Introduction to Genetics

Question 1

How many pairs of chromosomes do genetically normal individuals have?

Answer 1

23:

• 22 pairs of autosomes
• 1 pair of sex chromosomes

Question 2

What are genes?

Answer 2

Segments of DNA withint he genome that encode messenger RNA and usually protein which make up out genotype.

Question 3

Define phenotype

Answer 3

Observed characteristics of the genotype (physiological, anatomical, psychological)

Genotype + environment

Question 4

What are the 3 classifications of genetic disease?

What tests are used to detect them/establish risk?

Give examples for each one

Answer 4

Monogenetic:

• Single gene abnormality
• Detected by molecular genetic tests (PCR)
• E.g. cystic fibrosis

Multifactorial:

• Multiple genes and environmental factors involved
• Molecular genetic tests detect risk
• E.g. heart disease

Chromosomal:

• Abnormality in chromosome structure or number
• Investigated by cytogenetic tests
• E.g. Down’s syndrome

Question 5

What is genetic polymorphism?

Answer 5

When different versions of a particular gene (alleles) exist within a population

Question 6

Define alleles

Answer 6

Different versions of a particular gene that exist within a population

Question 7

Define wild type

Answer 7

The most common allele of a particular gene to exist within a population.

(usually not associated with disease)

Question 8

Define homozygous

Answer 8

2 identical alleles within a chromosome pair

Question 9

Define heterozygous

Answer 9

2 different genes within a chromosome pair

Question 10

What is the effect of dominant alleles?

Answer 10

They are expressed: determine the phenotype in heterozygous individuals

Question 11

What are Mendel’s laws of inheritance?

Answer 11

Law of uniformity: if homozygous dominant and homozygous recessive are mates, the offspring are identical to eachother, there is no blending of dominant and recessive.

Law of segregation: during gamete formation, members of each gene pair separate so that each gamete only contains one copy of the gene.

Law of independent assortment: each gene pair segregates independently of other gene pairs.

Question 12

What is the pattern of inheritance for autosomal dominant genes?

Answer 12

An affected person must also have an affected parent.

Question 13

What is the distribution of inheritence of autosomal recessive and dominant genes amongst males and females?

Answer 13

Inherited equally and both exhibit dominant phenotype

Question 14

What would be the risk of disease in the offspring of a dominany heterozygote and a healthy homozygote?

Answer 14

50%

Question 15

When do recessive alleles determine the phenotype?

Answer 15

If 2 copies are inherited (homozygous recessive individuals)

Question 16

What occurs with the function of proteins encoded by dominant and recessive genes?

Answer 16

Dominant = gain of function

Recessive = loss of function

Question 17

What is the risk of an offspring being affected by a gene carried by 2 unaffected parents?

Answer 17

25%- the parents must be heterozygotes as they are unaffected, the chance of the offspring inheriting both disease causing recessive alleles is therefore 25%

Question 18

What is consanguinity? How does it affect risk of inheriting recessive alleles?

Answer 18

‘Mixing of blood’: mating of 2 individuals with at least one common ancestor as far back as great-grandparents.

Increases the risk of expression of recessive disease causing allele as the likelihood of offspring inheriting 2 recessive alleles for the same disease is increased.

Question 19

What is genetic load?

Answer 19

Recessive alleles for harmful disorders of which are carriers.

Question 20

What are exceptions to Mendel’s Laws of Inheritance?

Answer 20

• Co-dominance
• Over-dominance
• Incomplete penetrance
• Genomic imprinting
• Sex-linked effects
• Mitochondrial inheritance
• Genetic linkage
• Dynamic mutation
• Mosaicism

Question 21

What is co-dominance?

Answer 21

Alleles for a particular gene are neither dominant or recessive, phenotype will be a blend of both alleles.

E.g. Blood types (A & B= dominant, O= recessive)

AA + BB = AB blood type

Question 22

What is over-dominance?

Answer 22

Homozygote dominant individuals have a more seve effect that heterozygote, often incompatible with life (early embryo may not be viable)

Question 23

What is incomplete penetrance?

Answer 23

Not all individuals with a disease causing genotype clinically exhibit the disease

Question 24

What is genomic imprinting?

Answer 24

Alleles have different effects depending on the parent of origin - may have no effect if inherited from one parent but effect if inherited from the other.

Question 25

What are sex-linked effects?

What will be the pattern of inheritance if it exists on both X and Y?

Are males or females more affected by X-linked genes? Why? Which parent are they inherited from?

Give an example of an x-linked genetic disease

Answer 25

When the genes exist on the sex chromosomes.

If the gene exists on both X and Y chromosomes it wil be inherited in the same way as autosomal genes.

Males more affected by X-linked genes as they do not have a nother X chromosome with a different copy of the disease to dominate. (x-linked diseases often recessive). Inherited from the mother.

• E.g. red-green colourblindness, Duchenne’s muscular dystrophy, haemophilia A, fragile X syndrome.

Question 26

What risk does X-linked inheritance give to females and males?

Answer 26

50% risk of carrier in females

50% of affected in males

Question 27

What is mitochondrial inheritance?

Answer 27

Mitochondria contain DNA which is inherited entirely from the mother.

Question 28

What is genetic linkage?

Answer 28

Combinations of genes tend to be inherited together as they are located close together on the chromosome.

Question 29

What is dynamic mutation?

Answer 29

Genetic diseases occur with increasing severity in consecutive generations due to expansion of a repetitive DNA sequence.

Question 30

What is mosaicism?

Answer 30

When cells of the same person contain different genotypes.

Usually caused by error in DNA replication.