Download ppt notes on genetics - Madeira City Schools

genetics
 The scientific
Modified from South Dakota Data
Center)
heredity
study of
heredity,
which is
controlled by
genes
Genes and
chromosomes
from one generation to the next
-- parent to child
Gene -- segment of DNA, codes for
single protein, all genes together carry
blueprint for organism
  chromosome -- carries genes,
extremely long molecule of DNA
Gregor Mendel
Mendel continued
 Passing on of characteristics
  Born in 1822 in
Czechoslovakia.
  Became a monk in
1843.
  Taught biology and
had interests in
statistics.
  After returning to the
monastery he
continued to teach
and worked in the
garden.
  Between 1856 and
1863 he grew and
tested over 28,000
pea plants
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Mendel’s Peas
  Easy to grow and to identify traits
  Quick and easy to produce many
offspring in many generations
  Selective breeding
easy
Genes and dominance
  Trait : a characteristic, determined by one or
more genes
  Mendel studied seven of these traits, such as
the color of the peas
  He crossed plants showing contrasting traits,
such as green peas crossed with yellow ones.
  He called the offspring the F1 generation or
first filial.
What will happen when pure
yellow peas are crossed with
pure green peas?
What happened when he
crossed pure yellow peas
with pure green peas?
All of the
offspring were
yellow.
What did Mendel conclude?
 1st conclusion: Inheritance
is determined by factors
passed on from one
generation to another.
 Mendel knew nothing
about chromosomes,
genes, or DNA. Why?
What were Mendel’s
“factors”
  The “factors” that Mendel mentioned
were the genes, which had not been
discovered yet.
  Each gene has different forms called
alleles
  Mendel’s second conclusion was that
some alleles are dominant and some
are recessive.
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Mendel’s second cross
  He allowed the F1 generation to self-
pollinate thus producing the F2
generation.
  Did the recessive allele completely
disappear?
  What happened when he crossed two
yellow pea hybrid (F1) plants?
Results:
 ¾ of the peas were yellow, ¼ of
the peas were green.
WHY???
Because of the Law of
Segregation:
During the formation of the sex
cells or gametes, the alleles
separated or segregated to
different gametes (pollen and egg)
But what does that have to do with
the color of peas?
 To explain, we need some
definitions…
Definitions
•  Genotype- the actual genes
or alleles of an organism for
one specific trait
•  Phenotype- the physical
appearance of a trait in an
organism
•  A Dominant allele or trait “hides”
the recessive trait in the phenotype
of an individual.
•  A recessive allele or trait is
covered over (or dominated) by the
dominant form and seems to
disappear.
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Definitions
Then what?
•  Homozygous= two alleles that are
the same for a trait (Pure -- YY or
yy)
•  Heterozygous= two different alleles
for a trait (Hybrid -- Yy)
• 
Example below: P=dominant purple
and p= recessive white The phenotype
The phenotype
for this flower is
violet while its
genotype (if
homozygous) is
PP.
for this flower is
white while its
genotype is pp
(to be white the
flower must
have two of the
recessive copies
of the allele).
Punnett Square
  Developed by
Reginald Punnett.
• 
We use two letters to represent the
genotype. A capital letter
represents the dominant form of a
gene (allele) and a lowercase letter
is the abbreviation for the recessive
form of the gene (allele).
Punnett Squares
  We use a Punnett
square to work out
what the possible
offspring of two parents
will be.
l 
This tool shows all the
possible combinations
and the probable
offspring ratios or
percentages.
Probability
  The likelihood of a
particular event
occurring, chance
  Can be expressed
as a fraction or a
percent.
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Probability and
statistics
Reading Punnett
squares
  No one event has a greater chance of
  Gametes are placed above and to the
occurring than another.
  You cannot predict the precise outcome
of an individual event.
  The more trials performed, the closer
the actual results to the expected
outcomes.
left of the square
  Offspring are placed in the square.
  Capital letters (Y) represent dominant
alleles.
  Lower case letters (y) represent
recessive alleles.
Let’s set up a Punnett Square…
Next, put the genotype of one parent
across the top and the other along
the left side.
We begin by constructing a grid of two perpendicular
lines.
Notice only
one letter
goes above
each box
• 
Y
y
y
Y
•  It does
not matter
which
parent’s
genotype
goes on
either side.
For this example, let’s consider a
genotype of YY crossed with yy for
the yellow and green peas.
Next, fill in the boxes by copying the column
and row head-letters down and across into
the empty spaces.
Y
y
y
Yy
Yy
Y
Yy
Yy
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What happened when he
crossed pure yellow peas
with pure green peas?
Now, let’s explain those “reappearing”
green peas. Take two heterozygous plants
from the 2nd generation and cross them:
Y
All of the
offspring were
yellow.
y
Y
y
How many of each type of pea do you get?
