IB DP Biology: SL复习笔记3.3.2 Inheriting Alleles

Segregation of Alleles

  • 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

Diploid Zygotes

  • 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

Dominant, Recessive & Co-Dominant Alleles

  • 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 Groups

  • 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

Worked Example

Show how a parent with blood group A and a parent with blood group B can produce offspring with blood group O.


Punnett square showing the inheritance of blood group

Exam Tip

Take good care when hand-writing ABO blood group genotypes and alleles. The expected notation for ABO blood group alleles is that the letter I is always written in UPPERCASE and all the alleles must be in superscript.Example IAIO