Transcript Introduction to Genetics - St. Croix Central School District

Introduction to Genetics
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What does genetics mean?
Branch of biology that studies heredity.
What is heredity?
Key to differences between species.
Makes species unique. Something that is
passed on from one generation to the
next.
First Theory of Heredity
• Blending Inheritance
– Get 50% from mom and 50% from dad
Is this TRUE or FALSE?
11.1 Gregor Mendel
• “Father of genetics”
• Became a priest in a monestary, teacher at high
school.
• In charge of the garden
• Understood the sexual mechanism of pea plants
-pea plants have both male and female parts
-self pollination-true breeding (identical offspring)
Mendel’s Experiment
• He wanted to see if self pollination could
be prevented or manipulated.
– He cut the male part off all the flowers of one
plant (stamen) and dusted pollen on the
female parts off another plant (pistil).
– Refer to page 264
What is this process called?
Cross Pollination
• Produces seeds that are offspring of two
different plants.
• What did the stamen say to the pistil?
– I like your style. Get it??
Terms
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Self pollination
Cross pollination
True breeding- produces purebreds
Purebred- allowed to self pollinate;
produce offspring identical to themselves.
• Traits/characteristics- size, shape, looks,
texture, color, etc.
Genes and Dominance
• Genes-sequence of DNA that codes for a protein
and thus determines a trait.
• Mendel crossed the seeds of tall plants with the
short plants; crossed yellow seeds with green
seeds.
• He produced HYBRIDS- organisms produced by
crossing with different characteristics.
• The result was not a 50/50 mix of tall and short
or yellow and green.
More terms
• Allele- different form of a gene. Given 2
letters to represent each trait.
– Example for tall: TT or Tt
– Example for short: tt
– Why are there always two alleles given?
• One from mom and one from dad.
Principle of Dominance
• Some alleles are dominant and others are
recessive.
• Dominance-when present, is the trait that
will always show up
• Recessive-will not show up when
dominant is present. Has to be two small
alleles only.
Heterozygous & Homozygous
• Heterozygous (different-one of each) Tt
• Homozygous (same) TT or tt
• Heterozygous tall plant crossed with a
homozygous tall plant.
• Tt x TT
• Purebred tall plant with a short plant
• TT x tt
Segregation
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Separation of alleles
What happens to the recessive traits?
P generation (parent generation)
F1 first filial or first generation.
F2 second generation –(offspring from the
F1 generation)
11.2 Genetics and Probability
• Probability- likelihood that a particular
even will occur
• Equation for probability=
# of times event occurs
# of times it could occur
Larger # of trials, the closer you get to the
expected ratio.
Punnett Squares
• Diagram that shows the outcome of a
particular cross.
Genotype
• The letters inside the boxes indicate probable
genotypes (genetic makeup) of offspring
resulting from the cross of these particular
parents. There are 4 boxes, and the genotypic
results can be written either as fractions or
percents. In this case, all 4 boxes out of the 4
are showing the Tt genotype. Therefore, each of
the offspring has a 4/4 or 100% chance of
showing the Tt genotype.
Phenotype
• We have also written the phenotype
(physical appearance) in each box under
the genotype. Remember, T = tall and t =
short (see step #1 above). Since a capital
letter indicates a dominant gene, T (tall) is
dominant over t (short). Therefore, each of
the offspring has a 4/4 or 100% chance of
being tall.
What are the possible genotype(s) of a tall plant?
What are the possible genotype(s) of a short plant?
What would be the phenotype of TT?
What would be the phenotype of tt?
Why is the phenotype of Tt tall and not
medium/average?
One Factor Cross
• Using only one trait or set of alleles
• TTx tt One trait…Height
• How many boxes would you need in your
punnett square?
• 2x2=4
Two factor cross
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Using two different traits and alleles
Example would be height and color
TT, Tt, tt would be alleles for height
BB, Bb, bb would be for color
T=tall t=short
B=black b=white
Practice with Punnett squares
• A heterozygous tall white weasel crossed
with a small heterozygous black weasel.
• Ttbb x ttBb
• How many squares would the Punnett
Square have?
• 4 x 4= 16
FOIL
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F = First terms
O= Outter terms
I = Inner terms
L = Last terms
• I know, I know…this isn’t math class, but
the same concepts do apply…math and
science go hand in hand…
Ttbb x ttBb
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First terms
Outter
Inner
Last
Tb
Tb
tb
tb
first
outter
inner
last
tB
tb
tB
tb
Ttbb x ttBb
11.3
• Independnet Assortment- genes for
different traits can segregate
independently during the formation of
gametes.
• Gametes- sex cells.
– Males=sperm
– Females=egg
– Zygote is a fertilized egg
Summary of Mendel’s Principles
• 1. traits are determined by genes from parents to
offspring.
• 2. when present a dominant trait will always
show up. 2 recessive traits must exist together
for recessive to show up.
• 3. each adult has two copies of a gene (one from
each parent). These genes segregate (separate)
from each other when gametes (sex cells) are
formed.
• 4. Alleles for different genes usually segregate
independently from on another.
Beyond dominant and recessive
• Incomplete dominance- one allele is not
completely dominant over another.
– RR x WW
– Pink flowers
R=red
W=white
• Codominance- when both alleles contribute to
the phenotype.
– Example: cows with an allele for red hair and white
hair, are roan cows…they look pink but close up you
can see the different hair colors.
– Example: speckled chickens have black and white
feathers.
– Example: Blood type A=IA B=IB O=ii
• Multiple alleles- more than two forms of
alleles, individuals cannot have more than
two alleles.
– C=full color (dominant)
– cch=chinchilla (partial defect, dominant to ch
and c alleles)
– ch=Himalayan (parts of body colored,
dominant to c allele)
– c=albino, no color (recessive)
• Polygenic traits- traits formed by two or
more traits.
– Example: skin color…more than four different
genes control this trait
Meiosis
• Process by which the # of chromosomes
per cell are cut in half through separation
of homologous chromosomes of a diploid
cell. (1/2 # of chrom. & different kinds)
• Homologous- two sets of chromosomes. A
male chromosome has a corresponding
female chromosome.
• Diploid-2N means two sets
• Haploid- N means one set
Phases of Meiosis
• Interphase I= cells undergo replication and
form duplicate chromosomes
Meiosis I
• Prophase I= chromosomes find homologous pair
(tetrad-4 chromatids)
• Metaphase I= spindle fibers attach to the
chromosome. Chromosomes line up at equator
and crossover-exchange different alleles.
• Anaphase I= fibers pull homologs to opposite
poles
• Telophase I and Cytokinesis= cell splits into two
daughter cells with different # & kind of
chromosomes.
Meiosis II
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Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
Gene Map
• Diagram showing the relative locations of
each known gene on a particular
chromosome.