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2
Basic Principles of Heredity
2.1
Mendel: The Father of Genetics
The basic principles of genetics had been discovered by Johann Gregor Mendel
(1822–1884). Mendel was born in what is now part of the Czech Republic. Although
his parents were simple farmers with little money, he was able to achieve a sound
education and was admitted to the Augustinian monastery in Brno in September
1843. After graduating from seminary, Mendel was ordained a priest and appointed
to a teaching position in a local school. He excelled at teaching, and the abbot of the
monastery recommended him for further study at the University of Vienna, which he
attended from 1851 to 1853. There, Mendel enrolled in the newly opened Physics
Institute and took courses in mathematics, chemistry, entomology, paleontology,
botany, and plant physiology. It was probably here that Mendel acquired the
scientific method, which he later applied so successfully to his genetics experiments.
After 2 years of study in Vienna, Mendel returned to Brno, where he taught school
and began his experimental work with pea plants. He conducted breeding
experiments from 1856 to 1863 and presented his results publicly at meetings of the
Brno Natural Science Society in 1865. Mendel‘s paper from these lectures was
published in 1866. In spite of widespread interest in heredity, the effect of his
research on the scientific community was minimal. At the time, no one seems to
have noticed that Mendel had discovered the basic principles of inheritance. The
paper was read by few and apparently understood by no one until 1900, long after
Mendel‘s death.
Meanwhile, Mendel tried unsuccessfully to repeat his observations with another plant
species, Hieracium, which, it turns out, forms seeds without a true meiosis. Mendel
knew nothing about meiosis, and the failure of these experiments must have
represented a great disappointment to him, and he undoubtedly came to regard
himself as a scientific failure.
The significance of Mendel‘s discovery was unappreciated until 1900, when three
botanists—Hugo
de
Vries,
Erich
von
Tschermak,
and
Carl
Correns—began
independently conducting similar experiments with plants and arrived at conclusions
similar to those of Mendel. Coming across Mendel‘s paper, they interpreted their
results in terms of his principles and drew attention to his pioneering work.
1
2.2
Mendel’s Success
Mendel‘s approach to the study of heredity was effective for several reasons.
Foremost was his choice of experimental subject, the pea plant Pisum sativum
(Fugure 2.1), which offered clear advantages for genetic investigation. Mendel,
however could easily succeed in formulating certain basic principles of heredity, for
the following reasons:
Figure 2.1: Mendel used the pea plant Pisum sativum in his studies of heredity. He examined
seven characteristics that appeared in the seeds and in plants grown from the seeds.
(1)
The pea plant is easy to cultivate, and Mendel had the monastery garden and
greenhouse at his disposal.
(2)
Peas grow relatively rapidly, completing an entire generation in a single
growing season.
(3)
Pea plants also produce many offspring—their seeds—which allowed Mendel to
detect meaningful mathematical ratios in the traits that he observed in the
progeny.
(4)
The pea plant which Mendel chose for conducting experiments, is most ideal for
controlled breeding, since it can easily be subjected to cross pollination.
(5)
He identified very clear contrasting characters in the pea plants.
(6)
He selected pure breeding plants for his experiments. He is said to have spent
about 2 years to ascertain this characteristic feature.
(7)
Mendel concentrated at a time only on the inheritance on one particular trait,
with the two contrasting conditions, instead of attempting the inheritance of
entire set of characters in the plant.
(8)
He maintained an accurate record of all the observations he made on the
breeding experiments that he had designed.
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(9)
He
pooled
the
data
obtained
from
similar
experiments
for
different
characteristics and analysed the results by using statistical methods and
applying the law of probability.
(10) He was able to effectively check the flowers under investigation from
contamination by unwanted pollen grains.
(11) Mendel was fortunate enough in choosing the seven pairs of contrasting
characters in pea plants. It was later discovered that the genes responsible for
these characters are located on separate chromosomes.
(12) Mendel was also fortunate in the sense that the characters he had chosen in the
pea plant did not show any interaction or linkage.
2.3
Monohybrid Crosses
2.3.1
Mendel’s Experiments of Monohybrid Crosses
Mendel chose to work with edible pea, Pisum sativum, since seeds for numerous
varieties were available from local seeds man. Mendel started with 34 varieties of
peas and spent 2 years selecting those varieties that he would use in his
experiments. He verified that each variety was genetically pure (homozygous for
each of the traits that he chose to study) by growing the plants for two generations
and confirming that all offspring were the same as their parents. He then carried out
a number of crosses between the different varieties.
