Genetic Vocabulary
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Genotype: The types of the genes of an
organism
Phenotype: The observable
characteristics of an organism
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Dominant Gene: A gene which, when
present on a chromosome, passes on a
certain physical characteristic.
Recessive Gene: A gene that is inferior
to another gene that controls the same
trait (the dominant gene). The inferior
gene does not get expressed in the
presence of a dominant gene.
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Allele: Location of a gene on a chromosome
Homozygous: The state of having two identical
alleles of a particular gene (eg AA, aa).
Heterozygous: The state of having two
different alleles of a particular gene (eg Aa)
The Fundamental Question
What is the relationship between genes (genotype) and observable
characteristics (phenotype)?
The answer?
Phenotype = Genotype + Environment.
Genes and Environment Determine Characters
Genetically identical hydrangeas growing in soils of different acidity
(different environments).
The phenotype = genotype + environment principle applies equally
to human traits.
Genes, Alleles, and Chromosomes
Consistency is Good
No matter what the
character, Mendel
observed a 3:1 ratio of
characters in the F2.
Characters
investigated by
Mendel
Monohybrid Crosses Yielded Consistent Results
Therefore, the Principle of Segregation indeed is a general principle of genetics.
What Works for Peas Also Works for Humans
In the cross Aa x Aa, where A is a
dominant allele for wild type
(standard) pigmentation and a is a
recessive allele for no pigmentation
(albinism), ¾ of offspring will be wild
type and ¼ will be albino.
An albino woman
Are Different Characters Like Color and
Shape Inherited Together or Inherited
Independently?
Mendel performed dihybrid crosses
to find out.
Mendel’s conclusion: Different
characters are inherited
independently.
Simple Dominance Inheritance – When one trait
is determined by one type of gene.
List of Simple Dominance Genes:
Tongue Rolling
Blue vs. Brown Eyes
Ear-Lobes
Arm Crossing
Hand Folding
PTC
Multi-Gene Inheritance – When one trait is
determined by many genes.
List of Multi-gene Inheritances:
Height
Skin pigmentation
Weight
Metabolism
Incomplete Dominance – two different alleles both possess
dominance, resulting in a mixture of the alleles’ traits.
Examples:
Green Eyes
Pink Roses
Wavy Hair
Co-Dominance – Two different alleles both possess dominance,
but the resulting trait features both phenotypes
Examples:
Type AB Blood
Spots
Sample problems:
A Brown-eyed man whose mom was blue-eyed marries a woman with
blue eyes. What is the probability of have a daughter with blue eyes?
What two types of eye color could you cross, that could produce
offspring with the 3 types of eye colors?
A black dog marries a white dog what are the chances that they will
produce a puppy with spots?