Genetic inheritance

• Some characteristics are controlled by a single gene
• . Each gene might have different forms, and these are called alleles.
• Alleles are different versions of the same gene. 
• For any gene, a person may have the same two alleles, known as homozygous or two different ones, known as heterozygous.
• • The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype.
• Alleles may be either dominant or recessive
• A dominant allele is always expressed, even if one copy is present
• Dominant alleles are represented by a capital letter, you only need one copy of this allele to have brown eyes. Two copies will still give you brown eyes.
• A recessive allele is only expressed if the individual has two copies and does not have the dominant allele of that gene. 
•  Recessive alleles are represented by a lower case letter, you need two copies of this allele to have blue eyes.
• • Homozygous alleles are both identical for the same characteristic, for example AA or aa.
• Heterozygous alleles are both different for the same characteristic, for example Aa.
• Cystic fibrosis is an inherited disorder of cell membranes that mainly affects the lungs and digestive system.
• cystic fibrosis is caused by two faulty recessive allele
• An individual who is homozygous (ff) with the recessive allele will develop cystic fibrosis. Someone who is heterozygous (Ff) will be a carrier of the recessive allele, but will not develop cystic fibrosis and have no symptoms.
• Polydactyly is an inherited condition in which a person has extra fingers or toes. It is caused by a dominant allele of a gene.
• Dominant allele

  An allele that will show in the phenotype even if there is only one copy present
• Recessive allele

  An allele that will only show in the phenotype if two copies are present, with no dominant allele present
• Homozygous


  Having two copies of the same allele
• Heterozygous


  Having two different alleles
• Genotype

  The alleles present in an organism
• Phenotype

  The characteristics caused by an organism's alleles
• Normal human cells contain 23 pairs of chromosomes
• One chromosome in each pair comes from the father, the other from the mother
• Allele

  A version of a gene
• Alleles for earwax


  • Allele for wet earwax (capital E)
  • Allele for dry earwax (lowercase e)
• The gene for earwax has two alleles
• Most characteristics are controlled by many genes acting together
• If a person has two copies of the allele for dry earwax

  They are homozygous for dry earwax
• If a person has one allele for wet earwax and one for dry earwax

  They are heterozygous
• If a person has one dominant allele for wet earwax and one recessive allele for dry earwax

  Their phenotype is wet earwax
• If a person has two recessive alleles for dry earwax

  Their phenotype is dry earwax
• Height is controlled by many genes acting together
• Cystic fibrosis

  An inherited disorder of cell membranes
• Cystic fibrosis

  • Controlled by a single gene
  • Has two alleles
• Normal cell membrane function allele

  Dominant, symbol capital C
• Defective cell membrane allele

  Recessive, symbol lowercase c
• To have cystic fibrosis, a person must inherit two copies of the defective allele
• Carrier

  A person with one defective allele and one normal allele
• Constructing a Punnett square for cystic fibrosis

  1. Write down genotypes of gametes

  2. Combine gametes during fertilization