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14) BASIC GENETIC CONCEPTS
Michel A. Wattiaux
Babcock Institute
WHAT IS GENETICS?
Genetics is a science that studies the
variation and transmission of features or
traits from one generation to the next. In
this definition, the word variation refers to
genetic variation; that is, the range of
possible values for a trait as it is influenced
by heredity. Heredity is the transmission of
traits from the parents to the offspring via
genetic material. This transmission takes
place at the time of fertilization in
reproduction—when the bull’s semen
unites with the cow’s ovum (egg) to
produce a calf with a unique genetic
makeup. Only identical twins have an
identical genetic makeup because they
come from one fertilized ovum that has
separated into two embryos during the
early phase of development.
WHAT IS ENVIRONMENT?
Environment is often thought of as an
animal's physical surroundings—light,
temperature, ventilation and other
parameters that may contribute to the
physical comfort of an animal. However, in
genetics, the word environment has a more
general meaning. The environment is the
combination of all factors, except the
genetic ones, that may affect the expression
of genes. For example, a cow’s milk
production is affected by age at calving,
season of calving, nutrition and many other
factors. Thus cows with similar or even
identical genetic makeups will produce
different amounts of milk when they are
subjected to different environments. For
example, the lactation performance of a pair
of identical twins will vary drastically if the
two calves are separated at birth and sent to
different countries. However, there may be
a great difference in milk yield between
these twins when they are placed on two
separate farms in the same area, each
having different management levels.
GENOTYPE AND PHENOTYPE
The genotype of an animal represents the
gene or the set of genes responsible for a
particular trait. In a more general sense, the
genotype describes the entire set of genes
inherited by an individual.
In contrast, the phenotype is the value
taken by a trait; in other words, it is what
can be observed or measured. For example,
the phenotype may be an individual cow's
milk production, the percentage of fat in the
milk or a classification score for
conformation.
There is an important difference between
genotype and phenotype. The genotype is
essentially a fixed characteristic of the
organism; it remains constant throughout
life and is unchanged by environmental
factors. When only one or a few genes are
responsible for a trait, the phenotype
usually remains unchanged throughout life
(e.g., hair color). In this case, the phenotype
gives a good indication of the genetic
composition of an individual. However, for
some traits, the phenotype changes
continually throughout the life of the
individual in response to environmental
factors. In this case, the phenotype is not a
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Dairy Essentials - Reproduction and Genetic Selection
reliable indicator of the genotype. This
usually occurs when many genes are
involved in the expression of a trait such as
milk yield. As a result, the milk yield of a
cow is often expressed as follows:
Phenotypic milk yield = G + E, where:
G is the genetic merit of the cow for milk
yield (the effect of the genes);
E refers to the effect of the cow’s
management and environment.
THE GENETIC MATERIAL
The genetic material is located in the
nucleus of each cell in the body. Except for
the reproductive cells (spermatozoa and
ova) and a few other exceptions (red blood
cells), cells contain two copies of an
animal’s complete genetic material. When
cells divide, the genetic material organizes
itself in a series of long threadlike
structures called chromosomes (Figure 1).
In body cells, each chromosome has a
counterpart that has the same length and
shape (except for the two chromosomes
that determine the sex) and contains genetic
information for the same trait. These two
chromosomes are two members of a
chromosome pair, one derived from the
sire, one from the dam. The number of
chromosome pairs is typical of a species
and it is usually abbreviated as the letter
"n." For example, in humans n = 23, in
swine n = 19 and in cows n = 30. Thus cells
in the bodies of humans, swine and cows
contain 2n = 46, 38 and 60 chromosomes,
respectively.
Genes are located along the
chromosomes.
A gene is the basic
functional unit of heredity; that is, it
contains the genetic information that is
responsible for the expression of a
particular trait. The entire length of a
chromosome can be divided into thousands
of these functional units, each responsible
for a particular trait.
A gene is composed of material called
deoxyribonucleic acid or DNA. The
function of the DNA is to carry the
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Figure 1: Chromosomes magnified
thousands of times
information necessary for the synthesis of
proteins. As proteins are synthesized and
DNA replicates itself, the number of cells in
the body increases (growth) and cells may
specialize into specific functions
(development) in which some genes are
turned on and others turned off. For
example, cells of the skin (a specialized
tissue) contain all the genetic material
needed to recreate an individual, but the
only specialized genes that are turned on in
these cells are the ones responsible for the
formation and color of hair.
TRANSMISSION OF GENETIC MATERIAL
Male or female
The testes of the bull and the ovaries of
the cow produce reproductive cells by a
special series of cellular divisions that halve
the normal number of chromosomes in a
cell. The spermatozoa and the ova contain
only one member of a chromosome pair.
