GENETICS
Vocabulary
 Genes
& Alleles
 Genotype vs. Phenotype
 Dominant vs. Recessive
 Homozygous vs. Heterozygous
 Pure-breeding vs. Hybrid
 Monohybrid vs. Dihybrid
 Punnett Square
 Generations: P, F1, F2, etc.
Gregor
Mendel’s
Peas
Dihybrid Cross



Mating of parents that are both heterozygous for two traits
Supports Mendel’s Laws
Exhibit a phenotypic ratio of 9:3:3:1
Di- and Trihybrid Crosses

The hard way…
BIG
PUNNETT
SQUARES

Di- and Trihybrid
Crosses

The easy way…
BRANCHING
OR TREE
METHOD

The Testcross

Crossing an unknown dominant
(homo/hetero?) with a recessive
The Law of Segregation
•
•
Separation of
alleles to gametes
During meiosis,
the two members
of each gene pair
must separate
into different
gamete cells so
that each gamete
receives only one
copy
The Law of Independent Assortment
•
•
Independent assortment of alleles
The different genes of different traits are
transmitted independently of each other
• Not true of linked genes
Exceptions to Mendelian Genetics
 Incomplete
Dominance
 Codominance
 Multiple Alleles & Polymorphism
 Gene-Environment Interaction
 Polygenic Inheritance
Incomplete Dominance

Heterozygotes
have an
intermediate
phenotype
– Often in flowers
(white as
recessive)

Dominance is not
an “all-or-none”
rule
Codominance


The heterozygotes display the phenotype of both
alleles of a single gene
Neither allele is dominant or recessive to the other
– Roan cattle: intermixed white and pigmented hairs
Multiple Alleles & Polymorphism
 Many
genes have more than two alleles
–Polymorphism: when more than two distinct
phenotypes are present in a population due to
multiple allelism
 Blood
 I A,
types: 3 alleles, 4 phenotypes
IB, i alleles(A&B=codominant, O=recessive)
 A/B glycoproteins on RBC
 Immune response to carbs
you don’t have
(antibody agglutination)
Gene-Environment Interaction
 Most
phenotypes are strongly influenced
by the physical environment as well as
their genotypes
 PKU (phenylketonuria)
–Causes dietary phenylalanine to accumulate
in the body, results in mental retardation
–Individuals placed on a low-phenylalanine
diet, however, develop normally
Polygenic Inheritance
 Interaction
of many genes to shape a
single phenotype
 Often results in continuous variation
(traits expressed as a range of
varieties), “quantitative” trait
 Skin color, height (most human
traits)
Model Organisms in Genetics
A good model organism:
–
–
–
–
–
Easy to maintain and breed
Reproduce quickly, high fecundity
Controllable sexual reproduction
Small genome (easy to manipulate DNA)
Easily recognizable traits
First: garden peas (Mendel)
 Common: E.coli, mice,
fruit flies (Morgan),
mustard plant

Morgan’s Fruit Flies
Drisophila
melanogaster
 He first identified
different phenotypes

– Wild-type: The most
common phenotype
for each trait
– Mutant: A change in
the phenotype
Morgan’s Experiments
Red eyes are wildtype and white eyes
are mutations
 Reciprocal cross
 Gender affected
inheritance patterns
 The gene for eye
color (Ww) is only on
the X chromosome
(X-linked)

– Females (XX) – inherit
2 copies
– Males (XY) – inherit 1
copy
Sex Chromosomes


Determine the sex of the
offspring
Humans and most mammals:
XY system
– Females XX

Produce all X gametes
– Males XY


Produce ½ X and ½ Y gametes
Other species have other
systems:
–
–
–
–
ZW in birds, others
XO in some insects
Haplodiploidy in bees, ants
Temperature-determined in some
reptiles
SRY Gene

Sex-determining Region Y gene
– Codes for a transcription factor
In mammals
 Initiates male sex determination
(development of testes)
 Significance

– The presence of a Y chromosome leads to
maleness, (even XXY, XXXY)
 If
the SRY gene is missing or defective, appear
female but infertile
– XX with translocated SRY gene – appear male
X Inactivation
 One
of the two copies of a female’s
X chromosomes is inactivated
(becomes a Barr body)
 Random, irreversible
 All cells in female mammals are not
functionally identical!
 Calico Cats:
– Fur color is X-linked; in
development its random
as to which X was inactivated
for that area of the body
Linked Genes


The physical association of two or more genes
found on the same chromosome
Predicted to be transmitted together during
gamete formation violating the law of independent
assortment
Morgan’s Experiment on Linked Genes
Recombinant Chromosomes

Linked genes are inherited together unless
crossing over occurs. When crossing over
takes place, genetic recombination occurs.
Linkage Mapping


Data on the frequency of
crossing over is used to
create a genetic map
showing the relative
positions of genes along a
particular chromosome
Map unit = % of time allele
is known to cross over
Add distances for frequency
Point Mutations
Single base substitution (A, T, C, G)
 Significance: May alter codon, amino
acid, protein structure
 May be advantageous, neutral, or
deleterious

Chromosomal Mutations
Deletion
Insertion or
Duplication
Inversion
Translocation
Basis for evolution:
- Duplications can lead to specialization of similar genes
- Inversions may alter mRNA splicing or the reading frame
Nondisjunction

Failure of chromosome pairs or chromatids to
separate properly during mitosis or meiosis
Aneuploidy
 Change
in part of the chromosome
set (abnormal chromosome number)
 Human genetic disorders:
– Monosomy 
Example: Turner Syndrome (XO female)
– Trisomy 
One missing chromosome
One extra chromosome
Example: Klinefelter Syndrome (XXY male)
Polyploidy
 More
than two complete sets of
chromosomes
– Fatal in human embryos
– Common in many flowering plants
 Commercial
advantages in plants
– Larger flowers, seeds, fruits
– Seedless fruits (bananas, watermelon)
– Disease resistance
Pedigrees
 Family
trees, showing:
–Relationships
–Sex of individual
–Presence of trait
 Determine
whether a trait is due to a
dominant or recessive allele and
whether the gene responsible is
located on a sex chromosome or an
autosome
Autosomal Recessive Traits

Individuals with the trait must be
homozygous
– Parents of an affected individual are
heterozygous carriers for the trait
Autosomal Dominant Traits
Expressed in any individual with at least
one dominant allele (either homozygous or
heterozygous for the trait)
 Ex: Huntington’s disease

Human Genetic Disorders

Autosomal Recessive
– Cystic Fibrosis


Thick mucous secretions in the lungs and digestive tract
Trinucleotide deletion (loss of 1 amino acid)
– Tay-Sachs


Neurological deterioration
Various point mutations
– Sickle Cell Anemia



Distorted hemoglobin
A-T point mutation
Autosomal Dominant
– Achondroplasia



Dwarfism
G-A Point Mutation
Homozygous Lethal
– Huntington’s Disease


Neurodegenerative
Trinucleotide repeat (CAGCAGCAG)
Sex-Linked Traits


Trait is not equally expressed between males and
females
X linked recessive
– Usually skips generations, males more likely affected
– Example: Hemophilia

X-linked dominant
– Rarely skip generations
– An affected male has all affected daughters but no
affected sons
Pleiotropy & Genetic Disorders


Pleiotropy = a single genotype that results in
multiple phenotypes
Marfan syndrome: A single gene results in a
connective tissue disorder, leading to increased
height and limb length, heart problems, etc.