Inheritance

• Chromosomes are made of DNA, which contains genetic information in the form of genes
• What is a gene?

  A length of DNA that codes for a protein
• What is an allele?

  An alternative form of a gene
• Sex is determined by an entire chromosome pair, females have the sex chromosomes XX, males have the sex chromosomes XY
• Sequence of bases in a gene determines the sequence of amino acids used to make a specific protein. Different sequences of amino acids give different shapes to protein molecules
• DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters.
• Protein synthesis:

  1. the gene coding for the protein remains in the nucleus
  2. Messenger RNA (mRNA) is a copy of a gene
  3. mRNA molecules are made in the nucleus and move to the cytoplasm
  4. The mRNA passes through ribosomes
  5. The ribosome assembles amino acids into protein molecules
  6. The specific sequence of amino acids is determined by the sequence of bases in the mRNA
• Most body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs
• What is a haploid nucleus?

  A nucleus containing a single set of chromosomes
• What is a diploid nucleus?

  A nucleus containing two sets of chromosomes
• In a diploid cell, there is a pair of each type of chromosome and in a human diploid cell there are 23 pairs
• What is mitosis?


  Nuclear division giving rise to genetically identical cells
• Mitosis is used in growth, repair of damaged tissues, replacement of cells and asexual reproduction
• Mitosis:

  1. The exact replication of chromosomes occurs before mitosis
  2. During mitosis, the copies of chromosomes separate, maintaining the chromosome number in each daughter cell
• Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions
• Meiosis is involved in the production of gametes
• Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells
• What is inheritance?

  The transmission of genetic information from generation to generation
• What is genotype?


  The genetic make-up of an organism and in terms of the alleles present
• What is phenotype?


  The observable features of an organism
• What is homozygous?

  Having two identical alleles for a particular gene.
• Two identical homozygous individuals that breed together will be pure-breeding
• What is heterozygous?

  Having two different alleles for a particular gene.
• A heterozygous individual will not be pure-breeding
• What is a recessive allele?


  An allele that is only expressed when there is no dominant allele of the gene present in the genotype
• What is a dominant allele?

  An allele that is expressed if it is present in the genotype
• What is codominance?

  A situation in which both alleles in heterozygous organisms contribute to the phenotype
• What is a sex-linked characteristic?

  A feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other, such as red-green colour blindness