Unit 1: Laws of Inheritance
Lesson 1.1
Mendelian Laws of Inheritance
Contents
Introduction
1
Learning Objectives
2
Warm Up
2
Learn about It!
An Overview of Genetics
Gregor Mendel and His Pea Plant Hybridization Experiments
Brief Background of Gregor Mendel
Pea Plant as Mendel’s Model Organism
Mendel’s Challenges and the Rediscovery of His Work
Review of Genetic Terminologies
Law of Segregation
Mendel’s Monohybrid Cross
Interpretations of the Monohybrid Cross
Using Punnett Squares
Law of Independent Assortment
Mendel’s Dihybrid Cross
Interpretations of the Dihybrid Cross
Laws of Inheritance and Gametogenesis
4
4
6
6
7
8
9
12
12
14
15
17
17
18
19
Key Points
25
Check Your Understanding
26
Challenge Yourself
27
Photo Credits
28
Bibliography
28
Key to Try It!
29
Unit 1: Laws of Inheritance
Lesson 1.1
Mendelian Laws of Inheritance
Introduction
What traits run in your family? What makes your family very distinct from that of your
friends and acquaintances? Have you also wondered why some features of your parents
are only present in some of your siblings? You may have inherited the complexion of your
mother, but have inherited the stature of your father. The hair type of your siblings might
also be different from both of your parents. You and your friends might even claim that
you have inherited the intelligence of your mother or father. Inheritance may also go
beyond our physical features. Some of you may probably be aware that a severe genetic
disorder runs in your family. It may be an increased risk for hypertension, obesity, or
diabetes, or a less lethal form, such as color blindness.
1.1. Mendelian Laws of Inheritance
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Unit 1: Laws of Inheritance
The diversity in the combinations of traits we have can be attributed to our sexual means
of reproduction. Because individuals from different sexes contribute half of their genetic
material, the resulting offspring is not genetically identical to either parent. Alongside the
increased variation, chance also comes into play. Different combinations of traits of the
mother and father will manifest in each of their children. In some cases, sets of characters
from both parents will not appear in their offspring. This chapter will discuss the very
foundation of the study and the underlying mechanisms of inheritance, tracing its roots
from the experiments of Gregor Mendel, the father of modern genetics.
Learning Objectives
In this lesson, you should be able to do the
following:
●
Explain the foundations and
DepEd Competency
Predict genotypes and phenotypes of
parents and offspring using the laws of
inheritance (STEM_BIO11/12-IIIa-b-1).
development of Mendelian genetics.
●
Describe and apply the Mendelian
laws of inheritance.
Warm Up
The Genetic Boat Is Sinking:
Survival By Chance
15 minutes
Look at your classmates. How do you think your class varies genetically? Which features do
you think your classmates inherited from their parents? How about you? What traits did you
get from your mother and your father? In this activity, the entire class will play The Boat is
Sinking but with a genetic twist.
Materials
●
paper lots with the genetic traits (accessible in the web link below)
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Unit 1: Laws of Inheritance
Procedure
1. You will be doing this activity as an entire class. Distribute yourselves around the
classroom. Note that you will play a game that is based on The Boat is Sinking.
2. The teacher shall access the link below before the class.
The Genetic Boat is Sinking: Human Genetic Traits Guide
Quipper Limited, “The Genetic Boat is Sinking: Human
Genetic Traits Guide,” (April 19, 2020),
https://drive.google.com/open?id=16tK8BezjmV66TvId1fCoFd
IpcKYLCsmb, last accessed on April 19, 2020.
3. Read the mechanics of the game carefully as follows.
a. The game shall consist of 10 rounds (or it may depend on class size).
b. At the beginning of each round, your teacher will describe a genetic trait
and its variations. For example, if the first trait is the widow’s peak, a
description will be provided first.
c. Make sure that you identify which of these traits that you have.
d. After giving the description, the teacher will specify the number of
“passengers” in a boat.
e. A signal will be given by the teacher. (Example: The boat is sinking. Group
yourselves into three and according to the presence of widow’s peak.)
f.
After the signal is given, you must group yourselves according to the number
and trait given. The groups that you choose must have similar traits based on
what was given by the teacher.
i.
For the earlier example, all of the three passengers in the boat shall
have a widow’s peak. Those without a widow’s peak should form a
group of three of their own.
g. All students who are not able to form full groups will be eliminated.
h. The game shall last for seven more rounds (with each round having a
different trait) or until there are three to four students remaining.
i.
Winners of the game may be given merit.
