Genetic variation

2.8.2 Genetic disorders

Genetic variation

  1. Structure

    1. The nucleus of a cell contains chromosomes, and in a human, there are normally 23 pairs
    2. Chromosomes are made of large molecules of DNA, deoxyribonucleic acid, which contains the coded information that determines inherited characteristics
    3. DNA is made of very long strands, twisted in the form of a double helix, which contain four different compounds called bases1
    4. A sequence of three bases is the code for a particular amino acid, and the order of bases determines the order in which amino acids are assembled to form a particular protein
    5. A gene is a small section of DNA, and each gene codes for a particular protein

  2. Variation

    1. Differences in the characteristics of organisms of the same kind may be due to genetic or environmental causes
    2. The information that results in organisms have similar characteristics to their parents is carried by genes, which are passed on in gametes
    3. A genotype is an organism’s genetic makeup
    4. A phenotype is an organism’s observable characteristics
    5. In human cells, one of the 23 pairs of chromosomes carries the genes that determine sex: in females, the sex chromosomes are the same - XX - and in males, the sex chromosomes are different - XY
    6. Different genes control the development of different characteristics of an organism

    7. Alleles

      1. Some characteristics are controlled by a single gene, and forms of that gene are called alleles
      2. If both chromosomes in a pair contain the same allele of a gene, the individual is homozygous for that gene, and if they contain different alleles, the individual is heterozygous for that gene
      3. An allele that controls the development of a characteristic whenever it is present is called a dominant allele
      4. An allele that controls the development of a characteristic only if the dominant allele is not present is called a recessive allele
    8. The possible outcomes of a monohybrid cross, the mating of individuals with different alleles of a single gene of interest, can be analysed in a Punnett square, wherein each of the 4 cells is the pairing of one allele from each parent, where the father’s alleles are listed at the top and the mother’s on the left, typically with an uppercase letter denoting a dominant allele and a lowercase letter denoting a recessive allele, and has an equal 25% chance of appearing
    9. The genotype inherited across multiple generations can be analysed using family trees
    10. Mendel2 theorised the basic rules of inheritance by breeding garden peas and observing the offspring of various different pairings of alleles with clear-cut phenotypes across multiple generations

  3. Reproduction

    1. Sexual reproduction is the fusion of male and female gametes and the mixture of the genetic information from the two parents leads to variety in the offspring
    2. Asexual reproduction only needs one individual as the parent but since there is no fusion of gametes, there is no mixing of genetic information and so no variety in the offspring, which are therefore known as clones
  1. The nucleobases are cytosine & guanine, and adenine & thymine ↩
  2. Gregor Mendel (1822-84), an Austrian monk ↩