Download Mendel’s work - Somerset Academy North Las Vegas

Mendel’s work
CO ULTE R
Gregor Mendel
• A priest in the mid 19th century tended a garden in the a European
monastery.
• Heredity is the passing of physical characteristics from parents to offspring.
• Mendel wondered why different pea plants had different characteristics.
• Some were tall, others were short, some plants produced green seeds, others had
yellow seeds,
• Traits: each different form of a characteristic
• Mendel observed that the pea plants’ traits were often similar to those of their
parents. Sometimes the plants had different trait from those of the parents
• Genetics: the study of heredity
Mendel’s experiment
• The flower petals surround the
pistil and the stamen.
• New organism begins to form
when egg and sperm join in a
process called fertilization.
• Before fertilization can occur
pollen (stamen) must reach the
pistil; this process is called
pollination.
Pollination
• Pea plants are self pollinating
plants.
• Pollen from the flower lands on
the pistil of the same flower.
• Mendel developed a method to
cross-pollinate pea plants.
• He removed pollen from the
flower of one plant.
• He brushed the pollen onto a
flower on a second plant.
Crossing pea plants
• Mendel decided to cross plants with contrasting traits.
• He started his experiment with purebred plants
• Purebred organisms are the offspring of many generations that have
the same trait.
Crossing pea plants: the F1 offspring
• Mendel crossed purebred tall
plants with purebred short
plants.
• Scientists call these parent
plants the parental generation,
or P generation.
• The offspring from this cross
are the first filial generation, or
the F1 generation.
• Filial is Latin for daughter or
son.
Crossing pea plants: F2 offspring
• When the plants of the F1
generation were fully grown,
Mendel allowed self-pollination
• Mendel was surprised to see
the F2 generation was a mix of
tall and short.
• The shortness trait reappeared
even though none of the F1
parent plants were short.
• About 3/4th were tall and 1/4th
were short.
Crossing pea plants: experimenting with
other traits
• In all of Mendel’s crosses, only one form of a trait appeared in the F1
generation. However in the F2 generation, the “lost” form of the trait
always reappeared in about 1/4th of the plants.
Dominant and recessive alleles
• Mendel reasoned:
• Individual factors, genetic information, must control the
inheritance of traits in peas.
• Factors that control each trait exist in pairs (female contributes one,
male contributes one).
• One factor in a pair can mask, or hide the other factor.
Dominant and recessive alleles:
genes/alleles
• Gene: the factor that controls the trait.
• Alleles: the different forms of a gene.
• An organism’s traits are controlled by the alleles it inherits from its
parents. Some alleles are dominant, while other alleles are recessive.
• Each pea plant inherits two alleles from its parents—one from egg and one from
sperm.
• Dominant allele: is one whose trait always shows up in the organism
when the allele is present.
• Recessive allele: is hidden whenever the dominant allele is present.
Dominant and recessive alleles: Mendel’s
crosses
• In Mendel’s height cross, the
purebred tall plant in the P
generation had two alleles for tall
stems. The purebred short plant
had two alleles for short stems.
• The F1 plants each inherited an
allele for tall stems and short
stems.
• The F1 plants are called hybrid
• Hybrid: organism has two different
alleles for a trait.
• Take a look at the F2 generation…
Symbols for alleles
• Dominant allele is represented by a capital letter.
• Example: tall stem is represented by T
• Recessive allele is represented by a lower case letter.
• Example: short stem is represented by t
• When a plant inherits two dominant alleles (purebred) it is represented as
TT
• When a plant inherits two recessive alleles (purebred) it is represented as tt
• When a plant inherits one dominant and one recessive (hybrid) it is
represented as Tt
Significance of Mendel’s work
• Mendel’s discovery of genes and alleles eventually changed scientists’ ideas
about heredity.
• Before Mendel, most believed trait were a blend of parents. So, if a tall and
short were crossed, offspring would be medium.
• Mendel discovered traits are determined by individual separate alleles
inherited from each parent.
• Unfortunately the importance of Mendel’s work was not recognized until
after his death.
• In the 1900, three different scientists discovered Mendel’s work.
• Mendel is often called the father or genetics.