INTRO TO GENETICS
Why
are we the way we are?
Early thinking about inheritance was based on blended
inheritance
you obtained a blend of characteristics of your parents
the reason was not known at the time
GREGOR MENDEL
THE WORK OF GREGOR MENDEL
Austrian monk born in 1822
first to look closely at inheritance
gained knowledge by studying biology and gardening
specifically learned about pea plants
reproduction of pea plants: self pollination
same plant produces pollen and eggs and fertilizes
seeds produced inherit all characteristics from
a single plant
Mendels Experiments
Mendel cut male
parts off all the flowers of one plant and female parts off another plant
He then
pollinated the 2 parts by sprinkling pollen from one plant onto the other
(cross pollination)
Seeds were
produced from the 2 different plants
He successfully
stopped self-pollination and was able to produce plants with different
characteristics
He later crossed
different pea plant characteristics to see what the offspring traits would be
RESULTS OF PEA TRAITS
RESULTS OF PEA TRAITS
Genes are in control
different forms = alleles
alleles that pass on characteristics are called dominate
alleles that do not pass on characteristics are called recessive
Mendel did NOT
discover HOW traits were passed from generations
he did discover patterns of
inheritance
What happens to the recessive genes?
To answer this question he self pollinated all 7
characteristics of the F1 generation (first generation)
example: hybrid tall x hybrid tall
produced: tall plant, tall plant, tall plant, short
plant
This was called a F1 cross
Mendel predicted the tall dominant allele would mask
the short recessive allele, but the recessive allele showed up
Why did the recessive allele show up?
SEGREGATION: seperation
of recessive alleles from dominant alleles
The Punnet Square was developed to help determine
offspring genotypes and help explain segregation
PUNNETT SQUARE
GENES ARE GIVEN SYMBOLS
CAPITAL LETTER = DOMINANT ALLELE
LOWER CASE = RECESSIVE ALLELE
SHOWS TYPES OF REPRODUCTIVE GAMETES THAT WILL PRODUCE
F1 GENERATION
SHOWS ALL GENE COMBINATIONS FOR F2
PUNNETT SQUARE EXAMPLE
F1 CROSS OF 2 HYBRID TALL PEA PLANTS
(Tt = HYBRID TALL)
definitions
Phenotype: physical characteristics
Genotype: genetic makeup
Homozygous: same identical alleles (tt, TT)
purebred
Heterozygous: 2 different allelles for same trait
hybrid
INDEPENDENT ASSORTMENT
2 FACTOR CROSS:
2 kinds of organisms are crossed
Results:
when a homozygous dominant of 2 traits is crossed with
an individual that is recessive for those traits - ALL offspring will be
heterozygous dominant!
Example: RRYY x rryy => RrYy
CONNECTION OF GENES
RrYy x RrYy
NOT CONNECTED GENES
If genes are not linked they can
assort independently - providing more possible combinations (4 possible
combinations; producing 16 F2 generations
Genetic Basics
All organisms have DNA.
Some of that DNA codes for certain instructions. Other parts code for nothing.
The parts that code are called GENES.
Each chromosome can have thousands of genes.
Chromosomes usually come in pairs. One came from each parent.
A cell that has
two of the same chromosome is called a diploid. (i.e. normal human cell)
An
abbreviation for diploid is 2n
A cell that has
only one chromosome is called a haploid. (i.e. normal gamete cell)
An
abbreviation for haploid is 1n or n
Each pair of
chromosomes helps to determine what the individual organism looks like.
These paired
chromosomes are called Homologous.
Each homologous chromosome codes for the same traits,
but each may have different alleles.
(look at page 270-271)
Meiosis: Produces
gametes that contain half the number of chromosomes of the parent.
Male and female gametes (sperm and eggs 1n each)
combine to create a zygote (fertilized egg).
A zygote is then a 2n cell.
MEIOSIS SUMMARY
HOMOLOGOUS:
refers to the corresponding chromosome
example: the male
has a homologous set for the female
DIPLOID (2N):
cell that contains both sets of homologous chromosomes (one set from each
parent) and 2 complete sets of genes
all of an
organisms cells except for gametes (sex cells) contain 2 alleles for a given
trait (2N)
HAPLOID (N):
cells that contain a single set of chromosomes
example: gametes (sex cells) 1N (sperm) + 1N (egg) = 2N
STEPS OF MEIOSIS
MEIOSIS I: segregation and independent assortment
MEIOSIS II: 4 daughter haploid cells are produced
Mitosis vs. Meiosis
Mitosis:
results in 2
identical cells
2N => 2N, 2N
Meiosis:
results in 4
cells that are genetically different
2N => n,n,n,n
Genetic Variations
Meiosis provides
a mechanism for shuffling chromosomes, which creates genetic variations
When homologous
chromosomes come together and form a tetrad (2 chromosomes held together at one
place, creating an X like structure) it is possible for the chromosomes to
actually exchange genetic material (DNA)
This is called
crossing over. (see Inside Story page 277)
Crossing over provides a source of genetic
recombination.
This way, you can pass on any combination of genes
Not just the genes you got from your mom OR you dad,
but any combination of the two.
Remember: only one chromosome per homologous pair is
passed to the gamete (law of independent segregation).
Another way that variation occurs is by actual errors
in Meiosis.
When paired chromosomes are not separated in meiosis,
then it is possible to pass on both chromosomes in a pair.
This failure to separate is known as NON-DISJUNCTION.
Trisomy: One extra chromosome
Trisomy of the
21st chromosome is called Downs Syndrome.
Monosomy: Missing one chromosome
Having one extra
chromosome (trisomy) normally allows a person to live, however, Monosomy is
almost always fatal.
One exception is
Turner syndrome, where a person only has one X chromosome. Physically the person is female, however,
they are sterile.
Triploidy: Is
when there is an additional chromosome for ALL of the chromosomes.
(fatal in
animals, but not always in plants)
Polyploid (more
the normal number of chromosomes) plants are normally larger and often more
valuable.
(Wheat
4n, Apples 3n
)