MCAT Biology #17
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17 terms
Terms | Definitions |
|---|---|
 About how often do twins occur | about 1% of normal births |
| What proportion of twins are fraternal | two-thirds |
| dizygotic twins | fraternal twins, two separate oocytes and two separate sperm are involved |
| The frequency of dizygotic twins increases with... | age of the mother |
| What can influence the frequency of dizygotic twins | the age of the mother and heredity |
| monozygotic twins | identical twins. A single oocyte and sperm are involved |
| is there an age correlation between the rate of monozygotic twins? | nope |
| If a first pregnancy results in twins, how does that affect subsequent twinning? | it's about three to five times greater than in the normal population |
| what is the key determining factor in determining the frequency of twin births | the genotype of the mother |
| when/where do the two monozygotic twins form | within a single trophoblast at the blastocyst stage |
| What do monozygotic embryos have in common? | placenta and chorionic cavity |
| How do monozygotic embryos differ? | amniotic cavities |
| What are identical in monozygotic twins, dizygotic twins | phenotypes and genotypes; nothing |
| What traits do identical twins have in common | sex, blood type, physical features |
| In which type of twins are the the zygotes totally different genetically | dizygotic twins |
| How often do triplet births occur? | about once in 8000 pregnancies |
| What can triplet births result from | one zygote (identical), two zygotes (one independent infant and identical twins), three zygotes (three independent individuals) |
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