Transcript theoretical genetics

GENETICS & INHERITANCE
INHERITED
CHARACTERISTICS
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Leaf shape in plants
Coat colour in guinea pigs
Tongue rolling ability
Hair & eye colour
These are all examples of the many
characteristics determined by genetic
information passed on from generation to
generation during sexual reproduction
INHERITED
CHARACTERISTICS
 The alleles inherited from parents may
be the same or different.
 A DOMINANT allele always shows in
the appearance of an organism
 A RECESSIVE allele only shows if the
organism has inherited two of them
*
Symbols
 To represent dominant and recessive
traits, we use the first letter of the allele,
using a capital letter for the dominant and
a small letter for the recessive.
 Eg mouse colour, Brown is dominant over
white coat colour.
 Brown =B
White = b
INHERITED
CHARACTERISTICS
BB or Bb
tt
TT or Tt
bb
If a pea plant inherited the allele for
tallness from each of its parents then its
genotype would be TT.
tt – inherited allele for dwarfness
Tt – inherited a dominant & recessive allele
for height
KEY WORDS
 DOMINANT – always shows up in appearance
of organism
 RECESSIVE – only shows if organism has
inherited two of the alleles
 GENOTYPE – the set of genes an organism
has, the alleles it carries for a characteristic
(usually written as letters)
 PHENOTYPE – the appearance of the
organism (blue-eyed, tall)
GENOTYPE & PHENOTYPE
 The phenotype of an organism depends
on the genotype!
 An organism which has inherited 2alleles
the same is called HOMOZYGOUS
(TT,tt)
 An organism which has inherited 2
different alleles is HETEROZYGOUS
(Tt)
PREDICTING PHENOTYPES
FROM GENOTYPES
 In humans the allele
for tongue rolling is
dominant (R) to the
non-rolling (r)
 In mice the allele for
black coat is dominant
(B) to white coat (b).
MONOHYBRID CROSS
 A monohybrid cross
is a breeding
experiment which
follows the
inheritance of one
characteristic.
 Organisms which are
true breeding pass
on the same
characteristic to
their offspring over
many generations.
Parents
X
YY yellow seeds
Gametes
Ovules Y
Pollen Y
F1 (first)
generation
Gametes
YY
X
Pollen Y
Ovules Y
Second
generation
All offspring genotype YY – yellow seeds
GENERATIONS OF A
MONNHYBRID CROSS
 How do we know
yellow is the
dominant seed
colour?
Parent
generation
Pea plant YY
yellow seeds
Pollen Y
X
Pea plant yy
green seeds
Pollen y
F1
generation
All offspring pea plants Yy with yellow seeds
THE F2 GENERATION
 To find out
what happened
to the recessive
characteristic
Mendel crossed
the F1
generation
 The second
generation –
offspring of
the F1
generation.
F1
Generation
X
Pea plant Yy – yellow seeds
Pea plant
Yy – yellow seeds
Punnet square
showing 2nd generation
Female gametes
F2
Y y
YY Yy
Yy yy
Male
gametes
Y
y
Providing a phenotypic ratio of 3:1
TEST CROSSES
 A practical problem for breeders is to be sure that
an organism is true breeding.
 Looking at phenotype is not enough.
 A test cross can be done to identify the genotype of
an individual with a dominant characteristic by
crossing it with a homozygous recessive individual.
CO-DOMINANCE
 It is possible for two contrasting
members of a pair of alleles to be in
equal dominance.
 This type of inheritance is called Codominance.
 Offspring turn out to have a phenotype
which is in between each parent.
CO-DOMINANCE
 Both black and
white coats breed
true when such
horses are crossed
but all offspring in
fact have both
black and white
hairs.
 These are called
grey roans.
X
GENOTYPES IN CODOMINANCE
 Neither alleles is recessive. Both are capital
letters.
Phenotype
Black coat
Roan coat
White coat
Genotype
BB
BW
WW
Both black and white phenotypes are true
breeding homozygous. The roan phenotype is
heterozygous
OTHER POSSIBLE CROSSES!?
RR
Red coat and white
coat are co-dominant
and intermediate
phenotype is light
red called red roan.
WW
X
RW
HOMOZYGOUS x
HETEROZYGOUS
X
PHENOTYPES
GENOTYPES
GAMETES
RED BULL
ROAN COW
RR
RW
ALL R
R or RW
Do a punnet square to work out the offspring off this cross.
F1 GENOTYPES
PHENOTYPES
RATIO =
RATIO =
OTHER POSSIBLE
CROSSES!?
 Blood group is determined by three alleles A, B,
and O. A and B are co-dominant to O the recessive
allele.
• A person with blood group A may have
the genotype AA or AO.
• A person with blood group AB must
have genotype AB as both alleles are
co-dominant.
EXAMPLES OF POLYGENIC
INHERIATNCE IN HUMANS
 Height, weight, skin colour, foot size etc.
show a pattern of polygenic inheritance.
 There are many different expressions of
these so they must be controlled by more
than 2 genes.
Skin colour
Height
Maize – kernel colour
COMPARING POLYGENIC &
SINGLE GENE INHERITANCE
TYPE OF INHERITANCE
SINGLE GENE
NUMBER OF ONE
GENES
NUMBER OF 2
PHENOTYPES
TYPE OF
DISCONTINUOUS
VARIATION
POLYGENIC
MORE THAN
ONE
MANY
CONTINUOUS
DROSOPHILA
MALE - ROUND
ABDOMEN
FEMALE POINTED
ABDOMEN
 Very variable so many phenotypes to investigate
 Short life cycle – 10-14 days
 Large offspring produced quickly become sexually
mature in short period of time (8-9hrs after hatching)
 Grey body (G) or ebony (black) (g)
Steps in Monohybrid Cross with
Drosophila
1) Parents are chose, one with a grey body
2)
3)
4)
5)
6)
and other with ebony body
Breed the parents
The F1 phenotype is observed
The F1 generation is self crossed
The F2 generation is examined and
phenotypes counted
The experiment is repeated