Download Exploring Mendelian Genetics

Height in Humans
Height in pea plants is controlled by
one of two alleles; the allele for a tall
plant is the dominant allele, while the
allele for a short plant is the recessive
one. What about people? Are the
factors that determine height more
complicated in humans?
Bell Work
1. Make a list of 10 adults whom you know. Next to the
name of each adult, write his or her approximate
height in feet and inches.
2. What can you observe about the heights of the ten
people?
3. Do you think height in humans is controlled by 2
alleles, as it is in pea plants? Explain your answer.
Exploring Mendelian Genetics
Key Concept

What inheritance patterns exist aside
from simple dominance


Incomplete dominance and Codominance
The influence of the environment on
genetic expression
A Summary of Mendel’s Principles:
Gregor
Mendel
concluded
that
experimented
with
Pea
plants
“Factors”
determine
traits
Some alleles
are dominant,
and some alleles
are recessive
which is
called the
Law of
Dominance
Alleles are
separated during
gamete formation
which is
called the
Law of
Segregation
A Summary of Mendel’s Principles:
1) The inheritance of biological characteristics
is determined by individual units know as
genes → genes are passed from parent to
offspring
Example: Offspring tend to resemble their
parents
A Summary of Mendel’s Principles:
2) In cases in which two or more forms (alleles)
of the gene for a single trait exist, some
forms of the gene may be dominant and
others may be recessive → this is known as
The Principle of Dominance
Example: Tongue Rolling → R = can roll
tongue (dominant), r = can’t roll tongue
(recessive)
A Summary of Mendel’s Principles:
3) In most sexually reproducing organisms, each
adult has two copies of each gene (one from
each parent) → These genes are segregated
from each other when gametes are formed
→ known as The Principle of Segregation
 Example:
Rr → body cell
/ \
 gametes→ R
r
The Principle of Independent Assortment

The principle of independent
assortment states that genes for
different traits can segregate
independently during the formation of
gametes

This helps account for the many genetic
variations observed in plants, animals,
and other organisms
A Two Factor Cross
Beyond Dominant and Recessive

Despite the importance of Mendel’s
work, there are important exceptions to
most of his principles


Some alleles are neither dominant nor
recessive
Many traits are controlled by multiple
alleles or multiple genes
Incomplete Dominance

One allele is not completely dominant
over another one

The heterozygous phenotype is
somewhere in between the two
homozygous phenotypes (both alleles
contribute to the phenotype)
Example: The Dosage Effect
Flower color in 4 o’clock
plants
R = red flowers
W = white flowers
P-generation: RR x WW
Codominance

Another situation in which both alleles
contribute to phenotype

A heterozygous phenotype shows distinct
characteristics from each allele
Example: Chickens

In chickens there is
an allele for white
feathers and black
feathers

A heterozygous
individual will have
both color feathers
Multiple Alleles

Genes that have more than two alleles
(individuals will still only have two
copies)
Example: Fig. 11-12 Coat color in rabbits
Polygenic Traits

Traits that are produced by the
interaction of two or more genes
Example: skin color in humans is
influences by at least 4 different traits
Applying Mendel’s Principles

Mendel’s principles don’t just apply to
plants
Example: Drosophila (the common fruit
fly) is a popular organism for genetic
research. Why?

Fruit flies are small, develop quickly, and
produce many offspring
Genetics and the Environment

The characteristics of any organism are not
solely determined by the genes it inherits →
characteristics are determined by the
interaction between genes and the
environment
Example: Many genes affect a Sunflower’s
height and color; however these traits are
also influenced by climate, soil conditions,
and the availability of water
Conclusion

Genes provide a plan for development,
but how that plan unfolds also
depends on the environment