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Multiple Allele and Dihybrid Crosses

Multiple Allele and Dihybrid Crosses

Multiple Allele Crosses

e.g. 3 blood types:

  1. IA = Blood group A (dominant)
  2. IB = Blood group B (dominant)
  3. Io = Blood group O (Recessive)

Dihybrid Crosses

  • The inheritance of two characteristics which are controlled by different genes
  • Each of the two genes will have different alleles
  • Dihybrid crosses can be used to determine the likelihood of offspring inheriting certain combinations of the two characteristics from particular parents
  • A dihybrid cross for two heterozygous parents leads to a 9:3:3:1 phenotypic ration
Monohybrid Inheritance

Monohybrid Inheritance

  • The inheritance of a characteristic controlled by a single gene

Monohybrid Crosses

  • Monohybrid Crosses show the likelihood of the different alleles of that gene (And therefore the different versions of the characteristics) being inherited by offspring of certain parents

e.g. The dominant allele (H) codes for ‘tall’ whereas the recessive allele (h) codes of “short”

Phenotypic Ratios

  • The ratio of different phenotypes in the offspring
  • Genetic diagrams can be used to predict this ration in F1 and F2 offspring
  • Commonly, two heterozygous parents give a 3:1 ratio of dominant: recessive characteristic in the offspring

Monohybrid Inheritance and Codominant Allele

  • In a codominant allele both genes are expressed
  • With two heterozygous parents involving codominant alleles it will commonly give a 1:2:1 ratio in the offspring#





Inheritance of Sex-linked characteristics

  • Sex linkage refers to an allele that code for a characteristic which is located on a sex chromosome
  • As the Y chromosome is smaller than the X, fewer genes are carried on it
  • Therefore, most sex-linked genes are found on the X chromosome (x-linked genes)
  • As males only have one X chromosome they therefore often only have one allele for sex-linked genes, and so there is a higher chance of a recessive gene being expressed
  • Ultimately, males are more likely to express a recessive gene relative to females
  • Genetic disorders caused by a faulty allele located on a sex chromosome include:
    • Colour blindness
    • Haemophilia

e.g. As haemophilia is sex linked, it is therefore on the X chromosome. Therefore, females would require 2 recessive alleles for expression, whereas males would only require one.

As the Y chromosome does not have either gene it is just represented as Y

  XH Xh

If a female carrier and a male non-carrier have offspring there is a 2:1:1 ratio (female without colour blindness : male without colour blindness : male with colour blindness). Overall there is a 50% chance of a male offspring to be colour blind. 

Linkage of autosomal Genes

  • Autosome is any chromosome that is not a sex chromosome
  • Autosomal genes are genes found on the autosomes
  • Genes found on the same autosome are linked (as they stay together during independent segregation of chromosomes in meiosis 1 and the allele will be passed to the daughter cell)
  • The will not occur is crossing over splits them linked genes first
  • The closer the two genes are on the autosome (the closer they are linked due to the decreased chance of crossing over)
Hardy-Weinberg Principle

Hardy-Weinberg Principle

Species: a group of similar organisms that can reproduce to give fertile offspring

Population: A group of organisms of the same species living in a particular area at a particular time

Gene Pool: Complete range of alleles present in a population

Allele Frequency: The frequency of an allele in a population (given as a %)

Hardy-Weinberg Principle

  • Used to predict the frequencies of alleles/genotype/phenotype within a population
  • It can also be test whether or not the Hardy-Weinberg principles applies to a particular allele in a particular population
    • E.g. if frequencies do change between generations in a large population then there’s an influence of some kind
  • The principle will work providing:
    • Large Population size
    • No immigration/Emigration
    • No natural selection
    • Random mating (so all possible genotypes can bread with all others)

Allele Frequency

p + q = 1

p = Frequency of one allele (usually dominant)

q = frequency of one different allele (usually recessive)

  • the frequencies of both p and q must equal to 1 (100%)

Genotype Frequency

p2 + 2pq + q2 = 1

p2 Frequency of homozygous dominant genotype

2pq = Frequency of heterozygous genotype

q2 = Frequency of homozygous recessive genotype

  • Total frequency of all genotypes within a population = 1 (100%)
  • Phenotype can also be determine providing that the relation between phenotype and genotype is known
  • Principle can also be used if two alleles are codominant or if whether the allele is recessive and which is dominant is unknown
    • By making p represent one allele and q represent the other


  • Evolution is the change in an allele frequency over time
  • Natural selection is one explanation of how evolution functions
  • Selection Pressure refers to factors such as food, predators, diseases and competition on a species which results in the frequency of an allele to alter within a gene pool
  • Gene Pool refers to the total number of all alleles within a population at a given time.

Types of Competition:

  1. Intraspecific Competition: Competition between the same species
  2. Interspecific Competition: Competition between different species

Over Production

  • Over production is the result of a high birth rate resulting in intraspecific pressures
  • Those with the allele which is advantageous provide them the limited resources to survive
  • Therefore the allele is passed on


  • The interaction of genes that are not alleles, in particular the suppression of a gene by a different gene.
  • E.g. windows peak:
    • Allele results in a “V shaped” hair
    • Another allele expressed results in baldness
    • Therefore, if carrier of both, as the person is bald the allele for widow’s peak is unclear

Phenotypic ratios for Epistatic Genes

  • Recessive Epistatic Alleles
    • If the epistatic allele is recessive then 2 copies of it will mask the expression of the other gene
    • Homozygous parent + homozygous parent = 9:3:4 phenotypic ratio

(dominant both: dominant epistatic/recessive: recessive epistatic (in F­2))

  • Dominant Epistatic Alleles
    • If the epistatic allele is dominant then 1 copies of it will mask the expression of the other gene
    • Homozygous parent + homozygous parent = 12:3:1 phenotypic ratio

(dominant epistatic: recessive epistatic, dominant other: recessive (in F­2))



  • Where both alleles are equally dominant
  • Therefore, both alleles are expressed in the phenotype

e.g. Snapdragon plant has one allele codes for enzyme which catalyses formation of red pigment and other allele codes for enzyme that lacks this catalytic activity

If alleles showed the usual pattern of 1 dominant and 1 recessive the flower would be either red and white

But as they are co-dominant three colours of flower are found:

Homozygous (1st allele): Both alleles code forthe enzyme and so plant is

Homozygous (2nd allele): No enzyme is produced and so plant is white

Heterozygous: Their single allele for the functional enzyme produces some red and so plant is pink

CR = Red pigment

CW= White pigment

e.g. CRC + CW CW


Phenotypic Ratio: 1

   e.g. CRCW + CRCW


Phenotypic ratio: 2:1:1

 Pink: Red: white