PATTERNS OF INHERITANCE

Key Genetic Terms

Understanding inheritance requires familiarity with several fundamental genetic concepts:

• Alleles: Alternative forms of a gene found at the same locus on homologous chromosomes.

• Dominant Allele: An allele that masks the expression of another allele at the same locus; its trait is expressed in the phenotype.

• Recessive Allele: An allele whose trait is only expressed when two copies are present (homozygous).

• Genotype: The genetic makeup of an organism, represented by the alleles present (e.g., HbA/HbS).

• Phenotype: The observable characteristics or traits of an organism, resulting from the genotype.

• Homozygous: Having two identical alleles for a particular gene (e.g., HbA/HbA or HbS/HbS).

• Heterozygote: Having two different alleles for a particular gene (e.g., HbA/HbS).

Genes, Chromosomes, and Inheritance

Genes are located on chromosomes, and inheritance patterns are determined by the behavior of chromosomes during meiosis and fertilization. Each parent contributes one set of chromosomes to their offspring, resulting in pairs of homologous chromosomes.

During meiosis, homologous chromosomes separate, ensuring that each gamete receives only one allele for each gene. Fertilization restores the diploid state, combining alleles from both parents.

Inheritance of the Sickle Cell Gene

Sickle cell disease is caused by a mutation in the hemoglobin gene. Individuals with the genotype HbA/HbS are carriers, while those with HbS/HbS have the disease. The inheritance follows Mendelian principles.

Each parent passes one allele to their child. The probability of inheriting the disease depends on the combination of alleles received.

Mendelian Patterns of Inheritance

Mendel's law of segregation states that each individual has two alleles for each gene, which segregate during gamete formation. Punnett squares are used to predict the outcome of genetic crosses.

• Punnett Square Example: If both parents are HbA/HbS, the possible genotypes for their child are:

  • HbA/HbA (normal)

  • HbA/HbS (carrier)

  • HbS/HbS (affected)

Non-Mendelian Patterns of Inheritance

Not all traits follow simple Mendelian inheritance. Several other patterns exist:

• Incomplete Dominance: The phenotype is intermediate between the two alleles (e.g., pink flowers from red and white parents).

• Codominance: Both alleles are fully expressed (e.g., ABO blood group).

• Pleiotropy: One gene affects multiple traits (e.g., sickle cell gene affects blood cells and other organs).

• Polygenic Inheritance: Multiple genes contribute to a single trait (e.g., skin color).

• Sex Linkage: Traits are associated with genes located on sex chromosomes (e.g., hemophilia).

Autosomal and Sex-linked Inheritance Patterns

Genetic disorders can be inherited in different ways, depending on whether the gene is located on an autosome or a sex chromosome.

• Autosomal Recessive: Both parents must carry the allele for the child to be affected. Example: Tay-Sachs, cystic fibrosis, sickle cell disease.

• Autosomal Dominant: Only one copy of the mutated allele is needed for the trait to be expressed. Example: Huntington's disease.

• X-linked Recessive: More common in males; females are carriers. Example: hemophilia.

• X-linked Dominant: Affected mothers can pass the trait to both sons and daughters; affected fathers pass it only to daughters.

Examples of Inherited Diseases

• Tay-Sachs Disease: Autosomal recessive; caused by a mutation in the HEXA gene. Individuals with two recessive alleles (tt) develop the disease.

• Cystic Fibrosis: Autosomal recessive; caused by mutations in the CFTR gene.

• Huntington's Disease: Autosomal dominant; caused by a mutation in the HTT gene. Only one copy of the dominant allele (H) is needed.

• Sickle Cell Disease: Autosomal recessive; caused by a mutation in the HBB gene.

Pedigree Analysis

A pedigree is a diagram that shows the occurrence of phenotypes in several generations of a family. Symbols include:

• Circle: Female

• Square: Male

• Shaded: Affected individual

• Half-shaded: Carrier

Pedigrees help determine the mode of inheritance and predict the risk for future generations.

Factors Affecting Distribution of Recessive Diseases

• Recessive diseases are not evenly distributed due to population structure, consanguinity, and genetic drift.

• Factors increasing the chance of passing on recessive traits include consanguineous marriages and small population size.

Chromosome Theory of Inheritance

The chromosome theory of inheritance states that genes are located on chromosomes, and the behavior of chromosomes during meiosis explains inheritance patterns.

Sex Chromosome Inheritance

• Males: XY genotype; pass their X chromosome to daughters and Y chromosome to sons.

• Females: XX genotype; pass one X chromosome to each child.

Practice Problems and Pedigree Interpretation

Pedigree charts and genetic crosses are used to analyze inheritance patterns and calculate probabilities for genetic disorders. Use the following notation:

• Uppercase letter: Dominant allele

• Lowercase letter: Recessive allele

• XR/Xr: X-linked dominant/recessive alleles

Probability calculations often use Punnett squares and pedigree analysis to determine carrier status and disease risk.

Summary Table: Modes of Inheritance

Additional info: Academic context was added to clarify inheritance patterns, disease examples, and pedigree interpretation.