Download Mendel and Heredity ppt

Mendel
and
Heredity
• Gregor Mendel (1850’s)– Austrian
monk that bred pea plants and
from his experiments he formed
the basis of GENETICS: study of
heredity
• Used peas because they had
easily distinguishable forms of
various traits: flower color, pod
shape/color, seed shape/color,
plant height and flower placement
• Easy to grow and matured quickly
• Traits are determined by genes
received from each parent
Mendel’s Experimental •
Design
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1. Allowed peas to self-pollinate
for several generations
Purple flowering plants would
produce only purple flowering
plants (same for white flowering
plants)
These were the P or parent
generation—Mendel started his
experiments with these
2. Cross-pollinated 2 varieties
that had contrasting traits (purple
flowers X white flowers)—pollen
from white plant was placed on
the stigma of the purple flower
Offspring of this cross would be
the F1 generation (filial)--only 1
flower color was present: purple
3. Allowed the F1 plants to selfpollinate, planted seeds and the
offspring were the F2 generation;
rec trait (white) showed up again
Experimental Design, cont.
• Mendel named the trait that “disappeared”
the recessive trait and the one that
showed he called the dominant trait
• After counting all the F2 offspring he found
that there was always a 3:1 ratio of
purple:white
• Mendel found this to be true of ALL the
pea’s identifiable traits
Mendel’s Theory of
Heredity
• Parents pass on genes to
offspring—not actual
traits
• For each trait, an
individual has 2 genes
governing that trait: 1
from Mom and 1 from
Dad
• If both genes carry the
same info (purple, purple)
then the individual is
HOMOZYGOUS for that
trait
• If the genes are different
(purple, white) then the
individual is
HETEROZYGOUS for
that trait
Mendel’s Theory,
cont.
• Each copy of a gene is called
an allele; set of alleles that an
individual has is called a
genotype : PP, Pp or pp—
shows genes from parents as
capital or lower case letters
• Capital letters are dominant
traits, lower case are recessive
traits (ALWAYS use the first
letter of the dom trait)
• Phenotype (purple/white
flowers) is the physical
appearance
• Dom allele (capital letter) is
expressed, rec allele (lower
case letter) is still present but is
unexpressed; this rec allele
CAN still be passed on to
offspring where it might be
expressed
Laws of Heredity
• Law of Segregation---alleles separate when
gametes are formed during meiosis and the
chromosomes separate
• Law of Independent Assortment—pairs of alleles
separate independently of one another during
meiosis—for example, if the gamete is
heterozygous (Pp) before meiosis, the dom
allele (P) goes into 1 new gamete and the rec
allele (p) goes into another
• What phase of meiosis would this occur?
Legend steps for Genetic Crosses
• 1. Read the word problem and determine WHAT is
being crossed
• 2. Determine dom and rec traits from the problem or
your text book—ALWAYS use the first letter of the dom
trait in the problem; capitalize it if dom, lower case if rec
• 3. Using steps 1 & 2 write the parents’ genotypes
• 4. Draw a punnett square to show the cross--♂’s
genotype goes on top of square, ♀’s on the side; fill in
the boxes with the offspring’s possible genotypes
• 5. Write the offspring’s possible genotypes in a ratio,
always starting with the homozygous dom, then hetero,
then homozygous rec
• 6. Write the possible phenotypes in a ratio
Let’s Practice!
• Mr. Jones, a pea farmer, crossed a plant that is homo. round
for seed shape with another plant that is homo. wrinkled for
seed shape. Show the steps and outcomes.
• 1. homo. round male X homo. wrinkled female
• 2. R= round
r= wrinkled
• 3. RR= male
rr= female
• 4.
R
R
r
Rr
Rr
r
Rr
Rr
• 5. 0 RR: 4Rr: 0 rr
• 6. 4 round seeds : 0 wrinkled seeds
Probability Lab or the PENNY LAB!
• Intro: Mendel’s crosses can be explained by the rules of
probability—the likelihood that a specific event will occur, or to
put it another way:
• # of 1 kind of possible outcomes
• Total # of all possible outcomes
• Ex : probability that a baby will be a girl? Kind of possible
outcomes is 1 and the total # of outcomes is 2 (either boy or
girl) so the probability is ½
• Purpose: To relate probability to genetic crosses
• Procedure: Pair up, make a chart, take 2 coins of the same
type, toss coins 100 times each, at the same time, record #’s
in the chart with tally marks, figure % error, place your results
in the class chart on the whiteboard
• Conclusion: 1. How does the probability change with the
increasing # of tosses?
2. What parent genotypes were present?
Penny Lab Chart
HH
Hh
hh
Obs.
Total
100
Exp.
100
Diff.
___ %error
100
Other types of crosses
• We have been
practicing monohybrid
crosses—those that
deal with only 1 trait
(flower color or pod
shape, etc)
• Dihybrid crosses
involve 2 different
traits; the steps are all
the same, except the
punnett square has
16 boxes instead of 4!
Let’s try one!
• In guinea pigs the allele for short hair (S) is dom over long
hair (s) and the allele for black hair (B) is dom over brown hair
(b). So if the guinea pig farmer mated a hetero short haired
brown male g.p. with a hetero short haired brown female g.p.,
the steps would look like this:
• 1. hetero short, homo brown X hetero short, homo brown
• 2..S=short, s=long B=black, b=brown
• 3. Ssbb X Ssbb
• 4. Use “foil” method to determine the parents’ gametes:
Sb
Sb
sb
sb
Sb SSbb
SSbb Ssbb
Ssbb
Sb SSbb
SSbb Ssbb
Ssbb
sb Ssbb
Ssbb
ssbb
ssbb
sb Ssbb
Ssbb
ssbb
ssbb
• 5. 4 SSbb : 8 Ssbb : 4 ssbb
• 6. 12 short haired brown: 4 long haired brown
Incomplete Dominance
• Complete dom is when the dom
trait completely masks the rec
trait (Rr = red)
• Incomplete dom is when you
have an intermediary trait in the
hetero genotype (Rr = pink)
• Common in some flowers like
snapdragons
• Book uses R and R‘ instead of
R and r– we will use R (red)
and r (white) and you will be
told that the problem is Inc dom
• Still supports Mendel’s Laws of
Heredity and the steps are the
same
• Cross a pink snapdragon with a
red one
Codominance
• With codominance you have
2 dominant traits that are both
expressed
• Both the letters are used and
both are capitals
• Roan coat in horses and cattle
is an example—Red (R) is
dom and so is white (W), so
when both are present in the
genotype (RW) the phenotype
is not spotted, but both colors
are expressed
• Try crossing a roan bull with a
white cow
• Blood types are this sort of
genetic problem—more about
that later……..