Although peas are normally self-fertilizing (each plant crosses with itself), Mendel
conducted crosses between different plants by opening the buds before the anthers
were fully developed, removing the anthers, and then dusting the stigma with pollen
from a different plant. Mendel began by studying monohybrid crosses—those
between parents that differed in a single characteristic. In one experiment, Mendel
crossed a pea plant homozygous for round seeds with one that was homozygous for
wrinkled seeds. This first generation of a cross is the P (parental) generation. After
crossing the two varieties in the P generation, Mendel observed the offspring that
resulted from the cross. When he examined their offspring, which he called the first
filial (F1) generation, he discovered that they were always like one of the parents.
When peas with round seeds were crossed with peas with wrinkled seeds, for
example, the F1 offspring always produced round seeds. Mendel proposed to
designate the traits expressed as dominant and the traits not expressed in the F1 as
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recessive. The full list of traits had the following relationships (Table 2.1) and the
results of the Mendel‘s monohybrid crosses are summarized in Table 2.2.
Table 2.1: Designation of the Mendel‘s experimental traits
_____________________________________________________________
Trait/Character
Dominant
Recessive
_____________________________________________________________
1. Seed shape
Round seeds
Wrinkled seeds
2. Seed colour
Yellow seeds
Green seeds
3. Seed coat colour
Coloured seed coat
White seed coat
4. Pod shape
Inflated pod
Wrinkled pod
5. Pod colour
Green pod
Yellow pod
6. Flower position
Axial flower
Terminal flower
7. Stem size
Long stem
Short stem
_____________________________________________________________
Table 2.2: The Results of Mendel‘s monohybrid crosses
___________________________________________________________________________
Trait/Character
Parental (P)
F1
F2 generation
Phenotypic
(Phenotype)
generation
(No. & Phenotype)
ratio
___________________________________________________________________________
1. Seed shape
Round x Wrinkled
Round
5,474
Round
2.96:1
1,850
Wrinkled
2. Seed colour
Yellow x Green
Yellow
3. Seed coat colour
Coloured x White
Coloured
4. Pod shape
Inflated x Wrinkled
5. Pod colour
6,022
2,001
Yellow
Green
3.01:1
705
224
Coloured
White
3.15:1
Inflated
882
299
Inflated
Wrinkled
2.95:1
Green x Yellow
Green
428
152
Green
Yellow
2.82:1
6. Flower position
Axial x Terminal
Axial
651
207
Axial
Terminal
3.14:1
7. Stem size
Long x Short
Long
787
Long
2.84:1
277
Short
___________________________________________________________________________
Mendel now allowed each F1 plants to self-pollinate, meaning that each stigma was
fertilized by its own pollen, and the resultant second filial (F2) generation proved
to include plants that displayed either the dominant or recessive trait. The key
operation performed by Mendel at this point was to count the number of plants of
each type. It is clean that in every cross, a ratio of three dominant to one recessive
was obtained in the F2. Thus the inheritance of each alternate trait obeyed a nonrandom, quantitative rule.
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2.3.2
What Monohybrid Crosses Reveal
Mendel drew several important conclusions from the results of his monohybrid
crosses.
(1)
First, he reasoned that, although the F1 plants display the phenotype of only
one parent, they must inherit genetic factors from both parents because they
transmit both phenotypes to the F 2 generation. He concluded that each plant
must therefore possess two genetic factors coding for a character. The genetic
factors that Mendel discovered (alleles) are, by convention, designated with
letters; the allele for round seeds is usually represented by R, and the allele for
wrinkled seeds by r. The plants in the P generation of Mendel‘s cross possessed
two identical alleles: RR in the round-seeded parent and rr in the wrinkledseeded parent.
(2)
A second conclusion that Mendel drew from his monohybrid crosses was that
the two alleles in each plant separate when gametes are formed, and one allele
goes into each gamete. When two gametes (one from each parent) fuse to
produce a zygote, the allele from the male parent unites with the allele from
the female parent to produce the genotype of the offspring. Thus, Mendel‘s F1
plants inherited an R allele from the round-seeded plant and an r allele from
the wrinkled-seeded plant. However, only the trait encoded by round allele (R)
was observed in the F1—all the F1 progeny had round seeds.
(3)
Those traits that appeared unchanged in the F 1 heterozygous offspring Mendel
called dominant, and those traits that disappeared in the F1 heterozygous
offspring he called recessive. When dominant and recessive alleles are present
together, the recessive allele is masked, or suppressed. The concept of
dominance was a third important conclusion that Mendel derived from his
monohybrid crosses.
(4)
Mendel‘s fourth conclusion was that the two alleles of an individual plant
separate with equal probability into the gametes. When plants of the F1 (with
genotype Rr) produced gametes, half of the gametes received the R allele for
round seeds and half received the r allele for wrinkled seeds. The gametes then
paired randomly to produce the following genotypes in equal proportions
among the F2: RR, Rr, rR, rr. Because round (R) is dominant over wrinkled (r),
there were three round progeny in the F2 (RR, Rr, rR) for every one wrinkled
progeny (rr) in the F2. This 3:1 ratio of round to wrinkled progeny that Mendel
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observed in the F2 could occur only if the two alleles of a genotype separated
into the gametes with equal probability.
