2. Mendel’s Law of Segregation states that there are two alleles
for every gene determining a specific characteristic, and these
alleles are segregated into separate gametes during
reproduction.
When the 2 different alleles occur together in one individual
(heterozygote), the dominant allele will be the one that is
expressed in the phenotype of the individual.
The Law of Addition is used to combine probabilities, when
there are 2 or more ways to arrive at a given outcome.
The Law of Multiplication is used to combine probabilities of 2
or more different events that need to occur in combination.
Mendel’s Law of Independent Assortment says that 2 or
more different genes, if found on separate chromosomes, are
determined independently of each other.
Many characteristics have been found to follow patterns of
inheritance that are modifications of Mendel’s rules.
3. • pure • forked line
• gene breeding • polygenes
• allele • hybrid • contributor
• dominant • homozygous y allele
• recessive • heterozygous • non-
contributor
• character • P (parental
y allele
generation)
• trait • continuous
• F1 (first filial variation
• phenotyp
generation) • discontinuo
e
• F2 (second us variation
• genotype
filial
generation)
• Punnett
4. CHARACTERS TRAITS
• Pre-Mendel beliefs in
genetics: blending
theory
• Augustinian monk and
science teacher
• Why Pisum sativum?
– Pea plants available in
many varieties
• character (heritable feature)
• trait (character variant)
– Perfect flowers
• cross-pollination and
self-pollination
– Short generation time
– Many offspring
5. MONOHYBRID CROSS
inheritance of a single trait
P Generation
(true-breeding
x
parents)
Purple White
flowers flowers
F1 Generation
(hybrids)
All plants had
purple flowers
self-pollination
F2 Generation
705 plants 224 plants
6.
7. Allele for purple flowers
Alternative versions
of genes exist
Homologous
(alleles)
Locus for flower-color gene pair of › Dominant
chromosomes
› Recessive
Organism inherites
Allele for white flowers 2 alleles
› Dominant is fully
expressed
› Recessive has no
visible effect
Law of segregation
› 2 alleles for one
character
separate and go
to different
gametes
8. True-breeding plants have identical P Generation
alleles. x
Purple flowers White flowers
Gametes each contain only one allele PP pp
for the flower-color gene. Every gamete Appearance:
produced by one parent has the same allele. Genetic makeup:
P p
Gametes:
F1 hybrids have a Pp combination.
Purple-flower allele is dominant, F1 Generation
all hybrids have purple flowers.
Purple flowers
Hybrid plants produce gametes, Appearance:
Genetic makeup: Pp
two alleles segregate: half the
gametes receiving the P allele and
1
/2 P
1
/2 p
Gametes:
the other half the p allele.
Punnett square: shows all possible F1 sperm
combinations of alleles in offspring P
p
from an F1 x F1 (Pp x Pp) cross. F2 Generation
P
Each square represents an equally probable PP Pp
product of fertilization. F1 eggs
p
Pp pp
Random combination of the gametes
results in the 3:1 ratio that Mendel
3 :1
observed in the F2 generation.
10. APPLICATION
An organism displaying the dominant phenotype can either X
be homozygous or heterozygous for the trait. A test-cross will
help determine which.
Dominant phenotype, Recessive phenotype,
unknown genotype: known genotype:
TECHNIQUE PP or Pp? pp
The individual with the unknown genotype is crossed with
a homozygous individual expressing the recessive trait
(white flowers in this example). By observing the If Pp,
If PP,
phenotypes of the offspring resulting from this then 1⁄2 offspring purple
then all offspring
cross, we can deduce the genotype of the purple-flowered
purple: and 1⁄2 offspring white:
parent.
p p p p
RESULTS
P P
Pp Pp Pp Pp
P p
Pp Pp pp pp
11. Illustrates the
inheritance of two
characters
Produces four
phenotypes in the F2
generation
Law of Addition -
combines probabilities
for mutually exclusive
events
The Law of
Multiplication -
combines probabilities
of 2 or more
independent events
that need to occur
together
14. 1. Codominance
– two dominant alleles affect
phenotype in separate
ways
– both alleles manifest
– e.g. roan coloring in horses
– both red and white
2. Incomplete dominance
– phenotype of offspring is
between phenotypes of
two parents
– e.g. red and white parents
give rise to pink offspring
3. Multiple alleles
– genes with more than two
alleles that control the
phenotype
– e.g. ABO blood type system
has 3 alleles--A,B,i. A and B
are codominant, i is
recessive to both