Autosomal Recessive Trait Pedigree

• Recessive: If neither parent has the characteristic phenotype (disease) displayed by the child, the trait is likely to be recessive.
• Matings between an affected and an unaffected parent will usually yield all unaffected offspring (unless the unaffected parent is heterozygous (carrier) for the trait).
• Autosomal: Male and female offspring are equally likely to inherit the trait.
• In an autosomal recessive inheritance pattern, a person must inherit two copies of a mutated gene (one from each parent) to express the trait or disorder. Individuals who inherit only one copy of the mutated gene are called carriers and do not typically exhibit the trait or disorder themselves.
• Affected individuals typically have two carrier parents.
• When two carrier parents have children, there is a 25% chance that any given child will inherit two copies of the mutated gene and, therefore, express the trait or disorder. There is also a 50% chance of the child inheriting one mutated gene and becoming a carrier like the parents and a 25% chance of inheriting two normal copies of the gene.
• Recessive traits generally skip generations.
• It mostly shows a horizontal pattern of inheritance, but it can be vertical if the trait is common in the population.
• Rare recessive traits are more prone to manifest within a family tree when marital partners share a blood relationship, such as being first cousins. This phenomenon arises from the fact that relatives have a higher likelihood of carrying the same genetic variants due to their shared lineage. Consequently, when they mate, the probability of their offspring inheriting two identical recessive alleles increases, leading to the expression of the trait. Autosomal recessive pattern of inheritance is shown in the following figure 5.8.3.
• Examples:
  • Cystic Fibrosis: This is a genetic disorder that affects the respiratory, digestive, and reproductive systems. Individuals with cystic fibrosis inherit two mutated copies of the CFTR gene.
  • Sickle Cell Anemia: This is a blood disorder where red blood cells are misshapen and can cause various health problems. It is caused by mutations in the HBB gene.

Watch this video about autosomal recessive inheritance.

Comparison of Autosomal Dominant and Autosomal Recessive Inheritance

In autosomal dominant inheritance, the presence of one mutated copy of the gene (heterozygosity) from either parent is sufficient to express the trait or disorder. This means that affected individuals typically have an affected parent, and there is a 50% chance of passing the trait or disorder to each child. Whereas, in autosomal recessive inheritance, two mutated copies of the gene (homozygosity) are required for the trait or disorder to be expressed. Affected individuals typically have carrier (heterozygous) parents who do not exhibit the trait or disorder themselves. The risk of an affected child is 25% when two carriers have children.

Sex-Linked Trait Pedigrees

X-Linked Recessive Inheritance

• X-linked recessive traits are carried on the X chromosome. In this pattern, males are more commonly affected because they have only one X chromosome. If a male inherits an X-linked recessive gene, he will express the trait because there is no corresponding X chromosome to mask the effect of the mutated gene. In females, two copies of the mutated gene (homozygosity) are needed to express the trait.
• In a pedigree for X-linked recessive traits, affected males often have unaffected mothers, as the mother must be a carrier (heterozygous) and pass the X-linked recessive gene to her sons. The trait may skip generations if carrier females have carrier daughters.

X-Linked Dominant Inheritance

• X-linked dominant traits are carried on the X chromosome, and only one copy of the mutated gene is needed to express the trait. Both males and females can inherit and express X-linked dominant traits.
• In the pedigree for X-linked dominant traits, all daughters of the affected fathers are affected, but none of their sons. Affected heterozygous mothers mating with unaffected fathers pass the trait to 50% of their daughters and sons.

Y-Linked Traits Inheritance

Y-linked traits are carried on the Y chromosome. Y-linked traits are exclusively passed from fathers to their sons. Females do not inherit Y-linked traits because they do not have a Y chromosome. In the pedigree for Y-linked traits, affected males will have affected fathers, and there is a direct father-to-son pattern of inheritance. The trait typically does not skip generations or affect females.