PATTERNS OF INHERITANCE
I.
The science of genetics has ancient roots
a. Hippocrates, known as the father of medicine, suggested an
explanation called pangenesis
b. PANGENESIS  particles called pangenes travel from each
part of an organism’s body to the eggs or sperm and are then
passed to the next generation
i. *NOT A CORRECT theory  body cells do not
influence sex cells
c. “BLENDING” HYPOTHESIS  male and female
characteristics mix in the formation of the offspring
i. *NOT A CORRECT theory  traits may “skip” a
generation
II.
Experimental genetics began in an abbey garden
a. Gregor Mendel (1860’s) discovered the fundamentals of
genetics
i. Stressed that inheritable factors retain their individuality
ii. Worked with garden pea plants
iii. SELF-FERTILIZE  sperm carrying pollen land on
carpel from same plant
iv. CROSS-FERTILIZE  fertilization of one plant by
pollen from a different plant
b.
c.
d.
e.
f.
g.
h.
1. Steps of cross-fertilization
a. He prevented self-fertilization by cutting off
the stamens of immature flowers
b. Dusted this female plant with pollen from
another plant
c. Carpel will develop into a pod, containing
seeds
d. He planted these seeds and allowed them to
grow
Mendel always knew the parentage of the plants involved; KEY
to drawing conclusions
TRUE-BREED  always return same offspring
HYBRIDS  offspring from two different varieties
CROSS  short for cross-fertilization
P GENERATION parental
F1 generation  filial (offspring)
F2 generation  filial 2 (offspring)
III.
Mendel’s principle of segregation describes the inheritance of a
single characteristic
a. MONOHYBRID CROSS  parent plants differ in only one
characteristic
P generation
Purple flowers
X
White flowers
(True-breeding

parents)
F1 generation
Purple flowers
 Fertilization among
F1 plants (F1 X F1)
F2 generation
Purple flowers
White flowers
(3/4 of plants)
(1/4 of plants)
b. There are alternative forms of genes, the units that determine
heritable traits. We call alternative forms of genes ALLELES
c. For each inherited characteristic, an organism has two genes,
one from each parent. These genes may both be the same
allele, or they may be different alleles
d. A sperm or egg carries only one allele for each inherited trait,
because allele pairs separate (segregate) from each other during
the production of gametes
e. When the two genes of a pair are different alleles and one is
fully expressed while the other has no noticeable effect on the
organism’s appearance, the alleles are called the DOMINANT
allele and the RECESSIVE allele, respectively
*Uppercase letters represent DOMINANT allele
*Lowercase letters represent Recessive allele
P Plants
Gametes
PP
X
pp


All P
All p

F1 plants
(hybrids)
Gametes
All Pp


½P
½p



F2 Plants
*Punnet Square of PP X pp
Phenotypic ratio  3:1
Genotypic ration  1:2:1
f. HOMOZYGOUS  a true-breeding organism, which has a pair
of identical alleles for a characteristic
g. HETEROZYGOUS  an organism with two different alleles
for a characteristic
h. PUNNET SQUARE  shows possible combinations of
gametes
i. PHENOTYPE  an organism’s expressed physical traits
j. GENOTYPE  an organism’s genetic makeup
k. How can the disappearance of a trait in one generation, then
reappear the following generation be explained?
i. PRINCIPLE of SEGREGRATION  pairs of genes
segregate (separate) during gamete formation; the fusion
of gametes at fertilization pairs genes once again
IV.
Homologous chromosomes bare the two alleles for each
characteristic
a. ALLELES  alternative forms of a gene reside at the same
locus on homologous chromosomes
V.
The principle of independent assortment is revealed by tracking
two characteristics at once
a. DIHYBRID CROSS  mating of parents differing in two
characteristics
b. Do traits get passed as a package or independently?
i. INDEPENDENTLY, most of the time
c. PRINCIPLE OF INDEPENDENT ASSORTMENT  each
pair of alleles segregates independently during gamete
formation
VI.
Geneticists use the testcross to determine unknown genotypes
a. TEST CROSS  a mating between an individual of unknown
genotype and a homozygous recessive individual
Testcross: Black Coat
Chocolate Coat
Genotypes
B_
bb
Two possibilities for the black dog
BB
or
Bb
Gametes
B
B
b
Offspring
All black
1 black: 1 chocolate
VII. Mendel’s principles reflect the rules of probability
a. Probability scale ranges from 0 to 1 with 0 representing the
chance an event will NOT occur and 1 presenting that an event
WILL occur
b. All probabilities must add up to 1
c. RULE OF MULTIPLICATION  the probably of a compound
event is the product of the separate probabilities of the
independent events
d. RULE OF ADDITION  the probability that an event can
occur in two or more alternative ways is the sum of the separate
probabilities of the different ways
e. Applying these rules allows us to predict probabilities for very
complex crosses that would require too complex a Punnet
Square
VIII. Genetic traits in humans can be tracked through family pedigrees
a. PEDIGREE  a family’s history for a trait assembled in a
visual map
b. CARRIERS  people who have one copy of the allele for a
recessive disorder and do not exhibit symptoms
IX.
Many inherited disorders in humans are controlled by a single gene
a. Some AUTOSOMAL Disorders in Humans
Disorder
Major
Incidence
Comments
Symptoms
Recessive Disorders
Albinism
Lack of
1 / 22,000
Very easily
pigment in skin,
sunburned
hair, and eyes
Cystic Fibrosis
Excess mucus
1 / 1,800
in lungs,
Caucasians
digestive tract,
liver; increased
susceptibility to
infections;
death in infancy
unless treated
Galactosemia
Accumulation
1 / 100,000
Treated by
of galactose in
eliminating
tissues; mental
galactose from
retardation; eye
diet
and liver
damage
Phenylketonuria
Accumulation
1 / 10,000 in
of
US and Europe
phenylalanine
in blood; lack of
normal skin
pigment, mental
retardation
Sickle-cell disease
Sickled red
1 / 500 African Alleles are
blood cells;
Americans
codominant
damage to
many tissues
Tay-Sach’s disease
Lipid
1 / 3500 Jews
accumulation in from Central
brain cells;
Europe
mental
deficiency;
blindness, death
in childhood
Dominant disorders
Achondroplasia
Alzheimer’s disease
Dwarfism
Mental
deterioration;
usually strikes
late in life
Huntington’s disease Mental
deterioration
and
uncontrollable
movements;
strikes in
middle age
Hypercholesterolemia Excess
cholesterol in
blood; heart
disease
X.
1 / 25,000
Not Known
1 / 25,000
1 / 500 are
heterozygous
Incomplete
dominance
Fetal testing can spot many inherited disorders early in pregnancy
a. AMNIOCENTESIS  extraction of amniotic fluid from a
pregnant woman (14 – 20 weeks)
b. CHORIONIC VILLUS SAMPLING (CVS)  physician
insects a narrow flexible tube and suctions off a small amount
of fetal tissue (chorionic villi) off the placenta
c. ULTRASOUND IMAGING  uses sound waves to produce
an image of the fetus
XI.
The relationship of genotype to phenotype is rarely simple
a. Mendel’s principles work for simple traits, but most traits are
not simple
XII. Incomplete dominance results in intermediate phenotypes
a. INCOMPLETE DOMINANCE  offspring that are hybrids
have an appearance IN BETWEEN the phenotypes of the two
parental varieties
P Generation
RR
X
rr


