Biology 30 Genetics Unit Mendel Notes
A discussion on Genetics must begin with by looking at the work of Gregor Mendel and a discussion of
independent events and probability.
The term independent events means that events are independent of each other if the probability of one
occurring does not affect the probability of the others. The probability of something is the measure of
how likely an event is. For instance, when my son was born there was a 1 in 2, or 0.50 probability that
Eli would be a boy. There was also a 1 in 2, or 0.50 probability that he would have been a girl and
hopefully would have a different name. So, if we were to have another child, would the probability still
be 1 in 2, or 0.5 for having another boy? Yes it would because the sex of one child has no impact on the
second child. So, these would be independent events.
11.1 - Gregor Mendel
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Developed the fundamental laws of heredity
He studied science and mathematics
Blended Concept of Inheritance vs Particulate Theory of
Inheritance
 Mendel chose garden peas (Pisum sativum)as his subjects as
they are easily grown and their pollination is easily controlled. He
controlled pollination by manually moving pollen between plants
 Developed True-breeding plants by self-pollination
 He could also allow the plants to self-pollinate.
 Mendel examined varieties of peas for heritable characters
and traits for his study. (stem length, pod shape, seed shape, seed
color..etc)
Mendel's Law of Segregation (MONOHYBRID CROSS)
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Mendel crossed true-breeding plants that differed
for a given character
A monohybrid cross involves one (mono) character
and different (hybrid) traits.
Pollen from true-breeding pea plants with purple
flowers (one trait) was placed on stigmas of truebreeding plants with white flowers (another trait).
The F1 seeds were all purple; the white flower trait
failed to appear at all.
Because the purple flower trait completely masks the
white flower trait when true-breeding plants are
crossed, the purple flower trait is called dominant,
and the white flower trait is called recessive.
The F1 plants were allowed to self-pollinate.
This step was the monohybrid cross. (or the F1 cross).
The progeny, called F2, were examined: roughly 1/4 were white, and 3/4 were purple.
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Mendel proposed that the units responsible for inheritance were discrete particles - particulate
theory
As Viewed by Modern Genetics
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During production of gametes, only one of the pair members for a given character passes to the
gamete. (LAW OF SEGREGATION)
When fertilization occurs, the zygote gets one from each parent, restoring the pair.
Mendel's units of inheritance are now called genes.
Different forms of a gene are called alleles.
Each allele is given a symbol: In the case of purple flowers, P might represent purple and p
white. By convention, uppercase P represents the dominant; lowercase p represents the
recessive. Never mix your letters! Choose one letter and use a capital to denote dominate and a
lowercase to denote recessive.
True-breeding individuals would have two copies of the same allele: Purple would be PP. Two
copies of same allele = homozygous. Homo means "the same"
White true-breeding would be pp. Two copies of same allele = homozygous.
Some purple-flowered plants could be Pp, although they would not be true-breeding. Individuals
that are purple, but had a white parent, are heterozygous: Pp. Hetero means "different".
When an organism is studied for three different genes and has the alleles AABbCC, it is
homozygous for A and C genes but heterozygous for the B gene.
The physical appearance of an organism is its phenotype. Purple-flowered would be a
phenotype.
The actual composition of the organism's alleles for a gene is its genotype: Pp is a genotype.
Organisms have many different genes some have thousands, and complex organisms have 10
times that number.
Task: Show the P, F1 and F2 generations of a true-breeding round seeded plant crossed with a true
breeding wrinkle-seeded plant.
The Punnett Square
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a simple box-like device that helps us to consider all genetic combinations and
show the expected frequencies of genotypes.
The P and p symbols represent the single allele each gamete receives.
Fertilization provides the two alleles for the new individual, one from the male
(sperm) and one from the female (egg).
The Punnett square shows that the genotypes and associated ratios for a
monohybrid cross are 1 PP :2 Pp : 1 pp.
Any progeny with a P would have the dominant (purple) phenotype, so the
phenotypic ratio is 3 purple to 1 white.
Now it is known that a gene is a portion of the chromosomal DNA that resides
at a particular site, called a locus (plural is loci). The gene codes for a particular
function or trait.
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Mendel arrived at the law of segregation with no knowledge of meiosis or chromosomes. The
mechanism of chromosome separation in meiosis I today explains his law of segregation.
One Trait Test Cross
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A test cross can determine the genotype (heterozygous or homozygous) of an individual with a
dominant trait.
It involves crossing the individual to a true-breeding recessive (homozygous recessive).
If the unknown is heterozygous, approximately half the progeny will have the dominant trait and
half the recessive trait.
If the unknown is homozygous dominant, all the progeny will have the dominant trait
WILD TYPE = refers
to the "normal"
genotype
Often designated
with a + symbol
Mendel's Law of Independent Assortment
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The second law describes the outcome of dihybrid (two character) crosses, or hybrid crosses
involving additional characters.
A dihybrid is an individual that is a double heterozygote (e.g., with the genotype RrYy - round
seed, yellow seed).
Mendel's second law states that the Rr alleles assort into gametes independently of the Yy
alleles.
The dihybrid, RrYy, produces gametes that have one allele of each gene.
Four different gametes are possible and will be produced in equal proportions: RY, Ry, rY, and
ry.
Random fertilization of gametes yields the outcome visible in the Punnett square. Note its 4x4
table construction to accommodate 16 possible phenotypes.
Filling in the table and adding the like cells reveals a 9:3:3:1 ratio of the four possible
phenotypes: (Round, Yellow - Round, Green - Wrinkled, Yellow - Wrinkled, Green.)
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The Law of Independent Assortment states that alleles of different genes assort independently
of one another during gamete formation.
In fact, this law is not always true - The law of independent assortment is accurate for genes
that are on separate chromosomes, but not necessarily for genes that are on the same
chromosome.
Genes that are close to each other on the same chromosome tend to stay together, but crossing
over during meiosis may separate them.
Dihybrid Cross: RrYy x RrYy
TIP: In any cross that is dihybrid (AaBb x AaBb) you will always get a 9:3:3:1 ratio, if you memorize this,
you can save the trouble of doing a giant square!
A Mathematical Alternative (LAWS OF PROBABILITY)
A punnet square is not needed to determine the ratios of genotypes and phenotypes. Simple statistics
and math can save you the trouble of filling out a square.
In a monohybrid cross Pp x Pp, each parent produced
P gametes and
If you wanted to determine how many of the offspring are pp:
x
p gametes
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Example 2: H is dominate for long hair (h = short) and B is dominate for black eyes (b = red eyes)
If the parents are: HhBb x hhBb
How many off the offspring will be short haired and red eyed?
Task: Use mathematical analysis to determine the number of short haired, black eyed offspring from the
cross above.
TWO-TRAIT TEST CROSS
Used to determine the genotype of an "unknown" by crossing it with an individual that is homozygous
recessive for both traits.
In flies (Long wings is dominant to short wings, Gray body is dominant to black)
A L __ G ___ is test crossed.
The offspring are 1:1:1:1 --> What is the genotype of the unknown parent?
If the offspring are half long winged & gray, and half long winged and black --> What is the genotype of
the unknown parent?
We will know complete some assignments with the use of punnett squares and our knowledge of
genetics.
1.
Please go to the following link and complete this assignment in your book:
http://www.biologycorner.com/worksheets/penny%20genetics.pdf
2.
We will try to complete the following assignment as a group:
http://biologycorner.com/worksheets/genetics_2traits_bio2A.html