Mendel and the Gene Idea
BASIC CONCEPTS IN GENETICS
WHAT IS HEREDITY
The passing on of characteristics from parents
to offspring
Trait
physical characteristic
Genetics
is the study of heredity
CHROMOSOME LOGICAL STRUCTURE



Gene – segments of DNA that code for the basic units of
heredity and are transmitted from one generation to the next
Allele – genes that reside at the same locus on homologous
chromosomes
Alleles are the result of mutation!!!!!!
GENOTYPES


At each locus (except for sex
chromosomes) there are 2
genes. These constitute the
individual’s genotype at the
locus. The alleles that are
present.
The expression of a genotype
is termed a phenotype. For
example, hair color, weight,
or the presence or absence
of a disease.
PHENOTYPES
GENOTYPE
Genotype
Gene(s) responsible
for the trait
The alleles that are
present on each
homologous
chromosome that
code for the trait
PHENOTYPE
Phenotype
Expression of the
characteristic
The trait
The way we “look”
Red hair or Brown hair
The expression of the
gene
PATTERNS OF INHERITANCE

Gregor Mendel
 (1822-1884)
is the
called the “Father of
Genetics”
 Researched with
garden peas
 Developed the ideas
that are the basis of
genetics
MENDEL USED PEAS…
Characters (inherited characteristic) are in two
distinct forms (such as white and purple color)
called traits.
Not many traits
Easy to keep track
The male and female gametes are enclosed
within the same flower – He could control the
fertilization process
Self-fertilization
Cross-pollination
The garden pea is small, grows easily, matures quickly and
produces many offspring.
MENDELIAN GENETICS
Mendel studied a number of characteristics in pea plants including:
•Height - short or TALL
•Seed color - green or YELLOW
•Seed shape - wrinkled or ROUND
•Seed coat color - white or GRAY
•Pod shape - constricted or SMOOTH
•Pod color - yellow or GREEN
•Flower position - terminal or AXIAL
MENDEL WAS A CAREFUL RESEARCHER




Carefully controlled experiments
Studied one trait at a time
Kept detailed data
Cross - combining gametes from
parents with different traits
 The offspring are called hybrids
 offspring of parents with
different traits
 A monohybrid cross is one that
looks at only one trait (let’s look
at plant height – tall or short)
 Cross fertilization
 Pollen from one plant to
fertilize another plant
MENDEL’S MONOHYBRID CROSSES

Step One:
 Mendel allowed the peas to self-pollinate for
several generations.
What Did Mendel Find?
MENDEL’S MONOHYBRID CROSSES
Step One:
Each variety was true-breeding for a particular
character.
tall plants only produced tall plants
These plants served as the parental generation.
The P generation is the first two individuals that
are crossed in a breeding experiment
MENDEL’S MONOHYBRID CROSSES

Step Two:
 Mendel cross-pollinated two P generation plants
with different traits
 The offspring were the first filial generation or
F1 generation
 Mendel recorded the traits of the offspring
What Did Mendel Find?
MENDEL’S MONOHYBRID CROSSES
 Tall
plant crossed with
short plants produced
all tall offspring
 Purple
flowers crossed
with white flowers
produced all purple
offspring
MENDEL’S MONOHYBRID CROSSES
Step Three:
Finally, Mendel allowed the F1 generation to selfpollinate.
He called the offspring of the F1 generation, the
second filial generation, or F2 generation
Again, Mendel recorded the traits of the offspring
What Did Mendel Find?
MENDEL’S MONOHYBRID CROSSES

The short plants reappeared!!!!!!

