The document outlines fundamental concepts of genetics, including the definitions of heredity, genes, alleles, and Mendelian principles derived from Gregor Mendel's experiments on pea plants. Key topics covered include dominant and recessive traits, genetic ratios, Punnett squares, and the laws of segregation, independent assortment, and dominance. Additionally, it discusses complex inheritance patterns such as incomplete dominance, co-dominance, multiple alleles, epistasis, pleiotropy, and polygenic inheritance.

2. • Genetics is derived from the
term genesis, which means
Creation, to give birth.
• Today, we can describe genetics as
the science which investigates the
formation of traits, their
development and their
transmission from generation to
generation.
• The study of inheritance is called
genetics.

4.  Heredity: The passing of traits from parents to their
offspring.
 Trait: A characteristic of an organism that is determined
gentically, enviromentally or by both. Traits can pass from
parents to their offspring by genes.
 Gene: A segment of DNA that codes a particular protein. It
is the basic unit of heredity. Genes are found on
chromosomes.

5.  Allele: is the one pair of genes, one coming from the
mother, the other from the father, in the chromosomes of a
living thing. Alleles can be similar or different. These gene
pairs control one trait.
 Homozygote: Having identical alleles for a given trait
(AA, or BB, cc, dd)
 Heterozygote: Having two different alleles for a given
trait. (Aa Bb Cc)

6. Dominant gene; It is always expressed and prevents the
expression of another gene. The dominant allele, is fully
expressed in the organism’s appearance. It is indicated by
writing the first letter of the trait as a capital letter.
Recessive gene; The recessive allele, has no noticeable
effect on the organism’s appearance. It is indicated by writing
the first letter of the dominant trait, but the letter is
lowercase. It is expressed only in homozygous condition (aa).

7. • Alleles
1. Alternative forms of genes.
2. Units that determine heritable traits.
3. Dominant alleles (TT - tall pea plants)
homozygous dominant
4. Recessive alleles (tt - dwarf pea plants)
homozygous recessive
5. Heterozygous (Tt - tall pea plants)

8.  Genotype: The complete set of genes of a living
thing or the genetic make up of organism.
Genotype is the arrangement of genes that produces
the phenotype.
1.tall pea plant TT = tall (homozygous dominant)
2.dwarf pea plant tt = dwarf (homozygous recessive)
3.tall pea plant Tt = tall (heterozygous)
 Phenotype: The physical, visible characteristics of
organism. Outward appearance or Physical
characteristics; 1.tall pea plant 2.dwarf pea plant

10.  Independent genes:
Alleles for different traits
located on different
chromosomes.
 Linked gene: More than
one gene on a single
chromosome.
 Principle of segregation:
During meosis, the alleles that
control one character
separate; one goes to one
gamete, the other goes to the
other gamete.

11.  P (Parental) Cross: Mating of the father and the
mother.
 F (Fillia:Generation): The individuals produced by
crossing of the father and mother.

12. Probability and Genetics
 The segregation of alleles into gametes and random
combination of alleles in fertilization demonstrate
simple rules of probability. Geneticists use probability to
predict the outcomes of crosses.
 The rule of independent events: Previous events do not
affect the probability of later occurences of the same
event.
 Suppose you toss a coin, and it comes up heads. What is
the probability of getting head if you toss the coin
again? The first time the coin is tossed, the probability of
getting heads is ½. On a second toss or any toss after
that, the probability of tossing heads is still ½. Each toss
of a coin is independent of any other toss of the same
nickel.

13.  The product rule: The probability of 2 independent
events happening together is equal to the product of their
seperate probabilities.
 When we toss 2 coins at the same time, the probability of
both coin coming up tails is ¼ , the probability of both
coming up heads is ¼ , and the probability of one head and
one tail is ½.

14. MENDEL AND THE GENE IDEA
•Many of your traits, including the color
and shape of your eyes, the texture of
your hair, and even your height and
weight, resemble those of your parents.
•The scientific study of heredity began more than a
century ago with the work of an Austrian monk named
Gregor Johann Mendel.
•Modern principles of inheritance are based on
Mendel’s work.

15. •Mendel experimented with
garden pea heredity by cross-
pollinating plants with different
characteristics.
•The patterns that Mendel
discovered form the basis of
genetics, the branch of biology
that focuses on heredity.

17. Useful Features in Peas
The garden pea is a good subject
for studying heredity for several
reasons:
1. Garden pea has many different visible traits and
types.
2. The male and female reproductive parts of garden
peas are enclosed within the same flower. This allows
you to easily control mating.
3. The garden pea is small, grows easily, matures
quickly, and produces many offspring.

