Download 01. Genetics

Genetics
What is genetics?
• Science of heredity and variation in
organisms
• Predicts what the next generation will be
like
• Improves the next generation
• Can prevent further genetic disorders
Background
There are two factors influencing affecting
“the next generation”
Nature vs. Nurture
Nature – genetics
Nurture – how we are influenced by
environment and those around us
Heredity History
• People knew a lot about inherited traits a
long time ago
• Pedigrees for horses are found on stone
tablets from Babylon (about 6000 years
ago)
• Chinese farmers had methods of
improving rice varieties long ago
• Some naturalists thought that a giraffe
came from a leopard and camel. Not
possible!
• Interspecific hybrids are the result from
two different species (zonkey).
• Humans mated selectively as well,
Cleopatra married her younger brother
• Spartans practiced infanticide (killing
babies with undesirable characteristics)
• One old theory said that the
sperm contained a fully
formed child (in miniature)
which would be incubated in
the female.
• Later, geneticists would
conduct breeding experiments
with mammals.
• They would describe both
parents, then examine the
offspring and try to decide
where the traits came from
Gregor Mendel
• An Austrian monk (1822 - 1884)
• Provided a basis for understanding of
heredity in general
• Used pea plants
• Why did Mendel choose the garden pea?
1. garden peas have a number of
characteristics that are expressed in 2
ways (contrasting, e.g. tall vs. short)
 tall plants + short plants
 yellow seeds + green seeds
colored seed coats + white coats
seed coats round (smooth) + wrinkled
axial flowers + terminal flowers
Inflated pods + constricted pods
2. The way the garden pea reproduces
(self-fertilize and cross-fertilize).
 Self-fertilization is when they fertilize
(pollinate) themselves.
 Cross-fertilization is when one organism
fertilizes another.
 He used pure tall breeds (TT, not Tt)
• People before Mendel thought that crossing a
tall and short would make a medium height plant
( or white and red make pink)
• When Mendel crossed a round (pollen) and a
wrinkled (egg)
all round offspring
• He thought that maybe the pollen determined
the offspring, so he tried a wrinkled (pollen) with
a round (egg). This made all round offspring as
well. He determined that round was dominant .
• He called the traits genes (seed colour,
seed shape)
• He knew there were alternate forms of the
genes (round vs. wrinkled) called alleles.
• Alleles are two or more alternate forms of
a gene. Alleles are located on pairs of
homologous chromosomes.
• Dominant – alleles of this type determine
expression of genetic traits of offspring
• Recessive – alleles overruled by dominant
From his work he came up with 2
explanations for inheritance
1. Law of Segregation – The Law of
Segregation states that when any individual
produces gametes, the copies of a gene
separate, so that each gamete receives only
one copy. A gamete will receive one allele or
the other.
2. Law of Independent Assortment –if
genes are located on separate
chromosomes, they are inherited
independently of each other.
Single-Trait Inheritance
• Genotype – alleles that an organism
contains for a trait (tall stem plant could be
TT or Tt)
• Phenotype – observable traits of an
individual (TT or Tt both produce a tall
organism)
• Homozygous – genotype that contains
two alleles that are the same (TT or tt)
• Heterozygous – used to describe the
genotype of an organism that contains two
different alleles (Tt)
• Hybrids - offspring that differ from their
parent in one or more traits
• Monohybrid cross – describes a cross
between a TT and a tt, because only one
trait is being tested
Punnett – developed a mathematical way of
predicting future generations
Punnett squares are charts used to show
possible combinations of alleles in offspring
e.g. monohybrid cross
If a pure tall pea plant crosses with a pure short
pea plant and Tall is dominant over short,
describe possible F1 and F2 generation offspring.
Pure tall and pure short mate (F1)
• Parents T T x tt = Tt (hybrid, not pure)
Pure short (tt) – t is not capital because
it is recessive
Pure tall (TT) – T is capital because it is dominant trait
F1
T
Phenotype (what you see) – all tall
T
Genotype (genes) – all hybrid tall (Tt)
t
Tt
Tt
t
Tt
Tt
Second Generation
offspring from pure tall and pure
short mate (F2)
F2
T
t
Phenotype (what you see) – 3 tall, 1 short
T
TT
Tt
t
Tt
tt
Genotype (genes) – 1 pure tall (TT)
2 hybrid tall (Tt)
1 pure short (tt)
• A fruit fly breeding pure for wild body,
mates with one pure for dumpy body, wild
body is dominant. Describe possible
describe possible F1 and F2 generation
offspring.
• Parents: WW x dd
F1
W
W
Phenotype (what you see) – all wild body
d
Wd
Wd
d
Wd
Wd
F2
W
d
Genotype (genes) – 4 hybrid wild body
Phenotype (what you see) – 3 wild body
1 dumpy body
W
WW
Wd
d
Wd
dd
Genotype (genes) – 1 pure wild body
2 hybrid wild body
1 pure dumpy body
Dihybrid
• Mendel also studied two-trait inheritance.
This he called the dihybrid cross.
• E.g. A pea plant breeding pure for round
seed and pure for inflated pod, crosses
with one plant breeding pure for wrinkled
seed and pure for constricted pod. Round
seed and inflated pod are dominant.
describe possible F1 and F2 generation
offspring.
• Parents: RR II x rr ii
F1
ri
ri
RI
Rr Ii
Rr Ii
RI
Rr Ii
Rr Ii
Phenotype (what you see) – all round seeds
and inflated pods
Genotype (genes) – all hybrid round seeds
and hybrid inflated pods
Rr Ii x Rr Ii
F2
Ri
RI
ri
rI
Ri
RR ii RR Ii Rrii
RrIi
RI
RRIi
RRII RrIi
RrII
ri
Rrii
RrIi
rrii
rrIi
rI
RrIi
RrII
rrIi
rrII
Phenotype –
9 round seed + inflated pod
3 round seeds+ constricted pod
3 wrinkled seed + inflated pod
1 wrinkled seed + constricted
pod
Genotype –
1 pure round seed + pure
constricted pod
2 pure round seed + hybrid
inflated pod
1 pure round seed + pure inflated
pod
2 hybrid round seed + pure
constricted pod
4 hybrid round seed + hybrid
inflated pod
2 hybrid round seed + pure
inflated pod
1 pure wrinkled seed + pure
constricted pod
1 pure wrinkled seed + pure
inflated pod
2 pure wrinkled + hybrid inflated
• Hand-out Questions