Mendelian Inheritance

Question 1

Describe the development of the concept of Mendelian inheritance

Answer 1

• Mendelian inheritance is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866
• The principles of Mendelian inheritance were named for and first derived by Gregor Johann Mendel, a nineteenth-century Austrian monk who formulated his ideas after conducting simple hybridization experiments with pea plants (Pisum sativum) he had planted in the garden of his monastery.

Question 2

How can mutations cause differences in protein function?

Answer 2

• A single mutation in a gene can lead to the production of non-functional, proteins with reduced function, truncated or missing proteins
• Can also be due to the number of each variant present, or just be its presence (recessive vs. dominant)

Question 3

Describe the mendelian inheritance of sickle cell disorders

Answer 3

Autosomal recessive

Question 4

What is a Punnett square?

Answer 4

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach. The diagram is used by biologists to determine the probability of an offspring having a particular genotype.

Question 5

Describe the mendelian inheritance of cystic fibrosis

Answer 5

Autosomal recessive

Question 6

What proportion of the general population of the UK are CF carriers?

Answer 6

1/25

Question 7

What proportion of the general population of the UK are affected by CF?

Answer 7

1/2500

Question 8

Describe characteristics of autosomal dominant inheritance

Answer 8

• Each child has 50% chance of inheriting the mutation
• No “skipped generations”
• Equally transmitted by men and women

Question 9

Describe characteristics of autosomal recessive inheritance

Answer 9

Both parents unaffected and homozygous - no children affected
Both parents unaffected but carriers - 1 affected child,2 carriers and 1 affected
Both parents affected - all children affected

Question 10

What is achondroplasia?

Answer 10

• Achondroplasia is a common cause of dwarfism. It occurs as a sporadic mutation in approximately 80% of cases (associated with advanced paternal age) or it may be inherited as an autosomal dominant genetic disorder.

Question 11

Describe the inheritance of achondroplasia

Answer 11

• Achondroplasia is a common cause of dwarfism. It occurs as a sporadic mutation in approximately 80% of cases (associated with advanced paternal age) or it may be inherited as an autosomal dominant genetic disorder.

Question 12

What is Huntingtons disease?

Answer 12

Huntington’s disease (HD), also known as Huntington’s chorea, is an inherited neurodegenerative disorder that results in death of brain cells. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unsteady gait often follow. As the disease advances, uncoordinated, jerky body movements become more apparent. Physical abilities gradually worsen until coordinated movement becomes difficult and the person is unable to talk. Mental abilities generally decline into dementia.

Question 13

Describe the mendelian inheritance of Huntingtons disease

Answer 13

HD is typically inherited from a person’s parents with 10% of cases due to a new mutation. The disease is caused by an autosomal dominant mutation in either of an individual’s two copies of a gene called Huntingtin. This means a child of an affected person typically has a 50% chance of inheriting the disease.

Question 14

What % of Huntingtons disease cases are due to a sporadic mutation?

Answer 14

10%

Question 15

Describe Duchenne muscular dystrophy

Answer 15

Duchenne muscular dystrophy (DMD) is a severe type of muscular dystrophy.The symptom of muscle weakness usually begin around the age of four in boys and worsens quickly. Females with a single copy of the defective gene may show mild symptoms.

Question 16

Describe the mendelian inheritance of Duchenne muscular dystrophy

Answer 16

The disorder is X-linked recessive. About two thirds of cases are inherited from a person’s parents while one third of cases are due to a new mutation. It is caused by a mutation in the gene for the protein dystrophin. Dystrophin is important to maintain the muscle fiber cell membrane. Genetic testing can often make the diagnosis at birth. Those affected also have a high level of creatine kinase in their blood.

Question 17

What is haemophilia?

Answer 17

Haemophilia is a mostly inherited genetic disorder that impairs the body’s ability to make blood clots, a process needed to stop bleeding. This results in people bleeding longer after an injury, easy bruising, and an increased risk of bleeding inside joints or the brain

Question 18

Describe the mendelian inheritance of haemophilia

Answer 18

Haemophilia is X-linked recessive, so a female carrying the defect on one of her X-chromosomes may not be affected by it, as the equivalent allele on her other chromosome should express itself to produce the necessary clotting factors, due to X inactivation. However, the Y-chromosome in the male has no other to replace it, so if the genes responsible for production of factor VIII or factor IX present on a male’s X-chromosome are deficient there is no equivalent on the Y-chromosome to cancel it out, so the deficient gene is not masked and the disorder will develop.

Question 19

Who are more likely to have haemophilia - men or women?

Answer 19

Since a male receives his single X-chromosome from his mother, the son of a healthy female silently carrying the deficient gene will have a 50% chance of inheriting that gene from her and with it the disease; and if his mother is affected with haemophilia, he will have a 100% chance of being a haemophiliac. In contrast, for a female to inherit the disease, she must receive two deficient X-chromosomes, one from her mother and the other from her father (who must therefore be a haemophiliac himself). Hence haemophilia is far more common among males than females.

Question 20

Describe X liked recessive inheritance

Answer 20

• Males affected more often than females, who have 2 copies of the X Chromosomes
• Transmitted through unaffected females
• Males cannot transmit disorders to sons
• Daughters of affected males will be carriers

Question 21

Describe X linked dominant inheritance

Answer 21

• Both sexes affected
• Females usually less severely affected than males
• Affected males cannot transmit disorder to their sons
• Daughters of affected males will be affected

Question 22

Describe Y linked inheritance

Answer 22

• Affects males only

Question 23

Descrive autosomal recessive inheritance

Answer 23

• Affected individuals in a single generation
• Generations “skipped”
• Both sexes affected in equal proportions
• Parents may be related

Question 24

Describe autosomal dominant inheritance

Answer 24

• Each generation is affected
• Both sexes affected in equal proportion
• Equally transmitted by both sexes
• Transmitted through affected individuals
• 50% chance of inheriting mutation (Only need 1 dominant allele)