Download Chapters 11-13: Classical Genetics

Chapters 11-12: Classical Genetics
I. Mendelian Genetics
A. Mendel’s experiments
1. pea plants with seven
distinct characteristics or
traits over many generations
and many years
2. utilized “pure” breeding
strains
3. crossed or mated plants by
transferring pollen (sperm)
from one strain or type to the
pistil (ovum) of the other
strain or type
4. collected and analyzed
data looking for patterns or
trends
B. Developed the basic laws
or principles of genetics
1. Principle of Unit
Characters:
2. Principle of Dominance
and Recessiveness:
3. Principle of Segregation:
4. Principle of Independent
Assortment:
C. Key terms and concepts
associated with Mendel’s
Laws
1. dominant vs recessive
2. homozygous vs
heterozygous
3. phenotype vs genotype
4. alleles
5. punnett squares
6. test cross
D. Monohybrid vs Dihybrid
crosses and punnett square
problems – problem solving
technique
E. Branching Diagrams –
problem solving technique
II. Non-Mendelian Genetics –
Other Patterns of Inheritance
A. Incomplete Dominance
B. Codominance
C. Epistasis – a gene at one
locus (position on a
chromosome) interferes with
a gene at a different locus,
both of which affect the same
trait
1. both loci must
have a functioning
(dominant) gene
for the dominant
phenotype to be
expressed
2. AAbb (white) x
aaBB (white) 
all purple (AaBb)
3. AaBb (purple) x
AaBb (purple) 
9/16 purple + 7/16
white
D. Pleiotropy – a single gene
affects more that one trait
Marfan’s syndrome (tall;
thin; long arms, legs, and
fingers; near sighted; wall of
the aorta is weak – Abraham
Lincoln?)
E. Multiple Alleles –
A,B,AB,O blood type
1. three alleles: IA,
IB, i
2. blood type –
antigens (on red
blood cells) and
antibodies (in
plasma)
3. O blood is the
universal donor,
AB blood is the
universal recipient.
4. To determine if a
blood transfusion
is feasible ask if
there is a match
between the cells of
the donor and the
antibodies of the
recipient. If there
is a match,
clumping will
occur leading to
clots and death.
F. Polygenic inheritance –
many genes affect the same
trait (height, hair color, eye
color, skin tones, etc.)
1. most human
traits are polygenic
2. how do you
know: large
variation in
phenotypes (“bell”
curve distribution)
G. Sex-linked (X-Linked)
Traits – traits whose genes
are found only on the sex
chromosomes – tend to be
more common in males
1. color blindness,
hemophilia in
humans
2. examples:
H. Gene Linkage – genes that
are linked are on the same
chromosome and therefore
inherited together
1. notation:
(normal): AaBb x AaBb
9/16 dominant/dominant;
3/16 dominant/recessive;
3/16 recessive/dominant;
1/16 recessive/recessive;
__ __
__ __
(linked): AB ab x AB ab
12/16 dominant/dominant;
4/16 recessive/recessive;
2. effect: reduces
genetic
combinations
possible
3. crossing over and
gene linkage can be
used to determine
gene map distance
units:
in #1: 100 offspring (70
were dominant for both
traits; 5 were dominant,
recessive; 5 were
recessive, dominant; and
20 were both recessive)
crossover frequency
(recombinant frequency)
= 10/100 or 10% = “A/a”
and “B/b” genes are 10
map units apart
I.Chromosomal Mutations
1. mutation = permanent
change in a gene or
chromosome that can be
passed on to other cells or
offspring if they occur in
gametes
2. nondisjunction = failure of
chromosomes to separate
during meiosis, may result
in abnormal chromosome
numbers
a. monosomy = only one
chromosome of a pair
(2n-1)
b. trisomy = three
chromosomes instead
of a pair (Down’s
syndrome – three #21
chromsomes)
(2n + 1)
c. polyploidy = multiple
sets of chromosomes;
i.e. 3n, 4n, 5n . . .
(uncommon in
animals, fairly
common in plants – up
to 50%, arises during
hybridization)
3. individual chromosome
abnormalities
a. inversion – a portion
is turned upside down
reversing the order of
the genes
b. translocation –
moving one part of a
chromsome to a
different, non
homologous
chromosome
c. deletion – part of a
chromosome breaks
off
d. duplication –
presence of a
chromosomal segment
more than once in the
same chromosome
J.
Probabilities and Genetics
1. rules:
a. All probabilities fall on
a scale from zero to one
b. The probabilities of all
the outcomes of an event
must add up to one
c. The probabilities of
independent events
occurring together is
found by multiplying
together their separate
probabilities
2. Using probabilities in
genetic problems
a. In a self cross of a pea
plant heterozygous for
both seed coat and
texture (SsYy), what is
the probability of an
offspring being both
smooth and green?
