The topic of today's lecture is the
genetics of animal reproductive traits.
This lecture is part of module one,
Animal Genetics, which is part of the
ISAGREED project. This
presentation was supported by an Erasmus
plus KA2 Partnerships for
Cooperation grant. Innovating the
structure and content of curricula in the
management of animal genetic and food
resources using digitalization.
Reproduction is essential for all
organisms because it is the key to
survival and evolution.
Reproductive traits are important for the
survival of species not only in natural
populations but also in livestock
populations, where they have a major
impact on the economic efficiency of
breeders and farms.
Reproductive traits are economically
important for sustainable food
production. Reproductive
traits, especially those related to
fertility, litter size and pre weaning
viability, are important components
to reduce the cost of animal meat
production. Therefore,
great efforts are being made to identify
ways of improving these traits.
This improvement in reproductive traits
is related to different areas, with
management, nutrition and genetics having
a major impact.
Fertility is a fundamental biological
and utilitarian trait of livestock.
It is a prerequisite for the maintenance
of the species as well as the production
of animal products.
Fertility is generally understood as the
ability to produce reasonably numerous
and constitutionally fit offspring.
Fertility is subject to considerable
environmental influences, and a
distinction must therefore be made
between potential and actual fertility.
Potential fertility is to be understood
as the ability of the female to release
eggs capable of fertilisation, regardless
of what happens to them next. So it can
largely be understood as hereditary
fertility, which is difficult to detect
without examination. By the action
of various environmental factors, by
which we mean not only the internal
environment of both sexes. But
also the external environment, hereditary
fertility in the sense of the above
definition is considerably reduced, so
that by actual fertility is to be
understood the number of live born
offspring. Potential fertility can
therefore be considered as an expression
of the genotype, and actual fertility
as an expression of the fertility
phenotype.
The most significant indicators of female
fertility include length of gestation,
number of pups, litter percentage into
period, length, service period, and
difficulty of delivery. Male
fertility indicators may include sperm
count, incidents of sperm malformations
and lebo sexualis, which is related to
sex hormone levels. The
importance of different reproductive
traits is not the same and also varies
between species. In cattle,
low female fertility has a very negative
impact on production as it causes
economic losses by increasing the need
for additional inseminations, higher
veterinary costs, significantly longer
lactations and fewer carves.
Also, carving ease is closely related
to reproductive success and viability of
mother and offspring. In meat sheet
production, litter size and days to
estrus are two of the most important
traits, but their inclusion in selection
schemes has been limited due to low
heritability. In pig
breeding, as in other multipair species
such as rabbits, litter size is
probably the most relevant and easily
measurable reproductive trait that
receives the most attention. Litter
size is determined by the interaction of
many physiological components, such as
the number of ovulated eggs, which
determines the maximum number of possible
offspring and the rate of prenatal
survival. Age at first
following is further considered an
important trait in evaluating the
reproductive performance of mares, with
results across the entire equine breeding
system. As it indicates the
beginning of the reproductive life of
individuals. Age at last
fall is a reproductive trait that is
considered to be a measure of the
cessation of reproductive activity of
individuals, thus indicating the
reproductive life of the population.
However, equine reproductive parameters
related to fertility are complex traits
influenced by a wide range of genetic,
environmental and management factors,
making it difficult to identify the
underlying factors directly related to
the animals themselves.
Factors affecting reproductive traits.
The reproductive traits of livestock are
influenced by internal and external
factors. Internal factors
are related to the genotype of the
animal, while external factors are
related to its environment.
Ovulation rate and litter size are
important reproductive traits in
livestock that have high economic value
and are genetically controlled.
Genetic improvement can be achieved by
identifying polymorphisms in genes
associated with reproductive traits.
Non genetic factors can be divided into
those that have measurable effects such
as parity, maternal weight and
carving period, and those that do not
have measurable effects such as
infections. Another external
factor affecting reproductive traits is
variation in photoperiods, which
control melatonin secretion.
Quantitative Genetics.
Reproductive traits are conditioned by
polygenes and their inheritance is dealt
with in quantitative genetics.
Quantitative genetic studies. The
inheritance of complex phenotypic traits.
Its essence is the study of phenotypic
variation between individuals,
focusing primarily on traits that take on
a continuous range of values.
Any phenotypic expression that has
different values in different individuals
and does not follow a pattern of simple
Mendelian inheritance can be referred to
as a quantitative trait.
Quantitative Traits. Are
conditioned by a large number of small
effect genes, otherwise called
polygenes. Environmental
factors have a large influence on the
variability of the traits in question.
They have smooth, uninterrupted
continuous variability that can be
objectively measured. In a
sufficient number of individuals, they
tend to have a normal distribution of
phenotypic classes and are ch
aracterised by a Gaussian curve.
Their variability is expressed in units
of measurement and evaluated by
statistical methods.
Quantitative traits are measurable. We
express the phenotype numerically and
their variability can be characterised
using statistical quantities.
With statistical quantities are
important. The mean?
The variance or covariance?
The main error. The standard
deviation and the coefficient of
variation.
Genetic parameters of quantitative
traits. To study quantitative
genetic variation, statistical methods
introduced by Fisher and Wright are used
to analyse the variance components of
phenotypic variation and describe the
similarity between related individuals,
often without knowledge of the genes
involved. Genetic parameters
of quantitative traits can be divided
into two groups into.
One primary genetic parameters,
which we estimate based on biometric
analysis of the population. They
represent the individual components of
the variance partitioned according to the
source that caused the variance.
These include. Genetic
variants. Environmental
variants. Covariance
The joint variability of two given
traits. Two
secondary genetic parameters are
calculated as functions of primary
genetic parameters. These
include. Heritability.
Repeatability. And the
genetic, environmental and phenotypic
correlations.
