Phenomenon
Sometimes family members resemble each other, sometimes they don't. This is particularly interesting in the case of biological siblings, who get their DNA from the same two parents.
Question
Why are siblings sometimes so much alike and other times so different? How is parental DNA distributed to offspring?
Model Ideas
- Sexually reproducing organisms have two chromosomes of each type, one from each parent.
- Parents contribute one chromosome of each pair to each offspring.
- Which chromosome is given from a pair is random.
- Traits are usually inherited independently from one other
- The number of possible chromosome combinations is 2n (n = # of homologous pairs for that species). In humans: 223 = >8 million possible combinations.
- Crossing over further increases variation.
Overview
Transition in: We know that offspring resemble their parents, but we are wondering how that works.
The phenomenon that motivates this model is the “Black and White Twins” —two girls that look very different and claim to be twin sisters. Is this possible? After presenting the phenomenon students will engage with the pipe cleaner activity where they reason how many chromosomes each parent must give to their offspring in order for the offspring to have the correct number of chromosomes. Students will work to explain the reason offspring have information from both parents and share some of that information with siblings. The approach stresses the underlying importance of the process – the need to make sex cells that contain half the information of a fertilized egg (23 chromosomes compared to 46) and that there must be one chromosome from each pair to have a complete set of information after fertilization.
Transition out: Now that we understand how the information is passed and distributed from parents to offspring, we wonder how that information controls our traits?
We are presented with a phenomenon of twin sisters from the same biological parents, who look nothing alike. We ask, are they really twins?
We observed a phenomenon of twin sisters who look quite different from one another and decided on a new driving question.
To help us make sense of this curious phenomenon of fraternal twins looking so different, we ask: How exactly does the heritable information get from the parents to the offspring? We then collect initial model ideas.
We posted our initial ideas for how traits are passed from parents to offspring.
Now that we have some idea that DNA (chromosomes) are involved with helping to pass on information, we use a pipe cleaner activity to figure out how to make gametes and a viable baby.
We made a baby! We figured out how to make gametes and combine them to make a viable baby. We recorded our “rules” for making a baby and now have an almost complete working model for Gamete Formation.
We use our model to help us reason about why meiosis is so important.
We figured out that sexual reproduction does require special haploid cells so that when the male and female gametes come together, they make an organism with a complete (diploid) set of chromosomes. We also have a better understanding that sexual reproduction helps with variation.
We now use our model to figure out how gamete formation increases variation through random assortment and crossing over.
Meiosis contributes to variation in several ways. It provides gametes for sexual reproduction. During the formation of gametes, there is a random assortment of chromosomes and at one-point pairs of chromosomes swap sections (crossing over).
We use our model to answer and discuss the challenge questions: In general, who has the possibility of sharing the most genetic traits: Parent/ Child or Sibling/Sibling?
A parent and sibling can only ever share ½ of their genetic traits, however, siblings have the possibility to share almost all or almost none of their traits.
Assessment. We now use our model to explain the Lucy and Maria phenomenon.
We have used our model for gamete formation to explain, in detail, how twins can look nothing alike and how non-twin sibling can look so much alike.
We talk about race and whether there is a biological basis to the concept of race.
Through a reading and guided discussion, we figured out that there is no scientific and genetic basis to the concept of race in the human species and that all human beings share 99.9% of their DNA due to common ancestry.
Download Resources
| Attachment | Size |
|---|---|
| All MBER-LE Biosynthesis Materials (Download) | 23.54 MB |