TOPIC 4: GENETIC CROSSES (GENETICS)

Definitions:

Alleles are different forms of the same gene

A dominant allele prevents a recessive allele from working

• Always represented by a capital letter

• The dominant allele will determine the characteristic of the offspring i.e. what the offspring will look like

A recessive allele will not work if a dominant allele is present

• Always represented by a lower case letter

• The recessive allele will only determine the characteristic of the offspring if there are two recessive alleles present

Genotype means the genetic make-up of an organism, i.e. what genes are present

• At the end of each cross you will have to list all the possible genotypes

Phenotype means the physical characteristic of the organism

• The genotype will determine the phenotype

• At the end of each cross you will have to list the matching phenotypes for each genotype

Progeny is the offspring produced

• Also known as the F1 offspring

Homozygous means that the two alleles on a chromosome are the same • The alleles will either both be dominant, i.e. BB or recessive i.e. bb

Heterozygous means that the two alleles on a chromosome are different • The alleles will have a dominant and a recessive allele, i.e. Bb

Here we will be looking at two types of crosses 1. Monohybrid 2. Dihybrid

Monohybrid crosses

• Are used to determine the possible genotypes and phenotypes of the offspring when studying one characteristic, i.e. hair colour • Parents will either be

● Homozygous (alleles are the same, i.e. both dominant or both recessive) ● Heterozygous (alleles are different, i.e. one dominant and

one recessive

• Each square in a Punnett square represents a 25% probability

• Offspring:

● Two homozygous parents will form a ratio of 1:0 ● Two heterozygous parents will form a ratio of 3:1 ● One homozygous and one heterozygous parent will form a ratio of 1:1