Decomposition of phenotypic variants.
When assessing the heritability of a
quantitative trait, it is essential to
determine to what extent the trait is
genetically determined and to what extent
it is environmentally determined.
The phenotype as an external
manifestation of a trait is the result of
the action of complex relationships
between the genotype and the environment.
The phenotypic value of an individual
is the sum of its genotypic value G
and its environmental value east.
P = G + E.
The variance of a phenotype which is
defined as GE is
characterised by the following
relationships. The value
V represents the variance of phenotypic
values. VG represents the variance of
genotypic values. V represents the
variance of environmental variance.
And represents the covariance between
genotype and environment.
Heritability. The most
common summary statistic in quantitative
genetics is the calculation of
heritability. Heritability
is a measure of the contribution of the
genotype to the phenotypic re
lisation of a atrait. It
determines the proportion of the
heritable component in the final
phenotypic expression of a trait.
Heritability refers only to the
population and environment in which they
were measured. Thus, the same
trait measured in different populations
may have different heritabilities.
The mathematical expression of
heritability is the coefficient of
heritability H2, which is the
ratio of genetic variation expressed by
the variance value to the total
phenotypic variation also expressed by
the variance value. Heritability
coefficient can be expressed in two ways
this. Heritability
coefficient in the broad sense.
Includes all components of genotypic
variants. Expresses the
extent to which the phenotype of an
individual is determined by the genotype.
Heritability coefficient in the narrow
sense. Includes the
component of genotypic variants caused by
the additive effect of alleles.
Expresses the extent to which the
phenotype of individuals is determined by
two independent sets of genes that they
acquired from their parents. It
determines the degree of similarity
between relatives. The
coefficient of heritability thus
determines the degree of similarity
between related individuals corresponding
to the general breeding value, the
proportions due to dominance, D and
interactions. I indicate
only how much certain trait combinations
increase or decrease the value of the
additive effect. The coefficient of
heritability is the most important
genetic parameter for populations of
quantitative traits. It expresses
the extent of variability over which a
population can be influenced.
It is a measure of the efficiency of
selection.
The values of the heritability
coefficient. Heritability
coefficient values vary from zero to
one or from zero to
100%. What is
important, however, is how the
heritability coefficient value is
practically understood in a particular
case, and what it is then used for in the
selection technique. A
heritability coefficient value close to 1
indicates that most of the observed
variation is due to variation in the
average effects of different alleles.
Conversely, while low heritability means
that VA is small, it tells us little
about VG because the genetic effects may
be largely non additive D and I.
This means that even a trait with the
value of coefficient of heritability
equals zero may still have considerable
genetic variation at locky,
contributing to the observed trait
variability. In general,
the higher the value of the heritability
coefficient I the more
heritability contributes to the overall
variability, the simpler the selection
methods we can use.
Conversely, the lower the value of the
heritability coefficient, IE the more
environmental factors contribute to the
variability of the trait, the more
complicated the selection methods we have
to choose. In practise,
this means that in the case of traits
with high heritability, we can expect the
offspring to have roughly the same
development as the parents for that
trait. The resulting value
of the heritability coefficient is
influenced by. The degree of
variability of the trait under study.
The degree of diversity of genotypes in
the population of interest.
And the degree of diversity of breeding
conditions.
Repeatability. When observing
quantitative traits and characteristics,
it is typical that many of them can be
observed and measured repeatedly in the
same individual throughout his or her
lifetime. Repeatability
refers to the degree of similarity in the
values of repeated traits or
characteristics in a given population.
Repeatability of production traits in
organisms is divided into four groups for
selection and breeding purposes.
Repeatability over time expresses the
extent to which the same trait is
repeated at different ages in the life of
an individual. Partial
repeatability expresses the extent to
which the same performance is repeated at
different periods and over the whole
period of production. Spatial
or topographic repeatability determines
the extent to which the same trait is
repeated on different parts of the animal
's body. Paratype
repeatability expresses the extent to
which the same performance is repeated in
different environments.
Repeatability coefficient Rop.
The measure of repeatability is expressed
numerically by the repeatability
coefficient ROP. The
coefficient of repeatability indicates
the proportion of variability caused by
genetic differences out of the total
phenotypic variability. It
represents the maximum level of
heritability, highlights the additive
component of the genotype, and
expresses the extent to which the same
performance level of performance is
repeated. The higher the value of
ROP, the higher the probability of the
same performance, so the trait is
genetically determined to a higher
degree. It includes the whole
component of genotypic variability,
but also variance conditioned by the
effect of the permanent environment and
variance conditioned by the interaction
between the permanent environment and the
genotype.
Heritability of reproductive traits.
Reproductive performance in both males
and females is the basis of profitable
livestock production systems.
Some of the variation in reproductive
traits is under genetic control, which
means that reproductive traits are
heritable and can help to extend
genetics within a population by improving
the intensity of selection and shortening
the generation interval. The
heritability of reproductive traits is
low to moderate, so that an extensive
record of data is needed to identify
specific selectable traits.
Male reproductive traits, including
ejaculate volume and progressive sperm
motility are moderately heritable
and could be used in selection
programmes. In
contrast, female reproductive traits are
low heritability and are currently the
limiting factor for genetic improvement.
Litter size and parturition interval are
the main constraints to increasing the
intensity of selection and shortening the
generation interval in a breeding
programme.
The following slides show the
heritability coefficients for selected
parameters for males and females of each
livestock species.
Estimation of heritability of bovine
reproductive traits. Males.
Estimation of heritability of bovine
reproductive traits. Females.
Estimation of heritability of
reproductive traits in pigs.
Estimation of heritability of
reproductive traits in sheep.
Estimation of heritability of
reproductive traits in horses.
Thanks for your attention.