Inheritance

When mutations are passed down from parent to child, we call it inheritance. The type of mutation, and where it occurs in the genome can influence the probability that it will be inherited.

To explore this, we must first understand how our genome is created.

The creation of a new genome

DNA is stored in densely packed structures called chromosomes. There are 23 pairs of chromosomes in a cell – so 46 in total. This is true for each type of cell in your body, bar one – your gametes.

An individual will make one of two kinds of gametes – sperm, or ova (eggs). Gametes have only 23 chromosomes, one from each pair, and are made through a process called meiosis. During meiosis, the pairs of chromosomes swap chunks of DNA with one another in a process called crossover, thus making a new, random assembly of DNA. These new chromosomes then split into separate gamete cells, to be inherited by the individuals offspring.

Upon fusion of a sperm and ovum, a zygote is created. Each of the gamete’s 23 chromosomes will find its pair, creating a new 46 chromosome genome. The zygote then starts dividing rapidly, creating exact copies of this new genome that will go on to form life.

X & Y Chromosomes

Of the 23 pairs of chromosomes, one pair are called “sex chromosomes”. A biological female will have two X chromosomes (XX), while a biological male will have an X and Y chromosome (XY). An XX   individual will mature to have female sex organs, and produce ova. An XY individual will mature to have male sex organs, and produce sperm. Since XX individuals only have one type of sex chromosome, their ova gametes will always have an X chromosome. When an XY individual’s sperm cells are being made, the gamete may retain either an X or Y chromosome during meiosis.

If a sperm cell containing an X chromosome fuses with an ovum, an XX biologically female zygote will be formed. If a sperm cell containing a Y chromosome fuses with an ovum, an XY biologically male zygote will be formed. This information is important when discussing X-linked genetic conditions, which will be explored later.

Alleles

An allele is simply a variant of a specific gene. If genes were like cars, then the alleles would be all the different makes and models – each allele, or variant of a gene, serves the same purpose, it just looks slightly different.

Because one chromosome in each of your 23 pairs comes from a separate parent, you will have two alleles of each gene in your cell. These alleles may be the same, or different, depending on your parents’ genetics and what DNA you inherited from them.

This serves an evolutionary purpose, as whichever allele is the “strongest” or more dominant, the cell will express, or make protein from. “Weaker” or recessive genes are not expressed. The logic behind this is that the cell is selecting the optimum performance options from the available selection in the genome.