7.3.1: Single Gene Disorders

Learning Objectives

1. Differentiate between autosomal recessive and autosomal dominant inheritance patterns.
2. Explain the role of sex-linked inheritance in increasing vulnerability to certain genetic disorders, especially among biological males.
3. Describe how genetic mutations occur during prenatal development.
4. Identify how these mutations can contribute to the variability in the expression of single gene disorders.

Single Gene Disorders

A single gene disorder is caused by one abnormal gene and can occur in one of two ways:

1. By inheriting a pair of recessive alleles from the parents, due to one or both parents being carriers of the disorder.
2. A mutation occurs during cellular division in the prenatal stage of development.

Inheritance of Single Gene Disorders

Recessive Disorders (Homozygous)

The individual inherits a gene change from both parents. If the gene is inherited from just one parent, the person is a carrier and does not have the condition due to the dominant gene suppressing the expression of the recessive allele that carries the single gene disorder. If both parents are carriers and the child inherits a pair of recessive alleles, the single gene order is expressed.

In some cases, single gene disorders are expressed because the recessive gene has no pair, so it is the only gene that can be expressed and does not have the possibility of being suppressed by a dominant allele in the pair. The Y chromosome has about 300 genes compared to the X chromosome, which has over 900. The XY chromosomes are our genetic information related to biological sex. In biological males, there is often no pair for many of the genes on the X chromosome to the Y, leaving biological males with an increased vulnerability to many single gene disorders.

Figure \(\PageIndex{1}\): The Y Chromosome. Image by The National Human Genome Research Institute is in the public domain.

Table \(\PageIndex{1}\) - Recessive Disorders (Homozygous)

Disorder	Description	Cases per Birth
Red-Green Blindness	A type of color vision deficiency where individuals have difficulty distinguishing between red and green hues. It is caused by a genetic defect that affects the cones in the retina, which are responsible for color detection.	This condition is usually inherited and is linked to a mutation on the X chromosome, making it more common in males than females. 1 in 12 men and 1 in 200 women
Hemophilia	A disorder that impairs the blood's ability to clot properly, leading to excessive bleeding after injury or surgery, and spontaneous bleeding episodes. There are three types of hemophilia: A (Classic), B (Christmas Disease), and C.	Type A affects about 1 in 5,000 male births Type B affects about 1 in 25,000 male births Type C affects both males and females but is much rarer, about 1 in 100,000 births
Phenylketonuria (PKU)	A metabolic disorder in which the individual cannot metabolize phenylalanine, an amino acid. Left untreated, intellectual deficits occur. PKU is easily detected and is treated with a special diet.	1 in 10,000
Albinism	When the individual lacks melanin and processes little to no pigment in the skin, hair, and eyes. Vision problems can also occur.	Fewer than 20,000 US cases per year
Duchenne Muscular Dystrophy	A weakening of the muscles resulting in an inability to move, wasting away, and possible death.	1 in 3500 males

Autosomal Dominant Disorders (Heterozygous)

The individual only needs to inherit the gene change from one parent to have the disorder.

Table \(\PageIndex{2}\) - Autosomal Dominant Disorders (Heterozygous)

Disorder	Description	Cases per Birth
Huntington’s Disease	A condition that affects the individual’s nervous system. Nerve cells become damaged, causing various parts of the brain to deteriorate. The disease affects movement, behavior and cognition. It is fatal and occurs at midlife.	1 in 10,000
Tourette Syndrome	A tic disorder that results in uncontrollable motor and vocal tics as well as body jerking	1 in 250
Achondroplasia	The most common form of disproportionate short stature. The individual has abnormal bone growth resulting in short stature, disproportionately short arms and legs, short fingers, a large head, and specific facial features features.	1 in 15,000-40,000

Mutation

Mutation occurs during the prenatal stage of development when genetic information is undergoing replication or mitosis. A process called deletion can occur. Deletion is when a portion of a chromosome is deleted because proteins are out of order or missing from one step to the next. This can happen when strands of DNA slip or loop out. Mutation is why there are varying degrees of severity and expression of the disorder from individual to individual. Mutation can occur spontaneously or, in most single gene disorders, due to inheriting a mutated allele from both parents (and could also be classified as homozygous).

Table \(\PageIndex{3}\) - Recessive Disorders Due to Mutation

Disorder	Description	Cases per Birth
Sickle Cell Disease (SCD)	A condition in which the red blood cells in the body are shaped like a sickle (like the letter C) and affect the ability of the blood to transport oxygen.	1 in 500 Black births 1 in 36,000 Hispanic births
Cystic Fibrosis (CF)	A condition that affects breathing and digestion due to thick mucus building up in the body, especially the lungs and digestive system. In CF, the mucus is thicker than normal and sticky.	1 in 3500
Tay Sachs Disease	Caused by an enzyme deficiency resulting in the accumulation of lipids in the nerve cells of the brain. This accumulation results in progressive damage to the cells and a decrease in cognitive and physical development. Death typically occurs by age five.	1 in 4000 1 in 30 American Jews are carriers 1 in 20 French Canadians are carriers

References, Contributions and Attributions

2.1: Heredity by Paris, Ricardo, Raymond, & Johnson is licensed CC BY 4.0.

Lifespan Development: A Psychological Perspective by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0