Inheritance (DP IB Biology: HL)

• Gregor Mendel was an Austrian monk
• He was trained in mathematics and natural history at the University of Vienna
• In the mid-19th century, Mendel carried out breeding experiments on large numbers of pea plants whilst looking after the monastery gardens
• He studied how characteristics were passed on between generations of plants
• Due to his extensive work on the understanding of inheritance, he is sometimes called the father of genetics 

Mendel's groundbreaking work

• Mendel carefully transferred pollen from one pea plant to the reproductive parts of another
• This technique eliminated any uncertainty from his data since he knew which plants were fertilized by which pollen
• He collected the pea seeds from these plants and grew them in favourable conditions to find out their characteristics
• He also cross-bred offspring peas in order to find out which, if any characteristics would appear in future generations
• Mendel investigated the height of pea plants, the colours of their flowers and the smoothness of their seed coat

Mendel's pea plant crosses

Mendel's Pea Plant Results Table

• Mendel found that characteristics were inherited in a predictable pattern
• All pea plants in the first generation had the same characteristic as one of the parental plants
• The offspring plants in the second generation had characteristics of both parent plants in a 3:1 ratio
• Without knowing it, Mendel had discovered genes, he referred to them as 'units of inheritance'
• He also discovered that some genes are dominant and some genes are recessive
• Different forms of the same gene are called alleles

NOS: Making quantitative measurements with replicates to ensure reliability; Mendel's genetic crosses with pea plants generated numerical data

• Mendel was not the first to investigate inheritance using plants
• However, he was the first to generate strong numerical (quantitative) data, as opposed to observations only (qualitative)
• Mendel also used a large number of pea plants in his studies, for example in one investigation he recorded the characteristics for over 7,000 pea seeds
• By recording data for such a large number of seeds he ensured reliability in his conclusions
• Many replicates are important in a scientific investigation because they allow for:
  • Ensuring reliability of results
  • Identification of anomalous points
  • Further statistical analysis to establish a significant difference

• Gametes are the sex cells of an organism
• For example, the sperm and egg (ovum) cells in humans
• The egg is larger than the sperm as most of its space contains food to nourish a growing embryo
• The sperm cell contains many mitochondria to release energy for its motion
• Gametes fuse during fertilization to form a zygote
• These sex cells are formed during meiosis and only have one copy of each chromosome and so are haploid cells
  • For humans, that means the sperm and egg cells contain 23 single chromosomes in their nucleus
  • As there is only one chromosome from each homologous pair there is only one allele of each gene present
    • This allele may be dominant, recessive or co-dominant

The structure of human gametes - the sperm and egg