INTRODUCTION: The work of Gregor Mendel, discovered in 1900, is a landmark in biology and marks
the birth of the science of genetics. Mendel’s studies with the pea plant enabled him to formulate two
principles, known as Mendel’s first and second principles. Mendel’s work is of the greatest significance to
biology since it firmly established the CONCEPT OF HEREDITY. According to this idea the hereditary
factors behave as if they were some sort of units which retain their individuality and persist from one
generation to the next. This picture of the genetic material replaced the previous BLENDING
CONCEPT, the idea that hereditary factors are some sort of fluids (blood for example) which come
together, blend, and lose their original identities.
Mendel knew nothing of chromosomes, genes, mitosis or meiosis. However, his explanations of
his results from breeding the pea plant sound as if he knew about such things. In order for us to grasp
Mendel’s principles, we must become familiar with several basic terms. We will define these, making use
of Mendel’s observations as well as information which has accumulated since his time.
For our purposes, we can think of a GENE as a unit of heredity which is concerned with a
particular function- Knowing nothing about genes, Mendel spoke of “factors.” We know today that a gene
occupies a given position on a chromosome, which is called the LOCUS of the gene. You will recall from
the exercise on meiosis that chromosomes occur in homologous pairs. This means that in a body cell, two
doses of a particular gene are typically present. For example, Mendel studied pea plants which fall into
two classes on the basis of height: tall and dwarf. Each plant carries in its body cells two doses of a gene
for height. This is so since body cells are diploid or 2N.
Let us represent the gene form for tallness by the letter “T” and the one for dwarfness by the small
letter “t”. * Any one plant, being diploid, can have a “T” or a “t” at the locus for height on either one of
the two homologous chromosomes which carry the gene for height. Therefore, any one plant may be TT,
Tt or tt with respect to its genetic makeup.
Mendel found that when a plant has the combination of factors (Tt) it is tall. We say that the
genetic factors or gene form “T” is DOMINANT to the one for dwarf, “t”. We say this because the gene
form “T” (for tallness) expresses itself when the contracting gene form “t” (for dwarfness) is also present.
The gene form “t” is said to be RECESSIVE, since it does not express itself when the contrasting form
“T” is also present. Mendel’s work demonstrated that the gene for height in the pea plant can exist in two
contrasting forms. We now call contrasting forms of a gene, ALLELES. We see here in our example that
the gene for height, which is found at a particular locus on a given chromosome, exists in tow forms or
alleles. The allele “T” for tallness in the pea is dominant to the allele “t” for dwarfness. (It is important to
realize that height in the human is inherited in a very different way from height in the pea.)
We noted above that any one pea plant may have any one of three combinations of the alleles for
height: TT, Tt, or tt. Those plants which are TT or Tt are tall since tallness is dominant to dwarf. A dwarf
plant must have the genetic constitution “tt” since the allele “t” can be expressed only when the
contrasting allele, “T”, is absent. Therefore, when we look at a plant, we may get some idea on the kinds
of alleles it carries in respect to height. The term PHENOTYPE refers to any feature of an individual
which we can detect or describe. We can say that pea plants fall into two phenotypes on the basis of
height: tall and dwarf. When we see that a plant is dwarf, we immediately know that it must carry two
doses of the recessive allele “t”. We use the term GENOTYPE to refer to the genetic constitution of any
individual. Therefore, the genotype of a dwarf plant must be “tt”. However, the genotype of any tall plant
may be either “TT” or “Tt”. When any dominant trait such as tallness is being expressed, we cannot be
sure on the genotype simply by observing the phenotype. More information is needed. This information is
usually derived from the phenotype/genotype of parents and offspring.
We say that a plant of genotype “TT” is HOMOZYGOUS in respect to the gene for height. It has
the same gene form (allele) represented two times. Similarly, any dwarf plant is homozygous since it must
have the recessive allele “t” represented twice. An individual whose genotype “Tt” is said to be
HETEROZYGOUS since it carries two contrasting forms of the gene for height. Therefore, any
individual may be homozygous or heterozygous with respect to any given gene. This is true for height in
the pea plant, eye color in humans, fur color in rats, etc.
Let us now follow a cross in peas between a homozygous tall plant (TT) and a dwarf plant which
must be homozygous for the recessive (i.e., “tt”). In any cross or mating, the first set of parents is called
the “P1? or “FIRST PARENTAL GENERATION.” According to MENDEL’S FIRST PRINCIPLE
OF SEGREGATION, any individual carries two doses of a given gene in the body cells. This is so since
each individual received one set of chromosomes from the female parent and one form the male parent
and is therefore diploid (2N). Although an individual is diploid in respect to body cells, the gametes are
haploid (N). In our study of meiosis, we learned how this reduction in chromosome number comes about.