R
r
Gametes

F1 Generation
Rr

Pink

R
r

F2 Generation
1 Red; 2 Pink; 1 White
Gametes
XIII. Many genes have more than two alleles in the population
a. Although each individual carries, at most, two different alleles
for a particular gene, in cases of multiple alleles, more than two
possible alleles exist in the population
b. ABO BLOOD GROUPS  three alleles for this characteristic
which produces for phenotypes; A, B, O, or AB
i. Named for the presence of a carbohydrate (A or B)
ii. Relevance in blood transfusions
c. CODOMINANCE  both alleles are expressed in
heterozygous individuals
Phenotype
Genotypes
O
ii
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
d. Blood type O  universal donor
e. Blood type AB  universal acceptor
XIV. A single gene may affect many phenotypic characteristics
a. PLEIOTROPY  the impact of a single gene on more than one
characteristic
i. Individual homozygous for sickle cell anemia

Sickle cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes, causing red blood
cells to become sickle shaped
XV. Genetic testing can detect disease causing alleles
a. CARRIER TESTING  used to determine if a person carries a
harmful allele
b. DIAGNOSTIC TESTING  can confirm or rule out an
existing disorder
c. PRENATAL TESTING  checks for disorders in unborn
babies
d. NEWBORN SCREENING  can catch disorders right after
birth; allowing infants to receive medical care
e. PREDICTIVE TESTING  used to determine a person’s risk
for developing on specific disorder on the future
XVI. A single characteristic may be influenced by many genes
a. POLYGENIC INHERITANCE  the additive effects of two or
more genes on a single phenotypic characteristic (*converse of
PLEIOTROPY)
i. This results in a continuum of phenotypes rather than two
distinct possible phenotypes (i.e. skin color, height)
XVII. Chromosome behavior accounts for Mendel’s principles
a. CHROMOSOME THEORY OF INHERITANCE  the genes
are located on chromosomes and that the behavior of
chromosomes during meiosis and fertilization accounts for
inheritance patterns
XVIII. Genes on the same chromosome tend to be inherited together
a. LINKED GENES  genes that are located on the same
chromosome
i. Since they are on the same chromosome, linked genes
tend to be inherited together so they defy Mendel’s
principles
XIX. Crossing over produces new combinations of alleles
a. Crossing over RECOMBINES linked genes into assortments of
alleles not found in parents
b. RECOMBINATION FREQUENCY  the percentage of
offspring that are recombinants (having a genotype not found in
either parent)
XX. Geneticists use crossover data to map genes
a. The greater distance between two genes on the same
chromosome, the higher the possibility that a crossover event
will occur
b. You can use the recombination frequencies to determine
approximate locations of genes on a chromosome
XXI. Chromosomes determine sex in many species
a. SEX CHROMOSOMES  a pair of chromosomes that
determine an organism’s gender (sex)
b. There are various chromosome systems
i. XY
ii. XO
iii. ZW
c. Some organisms like insects lack sex chromosomes, sex is
determined by chromosome number
d. MONOECIOUS  plants that produce both egg and sperm
e. HERMAPHRODITES  animals that produce both egg and
sperm
XXII. Sex-linked genes exhibit a unique pattern of inheritance
a. SEX-LINKED GENES  any gene located on a sex
chromosome
XXIII. Sex-linked disorders affect mostly males
a. RED-GREEN COLOR BLINDNESS  a common sex-linked
disorder characterized by a malfunction of light sensitive cells
in the (eyes?)
b. HEMOPHILIA  a sex-linked recessive trait characterized by
excessive bleeding due to a defective gene involved in blood
clotting
c. DUCHENNE MUSCULAR DYSTROPHY  a sex-linked
recessive disorder characterized by a progressive weakening
and loss of muscle tissue