
Simple Medelian Genetics
Competency 12.00
Genetic Terminology
o DNA
 Deoxyribonucleic Acid
 The major nucleic acid in organisms

o Gene

o Allele

 Example: Eye color alleles are blue, green, hazel, brown

o Homozygous Allele
 Organism with
 Can be
 Example: TT or tt
alleles for a given trait
o Phenotype

 Determined but not always indicative of the genetic makeup of the
organism
 Example: tall or short
o Genotype

 Often cannot be determined by looking at an organism
 Example: Tt or TT, both are tall
o Recessive Gene/Allele


Heterozygous individuals are carriers for recessive alleles.
o Dominant Gene/Allele

 Most
forms in natural populations
 Some traits can be co-dominant or exhibit incomplete dominance
o Chromosome


Form cells that when split, create an exact copy of DNA in the daughter
cell
o Chromatid Pairs

structures that serve as the mechanism for the
transmission of genetic material during cell division.
 They are pulled apart in the process of
o Heterozygous Allele

 Example: Tt
Heredity

o 1863
o Austrian monk who conducted the first genetics experiments using
in the mid 1800s.
o Often considered
and heredity.

What is Heredity?
o Heredity is best described as
o A direct outcome of the RANDOM genetic recombination resulting during
reproduction
 Ensures genetic diversity
o Determines the genetic potential of an animal, but . . .
o Heredity and environmental influences determine the overall quality of the
animal.
o

Types of Heredity
o Simple Heredity
o Complex Heredity
 Polygenic Inheritance
 Incomplete Dominance
 Codominance

Simple Heredity
o
 Example: height and color in pea plants

Complex Heredity
o
Inheritance

 Genes may even be located on different chromosomes

This is a slow process requiring many generations to achieve desired
results.
 Example:
o Codominance

 Example: RR (Red Flower) x WW (White Flower) = RW (Red
and White Striped Flower)
o Incomplete Dominance


Example: RR (Red Flower) x WW (White Flower) = RW (Pink
Flower)
Heredity in Agriscience
o Heredity is a huge factor in successful agricultural selective breeding programs.
o Heredity is manipulated to
o Plants and animals are inbred through several generations to isolate a specific trait
or traits.

o 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
o Hybrid vigor usually lasts only one generation, as hybrid organisms rarely express
traits
in offspring.
 Alternative forms of the gene resurface in the new cross.
o Punnett Squares, Pedigree Charts, Genetic Mapping and DNA analysis can be
used to predict heredity.
Punnett Squares
 What is a Punnett Square?
o


Must know the genotype of parents and the inheritance pattern of the trait.
Using Punnett Squares
o When using Punnett Squares the Dominant Trait is always represented by an
o Recessive is
 Example: T=Tall, t=short
o A box should be drawn with one space for each allele expressed by both parents.
o In simple heredity boxes are 2x2.
o The alleles for one parent should be placed above each column at the top, with the
alleles for the other placed beside each row on one side.
o The alleles of each parent should be distributed across and down the box.
 TT-Homozyous Dominant
 Tt-Heterozygous
 Genotype Ratio:
 3:1:0
 Phenotype Ratio:
 4:0
o When crossing using complex heredity boxes are 4x4 and two different traits are
being crossed.

Example of a Complex Heredity Punnett Squares