Transcript Genetics - Midway ISD

Chapter 11
Genetics
Gregor Mendel
• Used pure breed pea
plants to understand
inheritance
• Used self pollination
to cross breed
– Pollen from the male
part of the pea plant
fertilizes the female
egg of the same
flower
Cross Pollination
• Cross Pollination
– Mendel cut away the male parts of the
plant and then dusted pollen from another
plant on the flower
– Seed that come from cross-pollination
inherit all of the characteristics from two
parents
Genes and Dominance
• A trait is a specific characteristic that
varies from one individual to another
• The chemical factors that determine
traits are called genes
• The different forms of a gene are
called alleles
• Video Clip!!
Mendel’s Work
• Mendel crossed two pure breed plants
with different forms of the same trait
– One plant was tall and one plant was short
(trait: plant height)
• The offspring plants were called hybrids
– Hybrids are the offspring of crosses
between parents with different forms of a
trait
Principle of Dominance
• The principle of dominance states that
some alleles are dominant and others
are recessive
– An organism with a dominant allele for a
specific trait will always show that form of
the trait.
– An organism with a recessive allele for a
specific trait will only show that form only
when the dominant allele for that trait is
not present.
Segregation
• Segregation is the
separation of alleles
during gamete
formation
• Gametes are
specialized cells
involved in sexual
reproduction
Probability and
Punnet Squares
• Principle of probability can be used to
predict the outcomes of genetic crosses.
• Probability is the likelihood that a
particular event will occur.
– Probability can predict the outcome of genetic
crosses because alleles segregate randomly.
Punnet Squares
• These can be used to predict and
compare the genetic variations that will
result from a cross.
T
T
TT
50%
T
TT
50%
t
Tt
50%
Tt
50%
Punnet Square, cont.
• Homozygous – TT or tt, organisms that
have two identical alleles for a
particular trait.
• Heterozygous – Tt, organisms that have
different alleles for the same trait
• Phenotype – physical characteristics,
– TT = tall, Tt = tall, tt = short
• Genotype – genetic makeup, TT, Tt, tt.
T
t
T
TT
(tall)
Tt
(tall)
t
Tt
(tall)
tt
(short)
Ratios:
Phenotype- 3 Tall: 1 Short
Genotype- 1 TT: 2Tt: 1tt
Probabilities Predict Averages
• Probabilities predict the average
outcome of a large number of events.
• Probability cannot predict the precise
outcome of an individual event
Mendelian Genetics
• Mendel found that the gene controlling
seed shape did not affect the gene
controlling seed color.
• Principle of Independent Assortment
– Genes for different traits can segregate
independently during the formation of
gametes
Independent Assortment
• Independent
Assortment
accounts for the
many genetic
variations observed
in plants, animals,
and other organisms
Beyond Dominant and
Recessive Alleles
• Some alleles are neither dominant nor
recessive, and many traits are
controlled by multiple alleles or multiple
genes.
Incomplete Dominance
• One allele is not
completely dominant
over the other.
• The heterozygous
phenotype is
somewhere between
two homozygous
phenotypes
Codominance
• Both alleles contribute to the
phenotype.
• For Example: In chickens, the allele for
black feathers is codominant with the
allele for white feathers. These
chickens will have both black and white
feathers.
Multiple Alleles/Polygenic Traits
• Multiple alleles are when there is more
than two possible alleles that exist for
each trait in a population.
Example: blood type (A, B, AB, O)
• Polygenic traits are traits that are
controlled by 2 or more genes.
Example: variations in human skin
color; height
Meiosis
• The daughter cells (haploid) produced
contain only HALF of the number of
chromosomes as the original parent cell
(diploid).
• How does this happen? Meiosis is a
process of reduction division—that is,
the number of chromosomes is cut in
half. No replication occurs.
Meiosis
Crossing-over
• During meiosis I,
chromosomes pair up with
their corresponding
homologous pair
chromosomes. They form
a tetrad.
• These tetrads exchange
alleles and form new allele
combinations in a process
known as crossing-over.
• Video Clip!!