1. Explain the concept of sex-linked inheritance and its significance in genetics.
Answer:
Sex-linked inheritance refers to the transmission of genes located on the sex chromosomes (X and Y chromosomes). Unlike autosomal genes, the inheritance patterns of sex-linked traits differ between males and females due to the difference in their sex chromosome composition (XX in females and XY in males). For X-linked traits, males are more likely to express recessive disorders since they have only one X chromosome. Examples include hemophilia and color blindness. This concept helps in understanding genetic disorders, their inheritance patterns, and their impact on different sexes.
2. Describe the genetic basis of hemophilia.
Answer:
Hemophilia is an X-linked recessive disorder caused by mutations in genes responsible for producing clotting factors. Hemophilia A results from a deficiency of clotting factor VIII, while Hemophilia B (Christmas disease) is due to a deficiency of factor IX. Since these genes are located on the X chromosome, males (XY) are more likely to inherit the disorder from their carrier mothers, as they lack a second X chromosome to compensate. Females (XX) are typically carriers unless they inherit two defective alleles.
3. What are the different types of hemophilia, and how are they diagnosed?
Answer:
The main types of hemophilia are:
- Hemophilia A: Caused by a deficiency of factor VIII.
- Hemophilia B: Caused by a deficiency of factor IX.
- Hemophilia C: A rarer form caused by a deficiency of factor XI.
Diagnosis involves:
- Family history analysis.
- Coagulation tests to measure clotting factor activity.
- Genetic testing to confirm mutations in the respective genes.
4. Explain why hemophilia is more common in males than females.
Answer:
Hemophilia is more common in males because it is an X-linked recessive disorder. Males have only one X chromosome (XY). If the X chromosome carries the defective gene, there is no second X chromosome to compensate. Females (XX), on the other hand, require two defective X chromosomes to express the condition, which is rare. Most females with one defective gene are carriers and do not show symptoms.
5. Describe the role of the clotting factors involved in hemophilia.
Answer:
Clotting factors are proteins essential for blood coagulation. In hemophilia:
- Factor VIII (in Hemophilia A) and Factor IX (in Hemophilia B) play a crucial role in the clotting cascade, helping convert prothrombin to thrombin, which forms fibrin clots.
- Deficiency in these factors leads to prolonged bleeding, as the clotting process is incomplete or delayed.
6. What is color blindness, and what are its types?
Answer:
Color blindness is an X-linked recessive disorder affecting the ability to perceive colors correctly. It primarily affects the cones in the retina. Types include:
- Red-Green Color Blindness: Most common, difficulty distinguishing red and green.
- Blue-Yellow Color Blindness: Rare, difficulty distinguishing blue and yellow.
- Complete Color Blindness (Achromatopsia): Inability to see any color.
7. Explain the genetic mechanism of red-green color blindness.
Answer:
Red-green color blindness occurs due to mutations in the opsin genes located on the X chromosome. These genes code for light-sensitive proteins in cone cells. In males, a single defective X chromosome results in the disorder. Females require two defective X chromosomes, making them less frequently affected.
8. How can a carrier mother transmit color blindness to her offspring?
Answer:
A carrier mother (XN^{N}NXCB^{CB}CB) has a 50% chance of transmitting the defective X chromosome to her offspring.
- If the defective X is passed to a son (XY), he will be color-blind.
- If passed to a daughter (XX), she will become a carrier.
9. Why are males more affected by red-green color blindness than females?
Answer:
Males (XY) have only one X chromosome. If it carries the defective gene, they express the disorder. Females (XX) need two defective X chromosomes to express the disorder, which is rare. Most females with one defective gene are carriers.
10. Compare and contrast hemophilia and color blindness as examples of X-linked disorders.
Answer:
| Feature | Hemophilia | Color Blindness |
|---|---|---|
| Cause | Deficiency in clotting factors | Defective cone cell proteins |
| Symptoms | Excessive bleeding | Difficulty distinguishing colors |
| Severity | Potentially life-threatening | Non-life-threatening |
| Affected Gender | Predominantly males | Predominantly males |
11. Discuss the inheritance pattern of hemophilia using a Punnett square.
Answer:
If a carrier mother (XH^{H}HXh^{h}h) mates with a normal father (XH^{H}HY):
| XH^{H}H | Y | |
|---|---|---|
| XH^{H}H | XH^{H}HXH^{H}H (Normal Daughter) | XH^{H}HY (Normal Son) |
| Xh^{h}h | XH^{H}HXh^{h}h (Carrier Daughter) | Xh^{h}hY (Hemophilic Son) |
12. What are the symptoms and complications of hemophilia?
Answer:
Symptoms:
- Prolonged bleeding from cuts or injuries.
- Spontaneous bleeding in joints and muscles.
- Bruising easily.
Complications:
- Joint damage due to recurrent bleeding.
- Internal bleeding causing organ damage.
- Increased risk of infections from blood transfusions.
13. How is hemophilia managed or treated?
Answer:
- Regular replacement therapy with clotting factors.
- Desmopressin for mild Hemophilia A.
- Gene therapy to correct defective genes.
- Preventative measures like avoiding injury and using protective gear.
14. Explain the role of the Ishihara test in diagnosing color blindness.
Answer:
The Ishihara test consists of a series of plates with colored dots forming numbers or patterns. Individuals with normal vision can identify the numbers, while those with red-green color blindness cannot. It helps diagnose the severity and type of color blindness.
15. What challenges do people with color blindness face in daily life?
Answer:
- Difficulty distinguishing traffic lights and signs.
- Challenges in careers requiring color discrimination (e.g., pilots, electricians).
- Difficulty in matching colors for clothing or design.
16. What is the significance of genetic counseling for X-linked disorders?
Answer:
Genetic counseling helps families understand the inheritance risks of X-linked disorders like hemophilia and color blindness. It provides guidance on:
- Carrier testing.
- Prenatal diagnosis.
- Family planning decisions.
17. Describe the historical significance of hemophilia in European royal families.
Answer:
Hemophilia was called the “royal disease” due to its prevalence in Queen Victoria’s descendants. Her gene mutation spread through European royal families, highlighting the impact of genetic inheritance and intermarriages.
18. How can modern genetic technologies help in managing X-linked disorders?
Answer:
- CRISPR Gene Editing: Potential to correct defective genes.
- Preimplantation Genetic Diagnosis (PGD): Ensures embryos free of disorders.
- Gene Therapy: Introducing functional genes to compensate for defective ones.
19. Explain the evolutionary advantage of carriers of X-linked disorders like color blindness.
Answer:
Carriers of color blindness may have better night vision or enhanced ability to detect camouflaged objects, providing an evolutionary advantage in certain environments.
20. Discuss the ethical considerations in genetic testing for sex-linked disorders.
Answer:
- Ensuring confidentiality of genetic data.
- Avoiding discrimination in employment or insurance.
- Balancing the benefits of prenatal diagnosis with the risks of selective termination.
