Twins - Genetics

1. How do the two types of twins in humans arise? - Three timeframes in gestation.

• Monozygotic (MZ) twins: Arise from a single fertilized egg that splits into two embryos. This typically occurs during the first week of gestation.

• Dizygotic (DZ) twins: Arise from two separate eggs fertilized by two separate sperm. This happens when two eggs are released and fertilized in the same menstrual cycle, occurring during the early first trimester.

2. Which type, MZ or DZ, is probably more genetically similar?

• Monozygotic (MZ) twins are more genetically similar because they come from a single fertilized egg, so they share 100% of their genes.

3. What percentage of genes does each of the two types of twins in humans share?

• MZ (Monozygotic) twins: 100% of their genes are shared.

• DZ (Dizygotic) twins: 50% of their genes are shared, just like regular siblings.

4. How can we use comparisons of concordance between MZ and DZ twins to determine the extent to which expression of a trait is influenced by genetics or environment?

• Concordance: The percentage of twins both showing the trait.

• Formula:

  • Heritability estimate = 2 × (concordance rate in MZ twins) − (concordance rate in DZ twins).

  • If MZ twins show higher concordance than DZ twins, this suggests a genetic influence. If both show similar concordance, the environment may play a larger role.

5. How are adoption studies useful to determine the effects of environment or genetics in the study of human characteristics?

• Adoption studies compare traits between biological parents, adoptive parents, and adopted children.

  • Genetic influence: If adopted children resemble their biological parents more than their adoptive parents.

  • Environmental influence: If adopted children resemble their adoptive parents more.

6. Compare the advantages and disadvantages of amniocentesis and chorionic villus sampling (CVS). At what time can each of these be done (and get results) during pregnancy?

• Amniocentesis:

  • Advantages: Can be done later in pregnancy (15-20 weeks), provides a more reliable sample, lower risk of miscarriage.

  • Disadvantages: Results take longer (2-3 weeks), risk of infection or miscarriage.

• CVS:

  • Advantages: Can be done earlier (10-13 weeks), provides faster results.

  • Disadvantages: Slightly higher risk of miscarriage, sample may not be as reliable as amniocentesis.

7. What activities does the GINA (Genetic Information Nondiscrimination Act) prohibit?

GINA prohibits:

1. Discrimination by employers based on genetic information.

2. Discrimination by health insurance providers based on genetic information. It does not cover life insurance, disability insurance, or long-term care insurance.

8. How did you interpret these data?

Given the concordance data:

• Right- or left-handedness: Both MZ (80%) and DZ (75%) twins have high concordance, suggesting that environmental factors may influence handedness but genetics also play a role.

• Manic depression: Higher concordance in MZ twins (80%) compared to DZ twins (20%), indicating a strong genetic influence on this condition.

9. If you were a genetic counselor, why would a pedigree be useful to determine risk?

• A pedigree shows the inheritance patterns of traits across generations, helping to identify genetic risks. By analyzing a family history, you can predict the likelihood of inherited conditions and counsel family members on potential genetic risks.

10. How does genetic anticipation in certain disorders modify expectations in a pedigree?

• Genetic anticipation occurs when a genetic disorder presents earlier in life or with increased severity in successive generations. This is common in trinucleotide repeat disorders, like Huntington’s disease, where the number of repeats increases in each generation.

11. What is the GINA of 2008? What does it cover, and not cover?

• GINA (2008) is a U.S. federal law that protects individuals from genetic discrimination.

  • Covers: Employers and health insurance providers.

  • Does not cover: Life, disability, or long-term care insurance.

12. How does epigenetics change our view of 'identical twins'?

• Epigenetics shows that while identical twins have the same genetic sequence, differences in gene expression (due to environmental factors, lifestyle, etc.) can lead to differences in traits, behaviors, and health outcomes.

13. What do Trinucleotide Repeat (Triplet) Repeat Disorders look like in a pedigree?

• Trinucleotide Repeat Disorders: Disorders where a sequence of three nucleotides is repeated excessively (e.g., Huntington’s disease).

• Pedigree: The trait often shows genetic anticipation, where the symptoms become more severe or appear earlier in successive generations.

14. What is Genetic Anticipation?

• Genetic Anticipation refers to the phenomenon where a genetic disorder becomes more severe or manifests at an earlier age in each successive generation.

15. How would this be different in Identical vs. Fraternal Twins?

• Identical Twins: Since they share 100% of their genes, any differences due to anticipation are likely to be observed due to epigenetic factors (such as environmental influences).

• Fraternal Twins: Share 50% of their genes, so differences in anticipation might be less pronounced, but environmental factors can still cause variable expression.