Download Fundamentals of Genetics

Mendelian
Genetics
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Gregor Mendel
(1822-1884)
“Father of
Genetics”
Genetics-Study
of how traits
are passed on
from parents
to offspring
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Gregor Johann Mendel
 Mendel found
that the
plants'
offspring
retained traits
of the parents
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Mendel’s Genetic Laws
• Mendel formed 3
genetic laws from his
study of pea plants &
genetic crosses:
1. Law of SegregationOrganisms carry alleles
for each physical trait,
and these alleles separate
randomly during the
formation of gametes
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Mendel’s Genetic Laws
2. Law of Independent
Assortment- Alleles
for different traits are
distributed
independently of alleles
of other traits; i.e.- just
because you received a
dominant allele for
height doesn’t mean
you will get a dominant
allele for flower color.
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Mendel’s Genetic Laws
3. Law of DominanceA dominant allele will
be expressed anytime
that it is inherited, and
a recessive allele can
only be expressed
when a dominant allele
is NOT present.
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Genetic Terms
 Trait - any
characteristic that
can be passed from
parent to offspring
(ex: yellow fur)
 Heredity - passing
of traits from
parent to offspring
 Genetics - study of
heredity
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Types of Genetic Crosses
Monohybrid
cross - cross
involving a
single trait
e.g. flower color
Dihybrid cross cross involving
two traits
e.g. flower color
& plant height
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Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
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• Did the observed ratio
match the theoretical ratio?
The theoretical or expected ratio of
plants producing round or wrinkled
seeds is 3 round :1 wrinkled
Mendel’s observed ratio was 2.96:1,
which is EXTREMELY CLOSE to the
expected statistical outcome!
The larger the sample the more nearly
the results approximate to the
theoretical ratio
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Generation terminology
• P1 Generation = the parental
generation in a breeding experiment.
• F1 generation = the first-generation
offspring in a breeding experiment.
(1st filial generation)
• F2 generation = the second-generation
offspring in a breeding experiment.
(2nd filial generation)
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Alleles - two forms of a
gene (dominant &
recessive)
Dominant – the one of
the two genes
expressed in the
hybrid; represented by
a capital letter (B)
Recessive - gene that is
“masked” in the
hybrid; represented by
a lowercase letter (b)
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Different Genotypes
 Homozygous
genotype - gene
combination involving
2 dominant or 2
recessive genes (e.g.
RR or rr); also called
pure
 Heterozygous
genotype - gene
combination of one
dominant & one
recessive allele (e.g.
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Rr);
two more terms
Genotype - gene combination for a
trait (e.g. BB, Bb, bb)
Phenotype - the physical features
resulting from a genotype; how
something looks. (e.g. black,
white, tall, short, etc.)
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Complete Dominance
Genotype of alleles:
R = red flower
r = white flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
white
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Incomplete Dominance
• A heterozygous
•
genotype yields an
intermediary
phenotype.
Heterozygous
phenotype is
“somewhere in
between” the
dominant and
recessive phenotypes
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Genotype & Phenotype in
Flowers (incomplete dominance situation)
Genotype of alleles:
R = red flower
r = white flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Phenotypes
RED
Rr
PINK
rr
WHITE
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Incomplete Dominance
• Hybrids have an
appearance
somewhat in
between the
phenotypes of
the two parent
varieties.
• Example:
snapdragons
• Rr yields a PINK
flower!
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Incomplete Dominance
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Codominance
Two alleles are expressed
(multiple alleles) in
heterozygous individuals.
Example: blood type
Human blood under the
microscope
1.
2.
3.
4.
type A =
type B =
type AB=
type O =
IAIA or IAi
IBIB or IBi
IAIB
ii
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A Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
A
B
o
o
Ao
Ao
Bo
Bo
1/2 = Ao (A)
1/2 = Bo (B)
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Sex-linked Traits
• Traits (genes) located on
the sex chromosomes
• Sex chromosomes are X & Y
• XX genotype for females
• XY genotype for males
• Most sex-linked traits
carried on X chromosome
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•
A trait carried on a
Sex-Linked Traits
sex chromosome is
termed “sex-linked”
• Males have only one
copy of each sex
chromosome…NO
BACKUP for a
defunct gene!
• Females have 2 X’s,
so can be “carriers”.
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Female Carriers
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Examples of Sex-linked diseases
• Hemophilia A
– Clotting factor 8 carried on
X chromosome
• Red-Green colorblindness
– Carried on the X
chromosome as well
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Human Genetics
• Humans have 46
chromosomes
– 44 autosomes
– 2 sex chromosomes
• XX=female
• XY=male
• Y chromosome contains
the SRY gene, which
determines masculinity
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• Germ Cell mutations:
Chromosomal
affect the offspring,
Mutations
but not the organism
Drugs such as
methamphetamines and
marijuana contain chemicals
that can mutate cells.
– THC
(tetrahydrocannibinol)
is the active ingredient
in marijuana, and is a
germ cell mutagen.
– Significantly higher
rates of birth defects
are found among the
children of drug users
than the normal
populace.
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Types of chromosomal
aberrations
• Deletion
– Part of a
chromosome breaks
off & dissolves
• Inversion
– Breaks off, flips
around, &
reattaches
• Translocation
– Part breaks off &
then reattaches on
a non-homologous
chromosome.
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Types of chromosomal
aberrations
• Nondisjunction
– Homologous
chromosomes fail to
separate during
meiosis.
– Results in too many
or too few
chromosomes
– Ex: Trisomy 21
(Down Syndrome)
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Down Syndrome (Trisomy 21)
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Progeria
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Single Gene diseases
• Huntington’s Disease
• Cystic Fibrosis
• Sickle-cell Anemia
• Hemophilia
• Hypertrichosis
• About 6,000 inherited diseases are
single-gene diseases
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Hypertrichosis
Polygenic Diseases and Disorders
• Polygenic diseases may have
a “family tendency”, but the
determining factor of
influence is environmental.
– Smoking and heart dis.
– Sunbathing and skin
cancer
– Drinking and liver cancer
– Etc.