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5.2Heredity

Meiosis and Genetic Diversity — AP BIOLOGY Revision Notes & Topical Questions

5 min read 10 parts Lesson 2 of 5 1 quiz

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5.2 Meiosis and Genetic Diversity — Notes — AP Biology notes: Explain how meiosis and sexual reproduction generate genetic diversity.Meiosis makes haploid gametes that, unlike mitosis products, are not identical.Three events add variation — crossing over in prophase I, random ori…

  • Independent assortment
  • Recombinant
  • Random fertilisation
  • Nondisjunction
  • Monosomy X causes Turner syndrome
  • Homologous chromosomes pair up in prophase I (synapsis) and lie very close together

Section 1 of 10Why meiosis produces variation

Why meiosis produces variation
Meiosis makes haploid gametes that, unlike mitosis products, are not identical.
Three events add variation — crossing over in prophase I, random orientation of homologous pairs in metaphase I, and random fusion of gametes.
Result — every zygote carries a unique combination of alleles, so siblings differ from each other and from their parents (apart from identical twins).

Section 2 of 10Correct and incorrect separation

Correct and incorrect separation
Correct separation — homologues separate in meiosis I and sister chromatids in meiosis II,
  • so each gamete gets a haploid (1n) set of maternal and paternal chromosomes.
Nondisjunction — incorrect separation makes gametes that are no longer haploid, giving aneuploidy.
  • Trisomy 21 causes Down syndrome.
  • Monosomy X causes Turner syndrome.

Section 3 of 10Crossing over

1
Homologous chromosomes pair up in prophase I (synapsis) and lie very close together.
2
Non-sister chromatids come into close contact.
3
Chromatids break at the same point and become entangled at a chiasma.
4
Corresponding segments are exchanged and rejoined.
5
Recombinant chromatids with new allele combinations are sorted into gametes.
Crossing over
Effect — it trades alleles between non-sister chromatids and increases diversity among the gametes.
Why non-sister — sister chromatids are identical before crossing over, so exchange between them changes nothing.

Section 4 of 10Independent assortment

Independent assortment
Principle — at metaphase I each homologous pair orients randomly with respect to the others,
  • so maternal and paternal chromosomes reach gametes in many combinations.
Orientation — each pair can face either way across the equator, and one pair's orientation does not affect any other pair.
Number of combinations — 2n, where n is the haploid number; for humans 223 is about 8.4 million, even before crossing over.

Section 5 of 10Random fertilisation

Random fertilisation
Principle — any sperm can fuse with any egg, and each gamete carries different alleles from every other gamete of the same parent.
Result — each zygote has a unique allele combination.
Number of zygotes — (2n)2, about 7 × 1013 for humans, before crossing over is counted.

Section 6 of 10Other sources and importance of variation

1
Mutation — the ultimate source of new alleles.
2
Linkage — genes close together on one chromosome tend to be inherited together unless crossing over separates them.
3
Natural selection — genetic variation is its raw material, and populations with more variation are more likely to survive environmental change.
4
Asexual reproduction — mitosis gives offspring identical to the parent except for mutations.
5
Common mechanism — meiosis is conserved across eukaryotes, evidence of shared ancestry.

Section 7 of 10Sources of variation compared

SourceWhenMechanismEffect
Crossing overProphase ISegments exchanged between non-sister chromatidsNew allele combinations on a chromosome
Independent assortmentMetaphase IRandom orientation of pairs223 gamete types in humans
Random fertilisationFertilisationAny gamete pairAbout 70 trillion zygotes
MutationAny timeDNA changeNew alleles

Section 8 of 10More from the specification

1
Meiotic division forming haploid daughter cells
  • Meiotic division forming haploid daughter cells homologous chromosome segregation reduces diploid mother cells into four genetically distinct haploid daughter cells.
2
Daughter cell
  • Cell produced by cell division.
3
Genetically identical
  • Having the same alleles
4
Spindle
  • Structure of microtubules that attaches to and moves chromosomes.
5
Anaphase
  • Stage in which centromeres divide and chromatids are pulled to opposite poles.
6
Cytokinesis
  • Division of the cytoplasm.
7
Telophase
  • Final stage in which chromosomes decondense and nuclear envelopes reform.
8
Diploid
  • Having two sets of chromosomes.
9
Centromere
  • Constricted region that joins sister chromatids and binds spindle fibres.

Section 9 of 10Key terms

TermMeaning
Genetic diversityvariation in alleles within a population
Independent assortmentrandom orientation of homologous pairs at metaphase I
Crossing overexchange of segments between non-sister chromatids
Chiasmapoint where non-sister chromatids cross and exchange segments
Recombinantchromosome or offspring with a new allele combination
Random fertilisationany sperm can fuse with any egg
Nondisjunctionfailure of chromosomes to separate

Section 10 of 10Exam technique

1
Confusing independent assortment (whole chromosomes, metaphase I) with crossing over (segment exchange, prophase I).
2
Saying meiosis creates new alleles — only mutation does.
3
Calculating combinations with the diploid number — use the haploid number n in 2n.
4
Saying sister chromatids cross over — exchange between sister chromatids creates no new combinations.

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