Transcript Chapter 5: Patterns of Inheritance

Chapter 5: Patterns of Inheritance
Early Ideas of Inheritance
Pangenesis (~300 BC)
• Greek philosopher Aristotle
proposed that the egg and sperm
contained “pangenes” derived from
all parts of the body.
• When fertilized, pangenes
developed into the parts of the
body from which they came
Early Ideas of Inheritance
• In the 1700’s Antony van
Leeuwenhoek believed that he
saw a complete person within
the head of a sperm.
• The theory was that the person
came from the father, but
developed in the mother.
All these theories were developed to explain different observations,
but there was no scientific evidence that proved them true.
Gregor Mendel
• Studied botany and
mathematics
• Successfully discovered
and proved the mystery
of inheritance using:
• the pea plant
The Pea Plant
• Use sexual reproduction, but
also self-breed
• Available in many varieties,
showing many traits
• A trait is a specific
characteristic of feature such
as colour
– Determined by alleles!
Mendel’s Experiments
• Mendel began each experiment with truebreeding plants
– These are plants that have self-fertilized to
produce offspring with identical traits each
generation.
• These were referred to a the parental or P
generation
True-Breeding Crosses
• He then took true-breeding plants with one form
of a trait (e.g. yellow seeds) and crossed it with
true-breeding plants with the other form of the
same trait (e.g. green seeds)
– A cross is the selective fertilization of female and male
gametes with specific genes.
– This is referred to as a monohybrid cross because
what is produced is a hybrid of two parents that differ
by one trait
• The offspring of these crosses were called the F1
Generation
True-Breeding Crosses
Results of the monohybrid cross:
P generation
F1 generation
x
100%
• For all of 7 traits studied, when true-breeding
organisms of opposite traits were crossed, the
offspring expressed only one form of the trait
F1 Crosses
• Then Mendel crossed two plants from the F1
generation, allowing them to self-fertilize
F1 generation
x
F2 generation
3 : 1
• Mendel realized that the green form of the
seed colour had merely gone unexpressed in
the F1 generation since it reappeared in F2
The Law of Segregation
• Mendel concluded that there must be two
hereditary “factors” for each trait he studied
– Each “factor” = allele
– Each trait = gene
• Law: traits are determined by pairs of alleles that
segregate during meiosis so that each gamete
receives one allele.
• This explains why the green seeds disappeared in
the F1 generation but reappeared in the F2
Dominant vs Recessive
The allele for yellow seeds was dominant
• This can be expressed by “Y”
While the allele for green seeds was recessive
• This can be expressed by “y”
Together, two allele designations are referred to
as a genotype (e.g. YY, yy, or Yy)
The physical trait observed (determined by the
genotype) is referred to as the phenotype
Genotypes
P cross
F1 generation
F1 Cross
F2 generation
yy
x
Yy
Yy
25% yy
YY
100%
x
Yy
:
Yy/YY 75%
Homework
• Pg. 205 #1, 3-6
• Pg. 207 #1-12 (except 3 and 5)