Overview of Human Heredity
Genetic traits in human beings come about through a variety of pathways. Some are caused by a single gene, while others come from multiple genes working together. Some conditions are sex-linked and recessive, so that more males than females express the gene in their phenotype. Chromosome abnormalities also influence some traits. Learning more about the way that traits are inherited and what genes cause them to be expressed will help cure genetic disorders.
Single Genes
Single genes influence many dominant-recessive combinations such as cleft chins, dimples, free earlobes, and freckles. Extra fingers or toes, such as a six-fingered hand, are also determined by a single gene. Some serious medical conditions are inherited in that pattern. The neurological disease Huntington disease, which was the cause of folksinger Woody Guthrie’s death, has symptoms of progressive loss of muscle control and deterioration, is the result of one dominant gene. PKU and sickle-cell anemia are the result of recessive genes.
Polygenic Traits
Many human traits are the result of a group of genes working together. Human blood type is the result of three different alleles, as well as a number of inherited factors. Skin color, eye color, and height are all controlled by genes that are neither dominant or recessive, but additive. Many autoimmune diseases, such as allergies and some forms of asthma, are probably polygenic.
Sex-Linked Traits
Many sex-linked traits are known, such as colorblindness and hemophilia. The genes for these disorders are carried on the X chromosome, so that when a male infant inherits the recessive form of the gene, he will not have another X chromosome to counteract it. The most famous family in that regard was Queen Victoria, who passed on the gene to royal families in Germany, Spain, and Russia. The hemophilia of Prince Alexis Romanov had international consequences.
Chromosomal Abnormalities
Some serious genetic conditions result from abnormalities of an entire chromosome rather than a single gene or group of genes. When a chromosome does not divide properly during meiosis, half the gametes are a chromosome short and the other half have an additional chromosome. Down syndrome, which can include degrees of mental retardation, is an example of a chromosomal abnormality.
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