Principles of Inheritance and Variation

Learning Outcomes:

1. Understanding Mendel’s Laws and their application in genetic inheritance.
2. Grasping the principles of inheritance for single and double gene traits.
3. Differentiating between types of genetic inheritance, including co-dominance and incomplete dominance.
4. Comprehending the mechanism of sex determination in various organisms.
5. Recognizing the causes and effects of mutations and genetic disorders.

The study of genetics provides scientific answers to several intriguing questions about inheritance and variation. Questions like why offspring resemble their parents but may also show differences, or how particular species reproduce within their kind, are addressed in genetics, a field that examines how traits are passed from parents to offspring (inheritance) and the differences among individuals (variation). The understanding of these processes has been refined since ancient times, notably with selective breeding by humans, but the scientific basis was not fully developed until Gregor Mendel.

Mendel’s Laws of Inheritance

Mendel’s work in the mid-19th century on pea plants laid the foundation for understanding inheritance patterns. His seven-year-long experiments demonstrated statistical analysis in biological phenomena for the first time. He observed contrasting traits like tall vs dwarf plants or yellow vs green seeds, systematically explaining how characters were inherited.

Key findings from Mendel’s experiments:

1. True Breeding Lines: Mendel worked with several generations of true breeding pea plants (plants that consistently produce offspring with the same traits).
2. Contrasting Traits: Mendel chose pairs of plants with contrasting traits for his experiments. Examples include smooth or wrinkled seeds and yellow or green pods.
3. Laws of Inheritance: He formulated principles later termed Mendel’s Laws, including:
   • Law of Dominance: Traits are controlled by discrete units (now called genes). For any trait, one form of the gene is dominant, and the other is recessive.
   • Law of Segregation: During gamete formation, alleles for a trait separate so that each gamete carries only one allele.

Important Notes:

Mendel’s Law of Dominance asserts that in heterozygous conditions, the dominant trait is always expressed, whereas the recessive trait remains unexpressed unless in a homozygous condition.

Inheritance of One Gene

Mendel’s monohybrid cross experiments involved studying a single trait. For example, he crossed tall and dwarf pea plants. In the F1 generation, all plants were tall, showing the dominance of the tall trait. Upon self-pollination of the F1 plants, the F2 generation resulted in a 3:1 ratio of tall to dwarf plants.

1. F1 Generation: The dominant trait is expressed.
2. F2 Generation: Both dominant and recessive traits reappear, maintaining the 3:1 phenotypic ratio.
3. Genes and Alleles: The unit of inheritance is the gene, with different forms known as alleles. For example, the tall trait is represented as T, and the dwarf trait as t.

Punnett Square: A tool to predict the probability of offspring genotypes. For a monohybrid cross between TT (tall) and tt (dwarf), the Punnett square predicts that the F1 offspring will all be Tt (tall).

Inheritance of Two Genes

Mendel expanded his experiments to study dihybrid crosses, where two traits are considered simultaneously. An example includes crossing plants with yellow, round seeds with plants having green, wrinkled seeds. The results showed the traits segregating independently of each other, leading to the formulation of the Law of Independent Assortment.

1. Dihybrid Cross: Cross between individuals with two different traits.
2. Independent Assortment: Traits are inherited independently, as demonstrated in the 9:3:3:1 ratio in the F2 generation, where combinations like yellow, round or green, wrinkled seeds appear.

Important Concepts:

Law of Independent Assortment states that the segregation of one gene pair is independent of another gene pair during gamete formation.

Incomplete Dominance and Co-dominance

Not all traits follow strict dominance. In some cases, incomplete dominance or co-dominance occurs:

1. Incomplete Dominance: In cases like the flower color in snapdragons, crossing red (RR) and white (rr) flowers results in pink (Rr) flowers. This shows that neither allele is completely dominant.
2. Co-dominance: In human ABO blood grouping, IA and IB alleles are co-dominant, meaning that both are equally expressed when present together (as in AB blood type).

Sex Determination

Sex determination mechanisms differ across species. For instance, humans follow the XY system, where males carry one X and one Y chromosome, and females have two X chromosomes. Fertilization involving an X or Y-bearing sperm determines the sex of the offspring.

1. Male Heterogamety: Seen in humans, where males produce X and Y gametes.
2. Female Heterogamety: Seen in birds, where males are ZZ and females are ZW.

Important Notes:

In humans, the sperm determines the sex of the offspring. The probability of having a male or female child is always 50%.

Mutation

Mutations are changes in DNA sequences, leading to alterations in the genotype and phenotype. These can be point mutations, involving single base changes, or more significant chromosomal mutations, where entire sections of DNA are added, deleted, or rearranged.

1. Point Mutations: Single base pair changes, such as in sickle-cell anemia, where a substitution in the beta-globin gene causes red blood cells to form a sickle shape under low oxygen conditions.
2. Chromosomal Aberrations: Larger mutations like deletions or duplications, often seen in cancer cells.

Genetic Disorders

Mendelian disorders arise from mutations in single genes, while chromosomal disorders involve changes in chromosome number or structure.

1. Mendelian Disorders:

• Haemophilia: A sex-linked recessive disorder affecting blood clotting.
• Sickle-Cell Anemia: An autosomal recessive condition caused by a mutation in the hemoglobin gene.
• Thalassemia: A disorder caused by reduced hemoglobin production.

2. Chromosomal Disorders:

• Down’s Syndrome: Caused by trisomy of chromosome 21, leading to developmental delays.
• Turner’s Syndrome: Affects females with a single X chromosome (XO), resulting in sterility.
• Klinefelter’s Syndrome: Males with an extra X chromosome (XXY), exhibiting mixed physical traits.

Pedigree Analysis

Pedigree analysis is crucial for tracing the inheritance of genetic traits through generations. This is particularly useful in tracking Mendelian disorders and understanding how traits are passed within families. It helps in predicting whether a trait is dominant, recessive, or sex-linked.

Example:

• Haemophilia: A sex-linked recessive disorder traceable through family pedigrees.

Important Notes:

Pedigree analysis provides insights into whether a trait is autosomal or sex-linked, and whether it is dominant or recessive.

Chromosomal Theory of Inheritance

The chromosomal theory of inheritance, proposed by Sutton and Boveri, confirmed that genes are located on chromosomes. The behavior of chromosomes during meiosis mirrors Mendel’s observations on segregation and independent assortment of genes. This theory was crucial in linking Mendel’s abstract genetic units with physical structures.

Key points:

1. Chromosomes and Genes: Both occur in pairs and segregate during gamete formation.
2. Experimental Verification: Thomas Hunt Morgan’s experiments with Drosophila (fruit flies) confirmed that genes located on the same chromosome exhibit linkage. The discovery of recombination further explained how new gene combinations are generated during sexual reproduction.

MCQ:

What is the genetic disorder caused by trisomy of chromosome 21?

• A. Turner’s Syndrome
• B. Down’s Syndrome
• C. Klinefelter’s Syndrome
  Answer: B. Down’s Syndrome

Through the study of **inheritance and variation

**, we understand how traits are transmitted across generations and the complexities involved, from simple Mendelian genetics to more intricate chromosomal behaviors. The principles of inheritance not only apply to plants and animals but also provide insights into *human genetic disorders* and the broader implications of genetic variation.