4. Look for a partner to discuss the answers to the guide questions below.
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Unit 1: Laws of Inheritance
Guide Questions
1. Which of the traits mentioned in the activity is/are genetic? What makes them
genetic?
2. Why do you think some of your classmates possess these traits while others do not?
3. Why do you think it is important for you to understand the genetic basis of biological
traits?
Learn about It!
An Overview of Genetics
Genetics is the subdiscipline of biology that focuses on heredity and genetic variation. In
biology, heredity, which is also called inheritance or biological inheritance, refers to the
transmission or passing down of traits from the generation of parents to their offspring.
Genetic variation, on the other hand, refers to the degree of difference in the
deoxyribonucleic acid or DNA among individuals of a population. As you can observe,
genetics is centralized on the study of genes or segments of DNA, how they are inherited,
and how different they are among the members of a species. Ultimately, the biological
diversity (as shown in Fig. 1.1.1) that we observe in nature is attributed to how genes have
been passed on and modified through time. The molecular aspect of genes and their
expression will be discussed in greater detail in the next unit.
Fig. 1.1.1. The different varieties of corn show genetic diversity.
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Unit 1: Laws of Inheritance
Genetics, just like any other discipline, has different subfields. Table 1.1.1. briefly describes
some of these subfields.
Table 1.1.1. Examples of fields under genetics
Field
Descriptions
Molecular genetics
Molecular genetics deals with the study of DNA and gene
expression and regulation.
Cytogenetics
Cytogenetics deals with the study of chromosomes and their
behavior during meiosis.
Population genetics
Population genetics focuses on how the forces of evolution
influence the frequencies of genes at the population level.
Transmission genetics
Transmission genetics deals with the different patterns of
inheritance.
Fig. 1.1.2. Transmission genetics is concerned with the possible outcomes of mating among
different plant and animal species. To determine these outcomes, one must understand
how genes behave to establish patterns or mechanisms of inheritance.
Transmission genetics will be the focus of this chapter. This field, being the oldest, is also
closely associated with classical genetics or Mendelian genetics. Transmission genetics
aims to make predictions about the outcomes of genetic crosses and matings (as in Fig.
1.1.2), as well as the possibilities of parents having children with traits that run in their
families.
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Unit 1: Laws of Inheritance
How did the experiments of Gregor Mendel lay the
foundation for the study of transmission genetics?
Gregor Mendel and His Pea Plant Hybridization Experiments
Brief Background of Gregor Mendel
Today, we recognize Gregor Johann Mendel (shown in Fig. 1.1.3) as the father of modern
genetics. Born in 1822 in Moravia located in the Czech Republic today (but was part of
Austria when Mendel was born), he grew up on a farm. He worked with his father to
improve the plants in their orchard by grafting trees, a technique wherein two vegetative
structures from two plants are joined together. Later on, he took the path to the priesthood
as he entered the Augustinian monastery of Saint Thomas and became a monk. He was
finally ordained in 1847. Thereafter, he decided to work as an educator. However, due to the
poor quality of his answers, he failed his exams in physics and natural history. Thus, he
enrolled at the University of Vienna to expand his knowledge in these areas. Finally, in
1856, he began his hybridization studies involving pea plants. His eight-year work on
experimental crosses was later recognized as a path-breaking achievement in the field of
biology.
Fig. 1.1.3. Gregor Mendel (1822–1884), the father of genetics, made vital contributions to
the science of heredity through his pea plant experiments at the Augustinian monastery.
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Unit 1: Laws of Inheritance
Pea Plant as Mendel’s Model Organism
Mendel’s experiments were actually borne out of his interest for ornamental flowers. He
even performed separate experiments involving honeybees. His study on garden peas or
Pisum sativum (shown in Fig. 1.1.4) brought him the greatest success in the field of
genetics.
B
A
C
Fig. 1.1.4. Garden pea or Pisum sativum is a type of legume. Its flowers may either be violet
(A) or white (B). Being a legume, its fruits (C) are elongated or pod-shaped, with seeds
attached to only one side of the fruit or pod wall.
Mendel chose Pisum sativum for various reasons, which are the following:
●
First, it exhibits more vigorous growth (approximately 70 to 80 days harvest period)
compared to other plants.
●
Second, these plants are easy to cross-fertilize. The flowers of peas contain both
male and female organs, particularly, the stamens and pistil. Obtaining pollen grains
from the anthers from one plant and delivering them to the stigma of another plant
will allow fertilization. If this is done between plants with different traits, then
hybridization is performed. The offspring of the mating, consequently, are referred
to as hybrids.
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Unit 1: Laws of Inheritance
●
Lastly, pea plants are also capable of self-fertilization through self-pollination. A
modified petal of the pea flower is capable of covering the reproductive structures.
