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Genetics
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262
Genetics
 Every
living thing has a set of
characteristics inherited from its parents
(sexual reproduction) or parent (asexual
reproduction).
 Genetics
is the study of heredity.
Gregor Mendel’s Peas
 Gregor
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Mendel
born 1822
Polish monk
lived in a monastery
taught
looked after the monastery’s garden.
In this garden, he did the work that would
change biology forever.
Observing his pea plants, Mendel knew that:
-the anthers of each flower produced pollen
which contained the sperm.
-the female portion of the flower produces
the egg cells.
 During
sexual reproduction, male and
female gametes join and a nucleus
containing both sets of genetic information
is formed, a process known as fertilization.
 Fertilization
produces a new cell, a zygote,
which develops into a tiny embryo. With
flowering plants, this embryo is encased in a
seed.
 Pea
plants usually self-pollinate, meaning
that they have a single parent. The pollen
fertilizes the egg of the same plant.
 True
breeding plants are self-pollinating
and therefore produce offspring identical
to themselves.
Mendel’s Pea Plants
 Mendel
observed that some of his pea
plants were tall, others short;
 some had smooth pods, some wrinkled;
 some had yellow pods, some green;
 some had round peas, some wrinkled;
 some had green peas, some white;
 some had flowers in an axial position,
some terminal;
 some had yellow cotelydons, some
green.
Each offspring always had the same traits as
its parent.
Mendel’s Experiments
 Mendel
wanted to see what would
happen if he produced seeds by
fertilizing the seeds from one plant
with the pollen from another plant
with different traits.
 He removed the pollen-producing
parts of one plant and dusted
pollen from another plant on the
stigma (check the diagram in slide
4).
 Mendel cross-pollinated the pea
plants.
 Mendel
studied 7 different pea plant traits. A trait is
a specific characteristic that varies from one
individual to another.

Genes are the molecular
factors (pieces of code along DNA)that
determine traits.
 The
different forms of a gene are called
alleles.
(In peas, genes can occur in two
contrasting forms. i.e. yellow pods/ green
pods)
Terminology
P:
Each original pair of plants
(the parental generation)
F1:
the offspring (first filial) of the
o F2:
the offspring of the F1
P
 Hybrids:
The offspring of crosses between
parents with different traits.
Ex. A yellow-podded pea plant that is
crossed with a green-podded pea
plant.
Gregor Mendel concluded 2 things from his
experiments:
 Heredity
is determined by the elements
that are contained in the cell contributed
by each parent of the organism.
 When
two characteristics are crossed (as
in cross pollination, one of the
characteristics is dominant over the other.
(this theory turned out not to be
applicable in all cases)
Dominance
Mendel’s principle of dominance states that :
 some alleles are dominant and some are
recessive.
 An organism with a dominant allele for a
specific trait will always exhibit that form of
the trait.
 An organism with a recessive allele for a
specific trait will only exhibit that trait if the
dominant allele is not present. (2 recessive
alleles)
Take the alleles for plant
height.
T = tall
t = short
First cross the pure-breeding P: all of the plants that are tall
with all of the plants that are
short.
1st cross
producing the F1
2nd cross producing the F2
Vocabulary:
 Homozygous:
Having the same alleles for
a gene. (TT, tt)
 Heterozygous:
a gene. (Tt)
Having different alleles for
 Locus:
Location of a particular gene on a
chromosome.
 Genotype:
Actual genetic makeup. What
kinds of alleles the organism has (Tt, TT, tt).
 Phenotype:
The trait displayed.
(tall, short)
 Genome:
genome)
 Diploid:
Totality of genes. (human
2n (2 sets of chromosomes)
 Haploid:
n (1 set of chromosomes)
Problem
A homozygous black-coated pig (B) is
crossed with a heterozygous pig.
What are the phenotypes of the F1
generation?
What are the possible phenotypes of the
F2 generation?
Problem
Mendel crossed pea plants that were
homozygous dominant for round yellow
peas with plants that were homozygous
recessive for wrinkled green peas.
What did he get in the F1 generation?
Segregation is the separation of alleles
during gamete formation.
Dihybrid cross F
Dihybrid cross F+
Segregation
The Principle of Independent
Assortment States:
genes for different traits can segregate
independently during the formation of
gametes. Independent assortment helps
to account for the many variations
observed in plants, animals, and other
organisms.
A
cross of an individual with a dominant
phenotype to a homozygous recessive
individual in order to find out its genotype
is called a testcross.
Genetic variation "reshuffling the genes" comes
from several sources with each meiosis and
fertilization like a 'roll of the dice’- so no two
individuals are alike!
Sources of genetic variation:
1. Crossing Over at prophase I
2. Independent assortment: Homologues line up or
"shuffle" randomly on the metaphase plate in Meiosis
I. With 23 chromosomes assorting independently,
there are 2^23, or 8 million, possible assortments of
chromosomes inherited for every cell!!
3. Random fertilization: The ovum has 8 million possible
chromosome combinations, so does the sperm cell. 8
million x 8 million = 64 trillion possible diploid
combinations in EACH AND EVERY zygote!
In other words, you're unique...(...just like everyone
else.....).