13.1 Gregor
Mendel
A. Gregor Mendel
1. Mendel was an Austrian monk.
2. Mendel formulated two fundamental
laws of heredity in the early 1860's.
3. He had previously studied science
and mathematics at the University of Vienna.
4. At time of his research, he was a
substitute science teacher at a local technical high school.
B. Blending Concept of Inheritance
1. This theory stated that offspring would possess traits intermediate between
those of different parents.
2. Red and white flowers produce
pink; a later return to red or white progeny was considered instability
in genetic
material.
3. Charles Darwin wanted to develop
a theory of evolution based on hereditary principles; blending
theory was
of no help.
a. The
blending theory did not account for variation and could not explain species
diversity.
b. The
particulate theory of inheritance proposed by Mendel can account for presence
of differences
among member of a population generation after generation.
c. Mendel's
work was unrecognized until 1900; Darwin was never able to use it to support
his theory of evolution.
C. Mendel's Experimental Procedure
1. Mendel did a statistical study (he had a mathematical background).
2. He prepared his experiments
carefully and conducted preliminary studies.
a. He chose
the garden pea, Pisum sativum, because peas were easy to
cultivate, had a short generation time,
and could be cross-pollinated.
b. From many
varieties, Mendel chose 22 true-breading varieties for his experiments.
c. True-breeding
varieties had all offspring like the parents and like each other.
d. Mendel
studied simple traits (e.g., seed shape and color, flower color, etc.).
3. Mendel traced inheritance of
individual traits and kept careful records of numbers.
4. He used his understanding of
mathematical principles of probability to interpret results.
13.2. The
Monohybrid Cross
A. Cross-pollination Monohybrid Crosses
1. A hybrid is the product of parent organisms that are
true-breeding for different forms of one trait.
2. A monohybrid cross
is between two parent organisms true-breeding for two distinct forms of a
trait.
3. Mendel tracked each trait through
two generations.
a. P
generation is the parental generation in a breeding experiment.
b. F1
(for filial) generation is the first-generation offspring in a breeding
experiment.
c. F2
generation is the second-generation offspring in a breeding experiment.
d. He also
performed reciprocal crosses of pollen on stigmas (e.g. tall-with-short and
short-with tall).
B. Mendel's Results
1. His results were contrary to those predicted by a blending theory of
inheritance.
2. He found that the F1 plants
resembled only one of the parents.
3. Characteristic of other parent
reappeared in about 1/4 of F2 plants; 3/4 of offspring resembled the F1 plants.
4. Mendel saw that these 3:1 results
were possible if
a. F1
hybrids contained two factors for each trait, one dominant and one recessive;
b. factors
separated when gametes were formed; a gamete carried on copy of each factor;
c. Random
fusion of all possible gametes occurred upon fertilization.
5. Results of his experiments led
Mendel to develop his first law of inheritance:
a. Mendel's
law of segregation: Each organism contains two factors for each trait;
factors segregate in
formation of gametes; each gamete contains one factor for each trait.
b. Mendel's
law of segregation is consistent with a particulate theory of inheritance
because many individual
factors are passed on from generation to generation.
c.
Reshuffling of factors explains variations and why offspring differ from their
parents.
C. As Viewed By Modern Genetics
1. Each trait in a pea plant is controlled by two alleles,
alternate forms of a gene that occur at the same
gene locus
on homologous chromosomes.
a. A dominant
allele masks or hides expression of a recessive allele; it is
represented by an uppercase letter (T).
b. A recessive
allele is an allele that exerts its effect only in the homozygous
state; its expression is masked
by a
dominant allele; it is represented by a lowercase letter (t).
2. Gene locus is
specific location of a particular gene on homologous chromosomes.
3. In Mendel's cross, the parents
were true-breeding; each parent had two identical alleles for a trait -- they
were
homozygous,
indicating they possess two identical alleles for a trait.
a. Homozygous
dominant genotypes possess two dominant alleles for a trait (TT).
b. Homozygous
recessive genotypes possess two recessive alleles for a trait (tt).
4. After cross-pollination, all
individuals of the F1 generation had one of each type of allele.
a. Heterozygous
genotypes possess one of each allele for a particular trait (Tt).
b. The
allele not expressed in a heterozygote is a recessive allele.
D. Genotype Versus Phenotype
1. Two organisms with different allele combinations can have same outward
appearance (e.g., TT and Tt pea
plants are
both tall; therefore, it is necessary to distinguish between alleles and
appearance of organism).