18. Mendel carried out his experiments in three steps:
Step 1 Mendel allowed each variety of garden pea to
self-pollinate for several generations to ensure that
each variety was true-breeding for a particular trait;
that is, all the offspring would display only one form of
the trait.
These true-breeding plants served as the parental
generation in Mendel’s experiments. The parental
generation, or P generation, are the first two individuals
that are crossed in a breeding experiment.

19. Step 2 Mendel then cross-pollinated two P generation
plants that had contrasting forms of a trait, such as
purple flowers and white flowers. Mendel called the
offspring of the P generation the first filial
generation, or F1 generation.
Step 3 Mendel allowed the F1 generation to self-
pollinate. He called the offspring of the F1
generation plants the second filial generation, or F2
generation.

22. Mendel’s Results
• Each of Mendel’s F1 plants showed only one
form of the trait.
• But when the F1 generation was allowed to
self-pollinate, the missing trait reappeared in
some of the plants in the F2 generation.
• For each of the seven traits Mendel studied,
he found a 3:1 ratio of plants expressing the
contrasting traits in the F2 generation.

23. • Mendel correctly concluded from his experiments
that each pea has two separate “heritable factors” for
each trait—one from each parent.
• When gametes (sperm and egg cells) form, each
receives only one of the organism’s two factors for
each trait.
• When gametes fuse during fertilization, the offspring
has two factors for each trait, one from each parent.
Today these factors are called genes.

24. Mendel’s Factors

25. • Mendel correctly concluded from his experiments that
each pea has two separate “heritable factors” for each
trait—one from each parent.
• When gametes (sperm and egg cells) form, each
receives only one of the organism’s two factors for
each trait.
• When gametes fuse during fertilization, the offspring
has two factors for each trait, one from each parent.
Today these factors are called genes.

26. The four hypotheses Mendel developed as a result of his
experiments now make up the Mendelian theory of
heredity—the foundation of genetics.
1. For each inherited trait, an individual has two
copies of the gene—one from each parent.
2. There are alternative versions of genes. Today the
different versions of a gene are called its alleles.

27. 3. When two different alleles occur together, one of
them may be completely expressed, while the other
may have no observable effect on the organism’s
appearance. Mendel described the expressed form of
the trait as dominant. The trait that was not
expressed when the dominant form of the trait was
present was described as recessive.
4. When gametes are formed, the alleles for each gene
in an individual separate independently of one
another. Thus, gametes carry only one allele for each
inherited trait. When gametes unite during
fertilization, each gamete contributes one allele.

28. concluded that
which is
called the
which is
called the
Gregor
Mendel
Law of
Dominance
Law of
Segregation
Pea
plants
“Factors”
determine
traits
Some alleles
are dominant,
and some alleles
are recessive
Alleles are
separated during
gamete formation
experimented
with

29. The Laws of Heredity
1-The Law of Segregation
The first law of heredity describes the behavior of
chromosomes during meiosis. The law of segregation, states
that the two alleles for a trait segregate (separate) when
gametes are formed. At this time, homologous chromosomes
and then chromatids are separated.

30. 2-The Law of Independent Assortment
• Mendel found that for the traits he studied, the
inheritance of one trait did not influence the
inheritance of any other trait.
• The law of independent assortment states that the
alleles of different genes separate independently of
one another during gamete formation.

31. 3- The Law of dominance
When an organism has two different alleles for a trait,
one allele dominates and other one is masked.
The expressed form of the trait as dominant.
The trait that is not expressed when the dominant
form of the trait is present, is described as recessive.
Let's say that "B" means that a cat will be BIG and
"bb" will make a small.
"BB" and "Bb" cats would show the dominant phenotype.
There is no difference between "Bb" and "bB“.
"bb" cats show the recessive phenotype.

32. MENDEL CROSSING and Punnett Squares
•A Punnett square is a diagram
used to show the possible
combinations of gametes in the
cross.
•The possible gametes that one
parent can produce are written
along the top of the square.
•Each box inside the square is filled in with two letters
obtained by combining the allele along the top of the box
with the allele along the side of the box.
•The possible gametes that the other parent can
produce are written along the left side of the square.

33. Monohybrid Cross
A breeding experiment according to the inheritance of
a single trait. In monohybrid cross, phenotype ratio is
3:1 , genotype ratio is 1:2:1.

34. Monohybrid Cross: Heterozygous Plants
. In monohybrid cross, phenotype ratio is 3:1 , genotype
ratio is 1:2:1.

35. Dihybrid Cross:
• A dihybrid cross is a cross
between two organisms in
which two different traits
are being studied.
• In dihybrid cross, the
phenotype ratio is 9:3:3:1.

