Introduction to Genetics: Definition and significance of genetics. Historical milestones in the field of genetics. Central Dogma of Molecular Biology: DNA replication. Transcription and RNA synthesis. Translation and protein synthesis. Genetic Material: Structure of DNA and RNA. Genetic code and codons. Mendelian Genetics: Principles of inheritance (laws of segregation and independent assortment). Punnett squares and genetic crosses. Terms: genotype, phenotype, homozygous, heterozygous. Non-Mendelian Inheritance: Incomplete dominance. Codominance. Polygenic inheritance. Chromosomes and Cell Division: Overview of mitosis and meiosis. Chromosome structure and organization. Sex chromosomes and sex determination. Genetic Variation: Mutation types (point mutations, insertions, deletions). Causes of mutations (chemical, radiation, genetic). Genetic Disorders: Single gene disorders (e.g., cystic fibrosis, sickle cell anemia). Chromosomal disorders (e.g., Down syndrome, Turner syndrome). Multifactorial disorders and gene-environment interactions. Human Genome Project: Purpose and goals. Achievements and implications for medicine. Molecular Genetics: DNA sequencing techniques. Recombinant DNA technology and genetic engineering. Genetic Counseling and Testing: Purpose and process of genetic counseling.

1. INTRODUCTION TO
GENETICS AND PRINCIPLES
OF BREEDING
Dr. Ssuna Paul
1

2. INTRODUCTION
• Genetics: is a branch of Science concerned with
heredity and variation. It involves the study of cells,
individuals, their offsprings and the population
within which organisms live.
• Definition of Breeding: An activity of exploiting genetic
(heritable) endowment or those characters of individuals
which can be passed on from generation to generation in
the population to improve that particular populations’
ability to satisfy human endeavors.
2

3. Branches of genetics
• Transmission genetics: identifying the genes that
affect a particular characteristic, and also the
patterns by which these genes are transmitted from
generation to generation, or from cell to cell.
• Molecular genetics: focuses on the structure and
function of the genetic units, i.e, the chemical
composition of genes and their expression in
determining the structure of proteins, the most
important functional components of cells.
• Population genetics: analyses the pattern of
distribution of genes in populations of organisms,
and changes in the genetic structure of populations.
3

4. Common terminologies used in
genetics
• Gene: The smallest functional
segment of inheritance on the DNA
thread that is usually responsible for
one trait or character.
• Chromosomes: dense DNA-protein
complexes. Each animal species has
a specific number of chromosomes
e.g. humans have 46, Horse 64,
Donkey 62, Cattle 60, Swine 38,
Sheep 54, Goat 60.
• Genome: Total complement of
genes in an organism or cell.
4

5. Common terminologies used in genetics
• Allele - an alternate form of a gene at the same point
on each one of the pairs of Xsomes e.g. Tt. Usually
there are two alleles for every gene, sometimes as
many as three or four.
• Homozygous - when the two alleles are the same e.g.
TT or tt.
• Heterozygous - when the two alleles are different, in
such cases the dominant allele is expressed.
• Dominant- a term applied to the trait (allele) that is
expressed regardless of the second allele.
• Recessive - a term applied to a trait that is only
expressed when the second allele is the same (e.g.
short plants are homozygous for the recessive allele).
• Complete dominance- when both the heterozygotes
and dominant homozygotes cannot be distinguished
phenotypically. That is, they have the same phenotypic
value.
• Genotype - the allelic composition of an organism.
• Phenotype - the physical expression of the allelic
composition for the trait, which is a sum of genotypic
value (G) and the environmental deviation (E): P= G+E
• Testcross: A genetic cross between a homozygous
recessive individual and a corresponding suspected
heterozygote to determine the genotype of the latter.
5

6. Mendelian genetics
• Mendel demonstrated that traits were passed from
each parent to their offspring through the inheritance of
genes (using his peas experiments).
• Mendel's work showed:
– Each parent contributes one factor of each trait shown in
offspring.
– The two members of each pair of factors segregate from each
other during gamete formation (Meiosis).
– The blending theory of inheritance was discounted i.e. a
mixture of sperm and egg did not result in progeny that were a
"blend" of two parents' characteristics: RED + WHITE = PINK
– Males and females contribute equally to the traits in their
offspring.
– Acquired traits are not inherited. 6

7. Mendel’s laws of inheritance
• Mendel’s first law: It states that two members of a
gene pair segregate from each other into the gametes,
so that half of the gametes carry one member of the
pair and the other half of the gametes carry the other
member of the pair.
• Mendel’s second law (Law of independent
assortment):It states that gene pairs assort
(recombine) independently during gamete formation.
• Mendel's third law (law of dominance) states that one
of the factors for a pair of inherited traits will be
dominant and the other recessive, unless both factors
are recessive.
7

