Genetics, DNA and inheritance
Biology · Key Stage 3 · The School
Nested, not separate things
These four words describe the same material at four scales. The NUCLEUS is the control centre of the cell. Inside it are CHROMOSOMES — humans have 23 pairs, one of each pair from each parent. A chromosome is one very long molecule of DNA. A GENE is a section of that molecule that carries the instruction for one characteristic. So a gene is part of a chromosome, and a chromosome is made of DNA.
Passing it on
HEREDITY is the passing of genetic information from one generation to the next. You inherit one copy of each gene from each parent, so you carry two copies of every gene. The two copies can differ, and one can mask the other, which is why a characteristic can disappear in one generation and reappear in the next. It also means you are not simply an average of your parents.
How the structure was found
The double helix was published by Watson and Crick in 1953, building a model that had to fit the evidence. The critical evidence was X-ray diffraction work done by ROSALIND FRANKLIN and Maurice Wilkins at King’s College London — Franklin’s Photo 51 showed the helical form and its dimensions. Franklin’s data was shown to Watson and Crick without her knowledge, and she was not credited at the time.
Brown eyes (B) are dominant over blue (b). Two parents are both Bb. Draw a 2×2 grid with B and b along the top and down the side. The four boxes are BB, Bb, Bb, bb. Three of the four have at least one B, so about 3 in 4 children have brown eyes, and 1 in 4 blue — even though NEITHER parent has blue eyes. The blue allele was carried, unseen, by both.