of plants, including peas. • Around 1857, Mendel began breeding garden peas to study
• Pea plants have several advantages for genetic study.
Pea plants are available in many varieties with distinct heritable
features, or characters, with different variant traits.
Mendel could strictly control which plants mated with which.
Each pea plant has male (stamens) and female (carpal) sexual
In nature, pea plants typically self-fertilize, fertilizing ova with
the sperm nuclei from their own pollen.
However, Mendel could also use pollen from another plant for
• Mendel tracked only those characters that varied in an “either-or”
manner, rather than a “more-or-less” manner.
For example, he worked with flowers that were either purple or
He avoided traits, such as seed weight, that varied on a
• Mbreeding.rted his experiments with varieties that were true-
Wthe same traits.g plants self-pollinate, all their offspring have
• In a typical breeding experiment, Mendel would cross-pollinate
(hybridize) two contrasting, true-breeding pea varieties.
The true-breeding parents are the P generation, and their
hybrid offspring are the F 1eneration.
• Mendel would then allow the F hybr1ds to self-pollinate to produce
an F 2eneration.
• It was mainly Mendel’s quantitative analysis of F pla2ts that
revealed two fundamental principles of heredity: the law of
segregation and the law of independent assortment.
By the law of segregation, the two alleles for a character are
separated during the formation of gametes.
• If the blending model was correct, the F hyb1ids from a cross
between purple-flowered and white-flowered pea plants would
have pale purple flowers.
• Instead, F h1brids all have purple flowers, just as purple as their
• Wgeneration included both purple-flowered and white-flowered 2
The white trait, absent in the F ,1reappeared in the F . 2
• Mendel used very large sample sizes and kept accurate records of
his results. Mendel recorded 705 purple-flowered F plants2and 224 white-
flowered F p2ants.
This cross produced a traits ratio of three purple to one white in
the F 2ffspring.
• Mpresent in the F 1lants, but did not affect flower color.s was
Pa recessive trait. is a dominant trait, and white flower color is
• The reappearance of white-flowered plants in the F gener2tion
indicated that the heritable factor for the white trait was not diluted
or “blended” by coexisting with the purple-flower factor in F 1
• Mendel found similar 3-to-1 ratios of two traits among F off2pring
when he conducted crosses for six other characters, each
represented by two different traits.
• For example, when Mendel crossed two true-breeding varieties,
one producing round seeds and the other producing wrinkled
seeds, all the F 1ffspring had round seeds.
In the F 2lants, 75% of the seeds were round and 25% were
• Mendel developed a hypothesis to explain these results that
consisted of four related ideas. We will explain each idea with the
modern understanding of genes and chromosomes.
1. Alternative versions of genes account for variations in inherited
The gene for flower color in pea plants exists in two versions,
one for purple flowers and one for white flowers.
These alternate versions are called alleles.
Echromosome.sides at a specific locus on a specific
The DNA at that locus can vary in its sequence of nucleotides.
The purple-flower and white-flower alleles are two DNA
variations at the flower-color locus.
2. For each character, an organism inherits two alleles, one from
A diploid organism inherits one set of chromosomes from each
Each diploid organism has a pair of homologous chromosomes
and, therefore, two copies of each gene.
These homologous loci may be identical, as in the true-
breeding plants of the P generation.
Alternatively, the two alleles may differ.
3. If the two alleles at a locus differ, then one, the dominant allele,
determines the organism’s appearance. The other, the
recessive allele, has no noticeable effect on the organism’s
appearance. In the flower-color example, the F pl1nts inherited a purple-
flower allele from one parent and a white-flower allele from the
They had purple flowers because the allele for that trait is
4. 4. Mendel’s law of segregation states that the two alleles for a
heritable character separate and segregate during gamete
production and end up in different gametes.
This segregation of alleles corresponds to the distribution of
homologous chromosomes to different gametes in meiosis.
If an organism has two identical alleles for a particular
character, then that allele is present as a single copy in all
If different alleles are present, then 50% of the gametes will
receive one allele and 50% will receive the other.
• Mendel’s law of segregation accounts for the 3:1 ratio that he
observed in the F g2neration.
• The F h1brids produce two classes of gametes, half with the
purple-flower allele and half with the white-flower allele.
• Drandomly.f-pollination, the gametes of these two classes unite
• This produces four equally likely combinations of sperm and ovum.
• A Punnett square predicts the results of a genetic cross between
individuals of known genotype.
• Let us describe a Punnett square analysis of the flower-color
• Wlowercase letter to symbolize the recessive allele.nt allele and a
P is the purple-flower allele, and p is the white-flower allele.
• What will be the physical appearance of the F offs2ring?
One in four F o2fspring will inherit two white-flower alleles and
produce white flowers.
Half of the F 2ffspring will inherit one white-flower allele and
one purple-flower allele and produce purple flowers.
One in four F o2fspring will inherit two purple-flower alleles and
produce purple flowers.
• Mendel’s model accounts for the 3:1 ratio in the F gen2ration.
