Genetic terms

• A gene is a sequence of bases on a DNA molecule that codes for a protein (polypeptide) which results in a characteristic.
• Alleles are one or more versions of the same gene. There can be many different alleles of a single gene, but most plants and animals, including humans, only carry two alleles of each gene, one from each parent. The order of bases in each allele is slightly different - because each allele codes for different versions of the same characteristic.
• How are alleles represented?

  using letters e.g. there are many alleles for eye colour - allele from brown eyes is shown using a B, allele for blue eyes is shown using a B
• Humans are diploid organisms meaning we have two copies of each chromosome - one from each parent. This is why we have two alleles of each gene. The allele of each gene is found at a fixed position - locus - on each chromosome in a pair
• What is the locus of a gene?

  the fixed position of each gene on a chromosome
• The genotype of an organism is its genetic constitution, all the different alleles an organism has
• The phenotype of an organism is 'the expression of the genetic constitution and its interaction with the environment' - what characteristics an organism has as a result of both its genes and the effect the environment has on its genes
• A dominant allele is always expressed in the phenotype, even when there's only one copy of it. Dominant alleles are shown by a capital letter.
• Recessive alleles are those with characteristics that only appear in the phenotype if two copies are present. They are shown by lower case letters.
• Codominant alleles are both expressed in the phenotype because neither one is recessive e.g. horses can have alleles for white hair or coloured hair. Neither allele is recessive, so a horse with one copy of each allele will have a roan coat - coat with amixture of wite and coloured hairs
• At each locus in a diploid organism, the genotype can be homozygous or heterozygous. If an organism carries two copies of the same allele, it is said to be homozygous at that locus. If an organism carries two different alleles for a gene it is heterozygous.
• Diploid organisms have two alleles for each gene. Gametes (sex cells) contain only one allele for each gene - haploid. When haploid gametes from two parents fuse together, the alleles they contain form the genotype of the diploid offspring that is produced.
• Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if two parents are crossed (bred).
• Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. Monohybrid crosses show the likelihood of the different alleles of that gene (and so different versions of the characteristic) being inherited by offspring of certain parents.
• The phenotypic ration is the ratio of different phenotypes in the offspring. Genetic diagrams allow you to predict the phenotypic ratios in F1 and F2 offspring
• Usually, when you do a monohybrid cross with two heterozygous parents you get a 3:1 ration of dominant:recessive characteristics
• What is the most common ratio in a monohybrid cross of two heterozygous parents?

  3:1

  dominant: recessive
• Whenever you do a monohybrid cross with two heterozygous parents involving codominant alleles - expect to see a 1 : 2 : 1 ration in the offspring
• Dihybrid inheritance is the inheritance of two characteristics, which are controlled by different genes. Each of the two genes will have different alleles. Dihybrid crosses can be used to show the likelihood of offspring inheriting certain combinations of the two characteristics from particular parents.
• Usually, when you do a dihybrid cross with two heterozygous parents you get a 9 : 3 : 3 : 1 ratio - 9 dominant both : 3 dominant first recessive second : 3 recessive first dominant second : 1 recessive both
• There are two types of linkage and both can affect the phenotypic ratios of monohybrid and dihybrid crosses
• The information for biological sex is carried on 2 sex chromosomes, in mammals females have XX and males have XY
• Some characteristics are sex-linked. This means that the alleles that code for them are located on a sex chromosome.
• The Y chromosome is smaller than the X chromosome and carries fewer genes. So most genes on the sex chromosomes are only carried on the X chromosome - X-linked genes
• Males only have one X chromosome and so they often only have one allele for sex-linked genes. So because they only have one copy, they express the characteristic of this allele even f it is recessive. This makes males more likely than females to show recessive phenotypes for genes that are sex-linked.
• Genetic disorders caused by faulty alleles located on sex chromosomes include colour blindness and haemophilia. The faulty alleles for both are carried on the X chromosome and so are X-linked disorders. Y-linked disorders do exist but are less common.