The genetic code for the entire body is contained within every cell nucleus. This genetic code is written on a long string of nucleotides on a molecule called deoxyribonucleic acid (DNA). Each gene is the result of a section of DNA that contains the instructions for creating specific proteins. These proteins determine how the body functions.
Each DNA molecule, which codes for hundreds to thousands of genes, is twisted into a chromosome. Chromosomes come in pairs. Therefore, each human has two copies of each gene, one on each chromosome in a pair (except for the sex chromosomes). We have 23 chromosome pairs, for a total of 46 chromosomes.
One of the pairs of chromosomes is the sex chromosomes. The sex chromosomes are the X and Y chromosomes. Males have an X and a Y chromosome, and females have two X chromosomes. The other 44 non-sex chromosomes are called autosomes.
Alleles refer to different versions of the same gene. For example, the ABO blood group gene has alleles that code for type A, type B, or type O blood.
Genotype refers to the versions of genes we have. For example, having the allele that codes for type A blood.
Phenotype refers to the physical expression of the genes that we have. For example, actually having type A blood.
Inheritance
A mother’s ovum contains half her DNA, and a father’s sperm contains half his DNA. Gametes (ovum and sperm) are formed in a process called meiosis, in which germ cells divide to form haploid cells (cells with half the normal genetic material), halving the chromosome number and shuffling genes between them.
The DNA that makes up the ovum and sperm cells is selected at random and cannot be predicted. The ovum and sperm combine to produce a complete set of genes and chromosomes. A person’s genetic code is the result of half the genes from their mother combined with half the genes from their father.
Dominant and Recessive Alleles
When a single gene codes for a physical feature, the phenotype results from the combination of both genes (one on each chromosome). Some alleles are more dominant than others, and the phenotype will reflect the more dominant alleles. This is called Mendelian inheritance.
Alleles that are more dominant over other alleles are known as dominant, and alleles that are not expressed when paired with more dominant alleles are called recessive.
The allele for type A blood is dominant over the allele for type O blood, which is recessive. If the mother passes on the allele for type A blood and the father passes on the allele for type O blood, the allele for type A blood will overpower the allele for type O blood, and the child will have type A blood. For a child to have type O blood, they would need two copies of the type O allele, one from their mother and one from their father.
The alleles for type A and type B blood are co-dominant (both are expressed when present together). A combination of type A and type B alleles leads to type AB blood.
Last updated November 2025
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