The conclusions that Mendel developed about inheritance from his monohybrid
crosses have been further developed and formalized into (1) the principle of
segregation and (2) the concept of dominance.
(1) The principle of segregation (Mendel‘s first law) states that each individual
diploid organism possesses two alleles for any particular characteristic. These
two alleles segregate (separate) when gametes are formed, and one allele goes
into each gamete. Furthermore, the two alleles segregate into gametes in equal
proportions.
(2)
The concept of dominance states that, when two different alleles are present
in a genotype, only the trait of the dominant allele is observed in the
phenotype.
Concepts: The principle of segregation states that each individual organism possesses two
alleles coding for a characteristic. These alleles segregate when gametes are formed, and one
allele goes into each gamete. The concept of dominance states that, when dominant and
recessive alleles are present together, only the trait of the dominant allele is observed.
2.3.3
Predicting the Outcomes of Genetic Crosses
One of Mendel‘s goals in conducting his experiments on pea plants was to develop a
way to predict the outcome of crosses between plants with different phenotypes. In
this section, we will first learn a simple, short-hand method for predicting outcomes
of genetic crosses (the Punnett square), and then we will learn how to use
probability to predict the results of crosses.
The Punnett Square
To illustrate the Punnett square, let‘s examine another cross that Mendel carried out.
By crossing two varieties of peas that differed in height, Mendel established that tall
(T) was dominant over short (t). He tested his theory concerning the inheritance of
dominant traits by crossing an F1 tall plant that was heterozygous (Tt) with the short
homozygous parental variety (tt). This type of cross, between an F1 genotype and
either of the parental genotypes, is called a backcross.
To predict the types of offspring that result from this cross, we first determine which
gametes will be produced by each parent (Figure 2.2a). The principle of segregation
tells us that the two alleles in each parent separate, and one allele passes to each
6
gamete. All gametes from the homozygous tt short plant will receive a single short
(t) allele. The tall plant in this cross is heterozygous (Tt); so 50% of its gametes will
receive a tall allele (T) and the other 50% will receive a short allele (t).
Figure 2.2: The Punnett square can be used for determining the results of a genetic cross.
A Punnett square is constructed by drawing a grid, putting the gametes produced
by one parent along the upper edge and the gametes produced by the other parent
down the left side (Figure 2.2b). Each cell (a block within the Punnett square)
contains an allele from each of the corresponding gametes, generating the genotype
of the progeny produced by fusion of those gametes. In the upper left-hand cell of
the Punnett square in Figure 2.2b, a gamete containing T from the tall plant unites
with a gamete containing t from the short plant, giving the genotype of the progeny
(Tt). It is useful to write the phenotype expressed by each genotype; here the
progeny will be tall, because the tall allele is dominant over the short allele. This
process is repeated for all the cells in the Punnett square.
By simply counting, we can determine the types of progeny produced and their
ratios. In Figure 2.2b, two cells contain tall (Tt) progeny and two cells contain short
(tt) progeny; so the genotypic ratio expected for this cross is 2 Tt to 2 tt (a 1:1
ratio). Another way to express this result is to say that we expect of the progeny to
have genotype Tt (and phenotype tall) and of the progeny to have genotype tt (and
phenotype short). In this cross, the genotypic ratio and the phenotypic ratio are the
same, but this outcome need not be the case.
Concepts: The Punnett square is a short-hand method of predicting the genotypic and
phenotypic ratios of progeny from a genetic cross.
7
2.4
Dihybrid Crosses
2.4.1
Mendel’s Experiments of Dihybrid Crosses
In addition to his work on monohybrid crosses, Mendel also crossed varieties of peas
that differed in two characteristics (dihybrid crosses). For example, he had one
homozygous variety of pea that produced round seeds and yellow endosperm;
another homozygous variety produced wrinkled seeds and green endosperm. When
he crossed the two, all the F1 progeny had round seeds and yellow endosperm. He
then self-fertilized the F1 and obtained the following progeny in the F2:
Round seeds and Yellow endosperm x Wrinkled seeds and Green endosperm
1 (32)
Wrinkled/Green plants,
3 (101/32)
Wrinkled/Yellow plants,
3 (108/32)
Round/Green plants, and
9 (315/32)
Round/Yellow plants.
Mendel recognized that these traits appeared approximately in a 9:3:3:1 ratio; that
is, of the progeny were round and yellow, were wrinkled and yellow, were round and
green, and were wrinkled and green.
