🧬 DNA and Inheritance

Why do you look like your parents? The answer is in the DNA inside your cells! In this episode we follow inheritance: how features pass from parents to offspring.

Inherited information is stored like boxes inside boxes: the nucleus inside the cell, chromosomes inside the nucleus, and DNA inside each chromosome. A small piece of DNA that controls one feature is called a gene.

DNA looks like a twisted ladder with two strands made of nucleotides. The rungs are pairs of bases: A always pairs with T, and G always pairs with C.

Mendel worked with pea plants. When he crossed a true-breeding yellow (SS) pea with a true-breeding green (ss) pea, the offspring were all yellow. That is because yellow is dominant (S) and green is recessive (s): when the dominant allele is there, the recessive one stays hidden.

Now let’s cross two hybrid (heterozygous) yellow peas: Ss × Ss. The Punnett square shows four boxes: SS, Ss and Ss are yellow, ss is green. The ratio is 3 yellow : 1 green.

A common mistake: “A yellow pea is always SS.” Wrong! SS and Ss are both yellow: the same phenotype but a different genotype. To look green, a pea needs two recessive alleles: ss.

A mutation is a permanent change in DNA. Radiation, UV rays or some chemicals can cause it. Albinism is caused by a mutation. If it happens in a sex cell, it can be passed on to offspring.

Environmental variation is caused by the surroundings. The same hydrangea flowers blue in acidic soil and pink in alkaline soil. The genes do not change, so it is not passed on to offspring.

Summary: a gene is a piece of DNA, and in DNA A pairs with T and G pairs with C. A Ss × Ss cross gives a ratio of 3 : 1. A mutation is inherited, environmental variation is not. Now try it in Genetics Garden!