Mendel’s Laws Of Inheritance

Genetics, the study of inheritance and variation, explores how traits are passed from parents to offspring. Gregor Mendel, through his seven-year study (1856-1863) of garden pea plants, laid the foundation for modern genetics by proposing the fundamental laws of inheritance. He meticulously conducted hybridisation experiments, applying statistical analysis to his data, which provided a rigorous, scientific basis for understanding heredity. Mendel selected pea plants with contrasting traits for a single character (e.g., tall vs. dwarf, yellow vs. green seeds) and studied their inheritance across generations. He identified 14 true-breeding varieties representing seven contrasting traits.

Inheritance Of One Gene

Mendel's experiments involving the cross-pollination of tall and dwarf pea plants revealed key principles of inheritance.

Law Of Dominance

When Mendel crossed true-breeding tall plants (TT) with true-breeding dwarf plants (tt), all the offspring in the first filial generation (F1) were tall. This indicated that one trait (tallness) masked the expression of the other (dwarfness). Mendel proposed that characters are controlled by discrete units called factors (now known as genes), which occur in pairs. In a pair of dissimilar factors (alleles), one is dominant over the other, which is recessive. In the case of height, 'T' for tallness is dominant over 't' for dwarfness. The F1 generation, with genotype Tt, expressed the dominant phenotype (tall) and thus resembled one of the parents.

Law Of Segregation

This law, based on the observation that factors (alleles) do not blend, states that during gamete formation, the alleles of a pair segregate from each other, with each gamete receiving only one allele. In the F1 generation (Tt), each parent contributes one allele (T from one, t from the other) to the gametes. When the F1 plants self-pollinate, the gametes carrying T and t combine randomly. This results in the F2 generation having genotypes TT, Tt, and tt in a ratio of 1:2:1. Phenotypically, this yields a ratio of 3 tall (TT and Tt) to 1 dwarf (tt), demonstrating the segregation of traits. The recessive trait reappears in the F2 generation due to the segregation of alleles.

Incomplete Dominance

In some cases, such as the inheritance of flower colour in Snapdragon (Antirrhinum sp.), dominance is not complete. When a true-breeding red-flowered plant (RR) is crossed with a true-breeding white-flowered plant (rr), the F1 generation (Rr) exhibits an intermediate phenotype – pink flowers. Upon self-pollination of the F1 pink-flowered plants, the F2 generation shows a phenotypic ratio of 1 red : 2 pink : 1 white. This indicates that the R allele is not completely dominant over the r allele, and the heterozygote (Rr) expresses a unique phenotype. This phenomenon occurs because the allele produces a non-functional or less efficient enzyme, and the phenotype depends on the functional allele's activity.

Co-Dominance

Co-dominance is observed when both alleles in a heterozygous state express their traits simultaneously. A classic example is the ABO blood grouping system in humans, controlled by gene 'I' with three alleles: IA, IB, and i. Alleles IA and IB produce specific sugars on red blood cells, while 'i' produces none. IA and IB are dominant over 'i'. However, when both IA and IB are present (genotype IAIB), both sugars A and B are expressed, demonstrating co-dominance. This results in individuals with blood group AB. The ABO blood group system also illustrates multiple alleles, where more than two alleles exist for a single gene within a population.

Another example is starch synthesis in pea seeds, controlled by gene B/b. BB individuals produce large starch grains (round seeds), bb individuals produce smaller grains (wrinkled seeds), and Bb individuals produce intermediate-sized grains (round seeds), showing incomplete dominance for starch grain size but dominance for seed shape.

Inheritance Of Two Genes

Law Of Independent Assortment

Mendel extended his studies to dihybrid crosses, where he examined the inheritance of two traits simultaneously. For instance, crossing plants with yellow, round seeds (RRYY) with plants having green, wrinkled seeds (rryy) showed that in the F1 generation, offspring were yellow and round (RrYy). In the F2 generation, a phenotypic ratio of 9:3:3:1 was observed (9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled). This led Mendel to propose the Law of Independent Assortment, stating that when two pairs of traits are inherited in a hybrid, the segregation of alleles for one trait is independent of the segregation of alleles for the other trait. This implies that gametes like RY, Ry, rY, and ry are produced in equal proportions (25% each) due to the independent assortment of chromosome pairs during meiosis.

Chromosomal Theory Of Inheritance

Although Mendel's work was published in 1865, it was rediscovered in 1900 by de Vries, Correns, and von Tschermak. Advancements in microscopy allowed scientists to observe chromosomes during cell division. Walter Sutton and Theodore Boveri noted the parallel behaviour of chromosomes and genes during meiosis – their pairing, segregation, and independent assortment. They proposed the Chromosomal Theory of Inheritance, suggesting that genes are located on chromosomes and their behaviour during meiosis explains Mendelian laws. Chromosomes, like genes, occur in pairs, and homologous chromosomes carry alleles at similar loci.

Linkage And Recombination

Thomas Hunt Morgan, working with fruit flies (Drosophila melanogaster), further elaborated on the Chromosomal Theory of Inheritance. He observed that certain genes, when located on the same chromosome, did not assort independently, deviating from Mendel's Law of Independent Assortment. Morgan termed this phenomenon linkage, describing the physical association of genes on a chromosome. He also observed that the frequency of recombination (generation of non-parental gene combinations) varied depending on the distance between genes on a chromosome. Tightly linked genes showed low recombination frequencies, while loosely linked genes showed higher frequencies. Morgan's student, Alfred Sturtevant, used these recombination frequencies to construct genetic maps, indicating the relative positions of genes on chromosomes. This concept is fundamental to genome sequencing.

