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Inherited Change - chapter 17
- Studying Inheritance
- Genotype
- the genotype is is
the genetic
make-up of an
organism. It
describes all the
alleles an
organism has.
- The genotype
determines the
limits within
which the
characteristics
will vary
- the genotype is is
the genetic
make-up of an
organism. It
describes all the
alleles an
organism has.
- Phenotype
- The observable or
biochemical
characteristics of an
organism. It is the
result of the interaction
between the expression
of the genotype and the
environment
- The
environment
can
alter
a
phenotype
- The observable or
biochemical
characteristics of an
organism. It is the
result of the interaction
between the expression
of the genotype and the
environment
- Genes
- A length of DNA (a sequence of
nucleotide bases) that normally
code for a particular polypeptide
- Genes exist in two, or
more, different forms
called alleles
- The position of a gene on
a DNA molecule is
known as a locus.
- A length of DNA (a sequence of
nucleotide bases) that normally
code for a particular polypeptide
- Alleles
- An allele is one of the
different forms of a
gene.
- An allele is one of the
different forms of a
gene.
- Dominant and Recessive
- An allele can be
dominant or recessive
and expressed by
homozygous (the
same) or hetrozygous
(different)
chromosomes
- Dominant - an allele that is always expressed in the
phenotype of an organism
- Recessive -
the effect of
an allele is
apparent in
the
phenotype
of a diploid
organism
only in the
presence of
another
identical
allele
- An allele can be
dominant or recessive
and expressed by
homozygous (the
same) or hetrozygous
(different)
chromosomes
- Genotype
- Monohybrid Inheritance
- The inheritance of a single gene
- The law of genetics - In diploid
organisms, characteristics are
determined by alleles that occur in
pairs. Only one of each pair of alleles
can be present in a single gene.
- When the hetrozygous organismof
the F1 generation are crossed with
another, the offspring are always
an approx. ratio of 3:1
- The inheritance of a single gene
- Dihybrid Inheritance
- When two characteristics,
determined by two
different genes located on
different chromosomes are
inherited
- *when doing a dihybrid cross,
the gentoypes are crossed
within each other e.g. RrGg = RG
Rg rG rg*
- Theoretical Ratio = 9:3:3:1
- Mendel's law of independent
assortment = each member of a pair
of alleles may combine randomly with
either of another pair
- When two characteristics,
determined by two
different genes located on
different chromosomes are
inherited
- Codominance and Multiple Alleles
- Codominance
- both alleles are expressed in
the phenotype
- Codominance occurs where instead of one
allele being dominant and the other
recessive, both alleles are equally dominant.
This means both alleles are expressed in the
phenotype
- e.g. snapdragon plants = one allele
codes for an enzyme that catalyses
the formation of a red pigment in
flowers. The other allele codes for
another enzyme that doesn't
produce a pigment. The flowers are
codominant so they can be red,
pink or whits
- e.g. snapdragon plants = one allele
codes for an enzyme that catalyses
the formation of a red pigment in
flowers. The other allele codes for
another enzyme that doesn't
produce a pigment. The flowers are
codominant so they can be red,
pink or whits
- both alleles are expressed in
the phenotype
- Multiple Alleles
- there are more than two alleles,
of which only two may be
present at the loci of an
individual's homologous
chromosomes
- An example of multiple alleles is blood type
- there are 3 alleles associated with gene I. They lead to the presence of
different antigens on the cell surface membrane of red blood cells
- there are 3 alleles associated with gene I. They lead to the presence of
different antigens on the cell surface membrane of red blood cells
- there are more than two alleles,
of which only two may be
present at the loci of an
individual's homologous
chromosomes
- Codominance
- Sex-linkage
- Sex-linkage in
humans
- The sex-chromosomes in humans are X and Y
- Females have two X
chromosomes so all the
gametes are the same in
that they contain a single
X chromosome
- Males have one X
and one Y
chromosome so
produce half X
gametes and half
Y gametes
- The sex-chromosomes in humans are X and Y
- Any gene that is carried on the X
or Y chromosome is said to be sex
linked
- The X chromosome is
much longer than the Y
chromosome meaning
that for most of the
length of the X
chromosome there is
no equivalent
homologous portion of
the Y chromosome
- Those
characteristics
which are
recessive will
appear more
frequently in
men as there is
no portion on the
Y chromosome
which may
contain the
dominant allele
- The X chromosome is
much longer than the Y
chromosome meaning
that for most of the
length of the X
chromosome there is
no equivalent
homologous portion of
the Y chromosome
- Pedigree charts
- Male = square
- Female = circle
- shading
indicates
presence of a
character
- Male = square
- Sex-linkage in
humans
- Autosomal Linkage
- Any chromosome which is not a
sex chromosome is an autosome
- The name given to the
situation where two or
more genes are carried
on the same autosome
is called autosomal
linkage
- Assuming there is no
crossing over, all the linked
genes remain together
during meiosis and so
pass into gametes
- They do not segregate in accordance with Mendel's
Law of Independent Assortment
- ratio = 9:3:3:1
- They do not segregate in accordance with Mendel's
Law of Independent Assortment
- Any chromosome which is not a
sex chromosome is an autosome
- Epistasis
- When the allele of one gene affects or masks the
expression of another in the phenotype
- Recessive epistasis = 9:4:3
- Dominant epistasis = 12:3:1
- Dominant epistasis = 12:3:1
- *exam questions = normally one
enzyme which is not present leads
to the inhibition of another
molecule*
- When the allele of one gene affects or masks the
expression of another in the phenotype
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