Transcript Mendel and Heredity

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
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
• 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
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?
Other types of crosses
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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
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5. 4 SSbb : 8 Ssbb : 4 ssbb
6. 12 short haired brown: 4 long haired brown
• 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 phenotype (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
Incomplete Dominance
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……..