Genetics - Inheritance

Question 1

What are the four types of Medelian inheritance?

Answer 1

Autosomal dominant

Autosomal recessive

X-linked dominant

X-linked recessive

Question 2

What is an autosomal dominant disease?

Answer 2

This means that the gene abnormality is located on a non-sex chromosome and a single copy of this mutation is enough to cause the disease

Question 3

Which individuals are affected by autosomal dominant disease?

Answer 3

It affects both homozygous dominant and heterozygous individuals

There is no carrier state

Question 4

What are the three other features of autosomal dominant disease?

Answer 4

It affects both males and females in equal numbers

It means that anybody affected will have an affected parent

If the parents do not show a disorder then no further generations will have the disorder either

Question 5

List four examples of autosomal dominant disease

Answer 5

Achondroplasia

Huntingson’s

Von Willebrand disease

HNPCC

Question 6

What is an autosomal recessive disease?

Answer 6

This means that the gene abnormality is located on a non-sex chromosome and two copies of this mutation are required to cause the disease

Question 7

Which individuals are affected by autosomal recessive disease?

Answer 7

It affects those that are homozygous recessive

Those that are heterozygous are carriers and may be partially affected

Question 8

What are the three other features of autosomal recessive disease?

Answer 8

It is expressed relatively rarely

It usually skips generations

It affects both males and females in equal numbers

Question 9

List four examples of autosomal recessive disease

Answer 9

Cystic fibrosis

Sickle cell anaemia

Thalassaemia

Wilson’s disease

Question 10

What is an X-linked dominant disease?

Answer 10

This means that the gene abnormality is present on a sex chromosome and only one copy of the mutation is enough to cause the disease

Question 11

What is an X-linked recessive disease?

Answer 11

This means that the gene abnormality is present on a sex chromosome and two copies of the mutation are required to have the disease

Question 12

Which individuals are affected by X-linked recessive disease (male, female)?

Answer 12

It will be expressed in a female who has two copies of the defective allele

It will be expressed in a male who has one copy of the defective allele

Question 13

What are the five other features of X-linked recessive diseases?

Answer 13

It affects many more males than females (males only have one X so there’s a greater chance that it will affect them)

It is not passed from an affected male to son (this is because the man would have given his Y chromosome if it was a son, not the X)

It means that usually the mother of an affected male is unaffected herself

It means that all daughters of affected fathers will either be carriers or be affected themselves

It means that the daughters who are affected must have a father which is affected and a mother who is at least a carrier

Question 14

What are the three mechanisms of non-medelian inheritance?

Answer 14

Mitochondrial inheritance

Trinucleotide repeat expansions

Genomic imprinting

Question 15

What is mitochondrial disease?

Answer 15

It refers to a group of disorders that affect the mitochondria - as a small amount of double stranded DNA is present (mtDNA)

Question 16

Describe the inheritance pattern observed in mitochrondrial diseases

Answer 16

The inheritance is only via the maternal line as the sperm contributes no cytoplasm to the zygote

Question 17

What % of children will inherit mitochondrial disease if the father is affected?

Answer 17

0%

Question 18

What % of children will inherit mitochondrial disease if the mother is affected?

Answer 18

100%

Question 19

What are homoplasmy cells?

Answer 19

These cells contain identical mtDNA

Question 20

What are heteroplasmy cells?

Answer 20

These cells contain a mixture of two populations of mtDNA

Question 21

How does the severity of mitochondrial disease vary in a family?

Answer 21

Heteroplasmy

This is the presence of more than one mitochondrial DNA within a cell

This means that as an individual who has a mitochondrial disease may only have one affected mitochondrion in each of its cells, however when they produce offspring, the number of affected mitochondria can vary with more mitochondria being potentially affected

This can continue throughout several generations, where the number of affected mitochondria continue to increase

The more affected mitochondria, the more affected the individual is by the disease

Question 22

What investigation is used to diagnose mitochondrial disease?

Answer 22

Muscle biopsy

Question 23

What is the histological feature of mitochondrial disease on muscle biopsy?

Answer 23

Red, ragged fibres

Question 24

What are trinucleotide repeat disorders?

Answer 24

They are genetic disorders caused by an abnormal number of repeats (expansions) of a repetitive sequence of three nucleotides

Question 25

How do trinucleotide repeat disorders present? What is this phenomenon known as?

Answer 25

They lead to an earlier age of onset in successive generations

Anticipation

Question 26

Name seven trinuclotide repeat disorders

Answer 26

Fragile X (CGG)

Huntington’s (CAG)

Myotonic dystrophy (CTG)

Friedreich’s ataxia (GAA)

Spinocerebellar ataxia

Spinobulbar muscular atrophy

Dentatorubral pallidoluysian atrophy

Question 27

What is genomic imprinting?

Answer 27

This is when the phenotype depends on whether the gene abnormality occurs on a gene inherited from the mother or father

For example, if a mother is a carrier of an autosomal recessive disorder, they can still pass on the disorder to their offspring if they pass on two mutated copies of the chromosome

Question 28

Describe the principle behind genomic imprinting

Answer 28

Individuals inherit two copies of their genes - one from their mother and one from their father

In most cases, both copies of each gene are active in cells

In other cases, only one of the two copies are active

The active copy depends on the parent of origin, with some genes being active when they are inherited from a person’s father and other being active when they are inherited from a person’s mother

Question 29

Name a disease that shows genomic imprinting

Answer 29

Prader Willi syndrome

Question 30

What are pedigree charts?

Answer 30

They are charts that show how a genetic trait is passed through several generations of a family

Question 31

What are punnet squares?

Answer 31

They are used to predict the potential offspring phenotype of two individuals

Question 32

How do we calculate the genotype frequency?

Answer 32

We divide the number of individuals that the genotype is present by the total number of individuals

Question 33

How do we calculate the allele frequency?

Answer 33

We add up all the number of individuals who have that allele and multiply it by how many alleles that they have

We then divide this number by the total number of alleles