Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain sex-linked inheritance with examples and solve genetic crosses.
• Describe autosomal linkage and the effect of crossing over.
• Predict phenotypic ratios for crosses with linked genes.
• Define mutation and distinguish between gene and chromosomal mutations.
• Explain how mutations can lead to genetic disorders.

Lesson summary

Genetics and inheritance are fundamental to understanding life itself. They explain why we resemble our parents, how diseases are passed down through generations, and how species evolve over time. Understanding genetics is crucial in South Africa for addressing health challenges like inherited diseases and HIV susceptibility, improving agricultural practices through selective breeding of crops and livestock, and appreciating the incredible diversity of our indigenous flora and fauna.

Furthermore, ethical considerations surrounding genetic testing and modification are becoming increasingly relevant in our society.

Lesson notes

2.1 Sex-Linked Inheritance: Definition: Sex-linked inheritance refers to the inheritance of genes located on the sex chromosomes (X and Y in humans). Most sex-linked traits are located on the X chromosome because it is larger and carries more genes than the Y chromosome.

X-linked traits: Traits controlled by genes on the X chromosome are called X-linked traits. Females (XX) have two copies of each X-linked gene, while males (XY) have only one. This difference leads to different inheritance patterns in males and females.

Examples: Common examples of X-linked recessive traits include haemophilia (bleeding disorder) and red-green colour blindness. These conditions are more common in males because they only need to inherit one copy of the recessive allele to express the trait, whereas females need to inherit two copies.

Inheritance Pattern: A male inherits his X chromosome from his mother. A female inherits one X chromosome from her mother and one from her father. A mother who is a carrier (heterozygous) for an X-linked recessive trait has a 50% chance of passing the allele to her sons, who will then express the trait. Her daughters have a 50% chance of becoming carriers. An affected father will pass the X-linked allele to all of his daughters but none of his sons (because sons inherit the Y chromosome from their father).