Intro to Genetics

Question 1

What is a phenotype?

Answer 1

Genotype + Environment

Question 2

What is a genome?

Answer 2

A complete haploid set of genetic material in an organism

[Genetically normal humans have 23 pairs of chromosomes - 22 pairs of autosomes + 1 pair of sex chromosomes]

Question 3

What is the central dogma of molecular biology?

Answer 3

DNA encodes messenger RNA, which encodes protein

Question 4

What is monogenetic disease?

Answer 4

Disease caused by a single gene abnormality - diagnosed via molecular genetic testing (e.g. PCR)

Example = Cystic Fibrosis

Question 5

What is multifactoral disease?

Answer 5

Disease caused by multiple genes and environmental influences - molecular genetic tests can be used but generally just to assess risk

Example = Cardiovascular Disease

Question 6

What is chromosomal disease?

Answer 6

Disease caused by an abnormality of chromosome structure or number - can be investigated using cytogenetics

Example = Down Syndrome

Question 7

What is genetic polymorphism?

Answer 7

Different versions of a gene exist in a population, meaning the gene is polymorphic - different versions are called alleles

Question 8

What is the ‘wild type’ allele?

Answer 8

Allele most common in a given population (usually not associated with disease)

Question 9

What is the difference between homozygous and heterozygous pairs?

Answer 9

Homozygous = identical pair of alleles making up a chromosome 
Heterozygous = different alleles making up chromosome (dominant allele will determine phenotype in this case)

Question 10

What laws are encompassed by Mendel’s laws of inheritance?

Answer 10

1. Law of uniformity
2. Law of segregation
3. Law of independent assortment

Question 11

What is Mendel’s law of uniformity?

Answer 11

If homozygous dominant and homozygous recessive individuals are mated, the offspring are identical to one another - there is no blending of dominant and recessive (i.e. you are one or the other)

Question 12

What is Mendel’s law of segregation?

Answer 12

During formation of gametes, the members of each gene pair separate so that each gamete carries only one copy of the gene

Question 13

What is Mendel’s law of independent assortment?

Answer 13

Each gene pair segregates independently of other gene pairs

Question 14

What is autosomal dominant inheritance?

Answer 14

Dominant alleles are expressed (e.g. determine phenotype) when present as a single copy (or in a pair)
‘Gain of function’ in the protein encoded by the allele (e.g. another allele cannot override)
Vertical pattern of inheritance (e.g. dominant alleles cannot skip generations)

Question 15

What is autosomal recessive inheritance?

Answer 15

Recessive alleles are only expressed in a homozygous pair
‘Loss of function’ in the protein encoded by the allele (e.g. it is easy for another allele to override)
Can be carriers - 2 carriers can have an affected child even without either expressing phenotype

Question 16

What is consanguinity?

Answer 16

‘Sharing of blood’ - refers to matings where the partners who at least one common ancestor (usually refers to as far back as great-grandparents) - higher risk of autosomal recessive disease (4% risk compared to 2% risk for non-consanguinous matings)

Question 17

What is genetic load?

Answer 17

Hidden detrimental component of our genome (e.g. harmful recessive disorders for which we are carriers or pre-natal mutations)

Question 18

What is co-dominance?

Answer 18

An exception to Mendel’s Laws of Inheritance
Heterozygous individuals can have blend of 2 phenotypes in some genes

Example = blood groups (A and B are co-dominant to one another, O is recessive)

Question 19

Why is O blood type recessive and not A or B?

Answer 19

A and B both have antigens, O does not

Question 20

What is overdominance?

Answer 20

An exception to Mendel’s laws of inheritance in which homozygous dominant individuals display more severe effects than heterozygous dominant individuals (e.g. more severe disease, often incompatible with life)

Question 21

What is incomplete penetrance?

Answer 21

An exception to Mendel’s laws of inheritance in which not all individuals with a disease-causing genotype exhibit the disease clinically

Question 22

What is genomic imprinting?

Answer 22

An exception to Mendel’s laws of inheritance in which alleles have different effects depending on the parent of origin so they may have no effect if inherited from one or other parent

Question 23

What are sex-linked effects?

Answer 23

An exception to Mendel’s laws of inheritance referring to genes on sex chromosomes (X/Y) - X recessive diseases most common so if carrier female mates with unaffected male, male offspring have a 50% chance of being affected

Question 24

Name 4 sex-linked recessive diseases

Answer 24

Red/ green colour blindness
Duchenne’s Muscular Dystrophy
Fragile X Syndrome
Haemophilia A

Question 25

What is mitochondrial inheritance?

Answer 25

An exception to Mendel’s law of inheritance referring to the fact that all mitochondria is inherited entirely from mother

Question 26

What is genetic linkage?

Answer 26

An exception to Mendel’s law of inheritance in which a combination of genes tend to be inherited together because they are located closely together on a chromosome pair

Question 27

What is dynamic mutation?

Answer 27

An exception to Mendel’s law of inheritance in which genetic diseases can occur with increasing severity in consecutive generations due to expansion of a repetitive DNA sequence

Question 28

What is mosaicism?

Answer 28

Occurs when different cells within the same individual have different phenotypes caused by error in DNA replication and chromosome separation in the very early embryo.
Abnormal effects may be masked due to normal cell lines so individual may not be aware until offspring is born who is clearly affected by the disease

Question 29

Name some common autosomal dominant disorders

Answer 29

Familial hypercholestroleamia
Adult polycystic kidney disease
Huntingdon Disease
Achondroplasia

Question 30

How are genetic histories recorded?

Answer 30

Family pedigree (male = square, female = circle, person of unknown sex = diamond, affected = shaded, unaffected = not shaded, carrier = half shaded, unknown genetic status = question mark)

Marriage or mating depicted using horizontal line between the two with offspring along horizontal line below (oldest on left)