Mendel found that 3 out of 4 (¾) of the
offspring were tall & 1 out of 4 (¼) were
short
MENDEL’S MONOHYBRID CROSSES

Mendel found the same 1:3 ratio
(1 out of 4) in the other traits as well!
MENDEL’S RESULTS

He discovered different laws and
rules that explain factors affecting
heredity
THEORY OF HEREDITY



Before Mendel, people thought offspring were a blend of
traits
 Tall x short = medium
Mendel’s experiments did not support this theory
Mendel’s work led him to the understanding that traits are
carried in pairs (one from each parent)
QUESTIONS
What did Mendel cross?
What are traits?
What are gametes?
What is fertilization?
What is heredity?
What is genetics?
MENDEL’S 4 HYPOTHESES

Hypotheses 1
 For each inherited character,
an individual has two copies
of each gene
 One on each chromosome
 Alternative versions of
genes account for variations
in inherited characters.
 Alleles are different versions
of genes that impart the
same characteristic.
MENDEL’S 4 HYPOTHESES

Hypotheses 2


Gametes (sperm or egg) carry only one allele as a
result of pair separation during meiosis
Offspring inherit 2 alleles, 1 from each parent for
each characteristic (i.e. height, color, etc.)
MENDEL’S 4 HYPOTHESES

Hypotheses 3

The Rule of Dominance
 Some genes (alleles)
are dominant and
others are recessive
 The dominant allele, is
fully expressed in the
organism's appearance
 The recessive allele,
has no noticeable effect
on the organism's
appearance.
MENDEL’S 4 HYPOTHESES

Hypotheses 4



The two genes for
each character
segregate during
gamete production.
The genes are sorted
into separate
gametes, ensuring
variation.
This sorting process
depends on genetic
recombination
MENDEL’S FIRST LAW OF HEREDITY
Law of Random Segregation
A parent randomly passes only one allele for each
trait to each offspring
MENDEL’S SECOND LAW OF HEREDITY

Law of Independent Assortment


Different gene pairs assort independently in gamete
formation.
This Law is only true for genes on separate
chromosomes!
LAW OF INDEPENDENT ASSORTMENT
•
P GENERATION -True
Breeding Parent Plants
–
–
•
•
Gametes will be either P or p
FIRST FILIAL GENERATION F1
•
•
All purple (PP)
All white (pp)
F1 are all purple because of
dominance (Pp)
SECOND FILIAL GENERATION
F2
•
F2 results in a mathematically
predictable 3:1 ratio
MENDELIAN INHERITANCE PATTERNS




Involve genes directly influencing traits
Obey Mendel’s laws
 Law of segregation
 Law of independent assortment
Include
 Dominant / recessive relationships
 Gene interactions
 Phenotype-influencing roles of sex and environment
Most genes of eukaryotes follow a Mendelian inheritance
pattern
PREDICTING INHERITANCE




To determine the chances of inheriting a given trait,
scientists use Punnett squares and symbols to
represent the genes.
Each allele is represented by a letter
 UPPERCASE/CAPITAL letters are used to represent
dominant genes.
 lowercase letters are used to represent recessive
genes.
Homozygous - the two alleles for a trait are the same
(AA or aa)
Heterozygous - the two alleles for a trait are different
(Aa)
PREDICTING INHERITANCE


For example:
 T = represents the gene for TALL in pea plants
 t = represents the gene for short in pea plants
So:
 TT & Tt both result in a TALL plant, because T is
dominant over t.
 t is recessive.
 tt will result in a short plant.
PUNNETT SQUARES
 A diagram that predicts the outcome of a genetic
cross
 It considers all the possible combinations of
gametes
 1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S
PUNNETT SQUARES
The alleles from one parent go
here.
The alleles from the other
parent go here.
PUNNETT SQUARES
T
t
t
T
PUNNETT SQUARES
T
t
t
t
t
T
PUNNETT SQUARES
T
T
t
t
t
t
t
t
PUNNETT SQUARES
T
T
t
Tt
Tt
t
t
t
PUNNETT SQUARES
T
T
t
Tt
Tt
t
Tt
Tt
PUNNETT SQUARES
F1 generation
T
T
t
Tt
Tt
t
Tt
Tt
INTERPRETING THE RESULTS
 The genotype for all the
offspring is Tt.
 The genotype ratio is:
 Tt – 4:4 = 100%
heterozygous
 The phenotype for all the
offspring is tall.
 The phenotype ratio is:
 tall – 4:4 = 100% tall
T
T
t
Tt
Tt
t
Tt
Tt
PUNNETT SQUARES
F2 generation
T
t
T
TT
Tt
t
Tt
tt
INTERPRETING THE RESULTS
 This time the ratios are different!
 The genotype ratio is:
 TT – 1:4
Tt – 2:4
 1:2:1
 2 – homozygous
 2 – heterozygous
 The phenotype ratio is:
 TT, Tt, Tt = 3 tall
 tt = 1 short
 3:1 – tall : short
Tt.
tt – 1:4
T
t
T
TT
Tt
t
Tt
tt
LAWS OF PROBABILITY HELP EXPLAIN GENETIC EVENTS