This allows peas to naturally self-pollinate.
The mechanism of self-pollination was particularly important for Mendel because all of his
crosses begin with true-breeding plants or strains. If plants breed true for a particular trait,
then many generations of mating will never produce new traits. For example, if a pea
plant breeds true for violet flowers, repeated mating among these true-breeding
violet-flowered plants will never produce offspring with other flower colors.
Mendel’s Challenges and the Rediscovery of His Work
Gregor Mendel made an outstanding achievement when he was able to determine the basic
patterns of inheritance. One primary reason for this is that, during his time, people had no
idea that genes are found in the DNA. He had no clue that the genetic material contains
the instructions for the expression of biological traits. Predating today’s knowledge on
heredity were different theories to explain inheritance, but have all since been debunked:
●
One theory was pangenesis. This attempted explanation was proposed by
Hippocrates in 400 B.C.E. According to him, “seeds” are produced in different organs
of the body. When it is time for an individual to reproduce, all of the seeds will gather
to form the offspring.
●
Later on, the invention of the microscope allowed people to observe sperm cells.
This made them propose that sperm cells may bear a homunculus or “little man,”
which will eventually develop in the womb of the mother.
●
Lastly, another long-held explanation was the blending theory of inheritance,
which states that both parents will equally contribute to the genetic traits of their
offspring. Also, traits were considered to always blend every generation.
Mendel’s statistical and mathematical prowess, alongside his patience and perseverance,
made him refute all prior notions about inheritance. In 1866, Mendel was able to publish
the fruits of his work through the paper entitled The Experiments on Plant Hybridization.
However, his work was ignored even until he died in 1884. It was only in 1900 that his paper
was separately rediscovered by Hugo de Vries of Holland, Carl Correns of Germany, and
Erich von Tschermak of Austria (as shown in Fig. 1.1.5).
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Unit 1: Laws of Inheritance
A
B
C
Fig. 1.1.5. Three individuals, namely, Hugo de Vries (A, 1848–1935), Carl Correns (B,
1864–1933), and Erich von Tschermak (C, 1871–1962) rediscovered Mendel’s work.
What makes Pisum sativum an ideal model for
genetic studies?
Review of Genetic Terminologies
In the discussion of transmission genetics, several terminologies will be repeatedly used.
You should also understand them well to improve your skills in solving genetic problems.
Genes (as in Fig. 1.1.6) refer to the basic unit of heredity. Mendel termed these genes as
“unit factors” that control the expression of biological characteristics. A characteristic is
any heritable feature of an organism, which is under the control of a particular gene.
Different genes control flower color, hair color, and skin color, which are examples of
characteristics. The definite position that a gene occupies in a chromosome is called a locus
(plural, loci). A gene for a particular characteristic usually has different alternative forms,
which are called alleles. In the figure given below, two gene loci are given: one for the
height of a pea plant and another for the texture of its pods. If you look at the gene for pod
texture, two versions exist—one for wrinkled pods and another for smooth pods.
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Unit 1: Laws of Inheritance
Fig. 1.1.6. Genes and alleles in Pisum sativum can be accurately represented by a
homologous pair of chromosomes.
Two chromosomes are used to represent genes and alleles above. This is to emphasize that
our chromosomes, similar to most organisms, occur in pairs called homologous
chromosomes. The members of each pair originate from each of our parents. Thus, for
animals and most plants, two genes control the expression of a particular
characteristic. For us, humans, we inherit each of these genes from our mother and father.
Also, in Fig. 1.1.6, the individual has identical alleles for height (both alleles for tall). Thus,
this individual is said to be homozygous for height. If this individual happens to have
inherited an allele for “dwarf” trait from its parent 2, then its alleles will become different
from each other, making it heterozygous (or hybrid) for height. This is the case for pod
texture because of the non-identical alleles. These combinations of alleles that an individual
possesses at a certain locus, which can be homozygous or heterozygous, refer to its
genotype. The actual manifestation of a genotype into observable traits is called the
phenotype. You usually describe your classmates by enumerating their phenotypes. In the
given figure above, if the actual phenotype of the plant is a round seed, then the allele
for round seed is said to be the dominant allele. The other allele in which the expression is
masked (i.e., wrinkled allele) is called the recessive allele. Table 1.1.2 summarizes this
discussion of genetic terminologies.
