Genetics: Variation

What is a gene mutation?

A change in the base sequence of a gene

How do gene mutations occur?

Happen spontaneously during DNA replication

Explain why gene mutations do not always produce a different amino acid?

Due to the degenerate nature of the genetic code (more than one triplet codes for the same amino acid)

How do mutations in the number of chromosomes arise?

Non-disjunction of chromosomes during meiosis

What are substitution mutations?

One base is replaced by another

What are deletion mutations?

One or more bases are lost from the DNA sequence

Why do deletion mutations often led to non-functional proteins?

Removing one base disrupts the reading frame

What is polyploidy?

When organisms have additional numbers of all chromosomes (e.g. 3 sets of each instead of two)

Name the two types of cell division

Mitosis and Meiosis

How many daughter cells are produced in mitosis?

2

How many daughter cells are produced in meiosis?

4

What is the diploid number in humans?

46

What is the halploid number in humans?

23

How many nuclear divisions occur in meiosis?

2

What happens in meiosis I?

The homologous chromosomes align along the equator of the cell - each chromosome in the pair consists of two sister chromatids
The homologous chromosomes separate with one chromosome from each pair going into a different daughter cell

What happens in meiosis II?

The sister chromatids are pulled apart

How does meiosis produce variation?

• Independent segregation of homologous chromosomes

• Crossing over

What are homologous chromosomes?

A pair of chromones, one material and one paternal that contain the same gene loci

How does independent segregation produce variation?

• Homologous chromosomes align randomly along the equator of the cell

• Therefore there are many possible combustions of chromosomes of maternal and paternal origin that could end up in each daughter cell when the chromosomes segregate

When does crossing over occur?

Meiosis I

When does independent segregation occur?

Meiosis I

Describe the process of crossing over

• Sister chromatids become twisted together

• This causes portions of the chromatids to break off

• The broken pieces of chromatids can join with the homologous partner

• This creates new combinations of alleles that were not present in either of the parental chromosomes

What equations show the possible chromosome combinations in the gametes due to independent segregation?

2ⁿ
n \= the number of pairs of homologous chromosomes (haploid number)

What equation shows the possible chromosome combinations in the offspring?

(2ⁿ)²
n\= the number of pairs of homologous chromosomes (haploid number)

Define genetic diversity

The total number of different alleles in a population

Define population

A group of individuals of the same species that live in the same place and can interbreed.

What is a gene pool?

All the genes present in the population

Describe the principles of natural selection

• Random mutation can result in new alleles of a gene -Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success.

• The advantageous allele is inherited by members of the next generation. -As a result, over many generations, the new allele increases in frequency in the population.

What is selection?

The process by which organisms that are better adapted to their environment tend to survive and bread, while those that are less well adapted tend not to.

What is directional selection?

Selection that favours individuals that vary in one direction from the mean of the population.

What affect does directional selection have on the characteristics of the population?

Changes the characteristics of the population

What is stabilising selection?

Selection that favours average individuals

What affect does stabilising selection have on the characteristics of a population?

Preserves the characteristics of the population

What type of selection is the development of antibiotic resistant bacteria?

Directional
Give three types of adaptation that result from natural selection