the factors which determine an organisms phenotype
sex-linkage
when the certain allele is located on a sex chromosome
females have two X
chromosomes and males
have one X and one Y
chromosome
the Y chromosome is smaller so fewer genes are carried on
it, so most genes are carried on the X chromosome
if a gene is carried on a Y
chromosome then a female will
not inherit the gene, therefore
it will not be expressed in her
phenotype
males only have one X chromosome so they often only have one allele for
sex-linkage, so they often express the characteristic in thei phenotype even if
it is recessive so males are more likely than females to show recessive
phentoypes for genes that are sex-linked e.g. colour blindness
dominant, recessive and codominant alleles
dominant alleles
an allele whose characteristic
appears in the phenotype even
when there is only one copy
recessive alleles
an allele whose characteristic only appears in the
phenotype if tow copies are present
codominant alleles
alleles that are both expressed in the phenotype
epistasis
many different genes can control the same characteristics - they
interact to form the phenotype
if genes are epistatic they can mask or be masked.
mutations
mutagenic agents
increase the rate of mutation
mutations occur randomly
Ultraviolet
radiation, ionising
radiation,
chemicals and
viruses
some chemicals called base analogs can
substitute for a base during replication,
changing the base sequence in the new DNA
some chemicals can
delete or alter bases,
changing the
structure
some types of rediation can change the
structure of DNA, which causes problems
during DNA replication
substitution - base/s
swapped for another
deletion - base/s removed
addition - base/s added
duplication - base/s repeated
inversion - a sequence of bases reversed
translocation - a sequence of bases moved from
one location to another on the genome
monohybrid or dyhibrid genes
monohybrid inheritance
the inheritance of a
characteristic controlled
by a single gene
dihybrid inheritance
the genes that determine the two characteristics are located on different pairs of
homologous chromosomes. Each of these genes can have two or more alleles.
autosomal linkage
genes on the same autosome are linked
they're on the same autosome so will
stay together during independant
segregation of chromosomes in meiosis 1
the alleles will be passed on
the the offspring together
crossing over
occurs in meiosis
1, and the closer
the genes are, the
less likely
crossing over will
split them up