2.4.2
The Principle of Independent Assortment
Mendel carried out a number of dihybrid crosses for pairs of characteristics and
always obtained a 9:3:3:1 ratio in the F2. This ratio makes perfect sense in regard to
segregation and dominance if we add a third principle, which Mendel recognized in
his dihybrid crosses: the principle of independent assortment (Mendel’s second
law). This principle states that alleles at different loci separate independently of one
another.
A common mistake is to think that the principle of segregation and the principle of
independent
assortment
refer
to
two
different
processes.
The
principle
of
independent assortment is really an extension of the principle of segregation. The
principle of segregation states that the two alleles of a locus separate when gametes
are formed; the principle of independent assortment states that, when these two
alleles separate, their separation is independent of the separation of alleles at other
loci.
8
Let‘s see how the principle of independent assortment explains the results that
Mendel obtained in his dihybrid cross. Each plant possesses two alleles coding for
each characteristic, so the parental plants must have had genotypes RRYY and rryy.
The principle of segregation indicates that the alleles for each locus separate, and
one allele for each locus passes to each gamete. The gametes produced by the
round, yellow parent therefore contain alleles RY, whereas the gametes produced by
the wrinkled, green parent contain alleles ry. These two types of gametes unite to
produce the F1, all with genotype RrYy. Because round is dominant over wrinkled and
yellow is dominant over green, the phenotype of the F1 will be round and yellow.
When Mendel self-fertilized the F1 plants to produce the F2, the alleles for each locus
separated, with one allele going into each gamete. This is where the principle of
independent assortment becomes important. Each pair of alleles can separate in two
ways: (i) R separates with Y and r separates with y to produce gametes RY and ry or
(ii) R separates with y and r separates with Y to produce gametes Ry and rY.
F1 Plants
Genotype
F1 Gametes
Rr Yy
RY
Ry
rY
ry
The principle of independent assortment tells us that the alleles at each locus
separate independently; thus, both kinds of separation occur equally and all four
type of gametes (RY, ry, Ry, and rY) are produced in equal proportions. When these
four types of gametes are combined to produce the F2 generation, the progeny
consist of round and yellow, wrinkled and yellow, round and green, and wrinkled and
green, resulting in a 9:3:3:1 phenotypic ratio.
2.4.3
Mendel’s Interpretation of His Experiments
9
The F1, he proposed, would again produce equal numbers of gametes carrying R
alone and r alone. In addition, each gamete would produce four classes of gametes:
RY, Ry, ry, and ry.
The results of self-fertilization in this case involves 4 x 4 = 16 possible combinations;
these are displayed in the following chart (Punnett square) where each box in the
grid represents the F2 plants resulting from the union of the egg on the left and
pollen on the top (Table 2.3).
Table 2.3: F2 Generation (Punnett square)
F1 pollen
RY
F1 eggs
Ry
rY
ry
F2 results,
9/16
3/16
3/16
1/16
RY
Ry
rY
ry
RRYY
Round/Yellow
RRYy
Round/Yellow
RrYY
Round/Yellow
RrYy
Round/Yellow
RRYy
Round/Yellow
Rryy
Round/Green
RrYy
Round/Yellow
Rryy
Round/Green
RrYY
Round/Yellow
RrYy
Round/Yellow
rrYY
Wrinkled/Yellow
rrYy
Wrinkled/Yellow
RrYy
Round/Yellow
Rryy
Round/Green
rrYy
Wrinkled/Yellow
rryy
Wrinkled/Green
based on random combination of phenotypic classes:
Round/Yellow seeds
Round/Green seeds
Wrinkled/Yellow seeds
Wrinkled/Green seeds
It should be possible to calculate the genotypic ratio of this cross (Table 2.4):
Table 2.4: Genotypic and phenotypic ratios of dihybrid crosses
______________________________________________________
F2 genotype
Fraction
Phenotype
Number
______________________________________________________
1.
RRYY
1/16
Round/Yellow
9
2.
RrYY
2/16
Round/Yellow
3.
RRYy
2/16
Round/Yellow
4.
RrYy
4/16
Round/Yellow
______________________________________________________
5.
RRyy
1/16
Round/Green
3
6.
Rryy
2/16
Round/Green
______________________________________________________
7.
rrYY
1/16
Wrinkled/Yellow
3
8.
rrYy
2/16
Wrinkled/Yellow
______________________________________________________
9.
rryy
1/16
Wrinkled/Green
1
______________________________________________________
10
Closer examinations of this 1:2:2:4:1:2:1:2:1 genotypic ratio reveals the 9:3:3:1
phenotypic ratio:
1/16
RRYY
2/16
RrYY
2/16
RRYy
4/16
RrYy
_______________
9/16
R-Y(Round/Yellow)
1/16
RRyy
2/16
Rryy
______________
3/16
R-yy
(Round/Green)
1/16
rrYY
2/16
rrYy
1/16 rryy
_________________ _________________
3/16
rrY1/16 rryy
(Wrinkled/Yellow)
(Wrinkled/Green)
Concepts: The principle of independent assortment states that genes coding for different
characteristics separate independently of one another when gametes are formed, owing to
independent separation of homologous pairs of chromosomes during meiosis. Genes located
close together on the same chromosome do not, however, assort independently.
