6.1.6 Genetic Inheritance

In this lesson, we will explore the concepts and terms related to genetic inheritance and understand how different alleles and genes influence the phenotype of an organism.

Terms

• Gamete: A gamete is a reproductive cell (sperm or egg) that carries half the genetic information of an organism.
• ChromosomeA structure made of DNA that carries genetic information.: Chromosomes are structures made up of DNA and proteins. They contain genes and are found in the nucleusA membrane-bound organelle in eukaryotic cells that contains DNA. of cells.
• Gene: A gene is a segment of DNA that contains instructions for the production of a specific protein or RNA molecule, which contributes to a particular trait.
• Allele: Alleles are different forms or variants of a gene. They occupy the same position (locus) on homologous chromosomes.
• Dominant: A dominant allele is always expressed in the phenotype, even if only one copy is present.
• Recessive: A recessive allele is only expressed in the phenotype if two copies are present, with no dominant allele present.
• Homozygous: Homozygous refers to having two identical alleles for a particular gene.
• Heterozygous: Heterozygous refers to having two different alleles for a particular gene.
• Genotype: Genotype refers to the genetic makeup of an organism, representing the alleles present for a specific gene.
• Phenotype: Phenotype refers to the observable physical or biochemical characteristics of an organism, influenced by its genotype.

Single Gene Characteristics

• Fur Colour in Mice: The fur colour in mice is an example of a characteristic controlled by a single gene. Different alleles of the gene determine the colour variation observed.
• Red-Green Colour Blindness in Humans: Red-green colour blindness is another example of a characteristic controlled by a single gene. Individuals with different alleles may exhibit varying degrees of colour vision impairment.

Dominant and Recessive Alleles

• Dominant Allele: A dominant allele is always expressed in the phenotype, even if only one copy is present.
• Recessive Allele: A recessive allele is expressed in the phenotype only if two copies are present, with no dominant allele present.

Homozygous and Heterozygous

• Homozygous: If an organism has two identical alleles for a specific gene, it is said to be homozygous for that trait.
• Heterozygous: If an organism has two different alleles for a specific gene, it is said to be heterozygous.

Multiple Gene Characteristics

Most characteristics are influenced by the interaction of multiple genes, rather than a single gene. This interaction can lead to a wide range of phenotypic variations.

Probability in Single Gene Crosses

• Mendelian Genetics: The principles of genetic inheritance were first described by Gregor Mendel through his experiments with pea plants. Mendel's work showed that the inheritance of certain traits can be explained by the segregation and independent assortment of single genes.
• Punnett Square: A Punnett square is a simple tool used to predict the possible combinations of alleles and their probabilities in offspring from a single gene cross.
• Genotype and Phenotype Ratios: By using the Punnett square, we can determine the expected genotype and phenotype ratios in the offspring of a cross. These ratios represent the probability of specific genetic outcomes.

Conclusion

In genetic inheritance, some characteristics are controlled by a single gene, where different alleles produce variations. Dominant alleles are always expressed in the phenotype, while recessive alleles are expressed only in the absence of a dominant allele. Organisms can be homozygous (two identical alleles) or heterozygous (two different alleles) for a specific trait. Probability is also a fundamental concept in predicting the outcomes of single gene crosses. However, it is important to recognise that most phenotype features are the result of the interaction of multiple genes rather than single gene inheritance.