Polygenic Inheritance

Polygenic inheritance describes traits influenced by multiple genes, often in conjunction with environmental factors, resulting in a continuous range of phenotypes rather than distinct categories. Human height and skin colour are classic examples. In these traits, the phenotype is additive, meaning each dominant allele contributes a certain degree to the expression of the trait. For instance, if three genes (A, B, C) control skin colour, with dominant alleles (A, B, C) contributing to darker skin and recessive alleles (a, b, c) to lighter skin, an individual with genotype AABBCC would have the darkest skin, while aabbcc would have the lightest. Intermediate genotypes would exhibit intermediate skin tones.

Pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits. This usually happens when a gene affects a metabolic pathway that contributes to various developmental processes. A prime example is phenylketonuria (PKU), a human genetic disorder caused by a mutation in the gene for the enzyme phenylalanine hydroxylase. This single gene defect leads to phenotypic manifestations such as mental retardation and reduced pigmentation in hair and skin.

Sex Determination

The mechanism by which the sex of an individual is determined is a fascinating area of genetics. Several chromosomal mechanisms have been identified:

Sex Determination In Humans

Humans exhibit XY sex determination. Out of 23 pairs of chromosomes, 22 are autosomes, and one pair are sex chromosomes. Females typically have two X chromosomes (XX), making them homogametic. Males have one X and one Y chromosome (XY), making them heterogametic. During sperm formation, males produce two types of gametes: 50% carry an X chromosome and 50% carry a Y chromosome. Females produce only one type of gamete, the ovum, which carries an X chromosome. The sex of the offspring is determined by the type of sperm that fertilizes the ovum: an X-carrying sperm results in a female (XX), while a Y-carrying sperm results in a male (XY). Thus, the male gamete determines the sex of the child, and each pregnancy has a 50% chance of resulting in a male or female offspring.

Another notable system is the **XO type**, found in some insects like the grasshopper. Here, females have a pair of X chromosomes (XX), while males have only one X chromosome (XO). Males produce two types of sperm: one with an X chromosome and another without it. Fertilization of an ovum by an X-bearing sperm results in a female (XX), and fertilization by a sperm without an X chromosome results in a male (XO).

In birds, sex determination is based on a **ZW system**. The female is heterogametic (ZW), producing Z and W gametes, while the male is homogametic (ZZ), producing only Z gametes. Therefore, the sex of the offspring is determined by the ovum.

Sex Determination In Honey Bee

Honey bees exhibit a unique haplodiploid sex-determination system. Females (queens and workers) are diploid (32 chromosomes), developing from fertilized eggs. Males (drones) are haploid (16 chromosomes), developing from unfertilized eggs through parthenogenesis. This system means males have no father but have grandsons, and produce sperm through mitosis.

Mutation

Mutation is a process that causes a permanent alteration in the DNA sequence, leading to changes in the genotype and phenotype of an organism. Besides genetic recombination, mutations are a primary source of genetic variation. These alterations can involve the loss (deletion), gain (insertion/duplication) of DNA segments, resulting in chromosomal aberrations. Point mutations, a change in a single base pair (e.g., sickle-cell anemia), and frameshift mutations (caused by deletions or insertions of base pairs) are also important types of mutations. Various physical and chemical agents, known as mutagens (e.g., UV radiation), can induce mutations.

Genetic Disorders

Pedigree Analysis

Pedigree analysis is a method used in human genetics to study the inheritance patterns of traits, abnormalities, or diseases across several generations of a family. Since controlled crosses are not possible in humans, analyzing family history through a "family tree" (pedigree chart) provides crucial information about the inheritance of genetic conditions. Standardized symbols are used to represent individuals, their relationships, and the presence or absence of a specific trait, allowing geneticists to determine if a disorder is autosomal or sex-linked, dominant or recessive.

Mendelian Disorders

These disorders are caused by alterations or mutations in a single gene and are inherited according to Mendelian principles. Examples include:

• Colour Blindness: An X-linked recessive disorder affecting the ability to distinguish red and green colours, caused by mutations in genes on the X chromosome. It is more common in males.
• Haemophilia: An X-linked recessive disorder where blood clotting is impaired due to the absence of clotting factors. It is typically transmitted from carrier females to their sons.
• Sickle-cell Anaemia: An autosomal recessive trait caused by a mutation in the gene for the beta-globin chain of haemoglobin. Heterozygous individuals (HbAHbS) carry the trait and have a 50% chance of passing it on. In homozygous recessive individuals (HbSHbS), abnormal haemoglobin causes red blood cells to sickle under low oxygen tension, leading to anemia.
• Phenylketonuria (PKU): An autosomal recessive metabolic disorder where individuals lack the enzyme phenylalanine hydroxylase, leading to accumulation of phenylalanine, mental retardation, and excretion of phenylpyruvic acid in urine.
• Thalassemia: An autosomal recessive blood disorder characterized by reduced synthesis of alpha or beta globin chains of haemoglobin, resulting in anaemia. It is a quantitative defect, unlike the qualitative defect in sickle-cell anaemia.

Chromosomal Disorders

These disorders arise from the absence, excess, or abnormal arrangement of chromosomes. They are often caused by errors in chromosome segregation during meiosis (aneuploidy) or changes in the entire set of chromosomes (polyploidy).

• Down’s Syndrome: Caused by trisomy of chromosome 21 (an extra copy), resulting in an individual with a small round head, broad face, furrowed tongue, partially open mouth, characteristic palm crease, and intellectual disability.
• Klinefelter’s Syndrome: Occurs in males with an extra X chromosome (XXY), leading to overall masculine development but also some feminine traits (e.g., breast development) and sterility.
• Turner’s Syndrome: Affects females who are missing one X chromosome (XO). These individuals are sterile, have underdeveloped ovaries, and lack secondary sexual characteristics.

Karyotype analysis is used to diagnose these chromosomal disorders.

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