Introduction
Inheritance is a fundamental concept in biology that explains how traits are passed from parents to offspring. This topic is covered in detail in the International Baccalaureate (IB) syllabus and includes various subtopics such as unlinked genes, gene linkage, dihybrid crosses, identifying recombinants, and the chi-squared test. Understanding these concepts is crucial for grasping the principles of genetics and evolution.
10.2.1 Unlinked Genes
Definition and Explanation
Unlinked genes are genes located on different chromosomes or far apart on the same chromosome, such that they assort independently during meiosis. This independent assortment follows Mendel's Second Law, which states that the inheritance of one trait does not affect the inheritance of another.
Example
Consider two genes: one for seed shape (R for round, r for wrinkled) and one for seed color (Y for yellow, y for green). These genes are unlinked. A dihybrid cross between two heterozygous plants (RrYy) will produce offspring in a 9:3:3:1 phenotypic ratio.
NoteMendel's laws were derived from his experiments with pea plants, where he observed patterns in the inheritance of traits.
10.2.2 Skills: Analyzing Dihybrid Crosses
Definition and Explanation
A dihybrid cross involves two pairs of contrasting traits. When analyzing these crosses, a Punnett square is used to predict the genotypic and phenotypic ratios of the offspring.
Steps for Analyzing a Dihybrid Cross
- Identify Parental Genotypes: Determine the genotypes of the parents.
- Determine Gametes: Identify the possible gametes each parent can produce.
- Punnett Square: Use a 4x4 Punnett square to combine gametes and predict offspring ratios.
- Analyze Ratios: Determine the phenotypic and genotypic ratios from the Punnett square.
Example
A cross between a heterozygous long-winged, black-bodied fruit fly (VvBb) and a short-winged, brown-bodied fly (vvBB) will produce offspring in the following ratio:
$$ \begin{array}{c|c|c|c|c} & vB & vB & vB & vB \ \hline Vb & VvBb & VvBb & VvBb & VvBb \ vb & vvBb & vvBb & vvBb & vvBb \ \end{array} $$
ExamplePredict the phenotype ratio of offspring from the cross mentioned above. The expected phenotypic ratio is 1:1:1:1 for long-winged black-bodied, long-winged brown-bodied, short-winged black-bodied, and short-winged brown-bodied flies.
10.2.3 Gene Linkage
Definition and Explanation
Gene linkage refers to genes located close together on the same chromosome that tend to be inherited together. This phenomenon violates Mendel's Law of Independent Assortment.
Historical Context
Mendel's work initially suggested that traits assort independently. However, later experiments by Bateson, Punnett, and Morgan revealed exceptions to this rule, leading to the discovery of linked genes.
Example
In Drosophila melanogaster (fruit flies), the genes for body color and wing shape are linked. A cross involving these genes often produces offspring with parental phenotypes more frequently than recombinant phenotypes.
NoteLinked genes do not assort independently because they are located close to each other on the same chromosome.
10.2.4 Skills: Identifying Recombinants
Definition and Explanation
Recombinants are offspring with a combination of traits different from either parent, resulting from the crossing over during meiosis.
