B13

• Asexual reproduction

  • Does not involve the fusion of gametes (sex cells)
  • All of the genetic information comes from one parent
  • All of the offspring are genetically identical to the parent so there is little variation
• Clones

  Identical copies produced by asexual reproduction, formed by mitosis
• Asexual reproduction

  A process resulting in genetically identical offspring from one parent
• Sexual reproduction

  • Involves the fusion of sex cells (gametes)
  • There is a mixing of genetic information so the offspring show variation
• Gametes

  • Formed by meiosis
  • Chromosome number is halved
  • When two gametes fuse to form a zygote, the full chromosome number is restored
• Sex cells in animals

  • Egg cells
  • Sperm
• Sex cells in plants

  • Egg cells
  • Pollen
• Offspring produced by sexual reproduction

  • Similar to both parents, but cannot be identical
  • Have a combination of two sets of genes, leading to variation
• In humans, an egg and sperm each have 23 chromosomes, which is half the usual number, when they fuse at fertilisation, the zygote has 46 again
• There are 23 different types of chromosomes which are received from both parents to make the 46
• Cell division in sexual reproduction

  1. Cells in the reproductive organs divide by meiosis to form sex cells (gametes)

  2. Body cells have pairs of chromosomes, one of each pair comes from each parent

  3. Meiosis: Chromosome duplication, first division separates chromosome pairs, second division separates chromosome arms

  4. Results in four haploid daughter cells
• Meiosis

  Reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
• Sexual reproduction


  Results in variation because the gametes from each parent fuse
• Zygote

  • Single body cell with new pairs of chromosomes formed when gametes join at fertilisation
  • New individual develops as this cell repeatedly divides by mitosis
• Meiosis


  The process by which diploid cells divide to produce haploid gametes
• Meiosis


  1. Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes

  2. First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome

  3. Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart

  4. A total of four haploid daughter cells will be produced
• Reduction division

  The halving of the chromosome number from diploid to haploid during meiosis
• The number of chromosomes must be halved when the gametes are formed
• Gametes are described as being haploid - having half the normal number of chromosomes because of the double division in meiosis
• Meiosis produces four haploid cells
• Sexual reproduction results in variation

  The gametes from each parent fuse, so half the genetic information comes from the father and half from the mother
• When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed (the zygote)
• A new individual then develops as the zygote repeatedly divides by mitosis
• Meiosis

  The process by which a diploid cell divides to produce four haploid cells
• Meiosis


  1. Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes

  2. First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome

  3. Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart

  4. A total of four haploid daughter cells will be produced
• Reduction division


  The halving of the chromosome number from diploid to haploid during meiosis
• Meiosis

  1. Chromosomes double themselves as in mitosis and line up in the centre of the cell

  2. Cells divide twice so that only one copy of each chromosome passes to each gamete
• Gametes are haploid, having half the normal number of chromosomes due to meiosis
• Sexual reproduction

  Results in variation because the gametes from each parent fuse, so half the genetic information comes from the father and half from the mother
• Fertilisation

  1. Gametes join to form a single body cell with new pairs of chromosomes (the zygote)

  2. A new individual then develops as this cell repeatedly divides by mitosis
• When a gene codes for a protein synthesised in the cell, the gene is said to expressed
• Mutations are new forms of genes resulting from changes in exsisting genes
• Variation is the differences in characteristics of individuals of same species in a populatioon
• Human development may be affected during pregnancy, if mother smokes or drinks a lot of alcohol, baby can have low birthweight
• Polydactyly is caused by dominant allele so can be passed on by 1 parent who has allele
• Cystic fibrosis is caused by recessive allele so can be passed on by 2 parentswho has allele