genetics yr11 mid topic test

Introduction to Genetics

Definition: Genetics is the study of the variation and inheritance of genes.

Definition: A gene is a section of DNA that codes for a protein that determines a particular trait.

Definition: An allele is an alternative version of a gene

Definition: DNA is a chemical that carries genetic instructions (as a code formed by the sequence of bases in DNA). DNA is stored as chromosomes inside the nucleus of a cell.

Definition: A chromosome is an (organised) structure of DNA (found inside the nucleus of a cell).

 Genes vs Alleles

A gene is a section of DNA that codes for a protein that determines a particular trait whereas an allele is an alternative version of a gene. 

Members of a species have the same genes but carry different alleles for these genes. 

For example, mice have a gene for coat colour. Some mice have alleles for white coat, some have alleles for brown coat colour.

4.  Describe the structure of DNA as a double helix. 

5.  Label a diagram of a nucleotide (deoxyribose sugar, phosphate and base).

6.  Describe the complementary base pairing rule A-T, C-G.

DNA – Deoxyribose Nucleic Acid

DNA is a double stranded molecule twisted into a double helix.

DNA is made up of repeating sub-units called nucleotides. 

Each nucleotide is made up of a phosphate, sugar (deoxyribose), and base joined together.

The phosphate of a nucleotide joins to the sugar of another nucleotide to form the backbone of each strand. 

Then the two strands are joined together by the bases on the different strands pairing together by complementary base pairing between bases on different strands. 

A (Adenine) only pairs with T (Thymine), and G (Guanine) only pairs with C (Cytosine).                         Full name of the base is not required - just learn the letters).

7.  Explain the role of Rosalind Franklin in the development of the discovery of the structure of the DNA molecule

The discovery of the structure of DNA in 1953 was made possible by Dr Rosalind Franklin's X-ray diffraction work at King's College London. 

Her creation of the famous Photo 51 allowed Maurice Wilkins, Francis Crick and James Watson to deduce the double-helix structure of deoxyribonucleic acid.

8.  Define homologous pair of chromosomes

9.  Draw a labelled diagram of homologous pair 

10.  Understand individuals inherit one copy of each pair from each parent.

Homologous Pair of Chromosomes

Definition: Homologous Chromosomes are pairs of chromosomes that are the same length, and carry the same genes at the same loci (position or location) as each other.

Individuals inherit two copies of each chromosome - one from each parent. Therefore every person carries two copies of each gene (one from each chromosome they inherit). Note - homologous chromosomes may carry different alleles.

Sister chromatids are duplicated chromosomes that are connected to each other at their center points (centromere). They have the same alleles as they are copies of one another. 

11. Describe the genetic code as the base sequence of DNA

12. Explain that the genetic code (base sequence) is read in triplets. Triplets code for amino acids. Amino acids are ordered according to the base sequence to form different proteins

13. Describe primary protein structure as the sequence of amino acids linked together to form a polypeptide chain

The Genetic Code

Definition: The genetic code is the base sequence (the order of bases (A, T, C and G) in DNA)

How does the genetic code result in differing phenotypes?

→ The genetic code is read in triplets. 

→ Triplets code for amino acids. 

→ Amino acids are ordered according to the base sequence to form different proteins. 

→ Hence different bases sequences produce different proteins (and different phenotypes).

→ Different amino acid sequences form different proteins and therefore different phenotypes.

For example hair is made of a protein called keratin. Different hair types and colours are due to different alleles altering the amino acid sequences.

14. Describe Variation

Definition: Genetic Variation is differences in the DNA of individuals within a species.

Sources of Genetic Variation Summary

15. Describe mutation as a source of variation and that mutations are the only source of new alleles

16. Define mutation

17. Explain how mutation produces variation in phenotype

Mutation causes differences in the DNA of individuals and so are sources of genetic variation between individuals in a species.

Mutations

Definition: A mutation is a random permanent change in the base sequence of DNA

Mutations are a permanent change in the base sequence of DNA, so they result in individuals with differences in their DNA and so are a source of genetic variation. Mutations are the only source of new alleles (and therefore genetic variation) in a species.

How mutations contribute to (produce) variation/why there is variation in a species:

Differences in the base sequence of DNA (mutations) result in different alleles for genes and so contribute to differences in the DNA of individuals and therefore genetic variation. The changed base sequence of an allele results in a code that reads differently and so codes for a different form/version of a protein, which then results in a different form of the trait being produced (a different physical expression of the trait) e.g. blonde or black hair colour etc. Therefore, mutations contribute to variation between individuals in a species.

Alleles can occur in different combinations that produce different physical appearances in individuals.

Individuals with different genotypes can have different physical appearances (phenotypes) and are genetically varied.

