Passing it on: traits, peas and flowers

Why does a black cat sometimes have a ginger kitten? Why do seeds saved from one pea plant grow into plants that are not all the same? The answer is in the instructions each living thing passes on.

Instructions in every cell

In the nucleus of almost every cell are chromosomes, long strands made of DNA. DNA carries the instructions for building and running the body. A gene is a section of DNA that affects a trait, like flower colour or pea shape. When plants and animals reproduce sexually, each parent makes special cells called gametes, such as pollen and egg cells in plants. Each gamete carries only half of that parent's instructions. When two join at fertilisation, the new living thing gets a unique mix: some traits like one parent, some like the other, and some that seem to skip a generation.

Mendel's peas

In the 1850s and 1860s a monk called Gregor Mendel grew about 28,000 pea plants. He crossed yellow-seeded plants with green-seeded ones and found all the offspring were yellow. But when he grew the next generation, green peas came back, about one for every three yellow. He worked out that each plant carries two 'factors' (we now call them versions of a gene), one from each parent, and that one can hide the other. Yellow is dominant; green is recessive.

How flowers make seeds

Pollination is when pollen reaches a stigma, carried by wind, insects or birds. Tūī and korimako carry pollen between kōwhai and harakeke flowers on their heads. The pollen grows a tiny tube down to an ovule, fertilisation happens, and a seed forms. Seeds then travel by wind, water or animals, and germinate when conditions are right.

The environment matters too. Two seeds from the same pod will grow differently in shade and sun. What a living thing looks like depends on its inherited instructions and the world it grows up in.