• An organism with two identical alleles for a character is
homozygous for that character.
• Organisms with two different alleles for a character is
heterozygous for that character.
• An organism’s traits are called its phenotype.
• Its genetic makeup is called its genotype. Two organisms can have the same phenotype but have
different genotypes if one is homozygous dominant and the
other is heterozygous.
• For flower color in peas, the only individuals with white flowers are
those that are homozygous recessive (pp) for the flower-color
• However, PP and Pp plants have the same phenotype (purple
flowers) but different genotypes (homozygous dominant and
• How can we tell the genotype of an individual with the dominant
The organism must have one dominant allele, but could be
homozygous dominant or heterozygous.
• The answer is to carry out a testcross.
The mystery individual is bred with a homozygous recessive
If any of the offspring display the recessive phenotype, the
mystery parent must be heterozygous.
By the law of independent assortment, each pair of alleles
segregates independently into gametes.
• Mas flower color.periments followed only a single character, such
Aheterozygous for one character.crosses were monohybrids,
A cross between two heterozygotes is a monohybrid cross.
• Mendel identified the second law of inheritance by following two
characters at the same time.
• In one such dihybrid cross, Mendel studied the inheritance of
seed color and seed shape.
The allele for yellow seeds (Y) is dominant to the allele for
green seeds (y).
The allele for round seeds (R) is dominant to the allele for
wrinkled seeds (r).
• Mendel crossed true-breeding plants that had yellow, round seeds
(YYRR) with true-breeding plants that has green, wrinkled seeds
• One possibility is that the two characters are transmitted from
parents to offspring as a package.
The Y and R alleles and y and r alleles stay together.
• If this were the case, the F1offspring would produce yellow, round
• The F o2fspring would produce two phenotypes (yellow + round;
green + wrinkled) in a 3:1 ratio, just like a monohybrid cross.
This was not consistent with Mendel’s results. • An alternative hypothesis is that the two pairs of alleles segregate
independently of each other.
The presence of a specific allele for one trait in a gamete has
no impact on the presence of a specific allele for the second
• In our example, the F of1spring would still produce yellow, round
• However, when the F s pr1duced gametes, genes would be
packaged into gametes with all possible allelic combinations.
Four classes of gametes (YR, Yr, yR, and yr) would be
produced in equal amounts.
• When sperm with four classes of alleles and ova with four classes
of alleles combined, there would be 16 equally probable ways in
which the alleles can combine in the F g2neration.
• These combinations produce four distinct phenotypes in a 9:3:3:1
• This was consistent with Mendel’s results.
• Mendel repeated the dihybrid cross experiment for other pairs of
characters and always observed a 9:3:3:1 phenotypic ratio in the
• Each character appeared to be inherited independently.
• If you follow just one character in these crosses, you will observe a
3:1 F 2atio, just as if this were a monohybrid cross.
• Tformation is now called Mendel’s law of independenturing gamete
• Mendel’s law of independent assortment states that each pair of
alleles segregates independently during gamete formation.
• Strictly speaking, this law applies only to genes located on
different, nonhomologous chromosomes.
• Genes located near each other on the same chromosome tend to
be inherited together and have more complex inheritance patterns
than those predicted for the law of independent assortment.
Concept 14.2 The laws of probability govern Mendelian
• Mendel’s laws of segregation and independent assortment reflect
the same laws of probability that apply to tossing coins or rolling
• The probability scale ranges from 0 (an event with no chance of
occurring) to 1 (an event that is certain to occur).
The probability of tossing heads with a normal coin is 1/2.
The probability of rolling a 3 with a six-sided die is 1/6, and the
probability of rolling any other number is 1 − 1/6 = 5/6. • When tossing a coin, the outcome of one toss has no impact on
the outcome of the next toss.
• Each toss is an independent event, just like the distribution of
alleles into gametes.
Like a coin toss, each ovum from a heterozygous parent has a
1/2 chance of carrying the dominant allele and a 1/2 chance of
carrying the recessive allele.
The same odds apply to the sperm.
• We can use the multiplication rule to determine the chance that
two or more independent events will occur together in some
Compute the probability of each independent event.
Multiply the individual probabilities to obtain the overall
probability of these events occurring together.
The probability that two coins tossed at the same time will land
heads up is 1/2 × 1/2 = 1/4.
Similarly, the probability that a heterozygous pea plant (Pp) will
self-fertilize to produce a white-flowered offspring (pp) is the
with a white allele.with a white allele will fertilize an ovum
This probability is 1/2 × 1/2 = 1/4.
• The rule of multiplication also applies to dihybrid crosses.
For a heterozygous parent (YyRr) the probability of producing a
YR gamete is 1/2 × 1/2 = 1/4.
We can use this to predict the probability of a particular F 2
genotype without constructing a 16-part Punnett square.
The probability that an F p2ant from heterozygous parents will
have a YYRR genotype is 1/16 (1/4 chance for a YR ovum and
1/4 chance for a YR sperm).
• The rule of addition also applies to genetic problems.
