Inheritance

Inheritance is the fundamental biological process that governs how genetic information is passed from one generation to the next. The study of inheritance is called genetics. The basic unit of heredity is the gene, a specific sequence of DNA that codes for a polypeptide or functional RNA. Different versions of the same gene are called alleles, and they are located at a specific position, or locus, on a chromosome.

The genetic makeup of an organism for a particular trait is its genotype (e.g., TT, Tt, or tt), while the observable characteristic is its phenotype (e.g., tall or short). An individual with two identical alleles for a trait is homozygous (e.g., TT), whereas an individual with two different alleles is heterozygous (e.g., Tt).

Meiosis: The Basis of Inheritance

The foundation of sexual reproduction and inheritance is meiosis, a specialized type of cell division that reduces the chromosome number by half, creating four genetically distinct haploid cells (gametes) from one diploid cell. Genetic variation is introduced during Meiosis I through two key processes:

1. Crossing Over: During Prophase I, homologous chromosomes pair up and exchange segments of their non-sister chromatids. This creates new combinations of alleles on the chromosomes.
2. Independent Assortment: During Metaphase I, homologous chromosome pairs align randomly at the metaphase plate. This means the combination of maternal and paternal chromosomes sorted into each gamete is random.

Mendelian Genetics

Gregor Mendel's experiments with pea plants laid the groundwork for modern genetics.

A monohybrid cross involves tracking the inheritance of a single characteristic. When a homozygous tall pea plant (TT) is crossed with a homozygous dwarf plant (tt), all offspring in the first filial (F1) generation are heterozygous (Tt) and tall. This shows that the tall allele (T) is dominant over the dwarf allele (t), which is recessive. When the F1 generation self-pollinates, the second filial (F2) generation exhibits a characteristic phenotypic ratio of 3 tall : 1 dwarf.

A dihybrid cross examines the inheritance of two different unlinked genes simultaneously, such as seed shape and seed colour. This demonstrates Mendel's Law of Independent Assortment. A cross between parents homozygous for two traits (e.g., round yellow seeds, RRYY, and wrinkled green seeds, rryy) produces an F1 generation that is heterozygous for both traits (RrYy). The F2 generation, resulting from an F1 self-cross, shows a characteristic phenotypic ratio of 9:3:3:1.

Beyond Mendelian Ratios

Not all inheritance follows simple dominant/recessive patterns. Codominance occurs when both alleles in a heterozygous individual are fully and independently expressed in the phenotype. A classic example is the human ABO blood group system, where alleles Iᴬ and Iᴮ are codominant. An individual with genotype IᴬIᴮ will have blood type AB, expressing both A and B antigens on their red blood cells.

Gene Linkage refers to genes located on the same chromosome, which tend to be inherited together as they are physically linked. This violates the law of independent assortment. Autosomal linkage involves genes on non-sex chromosomes. During crossing over, these linked genes can be separated, producing recombinant offspring with different allele combinations from the parents. The closer two genes are on a chromosome, the less likely they are to be separated by crossing over, resulting in a lower recombination frequency.

Sex linkage describes genes located on the sex chromosomes (X or Y). Traits determined by genes on the X chromosome are X-linked. Since males (XY) have only one X chromosome, they will express any recessive X-linked allele they inherit. This is why X-linked disorders like haemophilia and red-green colour blindness are far more common in males than females (XX).

The Chi-Squared (χ²) Test

To determine if the results of a genetic cross fit an expected Mendelian ratio, a statistical test called the chi-squared (χ²) test is used. It assesses the 'goodness of fit' between observed (O) and expected (E) results. The null hypothesis states that there is no significant difference between the observed and expected results, and any deviation is due to chance. The formula is:

χ² = Σ [ (O - E)² / E ]

Where 'Σ' means 'sum of', 'O' is the observed frequency, and 'E' is the expected frequency for each phenotype. After calculating the χ² value, you determine the degrees of freedom (df), which is the number of phenotypic classes minus one (df = n - 1). By comparing the calculated χ² value to a critical value in a probability table (typically at a probability level of 0.05 or 5%), a conclusion can be drawn. If the calculated χ² value is greater than the critical value, the null hypothesis is rejected, suggesting a statistically significant difference (e.g., the genes may be linked). If it is less, the null hypothesis is accepted.