Simple genetics example

Simple genetics example • Albino rabbits have white coats and red eyes. • This is due to lack of the pigment melanin • Albinos are actually rather rare in the wild, but common in pet shops! • The original albino rabbit arose as a result of a mutation – a very rare event – due to a change in the DNA of the coat colour gene.

Simple genetics example • Brown rabbits are the normal colour in the wild. The pigment melanin gives them colouration which enables them to blend in with their surroundings, and protects them from UV radiation.

Parents We shall consider what happens if we cross an albino rabbit with a brown rabbit. • It does not matter which one is the male, and which is the female • The result is the same…

F1 offspring • All the offspring are the same: they look brown

Using F1 offspring as parents If two of these rabbits are allowed to reproduce…

F2 offspring The result is: • most of the offspring are brown • some are albino

F2 offspring Actually the proportion is: 3 brown : 1 albino (as a ratio)‏ 75% brown, 25% albino (as a percentage)‏

F2 offspring We can also use these figures to give the chance of an individual rabbit being one colour or the other

F2 offspring There is a 3 in 4 (¾) or 75% chance of any baby rabbit being born brown

F2 offspring There is a 1 in 4 (¼) or 25% chance of any baby rabbit being born albino

Some genetics terms(dominant/recessive)‏ • Since all the F1 offspring are of one type, we have a straightforward situation. • We use the term dominant to describe the characteristic seen in the F1, and most of the F2 – brown • Because they seem to recede and are not seen in the F1, we use the term recessive for the albino forms

Some more genetics terms(phenotype/genotype)‏ • Phenotype is a word used to describe an organism by its observable characteristics, i.e. what it looks like, and we use ordinary words to describe it • Genotype is defined at the genetic level, i.e. what genes it contains. This is usually not directly visible, and we describe it using a code

Some more genetics terms(genes/alleles)‏ In rabbits, the two colour forms are caused by 2 versions of a gene. We say that the coat colour gene has 2 alleles: brown and albino

Genetics conventions • It is normal to choose a particular letter to stand for the gene we are considering (could be A or B). • We use upper case (CAPITALS) for the dominant form and lower case for the recessive form

Genetics conventions • The original brown parent is described using the genotype BB (could be AA)‏ • The original albino parent is described as bb (could be aa)‏ • These letters are in pairs because chromosomes are in pairs in adult cells

Genetics: explanations • The brown rabbit (genotype BB) produces gametes (sperms or eggs, depending on its sex) containing only the allele B. • The albino rabbit (genotype bb) produces gametes (eggs or sperms, depending on its sex) containing only the allele b. • This is because meiosis separates chromosomes so that only one of each set is present in the sex cells

Genetics: explanations The brown rabbits of the F1 all have the genotype Bb. Their phenotype is brown because they contain a dominant allele B, and only one dominant allele is enough to give this colouration. They have the same phenotype as their parent (brown) but a different genotype.

Genetics: explanations The white rabbits of the F2 all have the genotype bb. Their phenotype is white because they contain a pair of recessive alleles bb, and two recessive alleles are required to give this colouration. They have the same phenotype as their grandparent (white) and the same genotype. Only double recessive genotypes can be forecast this way.

Genetics: more terms Because rabbits with the genotype Bb contain two different alleles they are called heterozygous, whereas rabbits with the genotype BB and bb are both called homozygous or true-breeding.

Genetics: predictions In heterozygous brown rabbits the recessive allele b is hidden, but it could reappear in the next generation…….

Simple genetics example