18. Define a point mutation.

19. Explain the three types of point mutation.

20. Relate point mutation to the genetic disorders e.g. cystic fibrosis, the BRCA1/BRCA2 mutation

Definition: A point mutation occurs when a single base or very few bases is changed which affects only a single gene.

Three Types of Point Mutations:

•                    substitution = base replaced

•                    deletion = base removed

•                    insertion = extra base added

Deletion and insertion result in reading “frameshift” i.e. all the triplets following the mutation are changed, resulting in a changed amino acid sequence after the mutation and a less or non-functional protein. This may be fatal. Substitution only changes 1 amino acid which may or may not be significant.

Point mutations can lead to genetic disorders such as sickle cell anaemia and increased risk of certain cancers developing e.g. a point mutation in the BRCA1 or BRCA2 genes  

22. Describe sexual reproduction as a source of variation 

23. Describe Asexual Reproduction

24. Describe Sexual Reproduction

25. Discuss the advantage of sexual reproduction over asexual reproduction.

26. Explain how fertilisation produces variation

Sources of Genetic Variation (Sources of differences in DNA)

Sexual reproduction causes differences in the DNA of individuals and so are sources of genetic variation between individuals in a species.

Definition: Asexual Reproduction is a type of reproduction in which a new offspring is produced by a single parent. The offspring produced are genetically identical to each other.

Definition: Sexual Reproduction is a type of reproduction in which a new offspring is produced by two parents. The offspring produced are genetically different to each other and the parents because it involves combining/mixing of DNA from the two parents.

Definition: Fertilisation is the random joining/combining of a male and a female gamete to form a new zygote/offspring/individual

Sexual reproduction results in genetic variation between individuals because it shuffles the genetic information of a parent when gametes are produced through meiosis, and it also mixes the genetic information of two parents through fertilisation, which is when a random gamete from each parent are joined together. This results in offspring being produced with differences in their DNA.

How fertilisation produces genetic variation

Fertilisation mixes the DNA of two parents when genetically unique gametes from each parent join together to form a zygote. It is random as to which two gametes from each parent join together. This means that fertilisation can result in many different combinations of gametes (and thus alleles) in offspring and thus leads to genetic variation in the population.

Compare Asexual Reproduction and Sexual Reproduction

The Advantage of Sexual Reproduction over Asexual Reproduction

An advantage of sexual reproduction is that it increases genetic variation between individuals. This is because the processes of meiosis and fertilisation shuffle the genetic information of parents, so that the offspring are genetically different from their parents and siblings. This means that within a sexually reproducing species, each generation will be genetically different to the last, and individuals in each generation will be genetically different from each other. Therefore, a species that reproduces sexually has increased genetic variation and so is more likely to be able to survive changes in its environment.

Asexual reproduction does not involve the production (meiosis) and fusion (fertilisation) of gametes, and so individuals produced by asexual reproduction have no genetic variation - they are genetically the same as the parent, and each other. Therefore, a species that reproduces asexually is less likely to be able to survive changes in its environment.

27. Describe the purpose of meiosis

28. Explain why meiosis as a reduction division is important

29. Explain how processes in meiosis (independent assortment and crossing over) produce variation

Note: stages of meiosis are not examined but the order and description may be examined

Definition: Meiosis is a cell division process that produces genetically unique gametes with half the number of chromosomes (only one set of chromosomes).

Reduction Division

Meiosis is known as a ‘reduction division’ as it reduces the number of chromosomes in the gametes it produces - from two sets in the parent cell to only one set in the gametes. 

If meiosis did not produce gametes with half the number of chromosomes, each generation of offspring would have double the number of chromosomes of the last generation. This is because during fertilisation the two gametes combine their chromosomes in the zygote (fertilised egg). If the gametes had two sets of chromosomes, the zygote would have four sets, and this would lead to a non-viable offspring. This is why it is important for meiosis to be a reduction division in order to produce a functioning offspring.

How meiosis produces genetic variation

During meiosis, the process of crossing over occurs, where homologous pairs exchange sections of DNA with each other - potentially swapping alleles with each other. This can result in new combinations of alleles on the chromosomes that will end up in the gametes. This results in gametes that are genetically different from each other, and from the parent cell, thus producing genetic variation.

Independent assortment is the random arrangement of homologous chromosomes during meiosis which results in new combinations of alleles in the gametes, and thus genetic variation.

Summary

Sexual reproduction produces genetic variation between individuals because the processes of meiosis and fertilisation shuffle and mix the genetic information of parents to produce offspring with differences in their DNA.

30. Describe the following terms: Population Genetics, Gene Pool, Allele Frequency and Gene Flow

31. Describe Migration 

32. Explain the effect of Migration on Genetic Variation 

The effect on migration on genetic variation:

Immigration = Individuals migrate into a population. 

May lead to an increase in the size of the gene pool, (increase in genetic variation) as new alleles may be introduced to a population.

