Biology
Class Notes
Lesson 22 Genetics
Objective: 3.2.2
Genetics: The study of traits and patterns of inheritance for the purposes of classification and medical
advancement.
Common Terms
Allele: A letter that represents the type or form a gene takes.
Ex: B=Brown, b = blue eyes, T = tall, t = short
Genotype: The combination of alleles an organism inherits, one from each parent.
Ex: CC, Cc or cc.
Phenotype: The form of the trait the organism displays.
Ex: Green eyes, black hair, attached earlobes, etc.
Related Laws:
Law of Segregation: Offspring get one allele for each trait from each parent.
Law of Independent Assortment: Genes for traits on different chromosomes separate
independently/randomly.
Dominant and Recessive Traits/Genes
*When paired up, one of the two traits is usually dominant.
Dominant: The allele/trait that is expressed. (abr. w/ capital letters)
Recessive: The allele/trait that is not expressed. (abr. w/ lowercase letters)
Trait
Dominant Gene
Recessive Gene
Hair Type (letter T/t)
Curly Hair (TT or Tt)
Straight Hair (tt)
Hair Color (letter C/c)
Dark (CC or Cc)
Light (cc)
Earlobe (letter E/e)
Free (EE or Ee)
Attached (ee)
Allele Combinations
Homozygous: Having two of the same alleles for a trait.
Ex: Homozygous Dominant (DD) and Homozygous Recessive (dd).
Heterozygous: Having one dominant and one recessive allele for a trait (Dd).
*The dominant allele is expressed with combos DD or Dd, and the recessive allele is expressed only
with combo dd.
Mendelian Genetics
 Gregor Mendel (1800’s Austrian Monk)
 Employed true breeding which produced pure-bred (i.e genetically identical through self-pollination)
pea plants for his cross pollination experiments.
*True bred genotypes are homozygous (PP or pp)
 Green pods (PP) x Yellow pods (pp) = All Green Pods
F2?
Test Cross
 Accomplished by crossing an individual with a known genotype with one that has an unknown
genotype.
Ex: Assume red flower color is dominant over white flower color. If a homozygous recessive
individual (rr) is crossed with individual that could be either homozygous dominant (RR) or
heterozygous (Rr), and all the offspring are red, what must have been the genotype of the
unknown individual?
Other Traits Examined by Mendel
Intermediate Traits
 Traits that are not clearly dominant or recessive.
 They may have more than two alleles.
 They may be controlled by more than one gene.
Incomplete Dominance
 Neither allele is dominant
 A heterozygous (Cc) genotype produces a phenotype that is a blend of the forms of a trait.
Ex: Red and white flowers crossing to produce pink.
Codominance
 Both alleles for a trait are phenotypically expressed in a heterozygous offspring
 Both alleles are expressed equally
Ex: Variegated Plants
Multiple Alleles
 An organism can have only two alleles for a trait, but more than two possible alleles exist in the
population.
Ex: Blood types: A, B, O
– Genotypes can be: AA, AB, BB, BO, AO, OO
– A and B are also codominant.
Polygenic Traits
 Traits controlled by two or more genes
Ex: Skin Color or height
Sex Linked Traits


Traits that are determined by the alleles on the sex chromosomes.
Most are recessive and are carried on the X chromosome. Therefore, males who have the trait
will express it, but females (which have two X chromosomes) can carry and pass on the trait
without expressing it.
Key Words:
Alleles
Dominant
Recessive
Homozygous
Genotype
Phenotype
True-breeding
Law of segregation
Law of independent assortment
Incomplete dominance
Codominance
Multiple alleles
Polygenic trait
Sex-linked trait
Karyotype