Determine the genotype:
(S ___ yy)
Determine the chance of
each separate event:
Smooth is dominant and
requires at least 1 S (Ss x
Ss  ¾)
Green is recessive and
requires two y’s (Yy x Yy - ¼)
¾ x ¼ = 3/16
b. For two separate events
each of which has 2
possible outcomes
(flipping a coin,
dominant vs recessive),
the probabilities of the
four possible outcomes
can be found by using the
binomial (p + q)2
(p + q) 2 = p2 + 2pq + q2
For three events: (p + q)3
(p + q)3 = p3 + 3p2q + 3pq2
+ q3
c. What is the probability
of three peas in a pod
from the cross Rr x rr
growing into 2 red
flowered and one white
flowered plant?
3p2q = 3 x (1/2)2 x ½ = 3/8
III. Human Genetics
A. Lethal alleles (cause the
death of the individual) and
other human “genetic”
diseases
1. dominant lethal alleles –
are rare since they will
be eliminated through
the death of the
individual
(example:
Huntington’s disease –
effects occur later in life
– neurological)
2. recessive lethal alleles
may be maintained in
the population in the
heterozygote (example:
brachydactyl –
shortened digits; in the
homozygous recessive,
individual dies during
embryonic development
and is reabsorbed into
the uterus – produces a
2:1 ratio)
3. sickle cell anemia – red
blood cells are
abnormally shaped
(recessive, but also
codominant)
a. Homozygous
dominant = normal
RBC’s
b. Heterozygous
dominant = both
normal and sickle
shaped RBC’s
c. Recessive = sickle cell
anemia
d. Under certain
conditions (prevalence
of malaria),
heterozygotes have an
advantage – resistant
to malaria
4. Tay Sach’s Disease –
metabolic disorder
resulting in deterioration
of the brain and early
death (recessive)
5. Cystic fibrosis – defective
in protein in lungs causes
mucus to build up,
alveoli stick together
causing lung damage and
early death (recessive)
6. PKU (Phenylketonuria) –
individuals lack an
enzyme that converts
phenylalanine into
tyrosine; excessive
phenylalanine builds up
in the body causing
organ damage and
mental retardation – can
be treated with a diet low
in phenylalanine
(recessive)
7. Albinism – lack of skin
pigmentation (recessive)
B. Using Pedigree charts to
follow the history of genetic
disorders
1. Male = box, female =
circle
2. Shaded circles and
squares are affected
individuals
3. Horizontal line between
individuals = mating;
vertical line between
individuals = children
4. dominant versus
recessive patterns, sexlinked (X), blood type
and other genetic traits
can be traced and
determined
C. Other human patterns of
inheritance
1. polygenic = many genes
affect the same trait
a. most human traits (eye
color, hair color, skin
color, height, etc.)
b. significant variation in
that trait
2. sex determination
a. male = XY
(heterozygous)
b. female = XX
(homozygous)
c. sex itself is a polygenic
trait with at least 19
different sets of genes
influencing its
expression (some are
autosomal)
d. in mammals, the Y
chromosome carries at
least one gene that
gives the embryo its
first “push” towards
becoming male 
secretes testes
determining factor
which causes the
immature gonads to
differentiate into testes
e. in birds and some
reptiles, female is
heterozygous (ZW)
and male is
homozygous (ZZ)
f. abnormal
arrangements of sex
chromosomes (XO:
Turner’s syndrome –
abnormal females,
infertile; XXY –
Klinefelter syndrome:
abnormal male)
3. sex-influenced – expression
of these traits depends upon
the level of sex hormones,
genes are usually located on
autosomes (male pattern
baldness in humans)
a. in males, male sex
hormones stimulate
expression of the
baldness allele even if
only one allele for
baldness is present
(acts as if it is
dominant)
b. in females, female sex
hormones reduce the
expression of the
baldness allele (the
same allele acts as if it
is recessive). Both
alleles must be present
for baldness in
females, and even if
present, the woman’s
hair is only “thinner”
IV. Genetic Counseling and Fetal
Testing
A. Blood tests and genetic
analysis of chromosomes can
help determine whether
prospective parents are
carriers for genetic disorders;
DNA testing
B. Chorionic villi sampling
and amniocentesis can be
used to obtain fetal cells
which can then be cultured
and examined for genetic
defects
1. human karyotype:
arrangement of
chromosomes in order
from largest to smallest
(1st 22 pairs – autosomes,
23 rd pair – sex
chromosomes)