2.5
Genetic Terminology
2.5.1
Homologous Chromosome/Locus/Alleles
Chromosomes occurring in pairs, one derived from each of two parents, normally
(except for chromosomes associated with sex) morphologically alike and bearing the
same gene loci are called homologous chromosomes. A diploid cell contains two
homologues of each chromosomes type possessed by the species, one of this
homologues derived from one gamete (e.g., an egg) and the other homologues
deriving from the other gamete (e.g., a pollen grain).
Each homologue in a pair contains the same kinds of genes and in the same order,
the physical location of any particular gene in the chromosome being called its locus
(pl. loci). However, if we examine the two homologues in any one nucleus, the genes
at a particular locus need not be identical. Specifically, in an Rr pea plant, one seed
locus will carry the round (R) gene, whereas the seed shape locus on its homologous
chromosomes will carry the wrinkled (r). The R and r genes are said to be alleles (a
term that derived from the Greek ―alleles‖ of one another), indicating that they are
two alternative genes that can occupy the same chromosomal locus.
2.5.2
Heterozygous/Homozygous
An organism, such as a Rr pea plant, with two alleles at a particular locus, is said to
be heterozygous at that locus (―hetero‖ means different). By contrast, its true
breeding round plant has a copy of the R gene at both of its seed-shape loci; this RR
parent is said to be homozygous (―homo‖ means the same) at the locus. Similarly,
11
the true breeding wrinkled parental
line used in Mendel‘s crosses is also
homozygous, carrying an identical ‗r‘ gene at each seed-shape locus.
2.5.3
Wild-type
We should note here that other systems of genetic notation are also used. In many
cases, the allele most often encountered at a locus in natural populations is said to
be wild-type gene. For example, if we examined peas growing in the wild and
determined that most carried the round allele at the seed-shape locus, we might
designate round as wild type and symbolize it as + or as r +. In this system, the less
common allele, which is often, but not always, recessive, might be designated r or r.
Therefore, a homozygote would be designated +/+ or r/r, whereas the heterozygote
would be +/r; alternatively, homozygote might be r+/r+ and r-/r-, with heterozygote
r+/r-.
2.5.4
Genotype/Phenotype
The genetic makeup or constitution of an individual, with reference to the traits
under consideration, is called genotype, usually expressed by a symbol, e.g., +, R,
rr, etc. Individuals of the same genotype bred alike. The appearance or discernable
character of an individual, which is dependent upon its genetic makeup, is called
phonotype. It is usually expressed in words, e.g., tall, dwarf, round, wrinkled, etc.
Identical phenotypes may not necessarily bred alike. The phenotype derives from the
Greek ―phain‖, to appear, and connotes the particular expression of a gene –
wrinkled seed, brown seed, brown eye – each particular feature being known a as
trait. The phenotype, in short, includes all the products or manifestations of the
organism‘s genes: the amino acid sequence of its proteins, its enzyme activities, its
appearance, and even its behaviour – whereas the genotype is synonymous with the
collection of genes in the organism‘s chromosome or chromosomes.
2.5.5
Dominant/Recessive
Dominant - the allele that expresses itself at the expense of an alternate allele; the
phenotype that is expressed in the F1 generation from the cross of two pure lines.
Recessive - an allele whose expression is suppressed in the presence of a dominant
allele; the phenotype that disappears in the F 1 generation from the cross of two pure
lines and reappears in the F2 generation.
12
2.6
Blending Inheritance Disproved
2.6.1
Complete Dominance
In the case of flower colour in the garden pea, Mendel found that the red flower trait
was dominant to that for white. Therefore, a cross between homozygous red (RR)
and white (rr) plants produces F1 hybrids (Rr) that are red flowered. Although the
white-flowered trait seems to disappear in the F1, a cross between two F1 red plants
(Rr x Rr) yields an F2 consisting of red and white-flowered plants in a ratio of 3:1.
The white-flowered plants are identical in phenotype to the original ones in the P1
and are in no way altered by the presence of the contrasting allele for redness in the
F1. Similarly, the allele for redness has in no way been diluted by the factor for
white. This is not certainly to be expected if fluids from two parental sources are
mixing; otherwise, the original qualities should become altered in some way, as if
paints of different colours had been blended.
