GEN 1: Key Terms and Concepts in Genetics and Genomics Flashcards
Observe the learning objectives of this session
Define genetics
- the study of genes and their inheritance
Define genomics
- the study of the genome
- the genome is the complete set of genetic material in a cell
Where are genes located in eukaryotes?
- on chromosomes
- most are in the nucleus
- there is one small chromosome in mitochondria
How many chromosomes is the human genome composed of?
- 23 pairs of nuclear chromosomes
and
- the mitochondrial chromosome
What is a haploid cell?
What type of cells is haploid?
- haploid cells have a single set of 23 chromosomes
- which is half of the 46 chromosomes in somatic cells
- gametes (sperm and egg cells) are haploid
What cells are diploid?
How do they become diploid?
- all somatics cells are diploid
- this happens when a cell is produced after a sperm fertilises an egg
- this cell then contains 46 chromosomes
How many protein-coding genes does the human genome contain?
- there are approx 3 billion nucleotide pairs of DNA
- ~ 20,000 protein-coding genes
What percentage of the whole genome codes for proteins?
- about 1.5% of the whole genome
- the 20,000 protein-coding genes make uo around 25% of the genome
- if we exclude introns, it is only 1.5%
Why is it more accurate to define genomes as ‘a complete set of genetic information than as a ‘complete set of genes’?
- genes make up only a portion of genomic DNA
- some of the remaining DNA is also functionally important
What is the transcriptome?
- the transcriptome is all the RNA in a specific cell or type of cells
- different definitions might encompass only protein-coding mRNA or also non-protein-coding RNA
Describe the variability of the transcriptome between cells
- it is highly variable between cells
- this is because not all of the ~20,000 protein-coding genes or non-protein-coding cells are expressed in all cells
Define the exome
- it is all the exons of the genome
- it is a subset (~1.5%) of genomic sequences
Describe the variability of the exome
- it does not vary from cell to cell
Define the proteome
- it is the complete set of proteins expressed in a particular cell type at a particular time
What is classical genetics?
- the oldest field of genetics, linked to Gregor Mendel
- it follows the inheritance of characteristics following sexual reproduction
Define phenotype
- the physical or biochemical appearance of a cell or organism
Define genotype
- the genetics constitution of an individual, either overall or at a specific gene or genetic locus
What is forward genetics?
What is it used for?
- when the phenotypes of successive generations are observed, deducing the genotype
- can help to find a gene that underlies a particular charactersistic
What is reverse genetics?
- to investigate the function of a gene, the genotype is manipulated and the phenotype change is observed
How did genetics become a molecular science?
- sickle cell disease was one of the first inherited diseases to be explained in molecular terms: by Linus Pauling in 1949
- the Glu to Val change in HbS (haemoglobin in sickle cell disease) caused it to aggregate and change the shape of red blood cells
- check e-module for more detail
Why are those with an Ss genotype for sickle cell anaemia have the phenotype ‘sickle cell trait’ not ‘asymptomatic carriers’ if the Hb-S allele is recessive?
- they are not completely asymptomatic
- small number of sickle cells are found in carriers
- when oxygen levels are low, these cells increase
- the symptoms might be mild because HbA interacts with HbS to inhibit aggregation
What are monogenic diseases?
- diseases that are caused by a mutation in a single gene
- sometimes referred to as Mendelian because they are inherited according to Mendelian rules
What is an autosome?
- any nuclear chromosome other than a sex (X or Y) chromosome
What are the five classes of monogenic disease and briefly describe them?
- autosomal recessive:
- Disease may not occur in every generation
- both parents of an affected person are carriers of the mutant allele (e.g. SCD)
- autosomal dominant:
- Every affected person has an affected parent (e.g. Huntington disease)
- X-linked recessive:
- males most often affected (e.g. haemophilia A)
- X- linked dominant:
- Females are most often affected but can affect both sexes in a generation (e.g. Rett syndrome)
- Mitochondrial:
- Passed on only by females but can affect both sexes and appear in every generation (e.g. Leber’s hereditary optic neuropathy)
What are polygenic/complex diseases?
- diseases that have on mutations in many genes
- these do not present Mendelian patterns of inheritance
- heart disease, diabetes, obesity etc are polygenic diseases
What are acquired genetic diseases?
- diseases that arise from other mutations that are not from parents
- e.g. viral infection, exposure to mutations
What influences phenotype apart from the genotype?
- the environment and epigenetic influences
What environmental issues cause phenotypic changes?
- chemical: e.g. pollutants
- physical: e.g. UV light
- biological: e.g. viruses
- these can mutate DNA
Define the epigenome
- the complete set of epigenetic tags on a particular genome at a particular time
How is the phenotype changed by epigenetics?
- epigenetics changes the transcriptome (the gene expression pattern) which changes the phenotype even when the nucleotide sequence remains the same
Can epigenetics effects be passed down through cell division?
- yes, as this means that when a specific cell divides, its daughter cells retain those characteristics
Are epigenetics modifications permanent?
- they are not permanent
- they may or may not be passed on through the germline
- the epigenome is ‘reprogrammed’
Read through this example of experimental reprogramming of Dolly the Sheep
What does a mean when a disease is multifactorial?
- when both environmental factors and multiple gene are looked at to determine the risk of a disease
Observe this diagram of how our phenotypes are determined by a number of different factors
