# Presentation: 228055

## Twins - Type

- Twins are identical genetically.

**Monozygotic**

- Arise from the fertilization of two different eggs by two different sperm.
- Twins share, on average, one half of their genes in common.

**Dizygotic**

**Polar Body**

- Arise from the fertilization of an egg and a polar body by two different sperm.
- Twins share, on average, one half of their genes in common.

**Twins - Type**

## Twins can also arise as a result of:

**Superfetation**** –**
**Twins arise as the result of the fertilization of an egg during an ongoing pregnancy. This results in a set of  twins where the pair members are of different gestational age.**
**Superfecundation**** –**
**Twins arise as the result of the fertilization of two different eggs by sperm from two different fathers. Twins arising in this way are only related to each other as half-siblings****.**

## Approximate Twinning Rates in Different Populations Ranked by DZ Rate

- Population

- 5
- 5
- 4

- 40
- 22
- 12

- 4
- 3
- 4

- 9
- 9
- 7

- 5
- 5
- 5

- 6
- 3
- 3

- *Data are average values primarily from tables in Bulmer (1970).

## Factors influencing theFrequency of Twin Birth

**Maternal Age-**

**Exclusive to DZ twinning-**
**probably related to increase**
**in gonadotropin level (FSH)**

**Fertility Drugs-**

**Have resulted in an increase**
**in the frequency of DZ twins**
**as well as a smaller increase**
**in the frequency of MZ twins. **

**Genetic Factors -**

**DZ twinning runs in families- this has not generally been **
**the case for MZ twinning.**

## Methods Used in Assigning Twin Zygosity

**  ****Genetic Markers**
**  ****Number of Fetal Membranes**
**  ****Questionnaires**
**  ****Physical Similarities**
**  ****Weinberg’s Differential Rule**

## Genetic Markers

- The efficiency of a given genetic marker
- is dependent upon the number of alleles
- and their frequency in the population. The accuracy of zygosity assignment increases with the number of informative markers evaluated.

## Twin zygosity was determined using 6 DNA markers that included:

**D1S495 **
**D2D1363**
**D3S1261**
**D4S1625**
**D51473**
**D6S477**

**(0.87)**
**(0.72)**
**(0.85)**
**(0.76)**
**(0.83)**
**(0.90)**

**Using these markers the probability of **
**correct zygosity assignment was 0.999**

## Number of Fetal Membranes

- Since only MZ twins can be monochorionic, the presence of a single chorion (placenta) is proof of monozygosity. This can only be
- determined by examining a cross-section
- of placental membranes.

## Questionnaires

**Cost effective for large studies, however**
**is not as accurate as use of **
**genetic markers.**

**1) Alike as two peas in a pod**
**2) Often confused with each **
**     ****other**

## Physical Similarities

**Misclassification rate is relatively high**
**(13-23%)**

**1) Finger ridge counts and patterns**
**2) Height**
**3) Weight**
**4) Hair and eye color**

## Weinberg’s Differential Rule

**Used when it is only possible to obtain the**
**sex of the twin pair. Assuming that males **
**account for 50% of DZ twins and the sex**
**of both members of a DZ pair is independently determined,  then the number of MZ twins is estimated as the number of unlike-sexed twin pairs minus the number of same-sexed twin pairs and the number of DZ twin pairs is twice the number of unlike-sexed twin pairs.**

## Twins in Research

**Advantages**

**Matched for age, race**
**Extremely efficient for determining if genetic**
**      ****factors are important**

**Disadvantages**

**May not be the same as singletons**
**More difficult to find than singletons**

## Ascertainment Methods

**Advertisements**
**Clinic Population**
**Population-based **
**Twin Registries**

## Ascertainment Methods

**Advertisements**
**Clinic Population**
**Population-based **
**Twin Registries**

## Ascertainment Methods

**Advertisements**
**Clinic Population**
**Population-based **
**Twin Registries**

## The rationale for the use of twin studies to detect the existence of genetic influences is that monozygotic twins are identical  genetically and dizygotic twins share only half their genes in common, MZ co-twins should be more similar for traits that are genetically determined than are DZ co-twins.

**The rationale for the use of twin studies to detect the existence of genetic influences is that monozygotic twins are identical  genetically and dizygotic twins share only half their genes in common, MZ co-twins should be more similar for traits that are genetically determined than are DZ co-twins.**

## Twin Study Design

**1.**	**Classical twin study**
**2.****	****Twins separated at birth and reared    ****	****apart**
**3.****	****Placentation type twin analyses**
**4.****	****Partitioned twin analyses**
**5.****	****Extended twin kindred study**

## Analytical Methods Used in Twin Studies to Detect the Existence of Genetic Effects

**Quantitative Traits**

- Correlation Analyses
- Paired Analyses of Variance

**Qualitative Traits**

- Tetrachoric Correlation Analyses
- Concordance Rate Analyses

## Pairwise Concordance Rate:

**[1/2(c+w)/[(1/2(c+w))+d)]**

## Probandwise Concordance Rate:

**[ 2c  + c  ] / [ 2c  +c  +d ]**

- 2

- 2

- 1

- 1