Chapter 3.4

Question 1

The Three Fundamental Laws of Inheritance

Answer 1

1. The law of segregation: The inheritance of each characteristic is controlled by a pair of alleles in an individual. The two alleles are separated during meiosis so that each gamete contains only one allele for each gene. Alleles are passed from one generation to the next as distinct units.
2. The law of independent assortment: The allele inherited for one trait does not affect which allele will be inherited for any other trait. A gamete contains one copy of each gene; which copy it receives during meiosis is the result of random orientation of homologous chromosomes during metaphase I.
3. The law of dominance: In an organism with two different alleles, one allele will determine the trait. The allele that determines the trait is dominant; the unexpressed allele is recessive.

Question 2

Exceptions to the Law of Segregation

Answer 2

There is an exception for genes found on the sex chromosomes. There is a further exception for mitochondrial DNA, which is inherited only from the mother.

Question 3

Exceptions to the Law of Independent Assortment

Answer 3

There is an exception for genes whose loci are close together on the same chromosome. These are called linked genes.

Question 4

The Law of Dominance

Answer 4

There are exceptions for patterns of inheritance other than dominant–recessive, e.g. co-dominance.

Question 5

Incomplete dominance

Answer 5

Where neither of the two alleles is dominant, and both impact the phenotype. Sometimes, when two alleles are present in an individual, the phenotype is different than in either homozygous state.

Question 6

Cystic Fibrosis

Answer 6

Cystic fibrosis is one of the most common genetic diseases. The recessive allele was formed by a mutation in the CFTR gene, which codes for a chloride channel in mucous membranes. The gene has been mapped on chromosome 7 and is involved in the secretion of sweat, mucus and digestive juices. The altered mucus builds up in the lungs, causing dangerous infections and damaging lung tissue. Mucus also builds up in the liver and pancreas, making it more difficult to digest food.

Question 7

Huntington’s disease

Answer 7

Huntington’s disease is a neurodegenerative disorder that usually starts to affect people between 30 and 50 years of age. It is caused by a dominant allele that has developed through the mutation of the HTT gene found on chromosome 4. Symptoms include loss of muscle coordination, cognitive decline and psychiatric problems. Once the symptoms appear, the affected person has a life expectancy of about 10 years, although it varies greatly by individual.

Question 8

Red-green colour blindness

Answer 8

The recessive allele for red-green colour blindness codes for a non-functional copy of one type of photoreceptor protein in the retina of the eye. Leading to colour blindness.

Question 9

Haemophilia

Answer 9

Haemophilia is a disease in which a protein needed for blood clotting, most commonly factor VIII, is not made. Without this factor, small injuries can cause patients to bleed excessively, and even die. The gene for clotting factor VIII is on the X-chromosome and the allele that causes hemophilia is recessive. For haemophilia, the normal allele has the notation XH and the allele for hemophilia has the notation the Xh.

Question 10

Mutagen

Answer 10

Any agent that causes or increases the frequency of mutations by triggering changes in the genetic material of an organism. Most mutations that form in an individual will not be passed to offspring. Mutations are only passed on if they are present in the DNA of cells that produce gametes.

Question 11

Mutations and radiation

Answer 11

Higher energy radiation, called ionising radiation, can break bonds between atoms including in DNA, leading to mutations. Radiation that can cause DNA damage includes ultraviolet (UV) light, X-rays, and alpha, beta and gamma radiation from the decay of radioactive elements.