2.6 How is cystic fibrosis inherited?

Question 1

Locus

Answer 1

The position of a gene on a chromosome. For example, a person with the gene disease thalassaemia has two copies of the recessive allele that causes the disease. Each allele will be at the same position or locus on one of the copies of chromosome 11.

Question 2

Homologous chromosomes

Answer 2

Similar chromosomes, i.e. a paternal chromosome and maternal chromosome, form a pair of chromosomes. Human diploid cells are made up of 22 pairs of homologous chromosomes and one pair of sex chromosomes. Homologous chromosomes will have the same sequence of genes and are capable of pairing with each other when a cell divides by meiosis

Question 3

CF

Answer 3

caused by a gene mutation in the CF gene, carrying the code to make the CFTR protein. The gene is located on chromosome 7 and at the same locus on its homologous chromosome. The mutation is passed on from parents to their children.

Question 4

Allele

Answer 4

One of the different forms of a particular gene. In humans, a gene codes for the protein that transports chloride ions across the cell surface membrane. We all have this gene, the CF gene. The CF gene has two alternative forms or alleles. The normal allele, F, produces a functioning protein. The other allele, f, produces a protein which is very slightly different and does not function. If a person has two copies of the f allele, he or she will have cystic fibrosis.

Question 5

Genotype

Answer 5

The genotype describes an organism in terms of the alleles it contains. In pea plants the allele for tall plants, T, is dominant to that for short plants, t. A short plant always has the genotype tt. This means each of its cells contains two copies of the t allele. The genotype of a tall plant, however, may be either TT or Tt.

Question 6

Homozygote, Homozygous

Answer 6

An organism in which the alleles of a particular gene are identical to each other. For example, in cystic fibrosis, F represents the normal allele and f represents the mutant allele. There are two possible homozygotes. A person with the genotype FF does not have cystic fibrosis while a person with the genotype ff has cystic fibrosis.

Question 7

Heterozygote, Heterozygous

Answer 7

An organism which has two different alleles of a particular gene. In cystic fibrosis, F represents the normal allele and f represents the mutant allele. A person with genotype Ff is a heterozygote. This person is a carrier of cystic fibrosis, but does not have the disease. They could pass the disease on to their children.

Question 8

Phenotype

Answer 8

The characteristics of an organism, which result from the genes the organism possesses and the environment in which it lives. Himalayan rabbits have white fur but black feet, ears and tail. Cross-breeding pure-bred Himalayan rabbits produces young rabbits with these characteristics. The black pigment will only develop, however, in parts of the body which are at a low temperature. A Himalayan rabbit must have the gene for making black fur. The rabbit must also be in the right environment for this black pigment to develop.

Question 9

Dominant allele

Answer 9

An allele is described as dominant if its effect is always shown. In peas, the allele for tall plants, T, is dominant to the allele for short plants, t. Because the allele for tall plants is dominant, plants with either the genotype TT or the genotype Tt will be tall.

Question 10

Recessive allele

Answer 10

An allele is described as recessive if its effect is only shown when the other allele of the pair is identical. In peas, the allele for short plants, t, is recessive to the allele for tall plants, T. Only homozygous plants with the genotype tt will be short.

Question 11

Monohybrid inheritance

Answer 11

Mode of genetic transmission in which a characteristic is determined by a single gene.

Question 12

Thalassaemia

Answer 12

A genetic disease caused by recessive alleles of a gene involved in the manufacture of haemoglobin. A number of different mutations can affect this gene. Someone who is homozygous for one of these conditions either makes no haemoglobin at all or makes haemoglobin that cannot carry out its function properly.

Question 13

Albinism

Answer 13

Genetic condition caused by a recessive allele that results in the non- production of pigment.

Question 14

Phenylketonuria

Answer 14

Genetic disease that results from a recessive mutation. Children with the condition develop low intelligence unless given a special diet.

Question 15

Achondroplasia

Answer 15

Genetic condition caused by a dominant allele. Someone who is heterozygous for this condition has very restricted growth.
homozygous for disease usually die

Question 16

Huntington’s disease

Answer 16

Genetic disease that results from a dominant allele. It only manifests itself in adulthood but results in loss of nervous control and early death.

Question 17

Incomplete dominance

Answer 17

neither allele is dominant and heterozygotes have an intermediate phenotype

Question 18

Carrier

Answer 18

In genetics, a carrier individual is
heterozygous and so carries (rather than
manifests) the recessive allele.