2.6.2
Incomplete Dominance
The common horticultural plant known as the four O‘clock can also have red or white
flowers. However, if red and white plants are crossed, the picture differ from the one
presented by Mendel‘s pea (Figure 2.3). Both red plants and white ones are
homozygous (RR and rr), and the F1 hybrids formed when they are crossed (Rr) are
always pink. When two pink plants are used as gametes, the results are always 1
Red : 2 Pink : 1 White. The existence of plants with the intermediate pink shade in
no way supports the blending concept of inheritance; on the contrary, it strongly
supports the particulate concept. Neither the factor for redness (R) nor the one for
whiteness (r) becomes altered in anyway by being present together in cells of an
individual. A cross such as this illustrates Mendel‘s first law beautifully, and differs
from that of the pea only in the question of dominance. In the case of the four
O‘clock, neither form of the gene for flower colour is dominant to the other. In the
hybrid (Rr), each allele expresses itself so that an intermediate effect results. Alleles
such as these show a lack of complete dominance. When dominance is not totally
complete, the 3:1 phenotypic ratio that typically results from a cross of two
monohybrids becomes modified to a ratio of 1:2:1, which is identical to the
genotypic ratio that arises from such a cross. This is so, since lack of dominance
permits the pink hybrids (Rr) to be distinguished phenotypically, unlike the hybrid
pea plants, which appear as red a the homozygote.
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Figure 2.3: Incomplete dominance in four O‘clock. Neither of the two alleles, R or r, is
dominant. Consequently the F2 phenotypic ratio is modified from 3:1 to 1:2:1, which is
identical to the ratio of genotype.
2.7
Dominance is not Always Absolute
Dominant and recessive are often used as terms of convenience, but strictly
speaking, they are not very precise.
2.7.1
Super Dominance
One example from human illustrates this point very well. A certain allele, D, when
present in a single dose, can produce a serious skeletal disorder, achondroplasia.
Persons heterozygous for this allele (Dd) have a dwarfed structure, short limbs, and
a large head with bulging forehead. The allele is generally referred to as a dominant.
However, persons homozygous for the allele (DD) have a much more abnormal
phenotype, and they do not usually survive infancy. Obviously, the terms dominant
and recessive are not 100% distinct in this case, because a double dose of the socalled dominant allele produces a far greater effect than a single dose in the
heterozygote. Many examples could be given in which heterozygotes appear on the
surface to be identical to homozygotes.
14
2.7.2
Incomplete Dominance
Nevertheless, the so-called recessive allele may actually be producing some effect.
The expression incomplete dominance refers to such situations in which both alleles
express themselves in the hybrid but the effect of one allele appears than that of the
other. In the absence of a refined examination, one allele may even appear to be
completely dominant.
2.7.3
Co-Dominance
Another commonly used term is co-dominance, which is also applied to cases in
which both alleles produce an effect in the heterozygote. However, in its strict sense,
co-dominance implies that a definite product or substance controlled by each allele
can be identified. The genetic factors that govern the A and B blood types in the
human are alleles. Each controls the formation of a different red blood cell substance
or antigen, antigen A in one case, antigen B in the other. Neither allele is dominant
over the other. The individual who is blood type AB is a heterozygote and has both
the alleles for antigen A and antigen B. Both of these substances, A and B, are easily
detected in equal amounts in the red blood cells. This is a clear cut case of codominance between a pair of alleles.
2.8
Misconceptions to be Avoided
Several important points concern the concept of dominance, which unfortunately, is
often misunderstood.
2.8.1
Misconception of Dominant Phenotype
Although Mendel showed that the factor for red flower colour in the pea plant is
dominant over its allele for white, the genetic factors for red and white flowers in the
four O‘clock are incompletely dominant. This example was given to illustrate that a
genetic factor that produces a certain phenotypic effect (red in the example) in one
species is not the same as one in a different species, even though the two genetic
factors appear to produce identical phenotypes (red flower colour in both the pea
and the four O‘clock). Simply because a particular factor in one group of animals or
plants expresses itself as a dominant in no reason to assume that a different factor
with a similar or even identical expression must also be dominant in another group.
In animals, a factor for black fur may be dominant in one species (as in guinea pigs)
but recessive in another (as in sheep).