Emigration = Individuals migrate out of a population 

May lead to an decrease in the size of the gene pool, (decrease in genetic variation) as certain alleles may become lost to the population.

Migration will have a more profound effect on SMALL POPULATIONS.

Allele frequencies are more likely to change due to any small changes such as migration in a small gene pool/ population.

Alleles are more likely to become lost (0%) or fixed (100%).

33. Describe natural selection the process whereby individuals with the most favourable phenotypes to the environment will survive and reproduce, passing these favourable alleles to their offspring

34. Identify environmental selection pressures in a given context e.g. predation, disease, availability of resources, climate, and competition

The Process of Natural Selection:

→ There is variation of phenotypes in the population and species produce more individuals than the environment can resource.

→ Individuals with the phenotype best suited to the environment have a survival advantage (are ‘fitter’),

→ so are more likely to survive to reproduce,

→ and therefore pass on their alleles for the favourable phenotype to offspring more often,

→ which means the frequency of the favourable/beneficial allele(s) increases in the gene pool. 

Selection Pressures:

Selection Pressure are environmental factors (abiotic or biotic) that affect the survival of an organism in its environment and therefore its reproductive success. 

Selection pressures select for favourable or against unfavourable phenotypes in the population.

Examples include: competition for food or light, diseases, predation, drought, high wind

35. Explain the process of natural selection in a given context

36. Discuss the importance of variation to natural selection when new diseases evolve or environments change (e.g. droughts)

37. Discuss the implications of Natural Selection on future population survival 

Peppered Moths:

When the Industrial Revolution Occurred: 

→ There was variation of phenotypes in the population as some moths were black, some peppered (white with black speckles) 

→ Individuals with the black phenotype were suited to the environment when the industrial revolution began and black soot was deposited on tree bark. Those with the black phenotype were camouflaged and less easily seen by birds therefore had a survival advantage over the peppered moths 

→ so the back moths were more likely to survive to reproduce 

→ and therefore pass on their alleles for the favourable black phenotype on to their offspring 

→ which led to the frequency of the black allele increasing in the gene pool. 

When the Clean Air Act Occurred: 

→ There was variation of phenotypes in the population as some moths were black, some peppered (white with black speckles) 

→ Individuals with the peppered phenotype were suited to the environment when the Clean Air Act was implemented and black soot was removed from the tree bark. Those with the peppered phenotype were camouflaged and less easily seen by birds therefore had a survival advantage over the black moths 

→ so the peppered moths were more likely to survive to reproduce 

→ and therefore pass on their alleles for the favourable peppered phenotype on to their offspring 

→ which led to the frequency of the peppered allele increasing in the gene pool.

Natural Selection - implication on future population survival:

Natural Selection selects for the most favourable phenotypes in an environment, overtime increasing the allele frequency of beneficial alleles. Therefore, this increases a populations (and species) future survival.

On the other hand, if the population is small and lacking in genetic variation, strong selection pressures may remove individuals. Eventually this further decrease in variation may result in population numbers that cannot recover and result in the extinction of a species.

Importance of Variation for natural selection: 

When new diseases evolve or when the environment changes there must be preexisting variation for selection pressures to work on. Beneficial phenotypes do not arise out of need. 

Without preexisting variation containing beneficial phenotypes selection pressures will decrease variation and may lead to the extinction of the species if only unfavourable phenotypes exist.  

38. Describe Genetic Drift 

39. Explain the effect of Genetic Drift on Genetic Diversity

40. Explain why Genetic Drift has more effect on smaller populations

Genetic Drift is the change in allele frequencies in a population due to random chance.

Genetic Drift reduces Genetic Diversity as some alleles become lost or fixed. 

Smaller populations are more likely to be impacted by genetic drift as every individual has a higher percentage of the total available alleles. 

41. Describe the Founder Effect

42. Explain the effect of Founder Effect on Genetic Variation (Allele Frequency and Gene Pool)

Founder effect occurs when a small number of individuals from a larger population establish a new, isolated population. 

Founder effect reduces Genetic Diversity as the smaller population will have a smaller gene pool and allele frequencies that are not representative of the larger population. 

43. Describe the Bottleneck Effect

44. Explain the effect of Bottleneck Effect on Genetic Variation (Allele Frequency and Gene Pool) 

45. Discuss the implications of Genetic Drift (including Founder and Bottleneck effect) on future population survival 

Bottleneck effect occurs when there is a significant reduction in population size due to an environmental event or human activity

Bottleneck effect reduces Genetic Diversity due to the sudden and dramatic reduction in population size. The gene pool drastically decreases. Alleles may be lost or fixed. 

Genetic drift reduces genetic diversity and therefore reduces future population survival due to:

• A lack of variation for natural selection to work on.

• Founder and Bottleneck reducing population size which results in increased genetic drift.

• Smaller populations have increased chances of inbreeding that can reduce the biological fitness of the population.