8. Hybrid
• Hybrid= offspring of parents genetically pure
for different characters/traits.
– Monohybrid cross is an offspring of two pure
breeding/true breeding parents for one
trait/character.
– Dihybrid cross is an offspring of two pure
breeding/true breeding parents for two
traits/characters.
– Heterosis/Hybrid Vigour: the tendency of a
crossbred individual to show qualities superior to
those of both parents
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9. Monohybrid cross
(can be shown using a tree method or punnet square
mtd
9
Parentalphenotype: Tall Short
Parentalgenotype(2n): TT X tt
Gamete(n) T T t t
F1Genotype(2n): Tt Tt Tt Tt
Phenotype: AllTall
P2phenotype: Tall Tall
P2genotype(2n): Tt X Tt (Selfing)
P2gamete(n) T t T t
F2Genotype(2n): TT Tt Tt t t
F2Phenotype: 3Tall 1Dwarf (3/4tall:¼dwarf)

10. Dihybrid cross
(seen better when using punnet square mtd)
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Round(R)isdominanttowrinkle(r),and
Yellow(Y)isdominanttogreen(y)
Parentalphenotypes: Round-yellow wrinkled-green
Parentalgenotypes(2n):
Gamete(n):
RRYY
RY
X rryy
ry
F1Genotype(2n): RrYy (Round-yellow)
R r
Y RY rY
y Ry ry
F1heterozygoteplantswereselfpollinatedtoproduceF2generationfromfourkindsof
gametes.
F1cross: RrYy X RrYy
Gametes(n) RY Ry rY ry
Note: Segregation of alleles (R, r, Y, y) and their independent assortment
(recombination) result to RY, Ry, rY and ry which are four possible arrangements of
allelesineachofthemaleandfemalegametes.

11. Dihybrid cross ctd
11
Male/Female Sperms
Eggs
RY Ry rY Ry
RY RRYY
Round-yellow
RRYy
Round-yellow
RrYY
Round-yellow
RrYy
Round-yellow
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
Genotypic
1/16 RRYY
2/16 RRYy
Phenotypic ratios
9/16 Round-yellow
Ratios
9
2/16 RrYY
4/16 RrYy
(R- Y-)
1/16 RRyy
2/16 Rryy
3/16 Round-green
(R- yy)
3
1/16 rrYY
2/16 rrYy
3/16 Wrinkled-yellow
(rr Y-)
3
1/16 rryy 1/16 wrinkled-green
(rryy)
1
F2 ratios

12. Dihybrid cross ctd
12

13. TEST CROSS
• A cross between an organism whose genotype
for a certain trait is unknown and an organism
that is homozygous recessive for that trait so
the unknown genotype can be determined
from that of the offspring
13

14. Exceptions to Mendel’s rules
• Incomplete dominance
• Multiple alleles
• Lethal genes
• Gene Linkage
• Sex-Linked Traits
• Quantitative Inheritance
14

15. Incomplete dominance
• Inheritance of a dominant and a
recessive allele results in a blending of
traits to produce intermediate
characteristics, so that heterozygotes
can be distinguished phenotypically
from the dominant homozygotes.
There are two types:
• Co-dominance-The phenotypic
expression of the heterozygote is
intermediate between the two
homozygotes e.g. as shown a side or
in a case of A cross of a black chicken
(BB) with a white chicken (WW)
produces all speckled offspring (BW).
• Over-dominance-Phenotypic
expression of the heterozygote
exceeds that of either homozygotes. 15

16. Multiple alleles
• A single characteristic may appear in several different forms controlled by 3 or more alleles
of which any two may occupy the same loci on the homologous chromosome. This is
known as multiple allele (multiple allelomorph) and control such characteristic such as
coat and eye colour in mice, and blood group.
• Inheritance of blood groups: Blood group is controlled by an autosomal gene encoding the
protein Isohaemaglutinogen (I)and there are 3 alleles represented by the symbols A, B, O.
A and B are equally dominant and O is recessive to both.
• Human blood group genotypes:
• Genotypes Blood group
• IAIA A
• IAIO A
• IBIB B
• IBIO B
• IAIB AB
• IOIO O
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 Presence of single dominant allele results in the blood producing a substance
called agglutinin which acts as an antibody E.g IAIO produces Agglutinin A

17. Lethal genes
• A single gene may affect several characteristics including
mortality. E.g. In chicken which are homozygous for an allele
controlling feather structure called ‘frizzled’, several phenotypic
effects results from the incomplete development of the
feathers. These chicken lack adequate feather insulation and
suffer from heat loss leading to high mortality rate especially if
they are homozygous for the frizzle gene.
17

