Download Simple Mendelian Genetics

Simple Medelian
Genetics
Competency 12.00
Genetic Terminology
DNA
• Deoxyribonucleic
Acid
• The major nucleic
acid in organisms
• Carries genetic
information and is
responsible for the
transmission of
traits.
Gene
• A segment
of DNA that
codes for a
specific trait
in an
organism.
Allele
• An alternative form of a
gene/trait.
• Example: Eye color alleles are
blue, green, hazel, brown
• Can be homozygous or
heterozygous
Homozygous Allele
• Organism with identical alleles
for a given trait
• Can be dominant or recessive
• Example: TT or tt
Heterozygous Allele
• Organism with different alleles
for a given trait
– Ex: Tt
Phenotype
• The physical appearance of a
trait in an organism
• Determined but not always
indicative of the genetic
makeup of the organism
• Example: tall or short
Genotype
• The genetic composition of an
organism for a given trait
• Often cannot be determined by
looking at an organism
• Example: Tt or TT, both are
tall
Recessive Gene/Allele
• Variation of a trait that can only
be expressed in the absence of
a dominant allele
• Heterozygous individuals are
carriers for recessive alleles.
Dominant Gene/Allele
• Variation of a trait that is
expressed over other variations
of the same trait
• Most common forms in natural
populations
• Some traits can be codominant or exhibit incomplete
dominance
Chromosome
• Long condensed
strand of DNA forming
in the nucleus of a cell
prior to cell division
• Form cells that when
split, create an exact
copy of DNA in the
daughter cell
Chromatid Pairs
• X-shaped structures that serve
as the mechanism for the
transmission of genetic
material during cell division.
• They are pulled apart in the
process of mitosis and meiosis.
Heredity
Gregor Mendel
• 1863
• Austrian monk who conducted
the first genetics experiments
using pea plants in the mid
1800s.
• Often considered the founder
of genetics and heredity.
What is Heredity?
• Heredity is best described as
the manner in which inheritable
characteristics (traits) are
passed from parents to
offspring.
Heredity
• A direct outcome of the
RANDOM genetic
recombination resulting during
reproduction
– Only functions in sexual
reproduction
– Ensures genetic diversity
Heredity
• Determines the genetic
potential of an animal, but . . .
• Heredity and environmental
influences determine the
overall quality of the animal.
• Nature versus Nurture
Types of Heredity
• Simple Heredity
• Complex Heredity
– Polygenic Inheritance
– Incomplete Dominance
– Codominance
Simple Heredity
• One gene controls one trait—
alleles are either dominant or
recessive.
– Example: height and color in
pea plants
Complex Heredity
• Polygenic Inheritance
– One trait is controlled by several
genes and possibly
environmental factors
– Genes may even be located on
different chromosomes
Complex Heredity
• Polygenic Inheritance
– This is a slow process requiring
many generations to achieve
desired results.
• Example: Height in humans
Complex Heredity
• Codominance
– Multiple alleles for a given trait
are not expressed over one
another, but in combination.
• Example: RR (Red Flower) x WW
(White Flower) = RW (Red and
White Striped Flower)
X
=
Incomplete Dominance
• Similar to codominance, except
characteristics of alleles blend
instead of remaining distinct
– Example: RR (Red Flower) x
WW (White Flower) = RW (Pink
Flower)
X
=
Heredity in Agriscience
• Heredity is a huge factor in
successful agricultural selective
breeding programs.
• Heredity is manipulated to
create high quality HYBRID
offspring.
Heredity in Agriscience
• Plants and animals are inbred
through several generations to
isolate a specific trait or traits.
– No more than 7 generations are
inbred to prevent genetic
disorders.
Heredity in Agriscience
• The final generation of two
different lines inbred for
different traits are crossed
producing offspring with the
beneficial traits of both lines.
– Resulting offspring possesses
hybrid vigor
Hybrid Vigor
Heredity in Agriscience
• Hybrid vigor usually lasts only
one generation, as hybrid
organisms rarely express traits
true to type in offspring.
– Alternative forms of the gene
resurface in the new cross.
Heredity in Agriscience
• Punnett Squares, Pedigree
Charts, Genetic Mapping and
DNA analysis can be used to
predict heredity.
Punnett Squares
What is a Punnett Square?
• A method utilizing the known
genotypes of parent offsprings
to predict the expression of a
given trait or traits in offspring.
– Must know the genotype of
parents and the inheritance
pattern of the trait.
Using Punnett Squares
• When using Punnett Squares
the Dominant Trait is always
represented by an uppercase
letter
• Recessive is lowercase
– Example: T=Tall, t=short
Using Punnett Squares
• A box should be
drawn with one
space for each
allele expressed
by both parents.
• In simple
heredity boxes
are 2x2.
Using Punnett Squares
• The alleles for
one parent
should be placed
above each
column at the
top, with the
T
alleles for the
other placed
beside each row t
on one side.
T
T
Using Punnett Squares
T
T
T
TT
TT
t
Tt
Tt
• The alleles
of each
parent
should be
distributed
across and
down the
box.
Using Punnett Squares
T
T
t
TT
Tt
T
TT-Homozyous
Dominant
TT
Tt-Heterozygous
Tt
Genotype Ratio:
2:2:0
Phenotype Ratio:
4:0
Using Punnett Squares
• When crossing using complex
heredity boxes are 4x4 and two
different traits are being
crossed.
Using Punnett Squares
Genotype
Ratio0:4:4:4:4:0
Phenotype
Ratio0:8:8:0
TG
Tg
tG
tg
Tg
TTGg
TTgg
TtGg
Ttgg
Tg
TTGg
TTgg
TtGg
Ttgg
Tg
TTGg
TTgg
TtGg
Ttgg
Tg
TTGg
TTgg
TtGg
Ttgg
Assignment
- 1. White hair (ww) x black hair
(WW)
- 2. blue eyes (bb) x brown eyes
(Bb)