Gregor Mendel: The father of genetics
Mendel’s Inferences

The basic units of genetics are material
elements (lengths of DNA)
 These elements come in pairs (homologous
chromosomes)
 These elements maintain their character through
many generations (chromosomes copy
themselves)
 Gene pairs separate during the formation of
gametes (meiosis, homologous chromosomes
line up and separate into gametes)
Peas are selfpollinating
plants.
Humans
can cross
pollinate
pea plants.
Terminology

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Recessive – A trait that remains hidden in a
certain generation
Dominant – A trait that appears in the same
generation
Phenotype – A physical feature, bodily
characteristic or behavior
Genotype - The phenotype is determined by the
underlying genetic makeup
Allele – the alternative form of a gene
Parental generation

Parental generation or P
 F1 – the first filial generation, the first generation
 Fx – the succeeding generation
Results of several of Mendel’s pea crosses
Mendel’s results show a 3:1 phenotypic ratio

Homozygous – When an organism has two of
the same alleles for a characteristic.


YY or yy
Heterozygous – When the organism has two
different alleles for a characteristic.



Yy
The characteristic in the heterozygous condition that
is expressed is the dominant one
The recessive trait in the heterozygous condition is
not expressed

Monohybrid crosses involve just one character.
 Dihybrid crosses involve two characters.

The question asked by Mendel was: Are these two
characters associated with each other? Do they
show up as pairs or are they independent of each
other?
Monohybrid cross
between homozygous
trait pea plants, results
in F1
Monohybrid
cross
between
heterozygous
trait pea
plants, results
in F2 and F3
Dihybrid Cross

P = smooth, yellow and wrinkled, green
 F1 = 100% smooth yellow
 F2 = 315 smooth yellow
101 wrinkled yellow
108 smooth green
32 wrinkled green
9:3:3:1 ratio
WHY?
Seed shape and seed color
are transmitted independently
of one another. The
transmission of one character
does not affect that of
another.
Phenotypic ratio 9:3:3:1
315 Smooth yellow
101 Wrinkled yellow
108 Smooth green
32 Wrinkled green
3 : 1 yellow/green
3 : 1 wrinkled/smooth
True breeding purple peas are bred to true-breeding
white peas. All F1 peas have purple flowers.
F1 flower is self-pollinated. The F2 generation has
3 purple flowered plants for every 1 white flowered plant
Dihybrid cross
Dihybrid cross: F1
Results of dihybrid cross showing F2 generation
(produced by self-crossing heterozygous F1 individual)
Mendel’s Laws
Law of Segregation – Differing characters in
organisms result from two genetic elements
(alleles) that separate in gamete formation,
such that each gamete gets only one of the two
alleles.
 Law of Independent Assortment - because they
are on separate chromosomes, they assort
independently at metaphase. This means they
are passed independently to future generations.

Incomplete Dominance

A genetic condition in which the heterozygous
phenotype is intermediate between either of the
two homozygous phenotypes
White + Red = Pink
Snapdragons
Incomplete dominance in snapdragons
Codominance

A condition in which two alleles of a given gene
have different phenotypic effects, with both
effects manifesting in organisms that are
heterozygous for the gene.
Human blood types, ABO
This is a blood typing system
that refers to the kind of
proteins found on the surface
of red blood cells. These
proteins come in the form of A
or B. A person could have two
A alleles, two B alleles, or one
of each, AB. Two alleles that
are inactive are type O.
Both alleles produce proteins.
Polygenic Inheritance

When a genetic character is determined by the
interaction of multiple genes, with each gene
having an additive effect. This produces a
continuous variation of the character.
 Multiple alleles – when three or more alleles of
the same gene exist.

Human height, weight, eye and skin color
Polygenic inheritance : Eye color
Continuous variation : Infant birth weight
The Bell Curve

A distribution of values that is symmetrical, and
largest around the average. Most biological
traits manifest this way.
Genes and the Environment
Genetic susceptibility – the genetic makeup of
some individuals makes them more susceptible
to an environmental influence.
 Genes and the environment work together to
create an outcome.
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Smoking is an environmental factor that influences
the development of lung cancer.
Some people are more susceptible to that risk
because of their genetic makeup.
Effect of Environment on Phenotype
Himalayan rabbits have an allele that makes production
of melanin (a dark pigment) heat-sensitive. Less melanin
is deposited in warmer areas of the body (such as the
trunk) and more in cooler areas (such as ears and limbs).
It is possible to demonstrate the phenotypic effect of
temperature experimentally by shaving a patch of fur
from a rabbit's back and applying an ice pack to the
shaved area.
The fur that grows back in the artificially cooled area is
dark. The lowered temperature caused increased melanin
production. A change in an environmental condition
(temperature) has altered the phenotype (fur color).
Effect of Environment on Phenotype
Arrowhead plants (Sagittaria sagittifolia) assume very
different-looking forms depending on whether they grow
on land or in water.

Genes and environment work together to create
the phenotypes we see in living things.
Pleiotropy

One gene has many effects.
Fragile-X Syndrome
An inherited cause of
mental retardation.
Called this because of
the break in the X
chromosome, a defect
in one single gene.
Symptoms include
mental retardation,
abnormally long face,
protruding ears, large
testicles (male only).
Fragile-X occurs in 1 out of 2000 male births
and 1 in 4000 female births.
Pleiotropic effects of sickle cell anemia
Sickle-cell anemia can cause deformation of the bones of the
skull, interfere with normal circulation, dilation of the heart and
heart failure, enlargement of the spleen, local failures in blood
supply, kidney damage and kidney failure, paralysis, damage
the lungs, damage to the gastrointestinal system and
abdominal pain.
Cancerous growth
Characteristics of cancerous cells