Chromosomes in the cell nucleus contain genes that code for characteristics, with two copies of each chromosome in sexually reproducing organisms. Sex chromosomes determine an organism’s sex, while the rest are autosomes. Autosomal pairs are usually identical in size and shape, with different alleles interacting to express traits. Autosomal diseases can be caused by abnormalities or gene loci, with autosomal dominant and recessive disorders having different inheritance patterns.
Inside the cell nucleus, DNA is organized into discrete structures called chromosomes. The cells of sexually reproducing organisms have two copies of each chromosome, called homologous chromosomes. One copy comes from the male parent and one from the female. Two chromosomes, or a pair, determine the sex of the organism, hence they are called sex chromosomes. The rest of the chromosomes are referred to as autosomes.
For example, humans have 46 chromosomes or 23 homologous pairs of chromosomes. A karyotype is a created image of all the chromosomes within a nucleus. In a karyotype of a human cell, pairs one through 22 are autosomal pairs, while pair 23 consists of the sex chromosomes. The paired autosomes are arranged from one to 22 in decreasing size. In a normal cell, an autosome pair of chromosomes is usually identical in size and shape and has the same genes in the same location.
Each chromosome has a specific set of genes that code for different characteristics, whether it’s a sex chromosome or an autosome. The genes are similar, but they don’t have to be identical, as one comes from each parent. Each autosome can contain a different type of gene or allele. Depending on the gene, there may be different alleles for it. Different alleles can interact in different ways, resulting in different expression of the trait.
Some alleles are always expressed on others, so they are said to be dominant alleles. Other alleles are expressed only if there are two copies of that allele. This type of allele is called recessive. Finally, some alleles can interact with each other and be partially expressed depending on which allele is the corresponding one. Some characteristics due to genes found on autosomes are called autosomal dominant or autosomal recessive based on the alleles and how the characteristic is expressed.
About half of genetic diseases in humans are caused by an autosome. The disease could be caused by an abnormality on an autosome, as is the case with Down syndrome. This disease is caused by an autosomal trisomy of chromosome 21. An autosomal trisomy occurs when a cell has three copies of a particular chromosome instead of two. Many of these types of abnormalities result in fetal death or a nonlive birth.
Other diseases are caused by the allele present at a particular gene locus. Autosomal dominant disorders are caused by an autosomal dominant gene, which means that if one parent is affected, there is a 50% chance that the children will also be affected. An example of an autosomal dominant disorder is Huntington’s disease.
With an autosomal recessive disease, the disease will only be present if both copies of the gene are recessive. Many people may have one copy of the recessive gene, so they won’t get the disease but will be a carrier. The offspring have a 25% chance of being transmitted the disease from an infected parent and a 50% chance of being a carrier. Cystic fibrosis is a common example of an autosomal recessive disease.
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