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2.8.2
Misconception of Frequency of Dominant Alleles
Another common misunderstanding of dominance concerns the frequency of
dominant alleles in a population. One often assumes that simply because a particular
factor is dominant it must, therefore, be more abundant in the population than its
recessive counterpart. Implicate in this erroneous idea is the belief that a dominant
factor must increase in frequency over its recessive allele. A moment reflection will
tell us in the simple case of eye colour in human that this is not so. Although the
factor for brown eye colour (B) behaves as a dominant to its allele for blue (r), there
may be more blue-eyed than brown-eyed people in the population. In Scandinavia
this would be so, whereas in Mediterranean countries we expect more people to be
brown-eyed. The frequency or abundance of an allele is not the same as its degree of
dominance. The allele for brown eye colour is dominant to the one for blue, which
means simply that the factor for brown eyes expresses itself in the presence of its
allele, whereas the factor blue does not. Which allele - the one for brown or the one
for blue - will be the more common is a very different matter. The frequency of any
allele, dominant or recessive, is related to the survival value it imparts to a
population and not just to its dominance or recessiveness.
2.8.3
Misconception of the Positive Effect of Dominance
Nor does the effect that an allele is dominant mean that it is necessarily better or will
somehow produce a stronger or more desirable effect. A long list of undesirable
dominant allele in various species could be presented; in humans the following seem
to depend on dominants: dystrophy, a type of dwarfism, and a form of progressive
nervous deterioration resulting in death. Such conditions are far from desirable and
certainly do not make the afflicted person stronger. It is also apparent that the
conditions, and hence their associated dominant alleles, are fortunately not as
common in the population as the normal conditions and the normal but recessive
gene forms. Moreover, these undesirable dominants are not increasing to the
disadvantage of the whole population. If their dominance made them increase,
everyone would eventually become afflicted.
2.8.4
Misconception of the Interpretation of Ratios
It is also essential to appreciate the correct use of the Punnett square and the
interpretation of ratios such as 3:1, 1:2:1 or any others. One reason for Mendel‘s
achievement was that he conducted his studies with large numbers of plants. Few of
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us would a cross between two heterozygous tall plants (Tt x Tt) to yield exactly three
tall and one dwarf if only four offspring were obtained. The ratio 3:1 (or any other)
tells us the probability of getting a certain phenotype or genotype when a particular
cross is made. Only after obtaining hundreds of F 2 plants would we expect to
approximate a ratio close to the ideal of 3:1. However, the chance of getting a tall
plant from any one fertilization when two heterozygote are crossed is 3 out of 4, and
the same value persists regardless of the number of offspring. No matter how many
offspring, even after hundreds of them, the 3:1 chance remains the same for
obtaining a tall at any way one trait. Likewise, the chance of obtaining dwarf remains
one out of four. It is very important to remember when any ratio is discussed that it
does not predict the exact results from a cross but instead gives us the chance at
any one event that a certain outcome (tall or dwarf, red or white, boy or girl) will
occur.
2.9
Test Cross and Its Value
Test cross is a simple method devised by Mendel to verify the genotype of the F 1
hybrid. When the F1 hybrid is crossed with the homozygous recessive parent, it is
called a test cross. Since, the F1 is crossed back with one of the parents, it is also
called a back cross. Test cross is also used for checking the correctness of Mendel‘s
law of segregation (using a monohybrid test cross) and the law of independent
assortment of characters (using a dihybrid test cross).
2.9.1
Monohybrid Test Cross
In a monohybrid cross of Tall x Dwarf, the F1 are all Tall (Figure 2.4). Let us see
what happens when this F1 Tall is test crossed with the homozygous recessive
parent, i.e., Dwarf with (tt) genotype. We aim to check and verify two things (i)
determine the genotype of F 1 Tall and (ii) check the correctness of the law of
segregation. Thus, F1 Tall is back crossed (test crossed) with the homozygous
recessive Dwarf parent and the progeny of the cross examined. We know that the
recessive dwarf with (tt) genotype will produce only one type of gametes [i.e., all
with (t) only]. However, as regards the F1 tall, there can be two possibilities:
(1)
If the F1 Tall is homozygous with (TT) genotype, it shall produce only one type
of gametes (i.e., all with (T) only). As a result, the progeny of the cross should
be all tall. (T) x (t) = (Tt) Tall.
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(2)
If, on the other hand, the F1 tall is heterozygous with (Tt) genotype and, if
Mendel's law of segregation of characters is correct, then the F1 should produce
two types of gametes, (T) and (t) in equal proportion. The recessive dwarf
parent produces only (T) type of gametes. The resulting test cross progeny
should be 50% Tall and 50% Dwarf or in 1:1 ratio as follows:
Figure 2.4: Monohybrid test cross.
The results obtained in the actual test cross experiments completely agree with the
theoretical expectations. Thus it is proved that F 1 Tall is a heterozygous dominant
(monohybrid) with (Tt) genotype and that the alleles segregate during gamete
formation.
