Download Chapter 11: Intro to Genetics

Chapter 11: Intro to Genetics
11-3
Other Patterns of Inheritance
Intro
• After showing that alleles segregate, Mendel also
wondered if they do so independently of each
other.
• Does the segregation of one pair of alleles affect
the segregation of another pair of alleles? Ex:
Does the gene for seed shape have anything to do
with the gene for seed color? Does a round seed
always have to be yellow?
• To answer this question, Mendel performed an exp
(2 factor cross) and followed 2 different genes.
Independent Assortment
• What was Mendel’s
experiment?
• The two factor cross.
• 1st Mendel crossed true
breeding plants that
produced only round yellow
peas (RRYY) with plants
that produced wrinkled green
peas (rryy)
• All offspring=round yellow
peas.
• Yellow round were dominant
over wrinkled green.
• Did not show if genes assort
independently. Provides
plants needed for next cross.
Independent Assortment
• All plants were heterozygous
(RrYy)
• Mendel wanted to know if the 2
dominant genes would stay
together or separate. He
crossed RrYy with RrYy.
• See fig 11-10 pg 271
• Results: 556 seeds produced,
315 round yellow, 32 wrinkled
green, 209 had combos of
phenotypes not found in either
parent.
• This meant the allele for shape
segregated independently for
color—this was known as.
independent assortment.
Independent Assortment
• What is the principle
of independent
assortment?
• Genes for different traits
can segregate
independently during the
formation of gametes.
Independent assortment
helps account for the
many genetic variations
observed in plants,
animals, and other
organisms.
A Summary of Mendel’s Principles
• How can Mendel’s
principles be
summarized?
• Inheritance of biological
characteristics is determined by
genes which are passed from
parents to offspring.
• When 2+ forms of gene for
single trait exits, some alleles
are dominant & some are
recessive.
• Each adult has 2 copies of each
gene—one from each parent.
These genes are segregated
from each other when gametes
are formed.
• Alleles for diff genes segregate
independently of each other.
Beyond Dominant & Recessive
• Do all genes show a
simple pattern of
dominant and
recessive?
• NO! Genetics is more
complicated in most
organisms because the
majority of genes have
more than 2 alleles.
• Many important traits are
controlled by more than 1
gene.
• Some alleles are neither
dominant or recessive, and
many traits are controlled
by multiple genes or
multiple alleles.
Beyond Dominant & Recessive
• What are the different
patterns of
inheritance?
• Incomplete
Dominance
• Codominance
• Multiple Alleles
• Polygenic traits
Beyond Dominant & Recessive
• What is incomplete
dominance?
• One allele is not
completely dominant over
another. The
heterozygous phenotype is
somewhere in between the
2 homozygous
phenotypes.
– A cross between red and
white flowered plants
produce pink colored
flowers.
Beyond Dominant & Recessive
• What is codominance?
• Both alleles contribute to
the phenotype of the
organism.
• Ex: In chickens, the allele
for black feathers is
codominant with the allele
for white feathers.
• Heterozygous chickens are
speckled with black and
white feathers.
Beyond Dominant & Recessive
• What are multiple
alleles?
• Genes have more than 2 alleles.
• Does not mean that the
individual has more than 2
alleles. It means that more than
2 possible alleles exist in a
population.
• Ex: Coat color in rabbits. A
rabbit’s coat color is determined
by a single gene that has four
different alleles.
• Ex: Human genes for blood
type, eye color in humans
Beyond Dominant & Recessive
• What are polygenic
trait?
• Two or more genes
control a trait.
• Three genes are involved
in making the reddishbrown pigment in the eyes
of fruit flies.
• Polygenic traits show a
wide range of phenotypes.
• Ex: skin color in
humans—more than 4 diff
genes control this trait.
Applying Mendel’s Principles
• Do Mendel’s
principles only apply
to plants?
• No! In the 1900s, Thomas
Hunt Morgan, a geneticist,
decided to look for a model
organism to study genetics.
• He wanted an animal that was
small, easy to keep in a lab, and
able to produce large numbers
of offspring in a short period of
time.
• He chose the fruit fly
(Drosophila melanogaster)
• Mendel’s principles also apply
to humans.
Genetics & Environment
• Are the characteristics
of organisms
determined solely by
genetics?
• NO! Characteristics are
determined by the
interaction between genes
and the environment.
• Ex: Genes may affect the
height of a plant. These
characteristics are also
influenced by climate, soil
conditions, & availability
of water.
• Genes provide the plan,
but how the plan unfolds
also depends on
environment.