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Unit 1: Laws of Inheritance
Table 1.1.2. Discussion summary for review of genetic terminologies
Elements
Descriptions
Gene locus 1
Pea plant height
Gene locus 2
Pea plant seed shape
Allele from parent 1 at locus 1
Tall allele
Allele from parent 1 at locus 2
Wrinkled allele
Allele from parent 2 at locus 1
Tall allele
Allele from parent 2 at locus 2
Round allele
Genotype at locus 1
Homozygous (tall, tall alleles)
Genotype at locus 2
Heterozygous (wrinkled, round alleles)
Phenotype
Tall, round seeded
The dominant allele for locus 1
Cannot be identified yet if homozygous
The dominant allele for locus 2
Round allele
How are the alleles of a gene transmitted from
parents to offspring?
In the pea plant hybridization of Gregor Mendel, seven characteristics were used, and
each of these characteristics exists in two alternative forms. These heritable traits include
plant height, flower color, the position of flower or inflorescence, the color of seeds, the
shape or texture of seeds, the color of pods, and the shape of pods. The alternative forms
for each are shown in Fig. 1.1.7.
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Unit 1: Laws of Inheritance
Fig. 1.1.7. Seven characteristics were used by Mendel to establish his laws of inheritance.
Law of Segregation
Mendel’s Monohybrid Cross
The core of the study of Mendel is the determination of the pattern in which the
characteristics mentioned above are inherited. The basic cross he performed was the
monohybrid cross. In this cross, only one characteristic is involved. However, the
parents should have different or contrasting phenotypes. For example, in Fig. 1.1.8, a
cross between tall and dwarf pea plants was made.
Note that in the monohybrid cross in Fig. 1.1.8., the initial individuals that are mated
comprise the P generation or the parental generation. Particularly, the P generation
consists of tall and dwarf parents. Note that P generations in all of Mendel’s crosses consist
of true-breeding or homozygous individuals. These parents, when cross-pollinated, will
give rise to the F1 generation or first filial generation, the offspring of P generation. In the
cross in Fig. 1.1.8., the F1 generation consists of only tall peas. Further, if F1 generation is
mated among each other, the F2 generation or second filial generation is obtained. In
other words, the offspring of the F1 generation is the F2 generation. In the above cross, 3/4
or 75% of the F2 are tall, while 1/4 or 25% are dwarf. Note that Mendel observed the same
ratio for the other six characteristics.
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Unit 1: Laws of Inheritance
Fig. 1.1.8. A monohybrid cross involving the height of pea plants will produce all tall
offspring in the F1 generation and 3/4 tall and 1/4 dwarf in the F2 generation.
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Unit 1: Laws of Inheritance
Interpretations of the Monohybrid Cross
Two major observations can be made from the monohybrid cross of Mendel. First, the F1
consists of only one phenotype, which is tall. Second, the dwarf phenotype reappeared
in the F2 generation. This implies that the tall trait is dominant over the recessive
dwarf trait. This is best explained by the Principle of Dominance. This states that when an
individual is heterozygous, the dominant allele tends to mask the expression of the recessive
allele. For you to understand better, let us establish notations to our monohybrid cross. Let
gene T code for height in the pea plant. This gene exists in two forms: the dominant T allele
for tall trait and recessive t allele for the dwarf trait. In the P generation, a TT (tall) parent is
crossed with a tt (dwarf) parent. In Table 1.1.3. below, note that the recessive allele is
masked in the heterozygous individuals in both F1 and F2 generations. Also, this explains
that the recessive phenotype is always true-breeding, whereas an individual with the
dominant phenotype may either be homozygous or heterozygous.
Table 1.1.3. The conceptual approach to Mendel’s monohybrid cross
Phenotypes
P
Tall × Dwarf
Genotypes
P
TT × tt
F1
Phenotypic Ratio:
100% or All Tall
F1
Genotypic Ratio:
100% or All Tt
F2
Phenotypic Ratio:
3/4 Tall : 1/4 Dwarf
F2
Genotypic Ratio:
1/4 TT: 2/4 Tt: 1/4 tt
Another major implication of the 3:1 ratio of the monohybrid cross is that the genes
segregate. According to the Law of Segregation, the two alleles of an individual segregate (or
separate) from each other during gamete formation. This process occurs at random. This
means that only one copy of the gene is present in each gamete or sex cell.
Consequently, the genetic makeup of the offspring will be determined by the alleles present
in the gametes that participate or fuse during fertilization. In Fig. 1.1.9 below, the tall parent
TT, will only produce gametes with the allele T, whereas the dwarf parent tt will only produce
gametes with the allele t. Thus, only one genotype is present in F1, which is Tt (tall hybrid).
