Recessive Calculators

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A recessive allele is an allele whose phenotypic effect is masked when paired with a dominant allele — its phenotype is only expressed when the organism is homozygous (has two copies of the recessive allele). In a heterozygote (Aa), the dominant allele (A) masks the recessive allele (a), and the organism shows the dominant phenotype. Only in the homozygous recessive genotype (aa) is the recessive phenotype expressed. Recessive alleles are frequently loss-of-function variants — one functional copy of the gene (from the dominant allele) is sufficient to produce the normal phenotype (haploinsufficiency is the exception where one copy is not sufficient).

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Dominance and Recessiveness

Dominant allele (A): phenotype expressed in AA or Aa genotypes. Recessive allele (a): phenotype expressed only in aa genotype; masked in Aa by the dominant allele. Molecular basis of recessiveness: most recessive alleles produce non-functional or absent protein. One functional copy (from the dominant allele) provides sufficient enzymatic activity, structural protein, or signaling function → heterozygote is phenotypically normal → carrier. Exception: haploinsufficiency — when one copy is NOT sufficient → the dominant allele shows incomplete dominance or the heterozygote is affected.

Autosomal Recessive Inheritance

Carrier × carrier (Aa × Aa) cross: 25% affected (aa); 50% carriers (Aa); 25% unaffected non-carriers (AA). The 3:1 phenotype ratio (Mendel's monohybrid cross). Both parents must carry at least one recessive allele for an affected child to be possible.

Common Autosomal Recessive Diseases

  • Cystic fibrosis (CFTR gene): 1:3,200 Caucasians
  • Sickle cell anemia (HBB gene): 1:500 African Americans
  • Phenylketonuria — PKU (PAH gene): 1:15,000
  • Tay-Sachs (HEXA gene): 1:3,500 Ashkenazi Jews

Hardy-Weinberg and Carrier Frequency

If disease frequency = q² (known from population data), carrier frequency = 2pq ≈ 2q (for rare alleles). Example: CF disease frequency = 1/3,200 → q = 1/56.6 ≈ 0.018 → carrier freq = 2 × 0.982 × 0.018 ≈ 1/27.

Glossary

Recessive Allele
An allele expressed only in the homozygous recessive (aa) genotype; masked by the dominant allele in the heterozygote (Aa); usually a loss-of-function variant; carrier status = Aa (normal phenotype).
Carrier
A heterozygous (Aa) individual who carries one recessive disease allele but shows the dominant (normal) phenotype; estimated from disease frequency using Hardy-Weinberg: carrier = 2pq.
Autosomal Recessive
A trait encoded by a recessive allele on chromosomes 1–22; requires two copies for expression; carrier × carrier gives 25% affected offspring; both sexes equally affected.

Frequently Asked Questions

A recessive allele is expressed phenotypically only when present in two copies (homozygous recessive, aa). In a heterozygote (Aa), the dominant allele (A) masks the effect of the recessive allele — the organism shows the dominant phenotype even though it carries one copy of the recessive allele. Molecular basis: most recessive alleles are loss-of-function mutations that produce no protein or a non-functional protein. In a heterozygote, the one functional copy (from the dominant allele) is sufficient to produce enough protein for normal function (dosage compensation). For a child to be affected by an autosomal recessive condition, they must inherit one recessive allele from each parent.

A carrier is a heterozygous individual (Aa) who has one copy of a recessive allele but shows the dominant (normal) phenotype — they can pass the recessive allele to offspring without being affected themselves. Two carriers (Aa × Aa) have: 25% chance of an affected child (aa); 50% chance of a carrier child (Aa); 25% chance of an unaffected homozygous dominant child (AA). Carrier frequency estimation using Hardy-Weinberg: if disease frequency = q² = 1/10,000, then q = 0.01 and p ≈ 0.99. Carrier frequency = 2pq = 2 × 0.99 × 0.01 ≈ 1/50. This means for every 1 affected person, there are ~50 carriers in the population. Common carrier frequencies: CF (Caucasians) ~1:25; sickle cell (African Americans) ~1:13.

Autosomal recessive: gene on chromosomes 1–22; both males and females equally likely to be affected; requires two copies of the recessive allele (aa); both parents must contribute a recessive allele; trait appears in siblings of affected individuals (~25% risk for each sibling if both parents are carriers). X-linked recessive: gene on X chromosome; males (XY) affected with just one copy (hemizygous); females (XX) are carriers with one copy; females need two copies to be affected (rare); trait transmitted through carrier mothers to sons; no father-to-son transmission; classically 'criss-cross' inheritance (grandfather → carrier daughter → affected grandson).

Complete dominance: in Aa heterozygote, the A phenotype is fully expressed; a is completely hidden. Incomplete dominance: the Aa heterozygote shows an intermediate phenotype between AA and aa — neither allele is dominant. Example: Antirrhinum (snapdragon) flower color: RR = red; rr = white; Rr = pink (intermediate). The pink heterozygote has 50% of the normal pigment production → visible intermediate color. This is not truly recessive — the r allele does contribute to the phenotype, just not at full strength. Other examples: hypercholesterolemia (FH) in heterozygotes have intermediate cholesterol levels (between normal AA and severely elevated aa). Codominance: both alleles expressed fully in the heterozygote (ABO blood type AB; MN blood group).