2.9.2
Dihybrid Test Cross
In the dihybrid cross of yellow/round x green/wrinkled the F1, dihybrid is a double
heterozygous yellow round with (YyRr) genotype. It is test crossed with the double
homozygous recessive parent, (i.e., green round with (yyrr) genotype). We aim to
test the correctness of Mendel's law of independent assortment of characters. If the
principle of independent assortment is correct, then as Mendel explained, the (RrYy)
dihybrid should produce four types of gametes: RY, rY, Ry and ry in equal
proportions. The recessive parent (rryy) shall produce only one type of gametes, i.e.,
ry type. It is expected that the maximum possible chance combinations between
these gametes should produce four kinds of phenotypes in the ratio of 1:1:1:1 as
follows (Figure 2.5).
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Figure 2.5: Dihybrid test cross.
The actual numbers of these four types of offspring obtained in one of Mendel's test
crosses were: Round/Yellow = 55, Wrinkled/Yellow = 49, Round/Green = 51 and
Wrinkled/Green = 52. This gives an approximate 1:1:1:1 ratio as per the theoretical
expectations and thus, confirms the law of independent assortment of characters.
2.9.3
Significance of Back Cross in Plant Breeding
(1)
It is a rapid method of improving a variety of crop.
(2)
It is an easier and quicker method of obtaining a desirable trait in a pure
homozygous condition (true breeding trait).
(3)
Back cross is also use frequently in hybridization experiments because of its
simple ratio. Moreover, this needs the analysis of fewer progeny.
References
1.
Genetics: A Conceptual Approach, First Edition. 2007. Benjamin A Pierce. WH Freeman &
Company, New York.
2.
Principles of Genetics, Sixth Edition. 2012. Snustad P and Simmons MJ. John Wiley and Sons Ltd.,
New York.
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2.10
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
(e)
11.
12.
13.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
14.
15.
16.
17.
18.
19.
Review Questions
Why was Mendel‘s approach to the study of heredity so successful?
What is the relation between the terms allele, locus, gene, and genotype?
What is the principle of segregation? Why is it important?
What is the concept of dominance? How does dominance differ from incomplete
dominance?
What is the principle of independent assortment? How is it related to the principle of
segregation?
Mendel spent his first two years verifying that his variants were true-breeding. Why was
this important?
What experiment disproved the then-popular blending theory of inheritance?
Which of Mendel's experimental results provided evidence for the particulate nature of
inheritance. Describe the evidence.
Distinguish between genotypic ratio and phenotypic ratio. Include in your answer values
for both ratios for the F2 generation of a monohybrid cross in which there is full
dominance.
What result would you expect for a test cross of each of the following:
(a) The F1 hybrid formed by crossing a true-breeding round pea with a true breeding
wrinkled.
(b) A true-breeding tall pea plant.
(c) A true-breeding short pea plant.
(d) The F2 progeny of a monohybrid cross in which there is full dominance when those
that exhibit the dominant phenotype are tested one at a time.
The F2 progeny of a monohybrid cross in which there is full dominance when those that
exhibit the recessive phenotype are tested one at a time.
Define each of the following terms and explain its significance in the study of genetics.
(a) Allele, (b) Genetic locus, (c) Self-fertilization, (d) Equal Segregation, (f) F2, (g) Test
cross, (h) Back cross, (i) Phenotype, and (j) Heterozygous.
What is a Punnett square and what is it used for. Include in your answer an example of
a Punnett square for an F2 generation in which there is full dominance.
A true-breeding strain of peas exhibiting unlinked dominant traits A and B is crossed
with a true-breeding strain exhibiting both recessive forms.
What will be the phenotype of the F1 progeny?
What will be the genotype of the F1 progeny?
What genotypes will be present and in what ratios in the F2 generation?
What phenotypes will be present and in what ratios in the F2 generation?
What phenotypic ratio will be obtained in a test cross of the F1 generation? Include in
your answer a description of each of the phenotypes you expect.
What fraction of the peas from the F2 generation that exhibit both dominant traitswill be
found to be true-breeding?
What fraction of all of the peas in the F2 generation will be found to be true-breeding?
(Think about this one carefully before you answer!)
What fraction of the peas from the F2 generation are heterozygous for both traits?
Use a diagram to demonstrate the origin of the 9:3:3:1 phenotypic ratio that is
characteristic of the F2 generation from a dihybrid cross with full dominance.
What is the difference between the principle of equal segregation and the principle of
independent assortment?
Explain how the 9:3:3:1 phenotypic ratio of a typical dihybrid cross is related to the
expected 3:1 phenotypic ratio for each of the genetic loci that are involved.
What genetic phenomenon does each of the following ratios bring to mind? Note that in
some cases there may be more than one correct answer.
(a) 1:1, (b) 2:1, (c) 1:2:1, (d) 3:1, (e) 9:3:3:1, and (f) 1:1:1:1
What is test cross and what is its value? Describe monohybrid and dihybrid test crosses.
What is back cross? What is the significance of back cross in plant breeding?
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