The individuals of F1, by contrast, produce two types of gametes: one with allele T and the
other with allele t. Crossing among F1 yields all possible genotypes in F2 at a ratio of 1:2:1.
1.1. Mendelian Laws of Inheritance
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Unit 1: Laws of Inheritance
Likewise, both phenotypes are also seen in F2 at a ratio of 3:1.
Fig. 1.1.9. The law of segregation explains why the monohybrid cross will have a
characteristic phenotypic ratio of 3:1 in the F2 generation.
Using Punnett Squares
To apply your knowledge of the principle of dominance and law of segregation in analyzing
other crosses, we can use the Punnett Square. This method, which was devised by
Reginald Punnett, is a basic technique that can be used to represent the segregation of
gametes in the parents and the fertilization to produce the possible offspring. By
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Unit 1: Laws of Inheritance
using the example earlier, Table 1.1.4. shows separate Punnett squares for P (tall × dwarf)
and F1 generations (tall × tall). Note that the first rows and columns below the parent
genotypes represent the segregated gametes (sperm and eggs). Combining them will
give rise to the possible genotypes of the offspring. The steps are summarized as follows.
1. Write down the genotypes of both parents for each cross.
a. P generation:
TT, tt
b. F1 generation:
both Tt
2. Write down the possible gametes for each genotype. Then, draw an empty version of
the boxes below where you can write the gametes and offspring.
a. TT:
only T
b. tt:
only t
c. Tt:
T or t
Table 1.1.4. Punnett squares for P and F1 generations in a monohybrid cross
P generation Cross
F1 generation Cross
Tall (TT) × Dwarf (tt)
Tall (Tt) × Tall (Tt)
♂
( )
♀
( )
Tt
(tall)
Tt
(tall)
Genotypic Ratio: 100% or all Tt
Phenotypic Ratio: 100% or all tall
1.1. Mendelian Laws of Inheritance
Tt
(tall)
Tt
(tall)
♂
( )
♀
( )
TT
(tall)
Tt
(tall)
Tt
(tall)
tt
(dwarf)
Genotypic Ratio: 1/4 TT: 2/4 Tt: 1/4 tt
Phenotypic Ratio: 3/4 tall: 1/4 dwarf
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Unit 1: Laws of Inheritance
Law of Independent Assortment
Mendel’s Dihybrid Cross
Fig. 1.1.10. The dihybrid cross of Mendel, similar to the monohybrid cross, will yield one
phenotype in the F1 generation. However, the F2 generation consists of four phenotypes,
which occur in 9:3:3:1 ratio. Particularly, these phenotypes are 9/16 round, yellow: 3/16
round, green: 3/16 wrinkled, yellow: 1/16 wrinkled, green.
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Unit 1: Laws of Inheritance
In Mendel’s dihybrid cross, the same pattern applies as in the monohybrid cross. However,
this time, two characteristics or two pairs of contrasting traits are involved. For
example, a dihybrid cross may involve seed shape and seed color (as in Fig. 1.1.10). His
monohybrid crosses already revealed that round is dominant over wrinkled, while yellow
seeds are dominant over green ones. We can let gene R represent seed shape and gene Y
for seed color. To start the dihybrid cross (as shown in Fig. 1.1.10), the P generation must
have round and yellow seeds (RRYY) and wrinkled and green seeds (rryy). This cross will
yield that characteristic F2 phenotypic ratio of 9:3:3:1.
Interpretations of the Dihybrid Cross
The F2 generation of the dihybrid cross of Mendel has a characteristic phenotypic ratio of
9:3:3:1. This result is best explained by Mendel’s Law of Independent Assortment.
According to this law, the alleles from different genes are sorted into the gametes independently
of each other. This also implies that genes are inherited independently of each other. For
example, in the given cross above, the result of the dihybrid cross means that seed color
and seed shape are inherited independently. The inheritance of one gene does not
influence that of the other. To better understand this, see Table 1.1.5.
Table 1.1.5. Application of Punnett square to P and F1 generations of a dihybrid cross
P generation Cross
F1 generation Cross
round, yellow (RRYY) ×
wrinkled, green (rryy)
round, yellow (RrYy) ×
round, yellow (RrYy)
ry
ry
RY
RY
RY
RrYy
round,
yellow
RrYy
round,
yellow
RY
round,
yellow
RrYy
RrYy
Ry
round,
yellow
rY
round,
yellow
ry
round,
yellow
round,
yellow
F1 generation results
Genotypic Ratio:
100% or all RrYy
1.1. Mendelian Laws of Inheritance
round,
yellow
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
round,
yellow
RRyy
round,
green
RrYy
round,
yellow
Rryy
round,
green
rY
RrYY
round,
yellow
RrYy
round,
yellow
rrYY
wrinkled
yellow
rrYy
wrinkled
yellow
ry
RrYy
round,
yellow
Rryy
round,
green
rrYy
wrinkled
yellow
rryy
wrinkled,
green
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Unit 1: Laws of Inheritance
Phenotypic Ratio:
100% or all round, yellow
What are the allele combinations of
F1?
