Zusammenfassung der Ressource
Pack 12 -
Inheritance
- Genotype:
- Genetic makeup of an
organism, all alleles
an organism has.
- Phenotype:
- Observable or
biochemical
characteristics
- What is a gene
- A sequence of nucleotide
bases coding for a
polypeptide or functional
RNA
- An allele is a different
version of the same
gene
- Each gene of diploid
cells should have 2
alleles, unless sex
chromosones
- The locus is the position
of a gene on a
chromosome
- Types of
chromosomes:
- Homozygous
- Alleles on each
chromosome are the
same
- Heterozygous
- 2 alleles on the
chromosome are
different
- Types of alleles:
- Dominant
- When present in
genotype it's always
expressed in phenotype
- Recessive
- Only expressed when
homozygous recessive
- Codominance
- This is when two alleles
both contribute to the
phenotype
- Monohybrid Inheritance
- This is the
inheritance
of a single
gene
- Pedigree Diagrams
- Diagrams show ancestral
relationships and
transmission of genetic
traits
- Exam hint:
- If asked whether an allele is dominant or
recessive, look for two parents that do not have
the characteristic but have children that do have
it
- Shows it is recessive
- Dihybrid Inheritance
- Two different
characters, determine
by different genes
located on two
different chromosomes
are inherited
- Mendel's Laws
- Law of segregation:
- Characteristics of an organism are
controlled by genes that occur in pairs,
only one of each pair can be carried in
a gamete
- Law of independent assortment
- Each member of a pair of
alleles may combine
randomly with either of
another pair
- In terms of meiosis:
- Homologous
chromosomes pair up
in prophase 1
- In metaphase 1:
- Homologous
chromosomes
line up on
equator in pairs
- Random segregation of c'somes
either side of equator leads to
different combinations and so
different alleles in gametes
- Homologous
chromosomes
separated in
Anaphase 1
- Autosomal linkage
- An autosome is a
chromosome that
is not a sex
chromosome
- Any two genes occuring
on the same chromosome
are said to be linked
- Autosomal linkage is
when two or more
genes are carried on
the same autosome
- Sickle cell anaemia
- Due to single base substitution.
- Results in one amino
acid being altered in
the 146 long Beta
polypeptide chain
- Multiple alleles
- Some genes have more than two alleles
- Only two of these alleles
can be present in an
individual at one time
- e.g. human blood groups
- Epistasis
- This is where an allele of one
gene affects or masks the
expression of another in the
phenotype
- Example: Coat colour in mice
- Gene A controls
melanin distribution
in hair
- Dominant =
black bands
- Recessive when
homozygous = uniformly
black
- Gene B determines the expression of gene A:
- Dominant allele
= production of
melanin
- Recessive = when
homozygous no
melanin produced.
- Sex inheritance in
humans
- Humans have 22
pairs of
autosomes and a
pair of sex
chromosomes
- Females have two X's
- Males have one X one Y
- Produce gametes
with X and Y
- Produce
gametes of only
X
- Sex Linkage
- All genes carried on
the sex
chromosomes are
transmitted along
with those during
sex
- Because Y is
smaller than X,
there is no
equivalent part
of X to half of Y
- This means that in males,
recessive alleles on
non-homologous part of X will
be expressed
- This is because there is
no equivalent
dominant allele
- Expressed as:
X^N X^a and
X^N Y
- Haemophilia
- This is the failure of the blood to
clot and is potentially fatal if not
treated.
- One cause is the
recessive allele of a
gene that codes for a
protein needed in the
clotting process
- Stats
Tests
- The chi-squared
test
- Used to test whether deviations
between observed and expected
numbers are significant or not
- Null Hypothesis:
- No statistically significant
difference between observed and
expected frequency, any difference
is due to chance
- Cut off point where p=0.05 is
whether you accept or reject
null hypothesis
- If X^2>critical value or equal
too, reject null hypothesis
- The probability that the differences
occured by chance is less than or equal to
0.05
- If X^2<critical value, accept null
hypothesis
- No significant difference,
probability that difference is due
to chance is greater and 0.05
- Then, calculate degrees of
freedom (number of
categories minus one)