Y
Y
y
YY
Yy
Genotype: YY: 1/4
Yy: 2/4
yy: 1/4
y
Y
Yy
Y
yy
y
Results:
 ¾ of the peas were yellow, ¼ of
the peas were green.
YY
Yy
Phenotype:
yellow: 3/4
green: 1/4
y
Yy
yy
Punnett Squares
 Now that we have learned the
basics of genetics, let’s walk
through some examples using
Punnett Squares.
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Usually write the
capital letter first
For flowers:
W- dominant white
w- recessive violet
W
w
WWW Ww
w Ww ww
Usually write the
capital letter first
For flowers:
W- dominant white
w- recessive violet
Parents in this cross are heterozygous (Ww).
Parents in this cross are heterozygous (Ww).
Note: Make sure your capital letters are different
from lowercase letters.
Note: Make sure your capital letters are different
from lowercase letters.
What percentage of the offspring will have violet
flowers?
What percentage of the offspring will have violet
flowers?
ANSWER: 25% (homozygous recessive)
Red hair (R) is dominant over blond hair (r). Make a cross
between a heterozygous red head and a blond.
R
r
r
r
Rr
rr
Rr
rr
What percentage of the offspring will have red hair? 50%
Black eyes (R) is dominant over red eyes (r)
in rats. Make a cross between a homozygous
rat with black eyes and a rat with red eyes.
r
R
Rr
R
Rr
r
Rr
Rr
Let’s try some more…
In pea plants, tall pea plants (T) are dominant
over short pea plants (t). Construct a Punnett
Square for a heterozygous tall pea plant and a
short pea plant.
T
t
t
Tt
tt
t
Tt
tt
What are the
percentage of
phenotypes?
50% tall
50% short
What about 2
characteristics?
 Mendel studied 7 traits.What
What is the possibility of
a red eye off springs?
0%
did he find out when he
followed 2 traits at once?
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He observed
independent assortment
Mendel tried 2 factor
crosses
  For example, he followed the color
 Alleles
separate
independently
during the
formation of
gametes.
and shape of peas.
  Round yellow mated with wrinkled
green -- RRYY x rryy
•  Offspring would all be hybrid for
both traits (RrYy)
Try it!
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Incomplete dominance
  A situation in which neither allele is
dominant.
  When both alleles are present a “new”
phenotype appears that is a blend of
each allele.
  Alleles will be represented by capital
letters only.
What happens when a
red flower is crossed
with a white flower?
  According to
Mendel either
some white and
some red or all
offspring either
red or white.
All are pink
How does meiosis explain
Mendel’s law of segregation?
In the formation of gametes, each pair
of chromosomes separates; different
alleles on matching (homologous)
chromosomes separate.
Examples: Hh mother can pass on
either H or h, but not both
Japanese four-o-clock
flowers
  Red flower plant genotype = RR
  White flower plant genotype = WW
  Pink flower plant genotype = RW
Meiosis
  An orderly method of dividing a cell’s
chromosomes so that the resulting
cells are haploid (have exactly 1 copy
of each chromosome)
  Meiosis produces gametes, which are
sex cells
How does meiosis explain Mendel’s
law of independent assortment?
•  Complex organisms have multiple
chromosomes, so it is likely that 2
different genes are on 2 different
chromosomes.
•  The chromosomes sort independently,
so most genes sort out independently
as well.
•  Genes only sort together if they are on
the same chromosome. In that case, we
say they are linked.
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Chromosome number
Pollen is haploid
  All cells of an
  Gametes are
organism contain a
specific number of
chromosomes.
  Most cells are
diploid (2n) meaning
they have two
copies of each
chromosome
haploid (1n)
meaning they
have only 1 of
each
chromosome
Human chromosomes
  gametes join to form a zygote (very
  Humans have 23
early embyo)
pairs of
chromosomes (46
total).
  the zygote has 2 copies of each
chromosome -- one from mother and
one from father
  One from each pair
comes from mom
and the other from
dad.
Human chromosomes
  One pair: sex
chromosomes -determine gender:
  XX - female
XY -- male
Sex-linked
  Genes on the X chromosome are sex-
linked.
  Sex-linked genes explain why
colorblindness occurs more often in
men than women.
  The other 22 pairs
are called somatic
chromosomes.
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References
http://www.athro.com/evo/gen/punnett.html
http://www.kidshealth.org/kid/talk/qa/what_is_gene.html
  http://brookings.k12.sd.us/biology/ch%2011%20genetics/
punnettpractice.ppt#1
http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/
sciber/punnett.htm
  http://www.biotechnologyonline.gov.au/images/contentpages/
karyotype.jpg
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