• Under the rule of addition, the probability of an event that can
occur two or more different ways is the sum of the separate
probabilities of those ways.
For example, there are two ways that F gam1tes can combine
to form a heterozygote.
▪ The dominant allele could come from the sperm and the
recessive from the ovum (probability = 1/4).
▪ Or the dominant allele could come from the ovum and the
recessive from the sperm (probability = 1/4).
▪ The probability of obtaining a heterozygote is 1/4 + 1/4 = 1/2.
• We can combine the rules of multiplication and addition to solve
complex problems in Mendelian genetics.
• Let’s determine the probability of an offspring having two recessive
phenotypes for at least two of three traits resulting from a trihybrid
cross between pea plants that are PpYyRr and Ppyyrr. There are five possible genotypes that fulfill this condition:
ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr.
We can use the rule of multiplication to calculate the probability
for each of these genotypes and then use the rule of addition to
recessive traits.ities for fulfilling the condition of at least two
• The probability of producing a ppyyRr offspring:
The probability of producing pp = 1/2 × 1/2 = 1/4.
The probability of producing yy = 1/2 × 1 = 1/2.
The probability of producing Rr = 1/2 × 1 = 1/2.
Therefore, the probability of all three being present (ppyyRr) in
one offspring is 1/4 × 1/2 × 1/2 = 1/16.
• For ppYyrr: 1/4 × 1/2 × 1/2 = 1/16.
• For Ppyyrr: 1/2 × 1/2 × 1/2 = 1/8 or 2/16.
• For PPyyrr: 1/4 × 1/2 × 1/2 = 1/16.
• For ppyyrr: 1/4 × 1/2 × 1/2 = 1/16.
• Therefore, the chance that a given offspring will have at least two
recessive traits is 1/16 + 2/16 + 1/16 + 1/16 = 6/16.
Mendel discovered the particulate behavior of genes: a review.
• While we cannot predict with certainty the genotype or phenotype
of any particular seed from the F 2eneration of a dihybrid cross,
we can predict the probability that it will have a specific genotype
• Mendel’s experiments succeeded because he counted so many
offspring, was able to discern the statistical nature of inheritance,
and had a keen sense of the rules of chance.
• Mendel’s laws of independent assortment and segregation explain
heritable variation in terms of alternative forms of genes that are
passed along according to simple rules of probability.
• These laws apply not just to garden peas, but to all diploid
organisms that reproduce by sexual reproduction.
• Mendel’s studies of pea inheritance endure not only in genetics,
but as a case study of the power of scientific reasoning using the
Concept 14.3 Inheritance patterns are often more complex than
predicted by simple Mendelian genetics
• In the 20th century, geneticists have extended Mendelian
principles not only to diverse organisms, but also to patterns of
inheritance more complex than Mendel described.
• In fact, Mendel had the good fortune to choose a system that was
relatively simple genetically. Each character that Mendel studied is controlled by a single
Each gene has only two alleles, one of which is completely
dominant to the other.
• The heterozygous F off1pring of Mendel’s crosses always looked
like one of the parental varieties because one allele was dominant
to the other.
• The relationship between genotype and phenotype is rarely so
• The inheritance of characters determined by a single gene
deviates from simple Mendelian patterns when alleles are not
completely dominant or recessive, when a gene has more than
two alleles, or when a gene produces multiple phenotypes.
• We will consider examples of each of these situations.
• Alleles show different degrees of dominance and recessiveness in
relation to each other.
• One extreme is the complete dominance characteristic of
• At the other extreme from complete dominance is codominance,
in which two alleles affect the phenotype in separate,
For example, the M, N, and MN blood groups of humans are
due to the presence of two specific molecules on the surface of
red blood cells.
People of group M (genotype MM) have one type of molecule
on their red blood cells, people of group N (genotype NN) have
both molecules present.ple of group MN (genotype MN) have
Tphenotypes but rather exhibits both the M and the N phenotype.
• Some alleles show incomplete dominance, in which
homozygotes.s show a distinct intermediate phenotype not seen in
Tseparable (particulate), as shown in further crosses.e
Ophenotypes: each parental and the heterozygote.three
The phenotypic and genotypic ratios are identical: 1:2:1.
• A clear example of incomplete dominance is seen in flower color of
A cross between a white-flowered plant and a red-flowered
plant will produce all pink 1 offspring.
Self-pollination of the F1offspring produces 25% white, 25%
red, and 50% pink F o2fspring. • The relative effects of two alleles range from complete dominance
codominance of both alleles.plete dominance of either allele, to
• It is important to recognize that a dominant allele does not
somehow subdue a recessive allele.
• Alleles are simply variations in a gene’s nucleotide sequence.
When a dominant allele coexists with a recessive allele in a
heterozygote, they do not interact at all.
• To illustrate the relationship between dominance and phenotype,
let us consider Mendel’s character of round versus wrinkled pea
Pea plants with wrinkled seeds have two copies of the
The seeds are wrinkled due to the accumulation of
converts them to starch.of the lack of a key enzyme that
Excess water enters the seed due to the accumulation of