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Genes and inheritance

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DNA and Inheritance

How characteristics are passed on

Genetics is the study of inheritance, that is, how characteristics are passed from one generation to the next. Gametes are haploid with only one of each chromosome and therefore one of each gene. When two gametes fuse at fertilisation the zygote now has two of each gene (that is a pair of alleles for each gene) one from the mother and one from the father. Where the alleles are different, one may be recessive and the other dominant and the dominant allele will determine the characteristic.

We can use genetic diagrams to show how a characteristic is passed on. Letters are used to represent the genes and a capital letter means an allele is dominant, and a small letter means a recessive allele.

The ability to roll the tongue is something an individual can or cannot do and is passed on genetically. It cannot be learned. In the diagram below, the parents are heterozygous for tongue rolling, that means they both have one copy of the allele that allows for tongue rolling, and one that does not. Make use of the animation to understand how alleles segregate and all the possible combinations, and their frequencies.

Note: in the animation below, dragged boxes will only stay in place when you have chosen the correct place for them. If another student wanted to do the exercise, or redo it, click on "Try again" button.

Another example of trait that is genetically inherited and which inheritance is well understood is blood groups. The inheritance of blood group can be represented by the simplified diagram below. It shows the possible blood groups of offspring when the parents are heterozygous A and heterozygous B.

The animation below shows an alternative way of illustrating the inheritance of a trait, as opposed to the tabular format used for the inheritance of tongue rolling.