A gene will have its effects only when it is paired with a similar gene on the other chromosome

Autosomal recessive is one of several ways that a trait, disorder, or disease can be passed down through families.

An autosomal recessive disorder means two copies of an abnormal gene must be present in order for the disease or trait to develop.

Inheriting a specific disease, condition, or trait depends on the type of chromosome that is affected. The two types are autosomal chromosomes and sex chromosomes. It also depends on whether the trait is dominant or recessive.

A mutation in a gene on one of the first 22 nonsex chromosomes can lead to an autosomal disorder.

Genes come in pairs. One gene in each pair comes from the mother, and the other gene comes from the father. Recessive inheritance means both genes in a pair must be abnormal to cause disease. People with only one defective gene in the pair are called carriers. These people are most often not affected with the condition. However, they can pass the abnormal gene to their children.

CHANCES OF INHERITING A TRAIT

If you are born to parents who both carry the same autosomal recessive gene, you have a 25% (1 in 4) chance of inheriting the abnormal gene from both parents and developing the disease. You have a 50% (1 in 2) chance of inheriting one abnormal gene. This would make you a carrier.

In other words, for a child born to a couple who both carry the gene (but do not have signs of disease), the expected outcome for each pregnancy is:

• A 25% chance that the child is born with two normal genes (normal)
• A 50% chance that the child is born with one normal and one abnormal gene (carrier, without disease)
• A 25% chance that the child is born with two abnormal genes (at risk for the disease)

Note: These outcomes do not mean that the children will definitely be carriers or be severely affected.

Genetics - autosomal recessive; Inheritance - autosomal recessive

Feero WG, Zazove P, Chen F. Clinical genomics. In: Rakel RE, Rakel DP, eds. Textbook of Family Medicine. 9th ed. Philadelphia, PA: Elsevier; 2016:chap 43.

Gregg AR, Kuller JA. Human genetics and patterns of inheritance. In: Resnik R, Lockwood CJ, Moore TR, Greene MF, Copel JA, Silver RM, eds. Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2019:chap 1.

Jones KL, Jones MC, Campo M. Genetics, genetic counseling, and prevention. In: Jones KL, Jones MC, Campo MD, eds. Smith's Recognizable Patterns of Human Deformation. 8th ed. Philadelphia, PA: Elsevier; 2022:chap 2.

Korf BR. Principles of genetics. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 35.

Updated by: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

• Since human cells carry two copies of each chromosome they have two versions of each gene. These different versions of a gene are called alleles.
• Alleles can be either dominant or recessive.
• Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous). For example, the allele for brown eyes is dominant, therefore you only need one copy of the ‘brown eye’ allele to have brown eyes (although, with two copies you will still have brown eyes).
• If both alleles are dominant, it is called codominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.
• Recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous). For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the ‘blue eye’ allele.

Illustration to show the inheritance of dominant and recessive alleles for eye colour.
Image credit: Genome Research Limited

What are sex-linked genes?

• Some genes are found on the sex chromosome, X.
• These genes are inherited with the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl).
• Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
• This means females have two alleles for X-linked genes while males only have one.
• Some genetic diseases, are caused by sex linked genes, for example haemophilia.
• The allele for haemophilia is recessive so two copies are needed for a female to have the disease
• However, because males only have one X chromosome, they only need one copy of the haemophilia allele to have the disease.
• This means haemophilia is much more common in males than in females.

For example:

Functioning allele = H

Haemophilia allele = h

XH XH = healthy female

XH Xh = carrier female

Xh Xh = haemophilia female

XH Y = healthy male

Xh Y = haemophilia male

This page was last updated on 2021-07-21

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