SaltersGenetics

Definitions

Questions

Gene

Gene: A sequence of bases in DNA (sometimes RNA) that codes for one polypeptide (protein). Genes are found in the same loci on chromosomes.

e.g. the human CFTR gene codes for a protein, which forms a chloride channel in the membranes of cells that line the passageways of the lungs, liver, pancreas, intestines, reproductive tract, and skin.

Locus

Locus, (plural loci): The position of a gene on a chromosome.

e.g. the human CFTR gene is located on the long arm (q) of human chromosome 7 . (7q31.2)

Locus of gene

Allele

Allele: Genes which code for the same characteristic and have the same loci; but which express the characteristic differently.

e.g. The gene for CFTR has many alleles and the protein it codes for is 1480 amino acids long. The f allele is one of the commonest and having two copies of this mutation (ff), (inherited from both parents,) is the leading cause of cystic fibrosis .

Gene Mutation

Mutation: A change in the sequence of bases in DNA. There are three main types: deletion, addition and substitution. A change in the base sequence may lead to a change in the amino acid sequence and to the shape and functioning of the protein the gene codes for.

One of the commonest alleles of the CFTR gene f (DF508) is caused by a specific mutation, a deletion of three base pairs (TTT). This codon codes for phenylalanine at the 508th position of the protein. Having two copies of this mutation (ff), (inherited from both parents,) is the leading cause of cystic fibrosis

DNA interactive, Genome, Chromosome packing and Chromosome tour

ChromosomeChromosome: Thread like structures found in the nucleus of Eukaryotic cells. They are composed of DNA, histone and non histone protein. The histone proteins help to package the DNA

e.g. Human Chromosome 7 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 7 spans more than 158 million base pairs and represents between 5 and 5.5 percent of the total DNA in cells. The estimated number of genes varies. But is likely to be between 1,000 and 1,400 genes.

Chromosome PackingShort region of DNA double helix 2 nm

‘Beads on a string’ chromatin chain 11 nm

Packed nucleosomes 30 nm

Section of extended chromosome 300 nm

Condensed section ofmetaphase chromosome

700 nm

Entire metaphase chromosome 1400 nm

The Metaphase ChromosomeHuman X and Y Chromosomes

Homologous Chromosomes

Homologous chromosomes: Chromosomes with the same types of genes in the same loci, however the alleles in these loci may be different. Homologous chromosomes pair in prophase 1 of meiosis and exchange genes in crossing over. They then segregate independently of one another.

An Address Book AnalogyHomologous chromosomes are like an address book. All address books

have two things in commen:i. They have the same letters D, Z, F, Q etc.

(this is like homologous chromosomes having the same genes)ii. All address books have their letters in the same order A, B C, D, E, etc.

(this is like each gene in a homologous chromosome having the same locus.)

However no two address books are identical, why?.Because the names and addresses put in may be different. (two homologous may have different alleles in the same locus)

n

Haploid Having one set of chromosomes

n

n 6

Diploid Having two sets of chromosomes

Triploid Having three sets of chromosomes

Tetraploid Having four sets of chromosomes

Hexaploid Having six sets of chromosomes

Polyploid Many sets of chromosomes, (more than 2)

2n 12

3n 18

4n 24

6n 36

WheatFruitFly

4

8

Homozygous

Homozygous: An organism that has the same alleles for a particular gene (characteristic).

e.g. In Humans homozygous individuals with alleles ff will have Cystic fibrosis. Whereas homozygous individuals with the alleles FF will not.

Heterozygous

Heterozygous: An organism that has the two different alleles of the same gene (characteristic).

e.g. In Humans heterozygous individuals with alleles Ff will be normal, but carry a cystic fibrosis allele.

Dominance

Dominance: When one gene masks or prevents the expression of another.

e.g. In humans the allele F is dominant to f . Heterozygous individuals Ff will be normal, but carry a cystic fibrosis gene.

Recessive

Recessive: When one gene is masked or prevented from expression by another.

e.g. In humans the allele f is recessive to F. Heterozygous individuals Ff will be normal, but carry a cystic fibrosis gene.

Co-dominance

Co-dominance: When neither of a pair of different alleles masks the other, both are expressed equally.

GenotypeGenotype: The genetic makeup of an organism, the genes it possess.

e.g. In humans normal individuals may have the genotype FF or Ff whereas genotype ff will produce individuals who suffer from cystic fibrosis.

Phenotype

Phenotype: The observable characteristics of an organism. A product of interactions between the genotype and the environment.

e.g. In humans individuals with the genotype FF or Ff have a normal phenotype .