• Meiosis is a form of?nuclear division?that results in the?production of?haploid?cells?from diploid cells
• During meiosis a diploid cell will?divide twice?to form?four haploid cells
• It produces?gametes?in plants and animals that are used in?sexual reproduction
• A diploid nucleus will contain?two?copies of each gene
• A haploid nucleus contains just?one?copy of each gene
• A diploid cell of genotype Yy will produce?two gametes carrying the Y allele?and?two carrying the y allele
• The separation of alleles into different cells during meiosis is called?segregation
• Segregation is important as it allows for?new allele combinations?in offspring

• Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele?or?different alleles
• Sexual reproduction is?a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilized egg cell) and the production of offspring that are genetically different from each other
• Fertilization is defined as the?fusion of gamete nuclei, and as each gamete comes from a different parent, there is?variation?in the offspring
• When a male and female gamete fuse their?chromosomes are combined
• This means the resulting zygote is?diploid
• The zygote contains two chromosomes of each type
• It will therefore also have?two alleles?of each gene
  • If the two alleles for a particular gene are the same then the?genotype?is described as?homozygous
  • If the two alleles for a particular gene are different then the genotype is described as?heterozygous

• A?gene?is a short length of DNA found on a chromosome that codes for a particular?characteristic?(by coding for the production of a specific protein)
• Alleles?are variations of the same gene
  • As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene
  • One of the alleles is inherited?from the mother?and the other?from the father
  • This means that the?alleles may not be the same
  • For example, an individual has two copies of the gene for eye colour but?one allele could code for brown eyes and one allele could code for blue eyes
• The?observable characteristics?of an organism (seen just by looking - like eye colour, or found – like blood type) is called the?phenotype
• The?combination of alleles that control each characteristic?is called the?genotype
• Alleles can be?dominant?or?recessive
  • A dominant allele?only needs to be inherited from one parent?in order for the characteristic to be expressed in the phenotype
  • A recessive allele needs to be?inherited from both parents in order?for the characteristic to be expressed in the phenotype.
  • If there is only one recessive allele, it will?remain hidden?and the dominant characteristic will show
• If the two alleles of a gene are the same, we describe the individual as being?homozygous?(homo = same)
• An individual could be?homozygous dominant?(having two copies of the dominant allele), or?homozygous recessive?(having two copies of the recessive allele)
• If the two alleles of a gene are different, we describe the individual as being?heterozygous?(hetero = different)

Alleles are different forms of the same gene. You can only inherit two alleles for each gene, and they can be the same (homozygous) or different (heterozygous). Alleles can be dominant or recessive.

• Co-dominant alleles?have a combined effect on the phenotype
  • Certain?red-flowered plants?can be crossed with?white-flowered plants?of the same species, and the offspring's flowers have a?pink colour
  • Speckled chickens?show co-dominance between an?allele for white feathers?and an allele that causes the feathers to be?black
  • The alleles are?both expressed to an equal extent?in the phenotype
• When completing genetic diagrams (punnet square diagrams), alleles are abbreviated to single letters
  • The dominant allele is given a?capital letter?and the recessive allele is given the same letter, but?lower case
  • For example a tall (phenotype) pea plant can have the genotype?TT?or?Tt; in this case 'T' represents the dominant tall allele and 't' represents the recessive dwarf allele

• Inheritance of blood group is an example of?co-dominance
• This is of critical importance when deciding to give?blood transfusions?following injury or illness
• Use of the wrong blood group can cause an immune response that coagulates (solidifies) blood, leading to clots and serious illness/death
• There are?three?alleles of the gene controlling a person's blood group instead of the usual two
  • I represents the?gene?and the superscripts A, B and O represent the?alleles
• Alleles IA?and IB?are?codominant
• IO??is?recessive
• IA?results in the production of?antigen A?on the surface of red blood cells
• IB?results in the production of?antigen B?on the surface of red blood cells
• IO?results in?no antigens?being produced on the surface of red blood cells
• These three possible alleles can give us the following genotypes and phenotypes

Blood Genotype & Phenotype Table

• We can use genetic diagrams to predict the outcome of crosses that involve the codominant alleles controlling blood groups

Punnett square showing the inheritance of blood group