Gregor Mendel performed experiments with pea plants to study inheritance patterns of traits. Through his work, he discovered three basic principles: traits are inherited independently, traits are inherited in dominant and recessive forms, and the inheritance of each trait is determined by discrete units (now known as genes). Mendel also studied monohybrid, dihybrid and trihybrid crosses to understand inheritance of single, double and triple traits. His work established the foundations of classical genetics.

1. Different crosses performed
by Gregor Mendel
AAPD2BB

2. Gregor mendel

3. 3 Hypothesis of Mendel
„ Hereditary determinants do not
“mix” or “contaminate each
other”.
„ Segregation is the method of
separating alleles; they are
independent from each other.
„ Gametes produced by segregation
come together in pairs at
random.

4. Mendelian laws of
Inheritance
1. Law of independent
assortment
-alleles separate
independently to each other.
2. Law of segregation
-the paired genes separate in
the formation of the
reproductive cells. It forms
together as a zygote.

5. 3 Generalization of
Mendel
1. The f1 holds the dominant trait . In
crossing a pure breeding variety , only
one trait will appear in the f1 and that is
the dominant trait.
2. In crosses it doesn’t matter which parent
variety contributes the
gametes, reciprocal crosses will yield
the same result
3. The trait that doesn’t appear in f1 will
appear in f2 in a 25% of each progency.
Wild type: common/ dominant

6. 2 traits:
„ Morphological trait
- It is the trait referring to the
external or visible
characteristics of traits; like
phenotype.
„ Molecular trait
- It is the intrinsic characteristics in
the organism; can’t be seen
externally.

7. Monohyb
rid cross

8. • is a method of determining the
inheritance pattern of a trait between
two single organisms.
• is a cross between parents who are true-
breeding for a trait
• Monohybrid inheritance is the inheritance
of a single characteristic. The different
forms of the characteristic are usually
controlled by different alleles of the
same gene.

9. Example:
A normal woman whose parents are both
heterozygous normal marries a normal
man whose mother is albino and whose
father is normal. He has an albino sister.
The couple has a normal and albino sons.
a. Construct a pedigree and give the genotypes
of all persons involved.
b. If the couple plans to have three more
children. What is the probability of having 2
normal and 1 abnormal?

10. Albinism: recessive A: NORMAL
Normal: dominant a: ALBINISM
a.

11. b.
n! 3! 3x2x1
a! (n-a) 1!(2)! 1 x (2 x 1)
3
3n 2 a 1 = 3 ( ¾ )2 ( ¼ ) = 27/
64
Aa x Aa
AAA Aa aa
a Albinism = carrier of
recessive trait

12. Dihybrid
cross

13. „ is a cross between F1 offspring of two
individuals that differ in two traits of
particular interest.
„ The cross between their offspring is
referred to as a dihybrid cross, in which
parents are both heterozygous.
„ is often used to test for dominant and
recessive genes in two separate
characteristics. Such a cross has a variety
of uses in Mendelian genetics.

14. Example:
• P1: yellow, round X green, wrinkled
• F1: all yellow, round
• F2: 9/16 yellow, round ; 3/16 yellow,
wrinkled ; 3/16 green, round ; 1/16 green,
wrinkled

15. Test cross
„ It is a test for genotypes;
-heterozygous - 1:1
-homozygous ‟ 1:0
„ This cross uses the recessive
traits.

16. Example:
1. D x dd
Dd Dd Dd Dd
= DD
2. D x dd
Dd Dd dd dd
= Dd

17. Trihybrid
cross

18. • The mating of two
individuals, organisms, or
strains that have different
gene pairs that determine
three specific traits or in
which three particular
characteristics or gene loci
are being followed.

19. Example:
23 = 8 * 8 = 64

20. Fork-line method
MEANING:
„ The fork line method can be used by
figuring the occurrence of each gene or
set of genes to be found in the gamete, and
then multiply them together.
„ This can be multiplied by figuring each
trait or combination of traits separate
for both the male and female, or each
gene individually regardless of sex.

21. P1: WWGG * ww gg
F1: Ww Gg * Ww Gg
F2: Ww * Ww Gg * Gg
WW Ww Ww ww GG Gg Gg gg
Phenotypes:
¾W ¾ G = 9/16 WG
¼ g = 3/16 Wg
¼ w ¾ G = 3/16 wG
¼ g = 1/16 wg
Phenotypic ratio:
9:3:3:1

22. Genotypes:
¼ GG = 1/16 WW GG
¼ WW 2/ Gg = 2/ WW Gg
4 16
¼ gg = 1/16 WW gg
¼ GG = 3/16 Ww GG
2/4 Ww 2/ Gg = 4/ Ww Gg
4 16
¼ g = 2/16 Ww gg
¼ GG = 1/16 ww GG
¼ ww 2/ Gg = 2/ ww Gg
4 16
¼ gg = 1/16 ww gg
genotypic ratio: 1:2:1:2:4:2:1:2:1

23. POLYHYBRID
CROSS

24. • Describing an individual, organism, or
strain that is heterozygous for more
than three specific traits or gene pairs
or that is the offspring of parents
differing in more than three specific
gene pairs.

25. 2 rules in solving
probability
• Addition rule
- If it is mutually exclusive; the same.
ex:
b+g
b+g
b2+ bg
bg+ g2
b2+2bg+g2

26. • Multiplication rule
- If it is independent; it is different.
ex:
DdAa * DdAa
DD Aa Dd dd
AA Aa Aa aa
(3DD + 1d ) (3A + 1a ) = 9DA