2. Genotype refers to
the alleles an individual receives at fertilization.
3. Phenotype refers to
the physical appearance of the individual.
E. Monohybrid Genetics Problems
1. First determine with characteristic is dominant; then code the alleles
involved.
2. Determine genotype and gametes
for both parents; an individual has two alleles for each trait; each gamete
has only on
allele for each trait.
3. Each gamete has a 50% chance of
having either allele.
F. Laws of Probability
1. Probability is the likely outcome a given event will occur
from random chance.
a. With each
coin flip there is a 50% chance of heads and 50% chance of tails.
b. Chance of
inheriting one of two alleles from a parent is also 50%.
2. Multiplicative law of probability
states that the chance of two or more independent events occurring together
is the
product of the probability of the events occurring separately.
a. Chance of
inheriting a specific allele from one parent and a specific allele from another
is 1/2 x 1/2 or 1/4.
b. Possible
combinations for the alleles Ee of heterozygous parents are the following:
EE = 1/2 x 1/2 = 1/4 eE = 1/2 x 1/2 = 1/4 Ee=
1/2 x 1/2 = 1/4 ee = 1/2 x 1/2 = 1/4
3. Additive law of probability
calculates probability of an event that occurs in two or more independent ways;
it is sum of
individual probabilities of each way an event can occur; in the above example
where unattached
earlobes are
dominant (EE, Ee, and eE), the chance
for unattached earlobes is 1/4 + 1/4 + 1/4 = 3/4.
G. The Punnett Square
1. Provides a simple method to calculate probable results of a genetic cross.
2. In a Punnett square,
all possible types of sperm alleles are lined up vertical, all possible egg
alleles are lined up
horizontally; every possible combination is placed in squares.
3. The larger the sample size
examined, the more likely the outcome will reflect predicted ratios; a large
number
of offspring
must be counted to observe the expected results; only in that way can all possible
genetic types
of sperm
fertilize all possible types of eggs.
4. We cannot testcross humans in
order to count many offspring; in humans, the phenotypic ratio is used to
estimate the
probability of any child having a particular characteristic.
5. Punnett square uses laws of
probability; it does not dictate what the next child will inherit.
6. "Chance has no memory":
if two heterozygous parents have first child with attached earlobes
(likely in
1/4th of children), second child born still has 1/4 chance of having attached
earlobes.
H. One-Trait Testcross
1. Mendel performed testcrosses by crossing his F1 plants with
homozygous recessive plants.
2. Results indicated the recessive
factor with present in the F1 plants; they were heterozygous.
3. A testcross is
between an individual with dominant phenotype and individual with recessive
phenotype to
see if the
individual with dominant phenotype is homozygous or heterozygous.
13.3. The
Dihybrid Cross
A. Dihybrid Crosses
1. A dihybrid cross is an experimental cross between two parent
organisms that are true-breeding for different
forms of two
traits; produces offspring heterozygous for both traits.
2. Mendel observed that the F1
individuals were dominant in both traits.
B. Plants to Self-Pollinate
1. Mendel observed four phenotypes among F2 offspring; he deduced second law of
heredity.
2. Mendel's law of independent
assortment states members of one pair of factors assort independently
of
members of
another pair; all combinations of factors occur in gametes.
C. Dihybrid Genetics Problems
1. Laws of probability indicate a 9:3:3:1 phenotypic ratio of F2 offspring
resulting in the following:
a. 9/16 of
the offspring are dominant for both traits;
b. 3/16 of
the offspring are dominant for one trait and recessive for the other trait;
c. 3/16 of
the offspring are dominant and recessive opposite of the previous proportions;
and
d. 1/16 of
the offspring are recessive for both traits.
2. The Punnett Square for Dihybrid
Crosses
a. A larger
Punnett square is used to calculate probable results of a dihybrid cross.
b. A
phenotypic ratio of 9:3:3:1 is expected when heterozygotes for two traits are
crossed and simple
dominance is present for both genes.
c. Meiosis
explains these results of independent assortment.
D. Two-Trait Test Cross
1. A dihybrid test cross tests if individuals showing two
dominant characteristics are homozygous for both
or for one
trait only, or is heterozygous for both.
2. If an organism heterozygous for
two traits is crossed with another recessive for both traits, expected
phenotypic
ratio is 1:1:1:1.
3. In dihybrid genetics problems,
the individual has four alleles, two for each trait