Genetic ratios are most properly expressed
as probabilities:

Probabilities range from
 0 - an event is certain NOT to happen
 1.0 - an event is certain to happen
PREDICTING OUTCOMES-PROBABILITY
 The

likelihood that a specific event will occur
Probability = # of one kind of possible outcome
total # of all possible outcomes



a coin lands on “heads”
 1 outcome
Total possible outcomes
 =2
 heads or tails
Possibility that the coin will
land on heads = 1/2
PRODUCT LAW
 For simultaneous outcomes (this AND that)
 What is the chance that you will roll snake eyes with two
dice? (1 and 1)
 Chance of rolling 1 with first die = 1/6
 Chance of rolling 1 with second die = 1/6
 Chance of rolling two 1’s = 1/6 X 1/6 = 1/36
SUM LAW
 For outcomes that can occur more
than one way (this OR that)
 What is the chance that you will roll either a 1 or a 6
with one die?
 Chance of rolling 1 = 1/6
 Chance of rolling 6 = 1/6
 Chance of rolling 1 or 6 = 1/6 + 1/6 = 2/6 = 1/3
PROBABILITIES
Multiplication Rule
The probability that two independent
events, A and B, are realized
simultaneously is given by the product of
their separate probabilities
What fraction would we
expect to be
Round AND Green
3/4
x 1/4
= 3/16
46
PROBABILITIES
Addition Rule
The probability that one or the
other of two mutually exclusive
events, A or B, is the sum of
their separate probabilities
What fraction would we
expect to be
(Round and Green)
OR
(wrinkled and yellow)
3/16 + 3/16 = 6/16
47
DIHYBRID CROSS




F1 produces equal amounts of
4 possible genotypes
F2 reveals even more
genotypic possibilities
(9:3:3:1)
Dihybrid cross is equivalent to
two monohybrid crosses (12:4
or 3:1)
Illustrates the Law of
Independent Assortment
 Many genes do not follow a Mendelian inheritance
pattern
 Incomplete Dominance
 Co-dominance
 Multiple alleles
 Pleiotropy
 Polygenic Inheritance
 Lethal Dominance
 Environmental Influence on Gene Expression
49
INCOMPLETE DOMINANCE

The phenotype of the
heterozygous genotype is
intermediate between the
phenotypes of the
homozygous genotypes



E.g. snapdragons
Heterozygotes differ from
homozygotes
Predictable 1:2:1 ratio


Different than “blending”
hypothesis
No testcross necessary
CODOMINANCE
 Both alleles are dominant and affect the phenotype
in two different but equal ways




Andalusian chickens show this pattern of inheritance.
If you cross a black (BB) chicken
With a white (WW) chicken
You get black+white speckled (BW) chicken
MULTIPLE ALLELES


More than two possible alleles controlling one trait
Example3 alleles in blood type – OAB
 4 possible phenotypes = O, A, B, AB
 6 possible genotypes
Note:
this is also an example
of co-dominance
MULTIPLE ALLELES
 Genes that have more than two alleles
 More than two alleles exist in a given population
 However, any one individual only has two of these
alleles
 Example - Coat color in rabbits (4 alleles)
Himalayan Rabbit
Full Color Rabbit
Albino Rabbit
Chinchilla Rabbit
PLEIOTROPY



