GEN 1: Key Terms and Concepts in Genetics and Genomics

Question 1

Observe the learning objectives of this session

Answer 1

Question 2

Define genetics

Answer 2

• the study of genes and their inheritance

Question 3

Define genomics

Answer 3

• the study of the genome
• the genome is the complete set of genetic material in a cell

Question 4

Where are genes located in eukaryotes?

Answer 4

• on chromosomes
• most are in the nucleus
• there is one small chromosome in mitochondria

Question 5

How many chromosomes is the human genome composed of?

Answer 5

• 23 pairs of nuclear chromosomes

and

• the mitochondrial chromosome

Question 6

What is a haploid cell?

What type of cells is haploid?

Answer 6

• 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

Question 7

What cells are diploid?

How do they become diploid?

Answer 7

• all somatics cells are diploid
• this happens when a cell is produced after a sperm fertilises an egg
• this cell then contains 46 chromosomes

Question 8

How many protein-coding genes does the human genome contain?

• there are approx 3 billion nucleotide pairs of DNA

Answer 8

• ~ 20,000 protein-coding genes

Question 9

What percentage of the whole genome codes for proteins?

Answer 9

• 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%

Question 10

Why is it more accurate to define genomes as ‘a complete set of genetic information than as a ‘complete set of genes’?

Answer 10

• genes make up only a portion of genomic DNA
• some of the remaining DNA is also functionally important

Question 11

What is the transcriptome?

Answer 11

• 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

Question 12

Describe the variability of the transcriptome between cells

Answer 12

• 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

Question 13

Define the exome

Answer 13

• it is all the exons of the genome
• it is a subset (~1.5%) of genomic sequences

Question 14

Describe the variability of the exome

Answer 14

• it does not vary from cell to cell

Question 15

Define the proteome

Answer 15

• it is the complete set of proteins expressed in a particular cell type at a particular time

Question 16

What is classical genetics?

Answer 16

• the oldest field of genetics, linked to Gregor Mendel
• it follows the inheritance of characteristics following sexual reproduction

Question 17

Define phenotype

Answer 17

• the physical or biochemical appearance of a cell or organism

Question 18

Define genotype

Answer 18

• the genetics constitution of an individual, either overall or at a specific gene or genetic locus

Question 19

What is forward genetics?

What is it used for?

Answer 19

• when the phenotypes of successive generations are observed, deducing the genotype
• can help to find a gene that underlies a particular charactersistic

Question 20

What is reverse genetics?

Answer 20

• to investigate the function of a gene, the genotype is manipulated and the phenotype change is observed

Question 21

How did genetics become a molecular science?

Answer 21

• 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

Question 22

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?

Answer 22

• 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

Question 23

What are monogenic diseases?

Answer 23

• diseases that are caused by a mutation in a single gene
• sometimes referred to as Mendelian because they are inherited according to Mendelian rules

Question 24

What is an autosome?

Answer 24

• any nuclear chromosome other than a sex (X or Y) chromosome

Question 25

What are the five classes of monogenic disease and briefly describe them?

Answer 25

- 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)

Question 26

What are polygenic/complex diseases?

Answer 26

- diseases that have on mutations in many genes - these do not present Mendelian patterns of inheritance - heart disease, diabetes, obesity etc are polygenic diseases

Question 27

What are acquired genetic diseases?

Answer 27

- diseases that arise from other mutations that are not from parents - e.g. viral infection, exposure to mutations

Question 28

What influences phenotype apart from the genotype?

Answer 28

- the environment and epigenetic influences

Question 29

What environmental issues cause phenotypic changes?

Answer 29

- chemical: e.g. pollutants - physical: e.g. UV light - biological: e.g. viruses - these can mutate DNA

Question 30

Define the epigenome

Answer 30

- the complete set of epigenetic tags on a particular genome at a particular time

Question 31

How is the phenotype changed by epigenetics?

Answer 31

- epigenetics changes the transcriptome (the gene expression pattern) which changes the phenotype even when the nucleotide sequence remains the same

Question 32

Can epigenetics effects be passed down through cell division?

Answer 32

- yes, as this means that when a specific cell divides, its daughter cells retain those characteristics

Question 33

Are epigenetics modifications permanent?

Answer 33

- they are not permanent - they may or may not be passed on through the germline - the epigenome is 'reprogrammed'

Question 34

Read through this example of experimental reprogramming of Dolly the Sheep

Answer 34

Question 35

What does a mean when a disease is multifactorial?

Answer 35

- when both environmental factors and multiple gene are looked at to determine the risk of a disease

Question 36

Observe this diagram of how our phenotypes are determined by a number of different factors

Answer 36