F2 generation results
Genotypic Ratio:
1/16 RRYY
2/16 RrYY
1/16 rrYY
2/16 RRYy
4/16 RrYy
2/16 rrYy
1/16 RRyy
2/16 Rryy
1/16 rryy
Phenotypic Ratio:
9/16 round, yellow
3/16 wrinkled, yellow
3/16 round, green
1/16 wrinkled, green
Is the law of segregation still applicable when two
genes are already involved? Why do you think so?
Laws of Inheritance and Gametogenesis
Both the law of segregation and the law of independent assortment provide the basic
mechanisms of the inheritance of traits. Note that both of these laws operate during the
anaphase I of gametogenesis (as shown in Fig. 1.1.11).
During segregation, the members of an allele pair separate as the members of
homologous chromosomes separate. Ultimately, each of these two alleles is distributed
randomly to each gamete. During independent assortment, at least two pairs of alleles
must be involved. In the figure below, if genes R and Y are said to be independently
assorting, they must be found on different chromosomes. This also implies that the
segregation of allele pair R and r is independent of the segregation of the allele pair Y and y.
1.1. Mendelian Laws of Inheritance
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Unit 1: Laws of Inheritance
Fig. 1.1.11. Both of Mendel’s laws of inheritance operate during the first anaphase of
meiotic division during gamete or sex cell formation.
How is the separation of homologous
chromosomes relevant to the laws of inheritance?
Did You Know?
The deposition of melanin in your skin, eye, and hair pigment is
controlled by the pair of alleles that follow the principle of
dominance.
1.1. Mendelian Laws of Inheritance
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Unit 1: Laws of Inheritance
For us to produce melanin, we need the enzyme tyrosinase, which
converts the amino acid tyrosine into melanin. Melanin is the
pigment that gives a dark color to our skin, and it is produced by
the cells called melanocytes. In fact, melanocyte overgrowth results
in the formation of moles in our skin. Tyrosinase gene is
dominant; thus, we only need one copy of it for us to have normal
skin pigmentation. If the tyrosinase gene becomes mutated, it
becomes
a
recessive
gene,
which
impairs
melanin
production—having two copies of the recessive gene for tyrosinase
results in the condition called albinism, the total lack of
pigmentation.
Albinism is characterized by the total lack of pigmentation of eyes, skin,
and hair. Eyes appear red because the light is reflected from the blood
vessels in the eyeball.
Let’s Practice!
Example 1
Brylle is fond of growing crops in his garden. One of the crops that he cultivates is the
garden pea (Pisum sativum). One strain of his pea plants is heterozygous for flower colors,
with genotype Mm. Another strain of his peas has smooth pods and axial flowers with
genotype AaBB. What are the gametes produced by each of these two plants with respect to
the indicated characteristics?
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Unit 1: Laws of Inheritance
Solution
Step 1:
You are asked to provide the types of gametes produced by the plants.
Step 2:
Write the given in the problem.
Plant 1 has a genotype of Mm.
Plant 2 has a genotype of AaBB
Step 3:
Identify the alleles produced by plants 1 and 2.
Plant 1: Mm
Gametes M and m.
Plant 2: AaBB
Gametes AB and aB.
Plant 1 (Mm) produces gametes with alleles M and m, while Plant 2 (AaBB) produces
gametes with allele combinations AB and aB.
1 Try It!
Nickson cultivated two different plants. The first plant is recessive for trait A, while
the second plant is homozygous dominant for trait B and heterozygous for trait C.
What are the allele combinations that can be produced by his first and second
plants?
Example 2
In pea plants, axial inflorescence is dominant over terminal inflorescence. If Laiza crossed a
parent plant that is heterozygous for inflorescence to another plant with terminal
inflorescence, what are the genotypic and phenotypic ratios of the offspring?
Solution
Step 1:
You are asked to provide the genotypic and phenotypic ratios of the cross.
Step 2:
The traits and the phenotypes of the parents are given.
The axial inflorescence is dominant over the terminal one in peas.
Then, we can assign alleles.
A
1.1. Mendelian Laws of Inheritance
:
axial inflorescence
22
Unit 1: Laws of Inheritance
a
:
terminal inflorescence
Parent 1 is heterozygous, while parent 2 has terminal flowers.