Thus the cells of cows and bulls contain 60
chromosomes (2n = 60), but the
spermatozoa in the semen and the ova in
the ovaries contain only 30 chromosomes (n
= 30, Figure 2). The two basic principles of
the transmission of a trait (e.g., sex) are as
follows (Figure 2):
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14 - Basic Genetic Concepts
Male or female?
How are chromosomes transmitted?
Parents
(2n)
2n=60
2n=60
Testis
Ovary
Spermatozoa (n=30)
Ova (n=30)
X
Y
X
X
Division
(Meiosis)
Reproductive
cells (n)
Spermatozoa
X
Y
Ova
X
Fertilization
X
X
Female
Zygote
(2n)
2n=60
X
Y
Male
Offspring
Figure 2: Chromosomes are transmitted with reproductive cells that contain only half the
normal number of chromosomes for a species. Chance at the time of fertilization is responsible
for specific traits inherited by the offspring (e.g., gender).
1) Separation of the paired chromosomes
during the formation of reproductive
cells;
2) Union of a spermatozoon with an
ovum to create a new cell with a
unique set of chromosomes.
For 29 of the chromosome pairs, both
members are visually identical. However,
for one of the pairs, one member is much
longer; it is called the X chromosome, and
the shorter member is called the Y
chromosome. All the ova carry the X
chromosome, but the spermatozoa can
carry either the X or the Y chromosome.
During cellular division to form the
reproductive cells, each member of a
chromosome pair goes into a separate cell.
As a result, 50% of the spermatozoa will
carry the X chromosome and the other 50%,
the Y chromosome.
If by chance a
spermatozoon carrying a Y chromosome
fertilizes an ovum, the offspring will be a
male. However, an offspring that receives
two X chromosomes develops into a female
(Figure 2). It is important to realize that it is
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impossible to predict the sex of an offspring
at the time of mating (insemination);
however, we can predict that, on the
average, 50% of all the offspring will be
male and 50% will be female.
Qualitative traits
Qualitative traits tend to fall into discrete
categories. Usually just one or a few genes
have a major effect on qualitative traits.
Environment usually has a minor role in
influencing the category into which the
animal falls. In this case, the phenotype of
the animal reflects its genotype. Examples
of qualitative traits in dairy cattle are:
• Hair color;
• Hereditary defects such as dwarfism;
• Presence or absence of horns;
• Blood type.
Quantitative traits
Quantitative traits differ from qualitative
traits in two important ways:
1) They are influenced by many pairs of
genes;
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Dairy Essentials - Reproduction and Genetic Selection
2) The phenotypic expression is more
strongly influenced by the
environment than is true for
qualitative traits.
Many of the economically important traits
in dairy cattle are quantitative traits:
• Milk yield;
• Milk composition;
• Conformation (also referred to as
type);
• Efficiency of feed conversion;
• Disease resistance.
The combined influence of many genes
and the effect of the environment on
quantitative traits make it much more
difficult to determine the genotype
accurately than in the case of most
qualitative traits. Sometimes the animal's
phenotype tells us very little about its
genotype. For example, a lactation record
only tells a portion of the information about
the genetic merit of a cow for milk yield.
What makes the genotype of a cow
unique?
When ova are formed, they receive one of
the two members of a chromosome pair.
Thus a particular chromosome in an ovum
can be like either the first member or the
second member of the parental
chromosome pair. There are only two
different kinds of ova for that particular
chromosome.
If instead of one
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chromosome pair, we now consider two,
what does the total number of different ova
become? In other words, what is the total
number of possible chromosome
combinations?
This situation is like
flipping two coins at the same time. The
number of possible combinations is: two
possible values for the first coin times two
possible values for the second coin = 2 x 2 =
22 = 4 different possibilities. The number of
different genotypes for an ovum is four and
the probability of any particular
combination of chromosomes is 1/4. This is
also true for the number of possible
genotypes in male reproductive cells. Thus
when one out of four possible kinds of
spermatozoa fertilizes one out of four
possible kinds of ova, the number of
possible genetically different offspring is 4
x 4 = 16 (i.e., 22 x 22 ). Thus the chance for
any particular genotype in the newborn
offspring is 1/16.
When the 30 chromosome pairs of a dairy
cow are separated during the formation of
the reproductive cells and then reunited at
fertilization, the total number of possible
chromosome combinations is 230 x 23 0 =
1,152,900,000,000,000,000, each being
unique. With this number of possibilities
for each mating, it is easy to understand
why no two individuals are alike in a
population, even when they have the same
parents.
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