Chapter 14
Genetic Variation
 Heritable variations observed daily in populations
 Transmitted from parents to offspring
 ‘Blending hypothesis’
 Genetic material mixes and eventually produces
uniformity
 Breeding experiments and everyday observances
disprove
 ‘Particulate hypothesis’
 Discrete units called ‘genes’ passed from parents to
offspring
 Retain separate identities generation to generation
Gregor Mendel
 Monk considered father of genetics
 Used peas to demonstrate ‘particulate’ mechanism
 Characters (hair color) with different traits (blonde or
brown)
 Controllable breeding

Hermaphrodites that commonly self-fertilize; could crossfertilize
 Lots of offspring with short generation time
 Chose ‘either or’ not ‘more or less’ characters
 Cross-pollinate (hybridize) contrasting true-breeding
lines


Parents are P generations and offspring are F1 generation
F2 generation to deduce 2 fundamental principles
 Law of Segregation and Law of Independent Assortment
Mendel’s Work
Useful
Genetics Terminology
Alleles
 Different versions of a gene
 Dominant mask other when both are present (P)
 Recessive are easily masked by others (p)


Dominance implies it determines phenotype, not superiority or
increased prevalence
Recessive traits often more common
 Actual combination determines genotype
 Resulting physical expression determines phenotype
Genotype versus Phenotype
 Homozygous
dominant (PP)
 2 dominant alleles
 Express dominant
 Heterozygous (Pp)
 1 dominant and 1
recessive allele
 Express dominant
 Homozygous
recessive (pp)
 2 recessive alleles
 Express recessive
Punnett Squares
 Predicts the allelic composition of offspring from a
cross between individuals of known genotype
 Testcross between an unknown dominant phenotype
and a recessive homozygote
 Monohybrid cross deals with only 1 character at a
time
 Dihybrid cross deals with 2 characters at the same
time
Testcross
Monohybrid Cross
 Each parent contributes only
1 type of allele to gamete
 F1 heterozygotes contribute
50% of each type of allele
 F2 offspring have a 3:1 ratio
Dihybrid Cross
 F2 offspring with 9:3:3:1
phenotypic ratio
 Same as monohybrid for
each of 2 characters
 Only for nonhomologs
Principles of Heredity
 Law of Segregation
 Two alleles for a heritable character separate during
gamete formation and end up in different gametes
 Law of Independent Assortment
 Each pair of alleles segregate independently of other
pairs of alleles during gamete formation
Reviewing Concepts
 What is a true-breeding individual and how does this
relate to allelic composition?
 Assume brown is the dominant character for eye color,
what case letter would represent the allele?
 What are the possible genotype(s) for a brown eyed
individual?
 If a person’s phenotype is blue eyes, what is their genotype
and what term most accurately applies to them?
Extending Mendelian Genetics
 Mendel used characters exhibiting complete
dominance, offspring like one of two parents
 Not applicable to all characters
 Genotype and phenotype relationship not all so simple


Single genes can have alleles that aren’t completely dominant
or recessive
Characters can have 1+ genes
 Basic principles of segregation and independent
assortment persist
Incomplete Dominance
 Offspring phenotype is
between both parental types
 Doesn’t support blending
 Each genotype has own
phenotype
Codominance
 Both alleles contribute to
the phenotype
 Blood type
demonstrates multiple
alleles too
 What blood type is the
universal donor?
 What blood type is the
universal acceptor?
Other Patterns of Inheritance
Epistasis: gene at one locus alters the
phenotypic expression of another gene at
a second locus
Polygenic Inheritance: an additive
effect of 2+ genes on one phenotypic
trait; opposite of pleiotropy (1 gene =
multiple phenotypes)
Pedigree Analysis
 Can’t control human
matings, so look at
existing
 Trait information from
phenotypes of a family
 Arrange into a pedigree
 Can deduce genotypes of
most from phenotypes
 Mendelian genetics and
logic
Is this a dominant or recessive trait?
Genetic Disorders
Albinism
 Recessive only affect homozygous
recessives
 Heterozygotes are carriers
 Prevents complete removal of
allele from a population
 Dominant affect all but
homozygous recessive
 Lethal forms less common
 When rare unlikely 2 carriers will
mate
 Inbreeding increases rate of
occurrence
 More likely both carrying
recessive allele
Achondroplasia
Autosomal Recessive
 Cystic fibrosis
 Most common lethal genetic disease in US
 2 recessive alleles have defective or lack chloride transport
channels
 Cause mucus build up in multiple organs, has pleiotropic
effects
 Sickle cell anemia
 Most common among people of African descent
 Only homozygous recessive have full blown disease
 Heterozygous only when O2 exposure down

Protection against malaria increases incidence of
Autosomal Dominant
 Can persist if death occurs
only after advanced age
 Genes passed on to
offspring already
 Huntington’s disease
 Degeneration of nervous
system
 No phenotypic effect till
35-45 years old
 Can now test for
genetically before
symptoms occur
Genetic Counseling
 Allows people with family histories of genetic disorders
to make informed decisions
 Many types of genetic disorders can now be tested for
 Potential problems inherent in medical records
 Fetal testing
 Amniocentesis extracts amniotic fluid to be tested
 Chronic villi sampling extracts cells from placenta, but
can be done sooner
 Karyotyping to view chromosomal abnormalities
 Ultrasounds produce anatomical images, no risks
 Biochemical testing for certain genetic markers