As you do the warm-up, take out your hw due today 

(Regents Packet #2)

Directions: Make a KWL chart in your notebook. Fill out the K and W column before the video. Fill out the L column during the video. 

Identical vs. Fraternal Twins (Short Reading)

Twins are two offspring born from the same pregnancy, but they are not always the same genetically. There are two main types of twins: identical twins and fraternal twins. The difference between them comes from how they are formed during early development.

Identical twins form when a single fertilized egg (one sperm and one egg) splits into two separate embryos. Because both embryos come from the same original cell, they have the same DNA and are genetically identical. This means they usually have the same eye color, hair color, and many physical traits. However, identical twins are not always exactly the same. One reason is a process called methylation, where small chemical groups are added to DNA. Methylation can turn certain genes “on” or “off” without changing the DNA sequence itself. Over time, differences in lifestyle, environment, and experiences can cause different patterns of methylation in each twin. This can lead to small differences in appearance, health, or behavior, even though their DNA is the same.

The placenta in identical twins can vary depending on when the original fertilized egg splits. If the split happens early, each twin may develop its own placenta. If the split happens later, the twins may share a single placenta. In some cases, they may even share the same amniotic sac. This is why identical twin pregnancies can differ from one another.

Fraternal twins, on the other hand, form when two different eggs are released by the mother and each is fertilized by a different sperm. This results in two embryos that are genetically different, just like regular siblings. Fraternal twins can have different eye colors, hair colors, and may even be different sexes. They share about 50% of their DNA, the same as any brother and sister.

Fraternal twins always develop with separate placentas and separate amniotic sacs because they come from two different fertilized eggs. Sometimes the placentas can grow close together and appear as one, but they are still separate structures.

In summary, identical twins come from one fertilized egg and share the same DNA, but can develop differences due to methylation and environmental factors, and may or may not share a placenta. Fraternal twins come from two different fertilized eggs, are genetically unique, and always have separate placentas.