Edexcel GCSE 9-1 Biology - SB3: Genetics

Paper 1 Specification: https://qualifications.pearson.com/content/dam/pdf/GCSE/Science/2016/Specification/GCSE_Biology_Spec.pdf

Explain some of the advantages and disadvantages of asexual reproduction, including the lack of need to find a mate, a rapid reproductive cycle, but no variation in the population

advantages:

• time + energy efficient (one parent is needed)

• population can be increased rapidly when the conditions are right

• can exploit suitable environments quickly

disadvantages:

• disease is likely to affect the whole population as there is no genetic variation

• the population is vulnerable to changes in conditions and may only be suited for one habitat

Explain some of the advantages and disadvantages of sexual reproduction, including variation in the population, but the requirement to find a mate

advantages:

• increases genetic variation

• the species can adapt to new environments due to variation, giving them a survival advantage

• disease is less likely to affect them due to genetic variation

disadvantages:

• time and energy is wasted to find a mate

• difficult for isolated members of the group to reproduce

Explain the role of meiotic cell division, including the production of four daughter cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes

• each daughter cell receives a mix of chromosomes from the two sets in the parent cell

• chromosomes in each matching pair swap some genetic material before they are parted in a process called crossing over

• these processes produce new combinations of genes in the sperm cells and egg cells

Describe DNA as a polymer made up of…

• two strands coiled to form a double helix

• strands linked by a series of complementary base pairs joined together by weak hydrogen bonds

• nucleotides that consist of a sugar and phosphate group with one of the four different bases attached to the sugar

Describe the genome

the entire DNA of an organism and a gene as a section of a DNA molecule that codes for a specific protein

Explain how DNA can be extracted from fruit

• mash medium with a pestle and mortar

• add DNA extracting mixture

• stand the tube in a water bath then cool

• filter into a second test tube

• pour ice cold ethanol down the side of the tube

• leave the tube undisturbed for some time until there is a layer formed on top of the extract

  • nucleic acids (DNA and RNA) will precipitate into the upper ethanol layer

Explain how the order of bases in a section of DNA decides the order of amino acids in the protein and that these fold to produce specifically shaped proteins such as enzymes

• the bases A, T, G and C get together in triplets

• each protein is made up of large numbers of amino acid molecules

• each triplet of bases codes for one particular amino acid

• amino acids are made in the number and order dictated by the number and order of base triplets

• amino acid molecules join together in a long chain to make a protein molecule

• number and sequence of amino acids determines which protein results

Describe the stages of protein synthesis, including transcription and translation

• RNA polymerase binds to non-coding DNA located in front of a gene

• RNA polymerase produces a complementary mRNA strand from the coding DNA of the gene

• the attachment of the mRNA to the ribosome

• the coding by triplets of bases (codons) in the mRNA for specific amino acids

• the transfer of amino acids to the ribosome by tRNA

• the linking of amino acids to form polypeptides

Describe how genetic variants in the non-coding DNA of a gene can affect phenotype

influences the binding of RNA polymerase and altering the quantity of protein produced

Describe how genetic variants in the coding DNA of a gene can affect phenotype

alters the sequence of amino acids and therefore the activity of the protein produced

Describe the work of Mendel in discovering the basis of genetics and recognise the difficulties of understanding inheritance before the mechanism was discovered

Mendel crossed tall pea plants and dwarf pea plants - offspring produced were all tall pea plants. Then bred two tall off spring together. He found that when offspring from the first cross where crossed together, three tall offspring and one dwarf. Produced a 3:1 ratio of tall:dwarf plants.

Explain why there are differences in the inherited characteristics as a result of alleles

• genes for the same characteristic can contain slightly different instructions that create variations

• different forms of the same gene are called alleles

• there are two copied if every chromosome in a body cell nucleus, a body contains two copies of every gene

• each copy of a gene may be a different allele

• there are many alleles and the different combinations of alleles in each person gives each of us slightly different characteristics

Explain the terms: chromosome, gene, allele

chromosome: a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes

gene: a section of DNA which controls part of a cell's chemistry - particularly protein production

allele: different versions of the same gene

Explain the terms: dominant, recessive

dominant: dominant alleles overrule recessive alleles (represented as a capital letter e.g. D)

• if alleles are DD or Dd then dominant characteristics will show

recessive: - represented with lower case letter e.g. d.

• recessive traits will only shown if both alleles are recessive "dd"

Explain the terms: homozygous, heterozygous,

homozygous: if both alleles an organism possesses are the same

• e.g. DD or dd

heterozygous: if both alleles an organism possesses are different

• e.g. Dd

Explain the terms: genotype, phenotype and zygote

genotype: the combination of alleles a person possesses

phenotype: physical attributes

zygote: a fertilised reproductive cell where an egg and sperm cell have fused together

Describe the inheritance of the ABO blood groups with reference to codominance and multiple alleles

There are four blood types - O, A, B and AB. The gene for blood type in humans has three different alleles - I^O, I^A and I^B.
A person with the genotype I^A I^B shows the effect of both alleles and has the blood type AB. They are both dominant alleles so I^A and I^B are codominant with each other.
I^O is recessive so bloody type O only happens when there are two recessive alleles (I^O I^O)

Explain how sex-linked genetic disorders are inherited - male

• Y chromosomes miss some genes found in the x chromosome

• this means a man (XY) will have only one allele for some genes on the X chromosome (because those genes are missing on the Y chromosome)

• if the allele for on of these X chromosome genes causes a genetic disorder, the man will develop this
  if a woman inherits the disorder allele, she may have a healthy allele on her other X chromosome

• if the disorder allele is recessive she will not get the disorder

• if she inherits two recessive disorder alleles she will develop the disorder

• probability of a woman developing it is lower a man

State that most phenotypic features are the result of…

multiple genes rather than single gene inheritance

Describe the causes of variation that influence phenotype

• genetic variation – different characteristics as a result of mutation and sexual reproduction

• environmental variation – different characteristics caused by an organism’s environment (acquired characteristics)

Discuss the outcomes of the Human Genome Project and its potential applications within medicine

HGP has:

• identified genetic disorders

• identify risks in developing disorders

• prevention + developing

• determine location of gene

• better understanding of disease

  • treatments better tailored to individuals

State that there is usually extensive genetic variation within…

a population of a species and that these arise through mutations

State that most genetic mutations have no effect on the…

no effect on the phenotype, some mutations have a small effect on the phenotype and, rarely, a single mutation will significantly affect the phenotype