Gene influences
multiple
characteristics
A singe gene
influences more
than one
phenotypic trait.
Genes that exert
effects on
multiple aspects
of physiology or
anatomy are
pleiotropic
POLYGENIC INHERITANCE

Multiple genes
have an additive
effect on a single
character in the
phenotype


Example: Skin
Color or height
Usually is
described by a
bell-shaped curve
with majority
clustered in the
middle
EXAMPLES OF POLYGENIC INHERITANCE
LETHAL DOMINANCE
 T/t x T/t =
 T/T
T/t
t/t
 1 : 2 : 1 ratio at conception
 0 : 2 : 1 ratio at birth
CHARACTERS INFLUENCED BY THE ENVIRONMENT
pH of the soil will change the color of
hydrangea flowers from blue to pink
CHARACTERS INFLUENCED BY THE ENVIRONMENT

Temperature will affect color change in fur
CHARACTERS INFLUENCED BY THE ENVIRONMENT
Body temperature affects color in Siamese cats
 Height is affected by nutrition
HOW DO WE DETERMINE INHERITANCE OF HUMAN TRAITS

In humans, pedigree analysis is used to determine individual
genotypes and to predict the mode of transmission of single
gene traits
61
SOME EXAMPLES OF DOMINANT AND RECESSIVE
TRAITS IN HUMANS (AT ONE GENE LOCUS)
AUTOSOMAL DOMINANT TRAITS





Huntington disease is a progressive nerve degeneration, usually
beginning about middle age, that results in severe physical and mental
disability and ultimately in death
Every affected person has an affected parent
Two unaffected parents will not produce affected children. (aa x aa)
Both males and females are affected with equal frequency.
Pedigrees show no Carriers.
EXAMPLES OF AUTOSOMAL DOMINANT
DISORDERS

Dwarfism

Polydactyly and Syndactyly

Hypertension

Hereditary Edema

Chronic Simple Glaucoma – Drainage system for fluid in the eye does not work and pressure builds up,
leading to damage of the optic nerve which can result in blindness.

Huntington’s Disease – Nervous system degeneration resulting in certain and early death. Onset in
middle age.

Neurofibromatosis – Benign tumors in skin or deeper

Familial Hypercholesterolemia – High blood cholesterol and propensity for heart disease

Progeria – Drastic premature aging, rare, die by age 13. Symptoms include limited growth, alopecia,
small face and jaw, wrinkled skin, atherosclerosis, and cardiovascular problems but mental development
not affected.
AUTOSOMAL RECESSIVE TRAITS







Albinism = absence of pigment in the skin, hair, and iris of the eyes
Most affected persons have parents who are “normal” (Aa x Aa)
The parents are heterozygous for the recessive allele and are called carriers (Aa)
Approximately 1/4 of the children of carriers are affected (aa)
Close relatives who reproduce are more likely to have affected children.
Both males and females are affected with equal frequency.
Pedigrees show both male and female carriers.
EXAMPLES OF AUTOSOMAL RECESSIVE TRAITS

Congenital Deafness

Diabetes Mellitus

Sickle Cell anemia

Albinism

Phenylketoneuria (PKU) – Inability to break
down the amino acid phenylalanine. Requires
elimination of this amino acid from the diet or
results in serious mental retardation.

Galactosemia – enlarged liver, kidney failure,
brain and eye damage because can’t digest
milk sugar

Cystic Fibrosis – affects mucus and sweat
glands, thick mucus in lungs and digestive tract
that interferes with gas exchange, lethal.

Tay Sachs Disease – Nervous system
destruction due to lack of enzyme needed to
break down lipids necessary for normal brain
function. Early onset and common in Ashkenazi
Jews; results in blindness, seizures, paralysis,
and early death.
Why we look the way we look...
MENDEL AND HEREDITY