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# Introduction to Genetics ## Chapter 17 ### Page 9 of 12 **In the Development of Male-Specific Characteristics** - The vast majority of genes on the X chromosome are not matched on the Y. - Males have only one copy of most genes on their sex chromosomes. - Traits coded for on the section of the...

# Introduction to Genetics ## Chapter 17 ### Page 9 of 12 **In the Development of Male-Specific Characteristics** - The vast majority of genes on the X chromosome are not matched on the Y. - Males have only one copy of most genes on their sex chromosomes. - Traits coded for on the section of the X chromosome that has no corresponding material on the Y are said to be sex-linked. - The gene that codes for normal colour vision is one example and is therefore carried on X chromosomes only. - This gene is dominant, while a recessive form codes for red-green colour blindness. ### Gender Inheritance Patterns - A female inherits a faulty copy of the gene and is statistically likely to have a normal gene on her other X chromosome, giving normal colour vision. - A female carrying the colour blindness gene may pass the faulty gene to her children and is said to be a carrier. - If the gene is abnormal in a male, he will be colour-blind because he has only one copy of the gene. **Inheritance of Colour Blindness Is Shown in Fig. 17.11:** - Illustrates the possible genetic combinations of the children of a carrier mother (one normal gene and one faulty gene). **Example Outcomes:** - 50% chance of a son being colour-blind. - 50% chance of a son having normal vision. - 50% chance of a daughter being a carrier and a 50% chance of a daughter being normal. ### Spot Check Questions 1. If the father is homozygous for the tongue-rolling gene and the mother is heterozygous, what is the chance that their child will be able to roll their tongue? 2. If a child's parents are both blood group O, what is the child's chance of being blood group A? ### Key Symbols - **X**: Normal gene on X chromosome - **X'**: Abnormal colour blindness gene on X chromosome - **Y**: Normal Y chromosome (no gene present) ### Figure 17.11 **Inheritance of Sex-Linked Red-Green Colour Blindness Gene Between Generations** **Ageing and Genetics** #### Learning Outcomes After studying this section, you should be able to: - Describe the main effects of ageing on the genetic material of cells. - Outline the genetic mechanisms of senescence. ### Ageing and DNA - Cumulative exposure over a lifetime to potential mutagens, as well as a diminishing ability to repair DNA, causes the cell’s genome to progressively accumulate mutations, increasing disease risk (e.g., cancer). - Mitochondrial DNA is more prone to mutations than nuclear DNA and as it ages, develops 'wear and tear' damage, causing progressive impairment of cell function. ### Cell Senescence (Ageing) - The number of times a cell can divide is somewhere between 50 and 60 divisions. - This is thought to relate to the effects of ageing on telomerase function. - Telomerase repairs telomeres (chromosome tips) following DNA replication. - Declines in function restrict the number of cell replications possible, as chromosomes become progressively shorter and the cell can no longer divide.