Therefore, any tall plant which is homozygous (TT) can produce sex cells of only one type. They must all
carry one dose of the allele “T” since meiosis separates homologous chromosomes and reduces the
chromosome number by one half. Similarly, the gametes of the dwarf plant must all be type “t” since the
dwarf is homozygous for the recessive allele (tt).
In his first principle, Mendel stated that hereditary factors exist in pairs and that at some time
previous to the formation of sex cells the two members of a pair separated from each other. As a result of
this separation or segregation, only one member of the pair of factors enters a sperm or an egg. Let us
assume in our cross between tall (TT) and dwarf (tt) plants that the tall parent is the female. This means
that all the eggs from this individual will be type “T”. The sperms from the dwarf parent will all be type
“t”. According to Mendel’s first principle, the two factors unite at fertilization, and the double number
(2N) is restored. However, in this generation (the “F1? or “FIRST FILIAL GENERATION”) all the
offspring are heterozygous in respect to height. The phenotype is tall, but the genotype is “Tt”. We would
not be able to tell this simply by looking at the plants. However, we know this is the case here because
one parent was homozygous (pure breeding) for tallness and the other was homozygous (pure breeding)
Let us now follow the F1 generation further and cross two F1 plants. A cross of two F1 plants
gives rise to the “F2? generation (“SECOND FILIAL GENERATION”). We know that the genotype of
the plants in the F1 is “Tt”. Mendel’s first principle again applies and tells us that since both parents are
“Tt”, each can produce two different kinds of gametes. Half of them will carry allele “T”, and half will
carry “t”. This makes perfect sense since we know that at meiosis homologous chromosomes separate
from each other. The chromosome carrying the allele “T” will go to one pole at anaphase I of meiosis.
The chromosome with “t” will travel to the other. Therefore, any parent who has the genotype “Tt”
produces two classes of gametes: “T” and “t”. These gametes can come together in three different
combinations at the time of fertilization: TT, Tt and tt. The checkerboard or Punnett square method is an
easy way to combine different kinds of gametes in all possible ways. In this method we put the gametes
from one parent on the top of the square and the gametes from the other parent on the side. When we
bring the single gametes together in each square, we see that we get: TT, Tt and tt. There gives a ratio of
3:1 (3 tall: 1 dwarf) for the phenotype. Note, however, that the ratio of genotypes is 1:2:1 [1 homozygous
for the dominant (TT): 2 heterozygotes (Tt): 1 homozygous for the recessive (tt)].
This 3:1 phenotypic ration is known as MENDELIAN MONOHYBRID RATIO. It means that
the phenotypic ratio will be 3:1 in the next generation when two heterozygotes are crossed and when just
one pair of alleles is involved. It must be pointed out, however, that this is true only when one allele, such
as “T” for tallness is dominant to the other allele such as the recessive “t” for dwarf. Mendel never stated
that dominance must always be the case. Today we know that only rarely is one allele absolutely
dominant to the other. We realize that upon close examination heterozygotes usually prove to be
somewhat different from those individuals which are homozygous for the dominant allele. Actually, in
many cases, there is no dominance operating at all. For example in the common garden flower, the 4
o’clock, red flower color (“R”) shows no dominance to white (“r”). Red flowered plants must possess
genotype “RR”, and white ones must have genotype “rr”. This is so since the heterozygote “Rr” is pink.
Lack of dominance in no way -violates Mendel’s concept that the hereditary factors are particulate. The
allele “R” and the allele “r” in the 4 o’clock remain the same even though they are present together in the
hybrid pink plant “Rr”. Such a pink plant can pass whiteness (r) or redness (R) down to the following
Now that we have these basic principles in mind, let us cover the following questions which
demonstrate aspects of Mendel’s first principle and the fundamentals of genetics. Answer all questions on
a clean sheet of paper. Be sure to show all your work.
