Genetics and Inheritance - Definitions

Alleles

• Versions of a gene, variants, occupying a specific position on a chromosome

Autosomes

• Chromosomes that carry information about the body but do not determine the sex of the individual

Chromosomes

• A length of DNA consisting of a number of genes

Codominance

• In heterozygotes where both alleles at a gene locus are fully expressed in the phenotype

Dihybrid

• The inheritance of two characteristics

Diploid number

• The total number of chromosomes in a body cell nucleus e.g. in humans the diploid number is 46

DNA

• Deoxyribonucleic acid; double helix found in the nucleus which carries the genetic code

Dominant

• A characteristic which is expressed in the phenotype whether the individual is homozygous or heterozygous for that allele

Gene

• A sequence of bases on a DNA molecule; it contains coding for a sequence of amino acids in a polypeptide chain that affects a characteristic in the phenotype of the organism

Genotype

• The genetic make-up of an organism with respect to a particular feature

Haemophilia

• A sex-linked genetic disease in which one of the factors needed for blood to clot is not made in the body

Haploid number

• The number of chromosomes found in the gametes of an organism, e.g. in humans the haploid number is 23

Heterogametic

• An individual who produces two types of gametes each containing different types of sex chromosome - in humans this is the male

Heterozygote

• An individual where the 2 alleles coding for a particular characteristic are different

Heterozygous

• Situation where the alleles are different e.g Aa (where one allele is dominant and one allele is recessive)

Homogametic

• An individual who produces gametes that contain only one type of sex chromosome - in humans this is female

homologous pairs

• matching pairs of chromosomes in an individual which both carry the same genes, although they may have different alleles

homozygote

• an individual where both alleles coding for a particular characteristic are identical

homozygous

• situation where all the alleles are both the same e.g. AA or aa

loci or locus

• the site of a gene on a chromosome

monohybrid

• the inheritance of 1 characteristic

monohybrid cross

• a genetic cross where only one gene for one characteristic is considered

mutation

• a permanent change in the DNA of an organism

phenotype

• the physical traits, including biochemical characteristics, expressed as a result of the interactions of the genotypes with the environment

recessive

• a characteristic that is only expressed when both alleles code for it, in other words, the individual is homozygous for the recessive trait

red-green colorblindness

• a sex-linked genetic condition which affects the ability to distinguish tones of red and green

sex-linked diseases

• genetic diseases that result from a mutated gene carried on the sex chromosomes - in human beings, on the x-chromosome

sex-linked traits

• characteristics which are inherited on the sex chromosomes

true-breeding

• a homozygous organism which will always produce the same offspring when crossed with another true-breeding organism for the same characteristic