inheritance and variation

• sexual reproduction

  • mixing of genetic material
  • formation of gametes involves meiosis
  • variety of offspring
  • fusing of gametes
• asexual reproduction

  • only one parent
  • no fusion of gametes
  • no mixing of genetic material
  • time efficient
  • genetically identical offspring
• some organisms reproduce asexually and sexually
• amoeba is a one-cell protozoan
• amoeba reproduce by dividing in two when it gets too large
• malaria reproduces asexually in a human host cell but sexually in a mosquito
• fungi reproduce asexually by spores and can reproduce sexually too
• many plants produce seeds sexually but also reproduce asexually by runners or bulb division
• meiosis is responsible for the production of gametes
• meiosis involves two cell divisions halving the number of chromosomes in gametes
• meiosis contributes to the production of variety in offspring produced by sexual reproduction
• DNA is the complex structure that carries genetic material
• DNA has a double helix structure
• DNA is a polymer
• DNA is contained in genes and chromosomes which are found in the nucleus of most cells
• Different forms of the same gene are called alleles
• many inherited disorders are caused by a damaged allele
• DNA bases

  • adenine pairs with thymine
  • guanine pairs with cytosine
• the bases in a DNA molecule carry the different codes needed for different amino acids (three bases code in a particular order called a codon)
• DNA


  Deoxyribose nucleic acid
• a gene is made up of sequences of DNA
• a gene is a small section of DNA on a chromosome
• each gene codes for a particular amino acid to make a specific protein
• causes for variation

  • genetic variation - genetic material inherited from parents
  • environmental variation - the environment you live in
• phenotype


  for every characteristic you have two forms of a gene, one from you mother and one your father
• a genome is an organisms complete set of genetic instructions
• each genome contains all of the information needed to build that organism and allow it to grow and develop
• the genome is made up of genes that code for proteins and other DNA sequences
• the combination of alleles you inherit will determine your characteristics / phenotype
• the genotype describes the alleles present
• alleles

  • recessive - yy (lower case)
  • dominant - YY (upper case)
  • heterozygous - Yy (different)
  • homozygous YY (the same)
• a homozygous individual has identical alleles for the same characteristic
• a heterozygous individual has two different alleles for the same characteristic
• humans have 23 pairs of chromosomes
• one pair of human chromosomes different to determine the sex of the offspring
• dominant phenotypes will be seen even if there is only one
• recessive phenotypes will only be seen if there are two
• when gametes are formed in meiosis all eggs have the x chromosome where half the sperm have the x and half have the y
• genes that produce light sensitive proteins are located in the x chromosome and mutations in these genes cause colour-blindness
• Huntington's disease is caused by dominant alleles so can be inherited