For each characteristic you have two alleles; one from your biological mother and one from your biological father. A particular chromosome from your biological mother. The homologous chromosome from your biological father. These homologous pairs of chromosomes have the gene for eye colour (located in this segment of the DNA) The chromosome of the biological mother has the brown eye allele. The chromosome of the biological father has the blue eye allele.

Alleles are given letters to represent them. E.g. Brown eye allele = B Blue eye allele =b

The combination of alleles is called the genotype ( ลักษณะตามพันธุกรรม ). E.g. This person’s genotype is: Bb

The genotype determines the phenotype ( ฟีโนไทป์ ) – the trait that is expressed (what we see). E.g. This person’s genotype is: Bb Will determine the person’s eye colour.

Some alleles can be dominant ( ตระหง่าน ) - always showing up in the phenotype) and some are recessive ( ลักษณะด้อย ) - these will only show up if both recessive alleles are present). E.g. This person’s genotype is: Bb

Dominant alleles are given capital letters and recessive alleles are given little letters. E.g. This person’s genotype is: Bb The blue eye allele is recessive The brown eye allele is dominant

Our phenotype therefore depends on which alleles we inherit. As the dominant allele is present in this person’s genotype, this person will have brown eyes. E.g. This person’s genotype is: Bb Phenotype = Brown eyes

If this individual had children with another individual of the same genotype, let’s work out the probability of their children having brown or blue eyes (next slides). E.g. This person’s genotype is: Bb