Genetics

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Genetics
Genetics Study of heredity. Heredity
Biological inheritance.

• Gregor Mendel father of genetics
• Studied pea plants in the
• monastery garden.
• Purebred plants were the
• basis of his experiments.
• Purebred If allowed to self pollinate,
• the purebred peas would produce offspring that
  were identical to themselves.
• Mendel studied a number of
• easily observable pea traits
• such as seed shape and
• seed color.

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Hybrids

• Mendel crossed plants with different
• characteristics to form hybrid offspring.
• Hybrid Organism produced by
• crossing parents with different forms of
• the same trait (alleles).
• Hybrid Heterozygous

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Genetics Definitions
P Generation The parental organisms. F1
Generation First generation produced by a cross
(offspring of P Generation). F2 Generation
Second generation (offspring of F1 Generation).
Segregation Separation of alleles during
gamete formation. Punnett Square Shows
possible gene combinations in offspring
that result from a cross. Gametes Reproductive
cells Independent Assortment- Genes for different
traits can segregate independently during the
formation of gametes.
Phenotype Physical characteristics (What you
see.) Genotype Genetic makeup (Alleles they
are carrying.) Homozygous Organisms that
have two identical alleles (purebred). Heterozyg
ous Organisms that have two different alleles
(hybrid). Incomplete Dominance- One allele is
not completely dominant over the
other. Codominance- Both alleles are expressed.
Sex Linked Traits- Gene is carried on a sex
chromosomes
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Fruit Fly Genetics
Why work with Drosophila? Part of the reason
people work on it is historical - so much is
already known about it that it is easy to handle
and well-understood - and part of it is
practical it's a small animal, with a short life
cycle of just two weeks, and is cheap and easy to
keep large numbers. Mutant flies, with defects in
any of several thousand genes are available, and
the entire genome has recently been sequenced.
Life cycle of Drosophila The drosophila egg is
about half a millimeter long. It takes about one
day after fertilization for the embryo to develop
and hatch into a worm-like larva. The larva eats
and grows continuously, moulting one day, two
days, and four days after hatching (first, second
and third instars). After two days as a third
instar larva, it moults one more time to form an
immobile pupa. Over the next four days, the body
is completely remodelled to give the adult winged
form, which then hatches from the pupal case and
is fertile within about 12 hours. (timing is for
25C at 18, development takes twice as long.)
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This is a female fruit fly Notice the
tiger-striped abdomen and the simple tag-like tip
on the abdomen.
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The tip at the end of the female abdomen is the
excretory opening. Just proximal to this opening
is the female genital opening.
Female Genitalia
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This is a mature male fruit fly Mature males
have a prominent black abdominal end. Males also
have tiny sex combs on their front pair of legs.
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Heres a close up of the male genital region.
Notice the metallic sheen of the genitals.
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One male and one female Find their
distinguishing features.
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Ventral view of a male and female Find their
distinguishing features
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Female laying an egg
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Tiny fly eggs hatch into tiny worm-like larvae.
The larvae eat and grow in length and girth. The
maximum size of the larval stage is called the
third instar. This larval stage becomes a pupa.
Anus
This is a third instar larva prior to pupation
Mouth
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Larva to Pupa Stages
Larvae are mobile
Pupal stages
With the onset of pupation the worm-like larva
contracts and a parchment-like pupa case hardens
around the metamorphosing fly.
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Sex Linked Crosses
Sex-Linked Gene Any gene located on a sex
chromosome (some related to sex determination,
some not). Sex-linked genes unrelated to
sex determination are often found on
X chromosomes ONLY! Ex Eye color in fruit flies
is sex-linked. Red Eyes Wild Type,
Dominant. White Eyes Recessive, Very Rare
Problem Heterozygous Female x Red-Eyed Male
X
R
Y
X
R
X
r
Genotypic Ratio 1 XRXR 1 XRXr 1 XRY 1 XrY
Phenotypic Ratio 2 Red-Eyed Females 1 Red-Eyed
Male 1 White-Eyed Male