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Patterns of Inheritance
The Vocabulary of Genetics
Heredity: the passing of traits from parents to offspring
Self-Pollination: the process by which plant pollen is transferred
from the male reproductive organs to the female reproductive
organs to form seeds. In flowering plants, pollen is transferred from
the anther to the stigma, often by the wind or by insects.
Cross pollination: sperm (pollen) from one plant fertilizes the eggs
of a different plant.
True-breeding plant: this plants offspring will have the same trait
as the parent
Characteristic: a feature that has different forms in a population
(hair color)
The Vocabulary of Genetics
Trait: a genetically determined characteristic or condition (red,
brown or blonde hair color)
Dominant trait: the trait observed in the first generation when
parents that have different traits are bred
Recessive trait: a trait that reappears in the second generation
after disappearing in the first generation when parents with
different traits are bred
Blending Inheritance
Classical assumptions (prior to Gregor Mendel--1866)
First, heredity occurs within species.
All species have been maintained without significant
change since time of creation
Second, traits are transmitted directly and independently.
Paradox - all members of same species should eventually
have the same appearance. Thus, if a brown rabbit mates
with a white rabbit, the offspring would be tan. Yet, in
reality when these two rabbits mate the offspring often
have brown fur, or one might have white fur.
hybrids differ in appearance
Gregor Mendel
Born in 1822 in Heinzendorf, Austria. He grew
up on a farm and learned a lot about flowers
and fruit trees. He entered a monastery in 1843
to study religion.
As a monk, Mendel put most of his energy into
plant research. He studied garden peas in
depth.
Pea plants have both male
and female reproductive
structures. And many
plants reproduce through
cross-pollination
Cross pollination: sperm
(pollen) from one plant
fertilizes the eggs of a
different plant.
Pollen may be carried by
insects, bats, birds, or the
wind.
Plant Structures
Pollination and Cross Pollination
During self-pollination, pollen from the
anthers (male) is transferred to the
stigma (female). Fertilization occurs
when a sperm from the pollen travels
through the stigma and enters the egg
in an ovule.
Self-pollination: A plant, that when selffertilized, only produces offspring with
the same traits. Thus, a plant with
purple flowers will have offspring with
purple flowers.
For cross pollination to occur most
plants need another plant to reproduce.
Pea plants can reproduce through crosspollination or self-pollination
Characteristics
Characteristics: a feature that has different forms in a
population. E.g. different hair colors in humans. The different
colors are called traits.
Traits: a genetically determined characteristic or condition. Traits
may be physical, such as hair color or leaf shape, or they may be
behavioral, such as nesting in birds and burrowing in rodents.
Traits typically result from the combined action of several genes,
though some traits are expressed by a single gene.
Mendel’s Experiments
Mendel used plants that had different traits for each of the
characteristics he studied. For example, he chose plants that had
purple flowers and plants that had white flowers. Mendel also
studied seed shape, pod color, and plant height.
Mendel also studied
seed shape, pod color,
and plant height.
Mendel’s Experiments
Mendel used plants that were
true-breeding. By choosing these
plants he knew what to expect
when they self-pollinated.
He also cross-bred two plants of
different characteristics. He
removed the anthers of one plant
and then fertilized it with the
pollen of another plant.
Mendel’s First Experiments
In the first experiments seven different
characteristics were examined. Mendel used
only true-breeding plants that were truebreeding for different traits for each
characteristic.
The offspring from such a cross are called firstgeneration plants. In Mendel’s experiment he
crossed purple plants with white plants and all
of the first generation offspring were purple.
Mendel got the similar results for each cross.
One trait seemed to disappear, while one trait
was always present in the first generation.
Mendel called this the dominant trait.
The trait that seemed to disappear was called
the recessive trait. Recede means “to go away
or back off.”
Mendel’s Second Experiments
Mendel’s second experiments
sometimes bred plants with
recessive traits. In this
example, for every three
plants that have purple
flowers, there is one plant
that has white flowers.
Mendel’s Second Experiments
Mendel then counted the
number of plants that had each
trait and that turned up in the
second generation.
Clearly, the recessive trait did
not show up as often as the
dominant trait. Mendel then
formulated a ratio of dominant
traits to recessive traits