Concise and pretty notes for AQA A-Level Biology topic/unit 7 - the control of gene expression.
Used to get an A* in A-Level Biology.
Concise and well written summary.
Topic 7: Genetics, populations, evolution and ecosystems
Inheritance
Keywords
Genotype - all of the alleles that an organism carries on its chromosomes.
Phenotype - observable characteristics of an organism which the result of the genotype as well as
environmental factors.
Most organisms are diploid so carry 2 alleles for each gene on each chromosome, alleles can be either:
-> Dominant - only single allele is required for the characteristic to be expressed in phenotype. Whether
the organism only has one dominant allele (heterozygous dominant) or has 2 dominant alleles
(homozygous dominant) the trait will be expressed.
-> Recessive - characteristic is only expressed if there is no dominant allele. There must be 2 recessive
alleles (homozygous recessive) present in order for phenotype to be expressed.
-> Codominant - both alleles are expressed equally and contribute to phenotype, with there either being
blend of 2 characteristic or both characteristics being seen together.
Pure breeding - homozygous pair of alleles.
Allele is alternative form of a gene and locus is specific position of gene on chromosome. 2 alleles for
the gene are both found at same loci on homologous pairs of chromosomes.
Monohybrid inheritance
Phenotype or trait is controlled by a single gene e.g., cystic fibrosis where the individuals with doubly
recessive phenotype are affected.
Diagram shows cross breeding between green pea plant (dominant) and yellow pea plant (recessive).
Dihybrid inheritance
2 characteristics are studied. It is determined by 2 different genes that are present on 2 different
chromosomes at the same time e.g., second diagram shows crossing round, yellow seeds (dominant)
with plant pure breeding for wrinkled, green seeds (recessive).
Codominance and multiple alleles
2 phenotypes are expressed e.g., when birds have
feathers that are both black and white.
See diagram below for roses having red petals and
the other having white petals.
Human ABO blood groups - 3 alleles that are
associated with immunoglobulin gene. These lead to
different antigens on their surface:
-> Allele IA - production of antigen A.
-> Allele IB - production of antigen B.
-> Allele I0 - production of neither antigen A or B.
There are 2 homologous chromosomes that cause
this and have 2 loci.
IA and IB are codominant whereas I0 is recessive
to both of these (see table).
, Sex linkage
Expression of an allele dependent on the gender of the individual as the gene is
located on a sex chromosome.
Humans have 22 pairs of autosomal chromosomes that are not involved in sex
determination and one pair of sex chromosomes either X or Y.
XY = males.
XX = females.
Most sex linked traits are located on X chromosomes with there being no
equivalent locus on the Y chromosome.
Females will carry 2 alleles of sex linked gene but males will only carry single
allele.
Sex-linked disease e.g., haemophilia where the blood clots slowly causing slow and
persistent internal bleeding especially around the joints. Caused by change to DNA
sequence resulting in faulty protein being created (see diagram.)
As a male can only obtain the Y chromosome from his father, it means that X
chromosome must come from his mother.
As this disease is not found on Y chromosome it means that this disease is
always inherited from the mother in males.
If the mother doesn’t suffer from the disease but is heterozygous with the alleles
then she is a carrier.
Autosomal linkage
2 or more genes are on same autosomal chromosome (see diagram).
When A and B are on 2 different chromosomes then there are 4 possible
combinations of alleles. However, if they are on the same chromosomes the there
are only 2 possible combinations of alleles.
Epistasis
Interaction of different loci on the gene, one gene locus affects the other gene
locus.
One gene loci can either mask or suppress the expression of another gene locus
e.g., 2 genes in mice can control fur colour.
-> Genotypes AA or Aa have black bands in their fur while genotype aa has solid
black fur.
-> Gene B controls expression of gene A.
-> Genotypes BB or Bb will allow expression of gene A but genotype bb will not.
-> Mice with genotype bb are all white called albino (see diagram).
Recessive epistasis occurs when presence of a recessive allele prevents
expression of another allele at second locus.
Recessive epistasis gives ratio 9:3:4.
Dominant epistasis is when dominant allele at one locus completely masks the
alleles at second locus.
Dominant epistasis gives ratio 12:3:1.
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