Transcript Mendel and Genetics

Mendel and Genetics
Terms and Protocols
Mendel’s Experiments
Probability
Modern Additions & Modifications
Mendelian Genetics and Humans
Terms and Protocols
• In any study of genetics there are some
terms you need to absolutely know
• P represents the parental generation
• F1 represents the first generation
produced by the parental generation
• F2 represents the next generation (often
produced by breeding two members of
the F1 generation together)
Terms and Protocols(cont)
• Alleles-contrasting forms of a gene located on
homologous chromosomes
• Gametes-sperm and egg
• Heterozygous- zygote or individual whose
genotype contains 2 different or contrasting
forms of an allele (like a gene for blue eyes
and a gene for brown eyes)
• Homozygous- zygote or individual with 2
identical genes for a trait.
Terms and Protocols (cont)
• Phenotype- the expression of genes or
appearance of an individual (blue eyes)
• Genotype-the genes or alleles an individual
has
• Dominant- an allele that fully expresses itself
whether it is homozygous or heterozygous
• Recessive-an allele that will only be fully
expressed if there are two recessive alleles in
the individual’s genotype
Terms and Protocols(cont)
• Monohybrid-a type of genetic cross involving
only one trait (ex. Eye color)
• Dihybrid – a type of genetic cross involving
two traits(ex. Eye color and hair color)
• Testcross- a type of cross in which an
individual of unknown genotype is crossed
with a homozygous recessive individual to
determine its genotype
• Punnett Square-a problem-solving device
used in genetics to predict outcomes of
crosses.
Terms and Protocols (cont)
• The set-up of a Punnett Square is based on
the events of Meiosis. The genetic
composition of each possible gamete that
might be made by each parent is placed on
each side of the square. Then you put the
gametes together to determine the genotype
of possible offspring.
• When you are finished you can predict the
percentage of offspring with a particular trait
by using the ratios you obtain.
Mendel’s Observations
• Alternative versions of genes account for
variety in organisms
• For each trait, an organism inherits two
alleles, one from each parent
• If an individual has two different alleles
(heterozygous)then the dominant allele will
be expressed and the recessive will have no
noticeable effect.(Law of Dominance and
Recessiveness)
• The two alleles separate during meiosis.(Law
of Segregation)
Mendel’s Observations (cont)
• Each pair of alleles segregates into
gametes independently(Law of
Independent Assortment)
Probability in Genetics
• Rule of Multiplication-Take the probability for
each individual event and multiply the
individual probabilities to get the overall
probability that these events will occur
together.
• Rule of Addition- take the ways an individual
result can occur and add them to get the
probability that a particular genotype may
result.
• See pages 254-5
Modern Additions & Modifications
• Mendel did not know about genes or
the complexities of molecular biology
and still made discoveries that are used
today to explain how traits are passed
from one generation to the next.
• The more we learn, however, the more
we realize that the relationship between
genotype and phenotype is not a simple
one.
Incomplete Dominance
• Where hybrids have an appearance
somewhere in between the phenotypes
of the two parents.
• Example: Snapdragons that are
heterozygous do not make enough red
pigment to make their flowers red.
They are pink. So only homozygous are
red or white. This is called Incomplete
Dominance.
Codominance
• Occurs when the alleles affect the phenotype
in separate but distinguishable ways.
• Example: M and N Blood Groups
• Each allele produces surface molecules on the
red blood cell.
• M produces 2 surface molecules and N
produces 2 different surface molecules
• If a cell has both M and N the two surface
molecules are different (LMM, LNN, and LMN)
Multiple Alleles
• Where there are more than two alleles
• Example: Human Blood Types
• Type A blood has a certain carbohydrate on
the surface of its red blood cells.
• Type B has a different carbohydrate on the
surface of its red blood cells.
• Type O has no carbohydrate on the surface
of its red blood cells
• Therefore there are 4 distinguishable blood
types
Polygenic Inheritance
• Traits vary in a population along a
continuum
• Traits are determined by several genes
and the genes have an additive effect
• Example: skin color in humans which is
probably controlled by at least three
genes
Environmental Effects on Phenotypes
• Phenotype depends on environment as
well as genes
• For example, if a person is genetically
destined to be tall, poor nutrition will
affect how tall they become. Without
proper nutrition, growth is impossible.
• Acidity in the soil affects the color of
Hydrangea flowers
Mendelian Inheritance in Humans
• There are many traits in humans that follow
simple dominant and recessive inheritance
patterns. But humans do not produce
offspring in large numbers so determining
genotypes is not so simple
• Geneticists use pedigrees which focus on
particular traits over several generations to
determine genotypes of family members
• Examples are on the next slide
Inherited Disorders in Humans
• Recessive disorders:
Cystic Fibrosis
Tay-Sachs Disease
Sickle-Cell Disease
• Dominant disorders:
Huntington’s Disease
Fetal Testing
• Recognition that two carriers for recessive
disorders have a much greater chance of
producing offspring with serious disorders has
led to the development of fetal testing and
genetic counseling
• Amniocentesis uses a sample of amniotic fluid
to test for disorders in the fetus
• Chorionic Villi Sampling takes fetal tissue
from the placenta for the same purpose