IB Biology HL 1 Genetics and Inheritance

Complete Dominance

A pattern of inheritance where the dominant allele completely masks the effect of the recessive allele in heterozygous individuals.

Incomplete Dominance

A blending of traits occurs when neither allele is completely dominant, resulting in a heterozygous phenotype that is intermediate between the two homozygous phenotypes.

Co-Dominance

Both alleles are fully expressed in the heterozygous condition.

Epistasis

One gene masks or modifies the expression of another gene.

Polygenic Inheritance

Traits controlled by multiple genes, leading to a range of phenotypes.

Pleiotropy

A single gene influences multiple, seemingly unrelated traits.

Epigenetics

The study of heritable changes in gene expression that do not involve changes to the DNA sequence.

X-Linked Traits

Traits that are carried on the X chromosome and often affect males more because they have only one X chromosome.

Autosomal Dominant Inheritance Pattern

Affected individuals appear in every generation.

Autosomal Recessive Inheritance Pattern

May skip generations, appearing only when two carriers have children.

X-Linked Recessive Inheritance Pattern

More common in males; affected fathers do not pass the trait to sons but may pass it to daughters who become carriers.

X-Linked Dominant Inheritance Pattern

Affected fathers pass the trait to all daughters but no sons.

Monohybrid Cross

A cross between two individuals that examines the inheritance of a single trait.

Dihybrid Cross

A cross between individuals that examines the inheritance of two traits.

Recessive Epistasis

A recessive allele at one locus masks the expression of alleles at another locus.

Dominant Epistasis

A dominant allele at one locus masks the expression of alleles at another locus.

Phenotypic Ratio of Incomplete Dominance

1:2:1 (Phenotype reflects blending of traits).

Phenotypic Ratio of Co-Dominance

1:2:1 (Both alleles are fully expressed).

Autosomal Recessive Disease Examples

Sickle Cell Anemia, Tay Sach’s Disease, Cystic Fibrosis

Autosomal Dominant Disorder Examples

Huntington’s Disease, Achondroplasia

X-Linked Disorder Examples

Red-Green Color Blindness, Hemophilia, Duchenne Muscular Dystrophy

Law of Independent Assortment

In Metaphase I, Genes for different traits can segregate independently during gamete formation, leading to genetic variation.

Law of Segregation

In Anaphase I, Each parent passes only one allele for each trait to their offspring, as allele pairs separate during gamete formation.