Then, we can assign genotypes.
Step 3:
Aa
:
Parent 1
aa
:
Parent 2
Draw a Punnett square and place the gametes of the parents.
A
a
a
a
Step 4:
Combine the gametes to form the genotypes of offspring.
You may also directly write the phenotypes.
A
a
a
Aa
(axial)
Aa
(axial)
a
aa
(terminal)
aa
(terminal)
The genotypic ratio of the cross is 1/2 AA: 1/2 aa. The phenotypic ratio is 1/2 axial: 1/2
terminal.
2 Try It!
If a parent pea plant that is hybrid for flower color is crossed with a plant that is
true-breeding for violet flowers, what are the genotypic and phenotypic ratios of the
F1 generation? Note that having violet flowers is dominant over having white flowers.
1.1. Mendelian Laws of Inheritance
23
Unit 1: Laws of Inheritance
Example 3
In pea plants, round seeds are dominant over wrinkled seeds, while the tall trait is dominant
over the dwarf trait. If you cross two plants that are both heterozygous for seed shape but
homozygous dominant for height, what are the expected genotypic and phenotypic ratios of
the offspring?
Solution
Step 1:
You are asked to provide the genotypic and phenotypic ratios of the cross.
Step 2:
The dominance of the traits, as well as the traits of the parents, is given.
Having round seeds is dominant over having wrinkled seeds. Also,
being tall is dominant over being a dwarf. We can assign the following.
A
:
round
B
:
tall
a
:
wrinkled
b
:
dwarf
Then, both parents are AaBB. The cross is AaBB × AaBB.
Step 3:
Determine the allele combinations of the parents.
Both parents have the same genotypes. They produce two gametes.
Gamete 1:
Step 4:
AB
Gamete 2:
aB
Draw a Punnett square and place the gametes of the parents.
AB
aB
AB
aB
Step 5:
Combine the allele combinations to form the genotypes of offspring.
You may also directly write the phenotypes.
AB
aB
AB
AABB
(round, tall)
AaBB
(round, tall)
aB
AaBB
(round, tall)
aaBB
(wrinkled, tall)
1.1. Mendelian Laws of Inheritance
24
Unit 1: Laws of Inheritance
The genotypic ratio of the offspring of the cross is 1/4 AABB: 2/4 AaBB: 1/4 aaBB. The
phenotypic ratio is 3/4 round tall: 1/4 wrinkled tall.
3 Try It!
Gene A codes for seed color, where having yellow seed is dominant over having a
green seed. Gene B codes for pod shape, where the smooth pod is dominant over
the constricted pod. Given the cross AaBB × AABb, what is the genotypic and
phenotypic ratio of the offspring?
Key Points
________________________________________________________________________________________________
● Genetics is the study of inheritance and variation in organisms. It has various
subdisciplines. Transmission genetics is the one that is particularly concerned
about the mechanisms or patterns of inheritance.
●
Gregor Mendel is the father of genetics. He performed experiments on garden pea
or Pisum sativum. This led him to formulate the laws of inheritance in his
publication, Experiments on Plant Hybrids.
●
Different genes control the expression of the characteristics of organisms. Each
gene exists in alternative forms called alleles.
●
In terms of expression, genes can either be dominant or recessive. According to the
principle of dominance of Mendel, in a heterozygous individual, the dominant
allele tends to mask the expression of the recessive allele.
●
Mendel’s monohybrid cross reveals the law of segregation. According to this law,
the alleles segregate during gametogenesis. This explains the characteristic 3:1
phenotypic ratio of F2 in monohybrid crosses.
●
Mendel’s dihybrid cross reveals the law of independent assortment. According
to this law, allele pairs from different genes separate independently during gamete
formation. This explains the characteristics ratio of 9:3:3:1 of F2 of dihybrid crosses.
1.1. Mendelian Laws of Inheritance
25
Unit 1: Laws of Inheritance
Transmission genetics serves as the pioneer field in genetics.
________________________________________________________________________________________________
Check Your Understanding
A. Determine the accuracy of each of the following statements.
Write true if the statement is correct and false if otherwise.
1. Transmission genetics deals with different patterns of inheritance.
2. The results of the hybridization experiments of Mendel received instant
recognition and acceptance two years after the publication of his paper.
3. Garden pea is an ideal model to study inheritance because it reproduces solely
through self-pollination.
4. Locus refers to the position of a gene in a chromosome.
5. The two alleles of the same gene are both acquired by one gamete.
6. Receiving different alleles for the same gene from each parent makes one
heterozygous for that gene locus.
