Segregation and Independent Assortment Ensure Genetic Diversity
- During meiosis, genetic material is shuffled and distributed to create genetically unique gametes (sex cells).
- Two key principles: segregation and independent assortment, explain how genes are passed from parents to offspring and how genetic diversity is generated.
Segregation Ensures Each Gamete Receives One Allele
Definition
Segregation
Segregation is the process where the two alleles of a gene separate during gamete formation, ensuring each gamete receives only one allele.
- How segregation works:
- Metaphase I: Homologous chromosome pairs (one pink chromosome with alleles R and Y, one blue chromosome with alleles r and y) line up at the cell's equator.
- Anaphase I: Homologous chromosomes are pulled to opposite poles of the cell.
- Result: Each gamete receives one chromosome from each pair, and therefore one allele for each gene.
Note
- Referring back to the diagram, in Metaphase I, you can see the pink chromosome (carrying R and Y) paired with the blue chromosome (carrying r and y).
- After separation in Metaphase II, each gamete receives either the pink chromosome OR the blue chromosome, never both.
Example
- For a person with blood type AB ($I^A I^B$):
- Half the gametes will carry the $I^A$ allele.
- The other half will carry the $I^B$ allele.
Independent Assortment Creates Unique Combinations of Traits
Definition
Independent assortment
Independent assortment refers to how alleles of different genes are distributed into gametes independently of one another.
- This principle applies to unlinked genes, which are located on different chromosomes or far apart on the same chromosome.
- How independent assortment works in meiosis:
- During Metaphase I, homologous chromosome pairs line up randomly at the cell's equator.
- Chromosome Arrangement 1 (left side of diagram): The pink R-Y chromosome aligns on one side, and the blue r-y chromosome aligns on the other side.
- Chromosome Arrangement 2 (right side of diagram): The chromosomes align in the opposite orientation—the blue r-y chromosome aligns where the pink was, and vice versa.
Analogy
- Think of independent assortment like flipping two coins.
- The outcome of one coin flip doesn't affect the other.
- Similarly, the inheritance of one gene doesn't influence the inheritance of another unlinked gene.
Example
- For a person with genotype $I^A H^A I^B H^S$:
- Gametes can have any of these combinations:
- $I^A H^A$
- $I^A H^S$
- $I^B H^A$
- $I^B H^S$
Connecting Meiosis to Dihybrid Crosses
- Dihybrid crosses examine the inheritance of two traits controlled by unlinked genes.
- The classic example is Mendel's cross between pea plants with round yellow seeds ( $RRYY$) and wrinkled green seeds ( $rryy$).
Steps in a Dihybrid Cross
- Parent Generation (P): Cross $RRYY$ (round yellow) with $rryy$ (wrinkled green).
- F1 Generation: All offspring are $RrYy$ (round yellow) because round and yellow are dominant traits.
- F2 Generation: When $RrYy$ plants self-pollinate, they produce four types of gametes: $RY$, $Ry$, $rY$, and $ry$.
Hint
Use a Punnett grid to visualize the combinations of gametes and predict offspring ratios in dihybrid crosses.
Self review
- What is the final result of segregation?
- What do dihybrid crosses examine?