1. In tomatoes, round fruit (R) is dominant to oblong (r). Write the genotypes of the following plants.
(a) a plant from a homozygous round fruited stock
(b) any plant with oblong fruit
(c) a plant with round fruit which had one parent with oblong fruit
(d) a plant with oblong fruit whose parents both produced oblong fruit
(e) a plant with round fruit whose parents both produced round fruit
2. Using information on fruit shapes in Question 1, give the genotypes and phenotypes to be expected in
regard to fruit shape from each of the following crosses:
(a) homozygous round x heterozygous round
(b) heterozygous round x heterozygous round
(c) oblong x oblong
(d) heterozygous round x oblong
3. In genetics, a cross of any individual to one which is expressing the recessive trait is known as a
TESTCROSS. It is important to remember this when doing genetics problems since test crosses are very
(a) What would be the result of a test cross using plant “a” from question #1?
(b) What would be the result of a test cross using plant “b” from question #1?
(c) What would be the result of a test cross using plant “c” from question #1?
(d) Can a testcross give you any information on plant “e” from question #1? Explain.
4. Two short haired female cats are mated to the same long haired male. Several litters are produced.
Female #1 produced 8 short haired and 6 long haired kittens. Female #2 produced 24 short haired kittens
and none with long hair. From these observations, what deductions can you make concerning hair length
in these animals? Represent the pair of alleles which is involved by the letters “A” and “a”. Give the likely
genotypes of the two female cats and the male cat.
5. In the garden 4 o’clock, “R” represents the allele for red flower color, and “r” stands for the allele for
white flower. There is no dominance involved. Consequently the heterozygotes are pink. Give the
genotypes of the following plants:
(a) a white flowered plant which had two pink parents
(b) a red flowered plant which had two pink parents
(c) a pink flowered plant which had a white parent
6. Give the expected results of crossing the following 4 o’clocks:
(a) a plant with white flowers and a plant with pink ones
(b) a plant with red flowers and a plant with pink ones
(c) two plants with pink flowers
7. In cattle, there is no dominance between the alleles for red coat (W) and white coat (C). The
heterozygote is intermediate or “roon” in color.
(a) give the genotypic and phenotypic ratios to be expected following a mating between two roon
(b) What are the expected genotypic and phenotypic ratios from a cross between a roon animal and a
8. In cattle, the hornless condition (H) is dominant to that for the possession of horns (h).
(a) A horned bull is mated to a hornless cow which is heterozygous. What kind of offspring are to be
expected and in what ratio?
(b) If the cow is next mated to a hornless bull which is also heterozygous, what is he chance that the
first calf will have horns?
(c) Assuming that the first calf has horns, what is the chance that the second calf will be hornless?
9. In the human, the alleles for normal hemoglobin (S) and for sickle cell hemoglobin (s) show a lack of
dominance. A blood examination can detect the healthy, heterozygous person since a certain percentage
of the cells of the heterozygote will show sickling. A person homozygous for sickle cell hemoglobin
suffers from the serious disease sickle cell anemia.
(a) A man whose cells show no sickling marries a healthy woman who is found to have a certain
percentage of sickle cells. What is the chance that they will have a baby with sickle cell anemia?
(b) The woman later marries a healthy man whose blood reveals sickle cells upon examination. They
produce three healthy children. What is the chance that the fourth child will have sickle cell
(c) What is the chance that any of the three healthy children carries the allele for sickle cells?
10. Assume that medical science finds a treatment for sickle cell anemia. When started in infancy, it
permits an otherwise doomed person to live a normal life without any serious effects of the anemia.
Suppose that two such persons who have been saved from the anemia later marry each other. What will be
the genotypes to be expected among their children? Would the children require treatment? Explain.
11. Very little hair is found on a Mexican hairless dog. A cross between a Mexican hairless and a dog
with typical coat usually produces litters of pups in which half of the animals are hairless and half have
hair. On the other hand, a cross between two Mexican hairless dogs produces litters in which two-thirds of
the pups are hairless and one third has hair. However, in addition to these surviving puppies, some pups
are usually born dead. Those dead pups are hairless and occur in about the same frequency as the live
pups with hair. Offer an explanation for these observations. Using the letters “M” and “m” to represent
the genotypes of the animals, diagram the crosses mentioned in this discussion.
12. In Guinea pigs, black fur (“A”) is dominant to the albino condition (“a”) which produces white fur. A
female from a homozygous black strain carries ovaries transplanted from an albino female. The albino
female has received in turn the ovaries from the homozygous black animal.
(a) What are the expected results when the black female is mated to a black male which has a white
(b) What results are to be expected if the albino female is mated to the same male? Diagram the cross.
(c) Suppose the black female used in the transplant study had a white parent. Would this change your
answers to parts (a) and (b) above? Explain.
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