7. An individual that is homozygous for a trait also breeds true for that trait.
8. A recessive allele can mask the expression of a dominant allele.
9. A cross between a tall, violet-flowered plant and a dwarf, white-flowered plant is
an example of a monohybrid cross.
10. The characteristic F2 phenotypic ratio of Mendel’s monohybrid cross is 3:1.
11. If the genes for seed shape and height of peas are independently assorting, then
they highly influence the inheritance of each other.
1.1. Mendelian Laws of Inheritance
26
Unit 1: Laws of Inheritance
12. The F2 generation is the offspring of the P generation.
13. Both the laws of inheritance operate during the first anaphase of meiosis.
14. The alleles for seed shape (round allele and wrinkled allele) should be found on
the same locus.
15. The characteristic F2 generation phenotypic ratio of Mendel’s dihybrid cross is
9:3:3:1.
B. Provide what is asked in each of the following items.
1. What are the alleles produced by an individual with genotype NN?
2. What are the alleles produced by an individual with genotype Bb?
3. What are the alleles produced by an individual with genotype Mmnn?
4. What is the genotypic ratio of the offspring of the cross AA × Aa?
5. What is the genotypic ratio of the offspring of the cross Bb × bb?
Challenge Yourself
Answer the following questions.
1. You crossed two true-breeding lines of violet-flowered and white-flowered peas. Is it
possible to establish a true-breeding line of the genotype found in the offspring of
your cross? Why or why not?
2. Would you automatically know the genotype of a recessive and a dominant
individual? Why or why not?
3. A nondisjunction is a rare event during meiotic division wherein the homologous
chromosomes fail to separate. If nondisjunction occurs in a chromosome pair, what
will be its effect on the gametes and the offspring in relation to the alleles present in
the chromosomes?
4. A pea plant with a dominant trait is crossed with a recessive individual. Their
offspring have only one phenotype. What do you think explains this outcome?
5. A plant with genotype MmNN is self-fertilized. What is the genotypic ratio of the
offspring?
1.1. Mendelian Laws of Inheritance
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Unit 1: Laws of Inheritance
Photo Credits
The Coral Reef at the Andaman Islands by Ritiks is licensed under CC BY-SA 3.0 via
Wikimedia Commons.
StThomasAbbeyBrno by No machine-readable author provided, Parmesan~commonswiki
assumed (based on copyright claims) is licensed under CC BY-SA 3.0 via Wikimedia
Commons.
Starr 081009-0043 Pisum sativum var. Macrocarpum by Forest & Kim Starr is licensed under
CC BY 3.0 via Wikimedia Commons.
Pisum sativum flowers J1 by Jamain is licensed under CC BY-SA 3.0 via Wikimedia Commons.
João Pedro - Albino Baby by Felipe Fernandes is licensed under CC BY-SA 2.0 via Flickr.
Bibliography
Brooker, J. Concepts of Genetics (1st ed.). New York, USA: McGraw-Hill Companies Inc., 2012.
Klug, W.S, and Cummings, M.R. Concepts of genetics (6th ed). Upper Saddle River, N.J:
Prentice-Hall. 2003.
Pierce, B. Genetics: a conceptual approach (8th ed). New York: W.H. Freeman. 2012.
Reece J., Taylor M., Simon E., and Dickey J. Campbell Biology: Concepts and Connections (7th
ed.). Boston: Benjamin Cummings/Pearson. 2011.
Snustad, D.P., and Simmons, M.J. Principles of Genetics (6th ed.). Hoboken, NJ: Wiley. 2012.
1.1. Mendelian Laws of Inheritance
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Unit 1: Laws of Inheritance
Key to Try It!
1. Plant 1, with genotype aa, can only produce gametes with allele a. Plant 2, with
genotype BBCc, can produce the gametes BC and Bc.
2. If we assign letter B for flower color, the genotypic ratio is 1/2 BB: 1/2 Bb, while the
phenotypic ratio is 100% or all violet.
BB × Bb
B
B
B
BB
(violet)
BB
(violet)
b
Bb
(violet)
Bb
(violet)
3. Given the allelic assignments, the genotypic ratio is 1/4 AABB: 1/4 AABb: 1/4 AaBB:
1/4 AaBb. The phenotypic ratio is 100% or all yellow seeded with smooth pods.
AaBB × AABb
AB
aB
AB
AABB
(yellow,
smooth)
AaBB
(yellow,
smooth)
Ab
AABb
(yellow,
smooth)
AaBb
(yellow,
smooth)
1.1. Mendelian Laws of Inheritance
29