Edexcel IGCSE Biology 复习笔记 3.2.3 Genetic Inheritance

• 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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