Inheritance (Genetics) Calculators

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Genetic inheritance describes how traits and alleles are transmitted from parents to offspring. Mendel established the foundational laws governing inheritance through his pea plant experiments: the law of segregation (alleles separate during gamete formation) and independent assortment. Beyond simple Mendelian inheritance, traits can show incomplete dominance (blending), codominance (both alleles expressed), multiple allelism, X-linked inheritance (sex-linked traits), autosomal dominant or recessive patterns, polygenic inheritance (quantitative traits), and maternal inheritance (mitochondrial genes).

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Inheritance Patterns

  • Autosomal dominant: One copy of dominant allele causes the trait; every generation affected; affected individuals typically have one affected parent; 50% offspring of affected × unaffected; examples: Huntington disease, achondroplasia, Marfan syndrome
  • Autosomal recessive: Two copies needed; parents often unaffected carriers; 25% risk when both parents are carriers; often appears in siblings but skips generations; examples: cystic fibrosis, sickle cell anemia, PKU
  • X-linked recessive: Gene on X chromosome; males (XY) affected if they inherit one X with the allele; females need two copies to be affected; males inherit from carrier mothers; examples: hemophilia A, Duchenne muscular dystrophy, color blindness
  • X-linked dominant: All daughters of affected father are affected; males typically more severely affected; examples: Rett syndrome, fragile X

Incomplete Dominance and Codominance

Incomplete dominance: heterozygote shows intermediate phenotype (Aa is pink if AA is red and aa is white — snapdragons, four o'clocks). Neither allele is completely dominant. Codominance: both alleles are fully expressed in the heterozygote (AB blood type — both A and B antigens present). Multiple alleles: ABO blood type has three alleles (Iᴬ, Iᴮ, i).

Polygenic Inheritance

Traits controlled by many genes (height, skin color, IQ, crop yield) show continuous variation. Each gene contributes additively. Normal distribution of phenotypes. Heritability h² = V_A / V_P quantifies the genetic component.

Glossary

Autosomal Dominant
Inheritance pattern where one mutant allele causes the trait; every generation affected; 50% offspring risk when one parent affected; examples: Huntington's disease, Marfan syndrome.
X-Linked Recessive
Gene on X chromosome; males affected with one copy; females must have two copies to be affected; mothers pass to 50% of sons; examples: hemophilia A, color blindness, Duchenne MD.
Polygenic Inheritance
Traits controlled by many genes each contributing small additive effects; shows continuous (normally distributed) phenotypic variation; heritability h² quantifies the genetic component.

Frequently Asked Questions

Autosomal dominant: one mutant allele causes the trait; ~50% of offspring affected when one parent is affected; examples: Huntington's disease, achondroplasia. Autosomal recessive: two copies needed; carriers are unaffected; 25% of offspring affected when both parents are carriers; examples: cystic fibrosis, sickle cell anemia. X-linked recessive: gene on X chromosome; males are more commonly affected (only one X); carrier females typically unaffected; examples: hemophilia, color blindness, Duchenne muscular dystrophy. Mitochondrial: maternally inherited (all mitochondria come from the egg); affects both sexes equally; examples: MERRF, LHON.

Incomplete dominance: the heterozygote (Aa) has a phenotype intermediate between both homozygotes — neither allele is completely dominant over the other. Classic example: red (RR) × white (rr) snapdragons → all pink (Rr). Codominance: both alleles are fully expressed in the heterozygote — you see both phenotypes simultaneously. Classic example: ABO blood type — a person with genotype IᴬIᴮ expresses both A and B antigens on red blood cells (blood type AB). In codominance, you can identify the heterozygote (AB) as distinct from either homozygote (AA or BB); in incomplete dominance, the heterozygote shows a blend.

X-linked genes are located on the X chromosome. Males (XY) have only one X — they are hemizygous for X-linked genes. For X-linked recessive traits: males with one copy of the recessive allele are affected (cannot be carriers). Females need two copies to be affected (can be unaffected carriers with one copy). Pattern: affected grandfather → carrier daughters → 50% of grandsons affected ('skip' a generation). X-linked dominant traits: affected fathers pass to all daughters (not sons — sons get Y from father); mothers pass to 50% of children. For autosomal genes: equal risk to males and females; fathers can pass to sons (possible in X-linked only through unaffected carrier mother).

Polygenic traits are controlled by many genes, each contributing a small additive effect. Examples: human height, skin color, blood pressure, intelligence, crop yield. Key features: continuous variation (normal distribution of phenotypes rather than discrete categories); no clear dominant/recessive distinction; sensitive to environmental effects; heritability h² = V_A/V_P measures the genetic component. Contrast with Mendelian traits: controlled by one or few genes; discrete phenotypic categories (tall/short); clear dominant-recessive relationships; relatively simple pedigree patterns. Most economically important traits in agriculture (yield, drought tolerance) and most common complex diseases in humans (diabetes, hypertension) are polygenic.