Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain sex-linked inheritance
• Use a Punnett square for a sex-linked trait
• Describe different types of genetic mutations
• Understand the effects of mutations on phenotype

Lesson summary

This week, we delve deeper into the fascinating world of genetics and inheritance, building upon your existing knowledge of DNA, chromosomes, and basic inheritance patterns. Understanding genetics is crucial because it underpins so much of what we see around us, from the biodiversity of our South African flora and fauna to the genetic predispositions to certain diseases within our communities. Genetic knowledge informs medical advancements, agricultural practices, and even our understanding of human evolution. Specifically, we'll be focusing on sex-linked inheritance, genetic mutations, and genetic engineering.

Lesson notes

2.1 Sex-Linked Inheritance Sex Determination: In humans, sex is determined by the X and Y chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During meiosis, females produce eggs containing one X chromosome, while males produce sperm containing either an X or a Y chromosome. The sperm that fertilizes the egg determines the sex of the offspring.

X-Linked Genes: Genes located on the X chromosome are called X-linked genes. Since males have only one X chromosome, they are hemizygous for these genes. This means that if a male inherits a recessive allele for an X-linked gene, he will express that trait, even if the allele is recessive. Females, on the other hand, have two X chromosomes, so they can be heterozygous for X-linked genes. Inheritance Patterns of X-Linked Recessive Traits: Males are more likely to be affected by X-linked recessive disorders than females. Affected males inherit the recessive allele from their mothers. Carrier females (heterozygous) do not express the trait but can pass the recessive allele to their sons. Affected fathers will pass the affected X chromosome to all of their daughters, making them carriers (if the mother is unaffected). Examples of X-linked recessive disorders include hemophilia and red-green color blindness.

X-Linked Dominant Traits: Much rarer than X-linked recessive traits. Affected males pass the trait to all their daughters but none of their sons. Affected heterozygous females will pass the trait to half of their children (both sons and daughters).

Y-Linked Genes: Genes located on the Y chromosome are called Y-linked genes. These genes are only found in males and are passed directly from father to son. Relatively few genes are found on the Y chromosome, mostly involved in sex determination and male fertility.