IB Biology SL

Revision Notes

3.3.2 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 every gene
  • A haploid nucleus contains just one copy of each gene
  • The separation of alleles into different cells during meiosis is called segregation
  • Segregation is important as it allows for new allele combinations in offspring

Identifying the Stages of Meiosis Table, downloadable AS & A Level Biology revision notes

A summary of the meiosis process

 

Diploid Zygotes

  • 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
Extended Only

Gametes & Zygotes

  • A gamete is a sex cell (in animals: sperm and ovum; in plants pollen nucleus and ovum)
  • Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells – we say they have a haploid nucleus
  • This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells
  • In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomes
  • When the male and female gametes fuse, they become a zygote (fertilised egg cell)
  • This contains the full 46 chromosomes, half of which came from the father and half from the mother – we say the zygote has a diploid nucleus
Extended Only

Advantages & Disadvantages of Sexual Reproduction

Advantages & Disadvantages of Sexual Reproduction table, IGCSE & GCSE Biology revision notes

  • Most crop plants reproduce sexually and this is an advantage as it means variation is increased and a genetic variant may be produced which is better able to cope with weather changes, or produces significantly higher yield
  • The disadvantage is that the variation may lead to offspring that are less successful than the parent plant at growing well or producing a good harvest

Key Definitions

  • 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, IGCSE & GCSE Biology revision notes

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

Table of key terms and definitions for Genetic Inheritance_1, downloadable IGCSE & GCSE Biology revision notes

Inheritance of Sex

  • 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

Sex chromosomes, IGCSE & GCSE Biology revision notes

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

Inheritance of sex, IGCSE & GCSE Biology revision notes

Punnett square showing the inheritance of sex

Codominance

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

Blood Phenotypes Table

Inheritance of Blood Group table, IGCSE & GCSE Biology revision notes

  • 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’

Blood group inheritance, IGCSE & GCSE Biology revision notes

Punnett square showing the inheritance of Blood Group

  • The parent with blood group A has the genotype IAIO
  • The parent with the blood group B has the genotype IBIO
  • 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

Monohybrid Crosses

  • Monohybrid inheritance looks at how the alleles for a single gene are passed on from one generation to the next
  • Known information about the genotypes, phenotypes and the process of meiosis are used to make predictions about the phenotypes of offspring that would result from specific breeding pairs
  • When two individuals sexually reproduce there is an equal chance of either allele from their homologous pair making it into their gametes and subsequently the nucleus of the zygote
    • This means there is an equal chance of the zygote inheriting either allele from their parent
  • Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made
    • These diagrams include a characteristic table called a Punnett square
  • The predicted genotypes that genetic diagrams produce are all based on chance
    • There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions

Worked Example

Worked example: Genetic diagram

  • One of the genes for the coat colour of horses has the following two alleles:
    • B, a dominant allele produces a black coat when present
    • b, a recessive allele produces a chestnut coat when present in a homozygous individual
  • In this example a heterozygous male is crossed with heterozygous female

Parental phenotype:   black coat x black coat

Parental genotype:     Bb                   Bb

Parental gametes:      B or b              B or b

Monohybrid punnett square with heterozygotes table

Monohybrid Punnett Square with Heterozygotes Table, downloadable AS & A Level Biology revision notes

  • Predicted ratio of phenotypes in offspring – 3 black coat : 1 chestnut coat
  • Predicted ratio of genotypes in offspring – 1 BB : 2 Bb : 1 bb

Codominance

  • When working with codominant alleles the genetic diagrams can be constructed in a similar way, however the genotypes are represented using a capital letter for the gene and superscript letters for the alleles (eg. IAIA)
  • There will be more possible phenotypes and so the predicted ratios will be different

Worked Example

Worked example: Codominance

  • The gene for blood type has three alleles:
    • A, a dominant allele produces blood type A
    • B, a dominant allele produces blood type B
    • O, two recessive alleles will produce blood type O
  • In this example a blood type A person is crossed with a blood type B person

Parental phenotype:   Blood type A x Blood type B

Parental genotype:     IAIO                  IBIO

Parental gametes:      IA or IO             IB or IO

Monohybrid punnett square with codominance table

Monohybrid Punnett Square with Codominance Table, downloadable AS & A Level Biology revision notes

Predicted ratio of phenotypes in offspring – 1 Blood type AB : 1 Blood type A : 1 Blood type B : 1 Blood type O

Predicted ratio of genotypes in offspring: 1 IAIB : 1 IAIO : 1 IBIO : 1 IOIO

Sex-linkage

  • Sex-linked genes are only present on one sex chromosome and not the other
  • This means the sex of an individual affects what alleles they pass on to their offspring through their gametes
  • If the gene is on the X chromosome males (XY) will only have one copy of the gene, whereas females (XX) will have two
  • There are three phenotypes for females – normal, carrier and has the disease, whereas males have only two phenotypes – normal or has the disease

Worked Example

Worked example: Sex-linkage

  • Haemophilia is a well known sex-linked disease
  • There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot
  • There are two alleles for factor VIII, the dominant F allele which codes for normal factor VIII and the recessive f allele which results in a lack of factor VIII
  • When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can’t clot normally
  • The genetic diagram below shows how two parents with normal factor VIII can have offspring with haemophilia

Parental phenotypes: carrier female x normal male

Parental genotypes:      XFXf                              XFY

Parental gametes:      XF or Xf                        XF or Y

Monohybrid punnett square with sex-linkage table

Monohybrid Crosses_1, downloadable AS & A Level Biology revision notes

Predicted ratio of phenotypes in offspring – 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting

Predicted ratio of genotypes in offspring: 1 XFXF : 1 XFXf : 1 XFY : 1 XfY

Exam Tip

Make sure to include all of your working out when constructing genetic diagrams. It is not enough just to complete a punnett square, you need to show that you have thought about the possible gametes that can be produced by each parent.

Also, remember to state the phenotype as well as the genotype of the offspring that result from the cross. Read the questions carefully when answering sex-linked inheritance questions – is the question asking for a probability for all children or is it asking about a specific gender (boys or girls).

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