Inheritance

What does ‘Haploid’ and ‘Diploid’ mean?

Haploid - having half of a set of homologous chromosomes

Diploid - Having the full set of homologous chromosomes

Explain the term ‘homologous chromosomes’

In diploid cells, the chromosomes are in pairs making them homologous

Each chromosome of a pair carries the same genes but can have different alleles so there are two loci for each gene

Why must the number of chromosomes be halved before fertilisation?

Each of the gametes contain half the number of chromosomes so when they fuse the diploid number is restored

If this did not occur the number of chromosomes would be doubled each generation

Name the process of cell division that gives rise to gametes

Meiosis

Describe the events of meiosis I

• Prophase I - Homologous chromosomes pair up and their chromatids wrap around each other and exchange information

• Metaphase I - Homologous chromosomes line up at the equator of the cell

• Anaphase I - Homologous chromosomes are separated and pulled to opposite poles by spindle fibres

• Telophase I - Nuclear envelope forms around the genetic material at each pole and the cell divides

Describe the events of meiosis II

• Prophase II - Centrosomes move to opposite poles of the cell

• Metaphase II - The chromosomes align separately along the equator of the cell

• Anaphase II - Centromeres divide and spindle microtubules pull the chromatids to opposite poles

• Telophase II - The nuclear envelope reform and four haploid daughter cells are formed

Explain how crossing over in meiosis increases genetic variation

During prophase I, a pair of homologous chromosomes twist around each other and are joined by chiasmata

Parts of the chromatids break off and may be rejoined with non-sister chromatids of the other chromosome, the same loci are exchanged and new combinations of alleles are made

Explain how random assortment in meiosis increases genetic variation

During metaphase I, the chromosomes align along the equator therefore there is a mix of maternal and paternal chromosomes, increasing genetic variation

Why is the random fusion of gametes important?

It increases genetic variation due to the random combination of chromosomes generated

Define the term ‘gene’

A sequence of DNA that codes for a polypeptide or RNA

What is meant by the term ‘locus’

A locus is the specific position of a gene on the chromosomes

What is an allele?

One of the different versions of a gene. With homologous chromosomes there are two loci for each gene and they may carry different alleles or the same

What is meant by homozygous and heterozygous?

Homozygous - the alleles on each homologous chromosome in a pair are the same

Heterozygous - the alleles in the homologous pair are different

State what is meant by dominant and recessive alleles

Dominant allele - form of a gene that will be expressed even if only one is present, it can ‘override’ recessive alleles

Recessive allele - Only be expressed if they are homozygous

What does ‘codominant’ mean?

Both alleles that are different are fully expressed and contribute to the phenotype

Explain what is meant by ‘linkage’

Two genes are said to be linked if they are close to each other on the same chromosome

What is a test cross?

A genetic cross to determine the genotypes of the parents and offspring.

What is a phenotype?

The observable features of an organism, it is a result of the interaction between the genotype and environment

Define ‘genotype’

The alleles an organism posseses

What is a monohybrid cross show?

The possible genotypes of the offspring for one gene only

What is autosomal linkage?

When two genes are positioned close together on the same autosome (except sex chromosomes) they are likely to be inherited together

What is sex linkage?

Sex-linked genes are any found on the X or Y chromosomes

Why are males at a greater risk of sex-linked disorders caused by recessive alleles?

• The X chromosome is much larger than the Y chromosome

• So for many of the genes on the X there is no match on the Y

• Recessive alleles on the X chromosome will appear more frequently in the phenotype in males because they only have one copy of that gene

Why is the chi-squared test used?

To test whether the difference between the observed and expected frequencies is significant or due to chance

Describe the types of gene mutations that can occur

Substitution - One nucleotide is swapped for another

Deletion - One or more bases are removed from the DNA sequence

Insertion - One or more bases are added into the DNA sequence

Explain how a gene mutation can affect the gene product

• Silent mutation - A change in one base does not affect the amino acid the codon codes for

• Nonsense mutation - The new base creates a stop codon, the strand is not fully produced

• Missense - The new codon creates a different amino acid so the change of the strand could change

• Frameshift

What is a frameshift?

When deletions or insertions shift the sequence. So every codon downstream is read differently, resulting in different amino acids and potentially a completely different protein

Describe how albinism can arise as a result of a mutation

One form of albinism is caused by mutations of the tyrosinase gene.

Mutations could alter the tyrosinase enzyme which is responsible for the production of melanin and this result in albinism

Outline how a mutation can cause sickle cell anaemia

There is a substitution of glutamine to valine in the HBB gene for beta haemoglobin and this changes how the Hb molecules interact and they form strands and produce sickle shaped RBCs

Describe how a mutation can lead to haemophilia

The F8 gene that codes for factor VII is mutated

The gene is found on the X chromosome implying that this mutation is sex linked and caused by a recessive allele

Explain how Huntington’s disease can arise from a muation

The HTT gene that codes for huntingtin contains a large number of CAG repeats within the DNA sequence - a ‘stutter’ can occur

Describe the difference between a regulatory gene and a structural gene

A regulatory gene controls the expression of other genes

A structural gene is one that codes for a protein or RNA

What is the difference between a repressible enzyme and inducible enzyme?

The synthesis of a repressible enzyme is only stopped when a repressor protein is activated

They synthesis of an inducible enzyme is only started when a substrate is added

What type of operon is a lac operon?

An inducible operon

How is the genetic control of protein production achieved using lac opeon?

• The expression of the genes in lac depend on the where lactose is present since lac repressor senses lactose

• When lactose is not present the repressor binds to the operator and prevents transcription of the genes

• When lactose is present it binds to the repressor and changes its shape and it can no longer block the transcription of lacZ, lacY and lacA genes which code for enzymes involved in lactose metabolism

Name the molecule that regulate gene expression in eukaryotes

Transcription factors

Explain how transcription factors control gene expression

Transcription factors bind to the promoter regions of genes

They may activate or repress the expression of a gene by changing how easy it is for RNA polymerase to access the DNA for transcription

Explain how gibberellin can activate the expression of genes

Gibberellin causes DELLA to break down and allow trans factors like PIF to bind to its target promoter, resulting in an increase in amylase production