• A?gene?is a short length of DNA found on a chromosome that codes for a particular?characteristic?(expressed by the formation of different proteins)
• 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 do not have to ‘say’ the same thing
  • 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 show up in the phenotype
  • A recessive allele needs to be inherited from both parents in order?for the characteristic to show up 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)
• When completing genetic 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

   

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).

• We?cannot always tell the genotype?of an individual for a particular characteristic just by looking at the phenotype – a phenotype associated with a dominant allele will be seen in both a dominant homozygous and a dominant heterozygous genotype
• If two individuals who are both?identically?homozygous?for a particular characteristic are bred together, they will?produce offspring with exactly the same genotype and phenotype as the parents?- we describe them as being?‘pure breeding’?as they will always produce offspring with the same characteristics
• A?heterozygous?individual can pass on?different alleles?for the same characteristic each time it is bred with any other individual and can therefore?produce offspring with a different genotype and phenotype than the parents?- as such,?heterozygous individuals are not pure breeding

Key Terms & Definitions for Genetic Inheritance Table

• Sex is determined by an?entire chromosome pair?(as opposed to most other characteristics that are just determined by one or a number of genes)
• Females?have the sex chromosomes?XX
• Males?have the sex chromosomes?XY
• As only a father can pass on a Y chromosome, he is?responsible for determining the sex of the child

Sperm cells determine the sex of offspring

• The inheritance of sex can be shown using a?genetic diagram?(known as a?Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes

Punnett square showing the inheritance of sex

• On occasion,?both alleles within a genotype are expressed?in the phenotype of an individual - this is known as?codominance
• Inheritance of blood group is an example of?codominance
• There are?three?alleles of the gene governing this instead of the usual two
  • I represents the?gene?and the superscript A, B and O represent the?alleles

   

• Alleles I^A?and I^B?are?codominant, but both are dominant to I^O
• I^A?results in the production of?antigen A?in the blood
• I^B?results in the production of?antigen B?in the blood
• I^O?results in?no antigens?being produced in the blood
• These three possible alleles can give us the following genotypes and phenotypes

Blood Phenotypes Table

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

‘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

• The parent with blood group A has the genotype I^AI^O
• The parent with the blood group B has the genotype I^BI^O
• We know these are their genotypes (as opposed to both being homozygous) as they are able to produce a child with blood group O and so the child must have inherited an allele for group O from each parent
• Parents with these blood types have a 25% chance of producing a child with blood type O

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