36. Formula:
Number of gametes = (n = number of heterozygotes)
Question: How many gametes will be produced for the
following allele arrangements?
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
1. RrYy: 2n = 22 = 4 gametes = RY Ry rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes = ABCD ABCd AbCD
AbCd aBCD aBCd abCD abCd
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

37. Trihybrid cross
 The cross between two induvidulas in which
three different traits are being studied. In a
trihybrid cross, the most important thing is
writing the gamete type.
 After writing gamete type, we can determine
the traits of the offspring by making a table.
 The phenotypic ratio in trihybrid cross is
27:9:9:9:3:3:3:1

38. The following is a cross between two people that are heterozygous for 3 traits.
AaBbDd X AaBbDd
the following gametes will be produced in equal numbers:
ABD, ABd, AbD, Abd, aBD, aBd, abD, abd
The following Punnett square shows the possible offspring for these 2
individuals:

40. •Scientists involved in breeding organisms often need to
know whether an organism with a dominant phenotype is
heterozygous or homozygous for a trait.
•In a test cross, an individual whose phenotype is
dominant, but whose genotype is not known, is crossed
with a homozygous recessive individual.

42. Mendel’s findings can be summarized as follows:
 The traits of a pea plant are related to its alleles, which
can be the same or different.
 Variety in organisms arises from the fact that, during
fertilization, the gametes combine randomly, allowing
the genes for various traits to form new combinations.

43. Gregor MENDEL- Father of genetics

44. • The relationship between
genotype and phenotype is
rarely so simple.
• Geneticist know many
examples in which an
organism’s appearance is the
result of

45. Extending Mendelian Genetics

46. Incomplete Dominance
• In some organisms, an individual displays a trait
that is intermediate between the two parents, a
condition known as incomplete dominance.
• It appears when both alleles for the development of
some trait have the same level of influence in
heterozygous individuals.
• The terms; dominant and recessive cannot be used
in incomplete dominance.
• The genotypic and phenotypic ratios are identical;
1:2:1

48. Co-Dominance
• For some traits, two dominant alleles are expressed
at the same time.
• In co-dominance, neither phenotype is dominant.
Instead, the individual expresses both phenotypes.
.
• The most important example for co-dominance is in
human blood types.

49. • The gene for blood types has three alleles: A, B, and i.
i causes 0 type and is recessive to both A and B.
• When a person has both A and B, that person has
type AB blood. This means, allele A and B are co-
dominant to each other.

50. • Another example for co-dominance involves cattle.
• If a homozygous bull and homozygous cow mate
(one being red and the other white), then the calves
produced will be roan-colored, with a mix of red and
white hairs.
• In horses,
The heterozygous horses(GW) is an
appaloosa horse (a white horse with gray spots on the
rump and loins).

51. Multiple Alleles
• In living organisms there are sometimes more than
two alleles governing the inheritance of a trait (alleles
T1' T2 and T3 for trait T).
• When more than two different alleles exist for the
same trait, this condition is called multiple alleles, and
the process of their inheritance is called heredity with
multiple alleles.
• Fur color of rabbits and human blood types are the
examples of the multiple alleles.

52. • There are series of alleles that
control fur color in rabbits. The C
allele is for silver color.
• Another allele, ch, when
homozygous causes white spots
on the body. When a fourth allele
cch is homozygous, it causes gray
spots on the body.

53. Epistasis
is a common type of gene
interaction in which the presence of
certain alleles of one locus can
prevent the expression of alleles of
a different locus.
• Epistasis should not be confused
with Dominance. is the
interaction between different genes
(nonallele). is the
interaction between different alleles
of the same gene.
Homozygous alleles for
albinism exhibit epistasis,
masking the expression of
alleles of other loci that
govern production of
melanin pigment. Albino
individuals, such as this
albino koala, occur
occasionally in nature.

54. Epistasis
– For example, in mice and many other mammals,
One, the epistatic gene,
determines whether pigment will be deposited in hair
or not. Presence (C) is dominant to absence (c).

56. Pleiotropy
• Sometimes a single gene affects more than one trait
rather than having a single affect.
• Albinism is an example of Pleiotropy. In addition to
lack of pigment melanin in the skin, albinism also
produces defects of vision.

57. • If sickle- cell alleles are inherited from both parents, then all the
hemoglobin will be abnormal.
• The abnormal hemoglobin deforms the red blood cells, starting
a cascade of symptoms throughout the body.
• Thus at the level of whole organism, the sickle-cell allele has
multiple phenotypic effects.

58. Polygenic inheritance
• When several genes influence a trait, the trait is said
to be a
• Polygenic inheritance is an additive effect of two or
more genes on a single phenotypic character (the
converse of pleiotropy, where a single gene affects
several phenotypic characters).

59. Imagine that each gene has two alleles, one light and
one dark, that demonstrate incomplete dominance.
An AABBCC individual is dark and aabbcc is light.
Individuals with intermediate skin shades would be the
most likely offspring, but very light and very dark
individuals are possible as well.