An animation introducing the basic concepts of cells, DNA, recessive inheritance, and Punnett squares.

All living things, including humans, are made up of cells. The human body has about 100 trillion cells. Inside every cell (except mature red blood cells) is a nucleus that contains all the genetic instructions that direct cell function and make each person unique. Inside the nucleus of every body cell are 46 chromosomes, arranged in 23 pairs. Each chromosome is made up of one long molecule of DNA, which has four building blocks: A, C, G and T. These "bases" are paired and stacked together to form a twisted ladder, or double helix. A gene is a section of DNA with a specific sequence of bases — a genetic code — that determines a cell function or a physical trait. A mutation is a change to one or more bases in DNA. This can cause variation in a trait (such as eye color) or can lead to genetic disease. Mutations can be passed down through generations as offspring inherit chromosomes from their parents. Sperm and egg cells each have a half set of 23 chromosomes. This sperm cell carries a mutation in a gene on chromosome 7 (red). Egg cells also have a half set of chromosomes. This egg cell carries a normal copy of chromosome 7. A sperm and egg cell join to form a new cell (a zygote), which will develop into an embryo. Each offspring inherits 23 chromosomes from the mother and 23 from the father. In this case, the offspring inherited the chromosome 7 with the gene mutation from the father. For the majority of genes, inheriting one intact, “dominant” copy from either parent is sufficient to maintain normal function. A recessive trait or disease occurs when an offspring inherits TWO copies of a mutated gene. A "gene “carrier" inherits one copy of an intact, dominant gene and one copy of a mutated gene, so does not have the trait or disease. A Punnett square follows the inheritance of each parent’s chromosomes. A chromosome with an intact gene is represented with an upper case letter ("A"), and a chromosome with a mutated gene with a lower case letter ("a"). Each parent donates one gene to an offspring. The Punnett square contains all possible gene combinations; each box shows a potential combination (or genotype) in one offspring. In this case, only one box contains two copies of the mutated gene. So there is a 1 in 4 (or 25%) chance of this couple having an offspring with this recessive condition. This 25% chance applies each time this couple has a child. Each child has the same 1 in 4 chance of inheriting two copies of the mutated gene. Tap OFFSPRING to see the outcome of another pregnancy.

DNA, gene, Punnett, recessive, inheritance, cell, nucleus, mutation, chromosome, double helix, nucleotide, base, trait, genetic disease, dominant, carrier