Lesson 12: Sex-linked Inheritance and Extra-nuclear Inheritance

I. Sex-linked Inheritance

1. Definition

Sex-linked inheritance refers to the inheritance pattern where genes responsible for a trait are located on sex chromosomes. Consequently, the expression of the trait depends on the sex of the individual.

2. Sex Chromosome Types

• XX – XO System:
  • Female: XX
  • Male: XO
  • Examples: Grasshoppers, crickets, butterflies
• XX – XY System:
  • Female: XX
  • Male: XY
  • Examples: Humans, fruit flies, birds, moths

3. Types of Sex-linked Inheritance

• Genes on the Non-homologous Region of the X Chromosome:
  • Example: Gene determining eye color in fruit flies (Morgan’s experiment)
• Genes on the Y Chromosome:
  • Example: Gene determining the presence of ear tufts in humans

4. Characteristics of Inheritance of Genes on Sex Chromosomes

• Genes on the Non-homologous Region of the X Chromosome:
  • Criss-cross inheritance: Sons inherit the X chromosome from their mother, daughters inherit an X chromosome from both their mother and father.
  • Different expression in the two sexes: Sons receive only the maternal allele, making them more susceptible to expressing recessive traits compared to daughters.
• Genes on the Y Chromosome:
  • Only expressed in the sex carrying the Y chromosome (usually males).
  • Direct inheritance: Sons inherit the Y chromosome from their father.

5. Determining if a Trait is Determined by Genes on Autosomes or Sex Chromosomes

• Distribution of the trait in the two sexes: If the trait is distributed evenly in both sexes, the gene determining the trait is likely located on an autosome. Conversely, if the trait distribution is uneven between sexes, the gene determining the trait might be located on a sex chromosome.
• Reciprocal crosses: Reciprocal crosses involve crossing a pure-breeding female with a pure-breeding male and vice versa. If the results of reciprocal crosses differ, the gene determining the trait is likely located on a sex chromosome.

6. Notes

• Genes located on the homologous region of X and Y chromosomes follow inheritance patterns similar to genes on autosomes.
• Genes on sex chromosomes can lead to genetic disorders. Examples include red-green color blindness and hemophilia in humans, caused by recessive genes on the X chromosome.

II. Extra-nuclear Inheritance

1. Definition

Extra-nuclear inheritance refers to the inheritance of genes located in the cytoplasm, primarily within mitochondria and chloroplasts.

2. Characteristics of Extra-nuclear Inheritance

• Maternal inheritance: Offspring always inherit their entire cytoplasm from their mother.
• Does not follow Mendel’s laws of inheritance.
• Susceptible to gene mutations and changes in the structure of DNA within the cytoplasm.

3. Examples of Extra-nuclear Inheritance

• Flower color in four o’clock plants: Flower color is determined by genes in the chloroplast.
• Huntington’s disease: Caused by a gene mutation in mitochondria.

4. Notes

• Extra-nuclear inheritance is a unique inheritance pattern independent of the sex of the individual.
• Genes within mitochondria and chloroplasts often play roles in respiration and photosynthesis.

III. Exercises

1. Organisms with XX sex chromosomes in females and XO in males.

• Examples: Grasshoppers, crickets, butterflies

2. Organisms with XX sex chromosomes in males and XY in females.

• Examples: Birds, moths

3. The gene determining eye color in fruit flies in Morgan’s experiment is located on:

• The non-homologous region of the X chromosome

4. In humans, the presence of ear tufts is determined by:

• A gene on the Y chromosome

5. In humans, a disorder caused by a gene on the Y chromosome:

• Syndactyly (fusion of fingers), although this disorder is extremely rare.

6. Traits determined by dominant or recessive genes on the non-homologous region of the Y chromosome are characterized by:

• Expression only in individuals with a Y chromosome

7. How to determine if a human disorder is caused by a recessive gene on an autosome or a sex chromosome:

• Observe the distribution of the disorder in both sexes. If the disorder is equally distributed, the gene is likely located on an autosome. Conversely, if the disorder appears only in one sex or with different frequencies in the two sexes, the gene might be located on a sex chromosome.

8. The cross that helps confirm if a gene determining a particular trait is located on an autosome or a sex chromosome:

• Reciprocal crosses

9. In a population with X and Y sex chromosomes, a locus with only two alleles but producing five different genotypes is located on:

• The non-homologous region of the X chromosome

10. Which characteristic exemplifies the inheritance pattern of extra-nuclear genes:

• Traits are always maternally inherited

11. In eukaryotic organisms, genes located in the cytoplasm are:

• Primarily transmitted from mother to offspring through the cytoplasm of the egg.

Note:

• The above information provides a basic understanding of sex-linked inheritance and extra-nuclear inheritance. For a deeper understanding, consult textbooks, specialized literature, or reliable websites.
• Sex-linked inheritance and extra-nuclear inheritance are complex phenomena with many exceptions, requiring further research to fully elucidate.
• Sex-linked inheritance plays a significant role in explaining the biodiversity of various species.
• Extra-nuclear inheritance influences human health and development.
• Researching sex-linked inheritance and extra-nuclear inheritance could provide valuable insights into genetic disorders and lead to more effective treatments.