18. Lethal genes ctd
• The effect of lethal gene is also clearly illustrated by the
inheritance of fur, in a condition known as agouti (wild
type coat).
• Some mice have yellow fur and are heterozygous for the
lethal agouti gene. Crossbreeding yellow mice produces
offspring in the ratio, 2 yellow: 1 agouti (Yellow is dominant
to agouti and all yellow coat mice are heterozygous).
• A ratio of 2:1 instead of the typical Mendelian ratio of 3:1 is
explained by the fetal death of the recessive homozygous
coat mice.
18

19. Gene Linkage
• An exception to independent assortment of genes
develops when genes appear near one another on
the same chromosome.
• When genes occur on the same chromosome, they are
inherited as a single unit and do not assort
independently. Genes inherited in this way are said to
be linked.
• For example, in fruit flies the genes affecting eye color
and wing length are inherited together because they
appear on the same chromosome.
• In many cases, genes on the same chromosome that
are inherited together produce offspring with
unexpected allele combinations from a process called
crossing over during meiosis. 19

20. Diagrammatic presentation of crossover
20

21. Sex-Linked/Influenced Traits
• Genes located on the sex chromosomes display different
patterns of inheritance than genes located on other
chromosomes.
• In human females, the sex chromosomes consist of two X
chromosomes (XX), while males have an X chromosome
and a shorter Y chromosome with many fewer genes (XY).
• A male’s X chromosome may contain a recessive allele
associated with a genetic disorder, such as hemophilia or
muscular dystrophy.
• In this case, males do not have a normal second copy of
the gene on the Y chromosome to mask the effects of the
recessive gene, and disease typically results.
• Red-green colour blindness in humans and baldness are
also sex-linked traits.
21

22. 22

23. Breeding
• An activity of exploiting genetic (heritable)
endowment or those characters of individuals which
can be passed on from generation to generation in
the population to improve that particular populations’
ability to satisfy human endeavors.
• Achieved through selection
• Selection can be natural (survival for the fittest) or
artificial
• Selection aims at identifying superior breeding
individuals that posses a large proportion of
superior genes for a desirable trait, or traits.
23

24. Methods of selection
• Individual performance testing-can be got from
the records of performance of the individual
• Pedigree testing- bases on the performance of
the relatives/parents
• Progeny testing- bases on the performance of
the offspring
• Sibling testing- Bases on the performance of the
siblings
24

25. Breeding schemes
• Once superior individuals are identified and selected, it is necessary
to devise breeding schemes (mating systems) which will give the
most genetic improvement. The different breeding schemes may be
grouped into:
• Pure breeding (mating of males and females of the same breed.
Purebred breeding aims to establish and maintain stable traits, that
individuals will pass to the next generation).
• Inbreeding/Close breeding: production of progeny by parents that are
more closely related than the average of the population from which they
came. May lead to inbreeding depression i.e. loss of hybrid vigor.
Responsible for transmission of undesirable traits in families e.g.
haemophilia
• Line breeding : involve matings between closely or distantly related
individuals from a common ancestor, but it does not emphasize
continuous sire-daughter, dam-son, or brother-sister matings. It reduces
chances of development of inbreeding depression compared to inbreeding.
• Outbreeding/Outcrossing: is the mating of males and females from
unrelated families in the same breed, with the aim of increasing genetic
diversity in a breed).
25

26. • Crossing breeding (mating of animals from
different breeds, with the aim of complementing
their traits)
• Two-Breed Cross: mating between a male and a female from
different purebreeds)
• Two-Breed Rotational Cross: Mating involving two males of
two purebreds and females of a pure breed; one of the
males is used to mate with the F1 females to produce a two-
breed rational cross.
• Three-Breed Rotational Cross: Like a two-breed rational
cross but includes three males of different breeds.
• Static Terminal Sire: F1 females are mated to a male of a
terminal breed
• Rotational Terminal Sire: Like a two – breed rational cross,
but gets rid of a certain percentage of parent stock and
maintains another percentage. 26

27. Pictorial presentation of production of a camborough
27
A and B are landrace and large white and the Duroc is the terminal sire

28. Why consider different combination?
• The Hampshire and Duroc enhance meat
quality and tend to be used as boars
• The Large White and Duroc can both improve
productivity and robustness
• The Landrace breed offers leanness and
increased productivity
• the Saddleback show good maternal traits.
28

29. Gene banks
• Genetic material can be stored for future use in
form of:
– Live individuals e.g. live animals/plants
– Semen
– Eggs(Oocytes)
– Embryos
– DNA
– Seeds
– Meristems/Explants (especially for non seeded plants)
– Pollen
29

30. Breeding value
• Breeding value refers to the value of an animal in a
breeding program for a particular trait.
• An animal's breeding value is estimated to be twice
the expected performance of its progeny.
• The reason for doubling the expected progeny
performance is that only half of the genes from the
individual are transmitted to any offspring (with the
remaining half coming from the other parent).
30

