Genotype is the genetic makeup of a cell or organism; phenotype is its observed characteristics.
● genotype The genetic makeup of a cell or organism; the particular combination of alleles present in an
○ Variation of genotypes of a population depend on its gene pool
● polymorphism Any genetic difference among individuals sufficiently common that it is likely to be
present in a group of 50 randomly chosen individuals (e.g. eye color).
○ Result of mutation; at one point every individual had same genotype
○ Ex: many have C-G where others have A-T
● phenotype The expression of a physical, behavioral, or biochemical trait; an individual’s observable
phenotypes include height, weight, eye color, and so forth.
○ Result of genotype and environment
The effect of a genotype often depends on several factors.
● alleles The different forms of a gene, corresponding to different DNA sequences (polymorphisms) in
each different form. (gene: hair color: different alleles: brown vs. blonde)
○ homozygous Describes an individual who inherits an allele of the same type from each parent,
or a genotype in which both alleles for a given gene are of the same type.
○ heterozygous Describes an individual who inherits different types of alleles from the parents, or
genotypes in which the two alleles for a given gene are different
● Harmfulness or benefits of sickle cell anemia S allele show us
○ It depends on homozygous/hteterozygous inheritance
○ Effect of a genome depends on the environment (homozygous/heterozygous doesn’t
automatically correspond to beneficial harmful)
Some genetic differences are major risk factors for disease.
● Mutations and polymorphisms can be risk factors for diseases like emphysema, making it more likely for an
○ Esp. when combined w/ environmental factors like smoking tobacco
● genotype-by-environment interaction Unequal effects of the environment on different genotypes, resulting
in different phenotypes. Combo of certain environment and certain gene can be especially bad (or good)!
○ Genetic and environmental factors combine to influence phenotype, combo of the two risk factors is
worse than either of their individual effects
Not all genetic differences are harmful.
● Most genetic differences are neutral, because of the large portion of noncoding genes in the human genome
and just mutations that don’t really affect survivability/reproductivity
○ Ex: The taster/nontaster polymorphism
A few genetic differences are beneficial.
● in human populations, beneficial mutations are often discovered through their effects in protecting against
○ Ex: sickle-cell gene that protects against malaria, or another that protects against HIV/AIDS
○ Other beneficial mutations can allow organisms to better adapt to their environment
16.1 Early Theories of Inheritance
Early theories of heredity predicted the transmission of acquired characteristics.
● Hippocrates theorized that each part of the body of a mature male collected in a reproductive organ that
determined inherited traits (charactristics) of offspring
○ Believed traits acquired during lifetime could be passed to offspring
● Aristotle concluded that the process of heredity transmits only the potential for producing traits present in the
parents, and not the traits themselves
Belief in blending inheritance discouraged studies of hereditary transmission.
● blending inheritance Darwin’s now-discredited model in which heredity factors transmitted by the parents
become intermingled in the offspring instead of retaining their individual genetic identities.
○ Black bunny+white bunny=grey bunny
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