Download Inherited Characteristics

GENOTYPE &
PHENOTYPE
CHAPTER 9
INHERITED CHARACTERISTICS
• Information
controlling inherited
characteristics is
passed on during
sexual reproduction
• E.g.
• leaf shape
• coat colour in dogs
• wing type in flies
• tongue rolling in
humans
TRUE BREEDING
• When a characteristic
is passed on
identically through
several generations
• E.g. flower colour
• Offspring are identical
to their parents for
that characteristic
• Also known as pure
breeding
SINGLE FACTOR (MONOHYBRID) CROSS
• Investigates the inheritance
of one characteristic only
• 2 true breeding species are
bred (crossed) together
• They only ever differ in one
characteristic
• PHENOTYPE: The physical
appearance of a
characteristic
MONOHYBRID CROSS (CTD.)
• When two true breeding species
are crossed, the offspring
produced all appear the same
• These are the first generation
(F1)
• When these F1 are interbred, the
offspring often produce a 3:1
ratio
• e.g. 3 red flower : 1white flower
• These are the second
generation (F2)
MONOHYBRID CROSS (CTD.)
• The characteristic that
comes through in the F1
generation is dominant
• E.g. purple flowers
• The characteristic that’s
masked is recessive
• E.g white flowers
• All monohybrid crosses
produce a 3 : 1 ratio
GENES & GENOTYPE
• Genes control each
inherited characteristic
• The complete set of genes
is known as the GENOTYPE
• At least 2 forms of each
gene exist within a species
• Different forms of a gene
are known as alleles
• every cell has two alleles,
one from each parent e.g
PP
• Every gamete has only
one allele e.g P
HOMOZYGOUS & HETEROZYGOUS
• When an organism
possesses two identical
alleles of a gene e.g. PP or
pp,
• - it is said to be
homozygous (true
breeding)
• If the alleles differ, the
genotype is said to be
heterozygous
• E.g Pp
• Organisms can have the
same phenotype but a
different genotype
OBSERVED VS. PREDICTED FIGURES
• Monohybrid crosses should always produce a 3:1
ratio in the F2 generation
• However in real life this rarely happens
• This is because fertilisation is a random process
• There is always an element of chance
HUMAN INHERITANCE
• Humans are unsuitable for
genetic study
• - too few offspring, too
many genes
• However a family tree can
trace inheritance
• E.g - red hair colour is
recessive
• - children can have red
hair even if their parents
don’t
• Element of chance always
applies
CO-DOMINANCE
• Some alleles are co-dominant
• That means no allele is fully
dominant or recessive
• E.g. blood group
• A & B are co-dominant to each
other
• A & B are completely dominant
over O
• So, AB genotype = AB
phenotype
• But, AO, or BO, genotype = A or
B phenotype
POLYGENIC INHERITANCE
• Many characteristics are
controlled by several
different genes
• Therefore there are a
range of phenotypes
possible
• E.g. height – varies
without distinct groups
• E.g. Skin colour – wide
range of colours
• These are controlled by
polygenic inheritance
PHENOTYPE AND ENVIRONMENT
• Some characteristics are solely
dependent on genotype
• E.g. blood group
• But, phenotype can be
influenced by environmental
factors
• Genotype + Environment ---
phenotype
• This can be shown by studying
clones or twins - genetically
identical
• Any differences must be due to
environmental factors
• If these species breed, offspring
will not inherit the physical
changes
NATURAL SELECTION
• Every species can potentially
produce far more offspring than
environment can support
• Members of a species all show
slight differences in phenotype
• E.g Peppered Moth
• Organisms with favourable
phenotypes will be better
adapted to their environment
• Their favourable characteristics
will be passed on to the next
generation
• Those with unsuitable
characteristics will die out and
will not get to reproduce
• Also known as SURVIVAL OF THE
FITTEST
EVOLUTION
• The present biodiversity on
the earth is due to natural
selection over millions of
years.
• This is known as EVOLUTION
• In time more and more
complex lifeforms will
continue to develop
• Provided that human activity
doesn’t wipe out too many
habitats