How Dominant Inheritance Affects the Passing on of Certain Genes
Genetics is the scientific study of heredity, the process in which a parent passes genes to their children. This includes hair, skin and eye colors. Other characteristics that are affected are mental abilities, natural talents and risks of inheriting certain diseases.
Humans are born with cells containing 46 chromosomes; two are sex chromosomes and 22 pairs of non-sex chromosomes. Males are “46, XY” and females are “46, XX”. These chromosomes are made of strands of genetic information, called DNA.
Genes carry necessary information to make a protein and are called sections of DNA. The location of the gene is called the locus. The pairs of autosomal chromosomes carry mainly the same information. The slight variances happen in less than 1% of DNA sequences and produces differences are called alleles.
An abnormal gene can lead to an abnormal protein or an abnormal amount of normal protein. Because the autosomal chromosomes are in pairs, there are two copies of each gene, one from each parent. In autosomal dominant inheritance, the defective genes appear in every generation. If only one gene is defective, the normal gene may make enough protein to compensate, so no disease is found. This disease is named the recessive disease. The defective gene is thought to be inherited in a recessive pattern. Persons born with only one recessive gene and show no signs of the disease are called carriers.
An abnormal trait called an anomaly can be passed down through families can have no effect on your well being, such as a birth mark, or be of minor importance as being colorblind. Or the anomaly can have an effect more serious such as quality or length of life. Genetic counseling and prenatal diagnosis is available for most genetic disorders.
Dependent on the disease, if only one recessive gene is needed, it is called dominant hereditary disorder. In this type of dominant disorder, the child will show signs of the disease. A person born with one abnormal gene, the abnormal gene is named Heterozygous.
A person born with two abnormal recessive genes, one from each parent, the defective genes are called Homozygous genes. Each child, born to both parents having one copy of a recessive disease gene, has a one in four chance (25%) of having the disease. If one parent has two copies of the disease gene and the other parent has one copy of the disease gene, the chances become one out of two (50%) and is termed homozygous.
For the X-linked genes to be deemed of dominant inheritance, a single dose of mutant allele must affect the phenotype of the female. Two doses of the mutant allele are required for a recessive X-linked gene to affect the female phenotype. The following list the characteristics of dominant inheritance:
1) The defective genes are never passed from father to son.
2) The defective gene is passed to all daughters of an affected father and a normal mother.
3) The defective gene from affected mothers and normal fathers is passed to 50% of daughters and 50% of sons.
4) The affected males are usually more severely afflicted than females.
5) Females are more likely affected than males in the general populatiom.
Dominant inheritance can affect chromosomes or alleles. The above scenarios are the basic forms. There are several other variations, but most autosomal dominant inheritance conditions follow these patterns. As more information is discovered these patterns may change.
Autosomal dominant inheritance disorders include: Neurofibromatosis, Achondroplasia, Huntington disease, Ehlers-Danlos syndrome and adult polycystic kidney disease.
References:
http://www.nlm.nih.gov/medlineplus/ency/article/002048.htm
http://www.slh.wisc.edu/wps/wcm/connect/extranet/genetics/basics_dominant.html
http://www.uic.edu/classes/bms/bms655/lesson6.html