31. Breeding value ctd
• The expected progeny performance is called
transmitting ability and is, therefore, half of the
breeding value.
• In other words, transmitting ability is the genetic
advantage an individual transmits to its offspring.
• Breeding values can be estimated based on the
animal's own records and the performance of known
relatives.
• These estimated breeding values divided by 2 may be
used to predict the performance of future offspring
and are termed as Predicted Transmitting Ability or
PTA or estimated breeding value (EBV).
31

32. Heritability of Quantitative Traits

33. Quantitative Inheritance
• This is the inheritance of a trait that is determined
by multiple pairs of genes.
• Each pair of genes has only a slight effect on the
trait, while the cumulative effect of all the genes
determines the physical characteristics of the trait
• For example: at least four pairs of genes control
human skin color and Multiple genes also control
many traits important in agriculture, such as milk
production in cows.
33

34. Genetic variation
• The allele frequency distribution in populations
can vary due to evolutionary processes:
1. Natural selection (survival for the fittest)
2. Genetic drift (dying or not reproducing)
3. Mutations
4. Gene flow e.g. due to migration
34

35. Hardy–Weinberg principle
• Also known as the Hardy–Weinberg equilibrium,
model, theorem, or law,
• States that allele and genotype frequencies in a
population will remain constant from generation
to generation in the absence of other
evolutionary influences.
35

36. Quantitative genetics
• provides tools to analyze genetics and evolution
of continuously variable traits
• Provides tools for:
1. measuring heritable variation
2. measuring survival and reproductive success
3. predicting response to selection

37. Measuring Heritable Variation
• When assessing heritability we need to make
comparisons among individuals. Cannot assess a
continuous trait’s heritability within one
individual
• Need to differentiate whether the variability we
see is due to environmental or genetic
differences
• Heritability = The fraction of the total variation
which is due to variation in genes

38. Types of Variation
• Phenotypic variation (VP) is the total variation
in a trait (VE + VG)
– Environmental variation. (VE) is the variation
among individuals that is due to their
environment
– Genetic variation (VG) is the variation among
individuals that is due to their genes

39. Total genetic variation (VG)
actually has two components
• Additive Genetic Variation (VA) = Variation
among individuals due to additive effects of
genes
• Dominance Genetic Variation (VD) = Variation
among individuals due to gene interactions
such as dominance
• VG = VA + VD

40. Measuring heritability
• heritability = =
• Heritability is always between 0 and 1
• A heritability close to one indicates that
almost all of the variability in a trait comes
from genetic differences, with very little
contribution from environmental factors.
VG
VP
VG
VG + VE

41. Heritability can be looked at
from two perspectives
 Broad sense heritability H2= VG / VP
 Is also known as True sense heritability
 It is the ratio of total genetic variance (Additive and
Dominance) divided by the total phenotypic variance.
 It explains the amount of phenotypic variation attributable
to genetics, including gene x gene interactions and dominant
effects
 Narrow sense heritability h2 = VA / VP
 Is the proportion of variance among phenotypes in a
population that can be attributed to additive genetic
variance
 Use of narrow sense heritability allows us to
predict how a population will respond to selection

42. Calculating h2-
Testing the relationship between parents
and offspring trait values:
 Plot midpoint value for the 2 parents on x axis
and mid-offspring value on y axis and draw a best
fit line.
 This slope which is calculated by least squares
linear regression is a measure of heritability
called narrow-sense heritability or h2
 h2 is an estimate of the fraction of the variation
among the parents that is due to variation in the
parent’s genes
 Looking at a hypothetical population…

43. If slope is near zero there is no resemblance
Evidence that the variation among parents is due to the
environment.
Mid parent height

44. If this slope is near 1 then there is strong resemblance
Evidence the variation among parents is due to genes

45. Need to make sure that the environment is not
causing some of the variation because
“environment runs in families too.”
• Any study of heritability needs to account for
possible environmental causes of similarity
between parent and offspring.
• Take young offspring and assign them
randomly to parents to be raised
• In plants, randomly plant seeds in a given field
• Example in Song Sparrows studied by James
Smith and Andre Dhondt.

46. Song sparrow
chicks (eggs or
hatchlings)
raised by
foster parents
resembled
their biological
parents
strongly and
not their foster
parents at all

47. Predicting Evolutionary
response
Once we know the heritability and the
strength of selection we can predict response
to selection
R = h2 S
* R = predicted response
* h2 = heritability
* S = selection differential

48. • The selection differential is the difference of
the base population mean and the mean of
the selected parents.
• The selection response is how much gain you
make when mating the selected parents.
48

49. Read about
• Selection intensity
• Heritability of threshold traits
• Breeding value of threshold traits
• Genetic correlation
END
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