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Cambridge IGCSE Biology · 0610

Chapter 17: Inheritance (Part 1)

Chromosomes, genes and proteins

Chromosomes
Thread-like structures of DNA located in the nucleus that carry genetic information in the form of genes.
Gene
A length of DNA that codes for a particular protein.
Allele
One of a number of different versions of a gene.
Inheritance of sex
In humans, one pair of chromosomes determines gender. Females have XX chromosomes and males have XY.
  • All female egg cells contain an X chromosome.
  • Male sperm cells can contain either an X or a Y chromosome; therefore, the gender of the baby depends on which sperm cell fertilises the egg.
Genetic diagram of sex inheritance showing XX female crossed with XY male and 1:1 ratio of male and female offspring
Diagram 1: Sex inheritance. A genetic diagram showing a cross between a female (XX) and a male (XY), resulting in a 1:1 ratio (50% chance) of male and female offspring.

Exam Traps

  • Do not say the mother determines the sex of the offspring — sperm carry either X or Y.
  • Avoid confusing a gene (section of DNA) with an allele (a version of that gene).
  • Do not place whole diploid genotypes (e.g. XX) in gamete boxes — gametes are haploid (X or Y).

Protein synthesis

DNA controls the function of the cell by regulating the production of proteins, including enzymes, membrane carrier proteins, and receptors for neurotransmitters.

The process
  1. The gene remains in the nucleus and is used to make an mRNA copy.
  2. mRNA is a single-stranded molecule similar to DNA but contains base U instead of T.
  3. The mRNA leaves the nucleus and travels to a ribosome in the cytoplasm.
  4. The ribosome passes the mRNA through and assembles a chain of amino acids into a protein molecule.
Specificity
The specific sequence of amino acids (which determines the protein’s shape and function) is determined by the sequence of bases in the gene/mRNA.
Gene expression
While almost all body cells contain the same genes, many are switched ‘on’ or ‘off’ (expressed or not expressed) so the cell only makes the specific proteins it needs for its function.

Exam Traps

  • Do not say protein synthesis occurs in the nucleus — ribosomes are in the cytoplasm.
  • Avoid claiming all genes are expressed in every cell — only those needed for that cell type are switched on.

Haploid and diploid nuclei

Diploid nucleus
A nucleus containing two sets of chromosomes (a full set). In humans, this is 23 pairs (46 total).
Haploid nucleus
A nucleus containing a single set of chromosomes (half the number). These are found in gametes (sperm and egg cells).
Fertilisation
The fusion of two haploid gametes produces a diploid zygote.

Exam Traps

  • Do not swap haploid and diploid — haploid is half, diploid is the full set.
  • Avoid saying body (somatic) cells are haploid — only gametes are haploid in humans.

Mitosis and meiosis

Mitosis
Nuclear division giving rise to genetically identical cells.
  • Role: Used for growth, repair of damaged tissues, replacement of cells, and asexual reproduction.
  • Process: Exact replication of chromosomes occurs before division. Copies separate so the daughter cells maintain the same chromosome number as the parent.
  • Stem cells: Unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.
Meiosis
A reduction division in which the chromosome number is halved from diploid to haploid.
  • Role: Involved in the production of gametes.
  • Outcome: Results in four genetically unique/different cells, which provides genetic variation for the species.

Examiner Report Insights

  • Meiosis introduces variation; mitosis does not — do not use mitosis to explain gamete formation.

Exam Traps

  • Do not say meiosis produces genetically identical cells or that mitosis forms gametes.
  • Do not confuse stem cells (unspecialised, divide by mitosis) with gametes (haploid, formed by meiosis).

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