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Genetics & Inheritance
Gregor Mendel
• Father of Genetics
• worked in a monastery
• Studied how traits were
passed from parents to
offspring
Experimental Approach
• Chose garden
peas because:
– Self-fertilizing
– True-breeding
Experimental
Approach
• self-pollinated
• cross-pollinated
by human
manipulation
btwn two diff.
plants
• Mendel cross-fertilized truebreeding garden pea plants having
clearly contrasting traits
Mendel’s Experiment
1. P Generation
• Self-pollinated for several generations
to get “true-breeding”
• Always produce offspring w/ the
desired trait
2. F1 Generation
• Cross-pollinated P individuals with
contrasting traits
• purple X white
3. F2 Generation
• Self-pollinated an
F1 individual
• purple X purple
Mendel Wondered:
Why does one form of the trait
disappear in the
first generation (F1 ),
only to show up in the
second generation (F2 )??
Some Terms Used In Genetics
• Dominant (G): When present,
“masks” the other trait
• Recessive (g): Has no effect on the
organism when the dominant allele
is present
Genetic
Terms
Basic Genetics
• Observable characters are the result of
genes that come in slightly different
molecular forms called alleles.
Genetic Concepts
• Genotype is the alleles present
(I.e. Aa, or aa) (this individuals
“genetic makeup”
• Phenotype is
what you see
(I.e. brown, tall)
Some Terms Used In Genetics
• Homozygous = alleles are the same
– dominant =“AA”
–recessive = “aa”
• Heterozygous = alleles differ
(“Aa”).
– One allele is dominant (A)
–Second allele is recessive (a)
º Dominant allele blocks expression
of recessive product
Probability
• Likelihood that a specific event will
occur
• May be expressed at fraction,
decimal, percent or ratio
• Every time an event happens it has
equal probability of happening regardless of # of times it has
happened before
Predicting results of
monohybrid cross
• Use a punnett
square
• Tracks only
one trait
Phenotypic/Genotypic
Ratios
• Phenotypic: Dominant:recessive (3:1)
• Genotypic: HH:Hh:hh (1:2:1) or
(1HH:2Hh:1hh)
• Totals must equal # of boxes in
punnett square
Punnett Squares
• Dihybrid cross: examines 2 traits
together
Testcross
• Cross an individual w/ an unknown genotype
(dominant pheno) w/ a homozygous
recessive - if any offspring are recessive, the
unknown must be heterozygous
• Plant produces all purple flowers, what is its
genotype? PP or Pp?
P P
P p
p Pp
p Pp
Pp
Pp
p
p
Pp
Pp
pp
pp
Complete Dominance
• Dominant allele completely masks
the expression of another
–Heterozygous and homozygous
dominant individuals show same
phenotype
X
Incomplete Dominance
• Dominant allele
cannot completely
mask the expression
of another
• Have intermediate
phenotype
– Example: red
snapdragons
crossed with white
ones yield pink.
Codominance
– A white horse and a black horse
produce horses with black and white
hairs – both alleles are expressed
–Roan horse:
Multiple Alleles
• Controlled by 3 + alleles of a gene
• Ex: Blood Types
• Phenotype
Genotype
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
Single Allele Traits
• Cystic fibrosis
• Hair on third
knuckle
• Earlobe
attachment
• Tongue rolling
Polygenic Traits
• Characteristic controlled by several
genes
– skin, hair, eye color
Complex Characters
• Influenced by genes & environment
– Polygenic traits
– Height, heart disease cancer
Effect of Gene Location
• Sex-linked genes & traits
– X Linked
• Traits found only on the X
chromosome
• Males get only 1 allele because they
have only 1 X
• Hemophilia
• Colorblindness (recessive)
• ALD (Lorenzo’s oil)
Sex-linked traits
Dihybrid Cross
• Predict results of 2 traits
Dihybrid Cross
Sex Chromosomes & Autosomes
• Sex C’some determine gender
– Male = XY, Female = XX
• Autosomes: all other c’somes
Male & Female Karyotypes
Mendel's
Law of Segregation
• Diploid organisms inherit two
genes per trait
• A pair of factors (alleles) is
segregated or separated during the
formation of gametes (meiosis –
anaphase II) so sperm or egg only
carry one allele for each trait
• If mom is Aa..you either get A or a
Law of Independent
Assortment
• Factors for different characteristics are
distributed to gametes independently of
one another
• Only true for genes on separate c’somes
or far apart on same c’some
• Ex: white flowers and smooth pods are
independent of each other OR dimples
and skin color are independent of each
other
Mutations= Ch 12
• Change in a gene
– may be due to copying incorrectly or
damaged
• Most are recessive
• Can be good, bad, or have no effect
• Somatic ones (body cells) affect only
the organism
• If occurs in germ cell (gametes), may
be passed to offspring
Chromosome Mutations
• Deletion - lose a piece of c’some
• Inversion - a piece flips around
• Translocation - piece of c’some
attaches itself to a different c’some
• non-disjunction - homologues don’t
separate during meiosis
– Trisomy 21 = Down Syndrome …..47X
– XXY = Klinefelters Syndrome in males
(feminine, infertile)…..47X
– X = Turners (female but
infertile)….monosomy….45X
Chromosome Mutations
• Klinefelters
• Turner Syndrome
Gene Mutations
Gene Mutations
• Involve one or more nucleotides in a strand
of DNA (not whole chromosome)
• Point mutations-ONE error
– Substitution
• THE DOG BIT THE CAT
• THE DOG BIT THE CAR
• Sickle Cell anemia: subs A for T 
defective hemoglobin
– Deletion-deletes a nucleotide
• THE CAT ATE THE FAT RAT
• THE ATA TET HEF ATR ATT
– Insertion-inserts a nucleotide
• THE CAT ATE THE FAT RAT
• THE CAT ATE THE FAT RATS
• Both cause frameshift mutations!
Gene Mutations
• Frameshift throws off
entire
sequence…
caused by
deletions or
additions
Detecting Genetic Disorders
• Genetic screening: examines your genetic
makeup & potential risk of passing
disorders to offspring followed by genetic
counseling to discuss results
• Before Birth on fetus
– Amniocentesis: analyze amniotic fluid looks
at fetal cells, proteins, do karyotype
– Chorionic Villi Sampling (CVS): from tissue
between placenta & fetus, do karyotype
Treating Genetic Disorders
• Symptom-relieving
– Ex: insulin injections for diabetic
• Gene Therapy
– Replace defective gene w/ copy of a healthy
one
• Somatic cell therapy alters only body cells
• Germ cell therapy alters sperm or egg
Linkage groups
• Genes located on the same
c’some are linked
• You inherit them together - unless
crossing over separates them=
chromosome maps
Crossing
Over
• The further
apart two genes
are from each
other on the
c’some the
likelier they are
to be separated
by crossovers
Genetic
Disorders
Pedigrees
• Way of tracing traits through a family
Male - has trait
Female - has trait
Male- doesn’t have trait
Male - carrier
Female - doesn’t have trait
Female - carrier
• Carrier: doesn’t express trait, but
can pass it on to offspring
Pedigree for Cystic Fibrosis