BIOL1020 Chap.9 Meiosis and Mendelian Inheritance
Meiosis and Mendelian Inheritance
Meiosis creates haploid gametes and reshuffles alleles through segregation, independent assortment and recombination. This chapter follows homologous chromosomes through the first division and sister chromatids through the second, then connects those physical events to Punnett squares, test crosses and gamete probabilities.
It also explains why triploidy disrupts balanced gamete production and how nondisjunction at different divisions creates different gamete sets. Ratios are treated as probability expectations rather than exact small-family guarantees.
What this chapter covers
- 01
Homologues and sister chromatids
- 02
Meiosis I and meiosis II
- 03
Segregation and assortment
- 04
Recombination between homologues
- 05
Test-cross inference
- 06
Nondisjunction and aneuploidy
Worked example · free
Inferring an unknown genotype by test cross
- +1The recessive tester contributes a recessive allele to every offspring.
- +1A wrinkled offspring must also receive a recessive allele from the unknown parent.
- +1The smooth parent therefore produces both dominant-bearing and recessive-bearing gametes.
- +1The unknown smooth parent is heterozygous rather than homozygous dominant.
Key terms
- Homologous Chromosomes
- A maternal and paternal chromosome pair carrying the same gene loci but potentially different alleles.
- Segregation
- Separation of two alleles into different gametes as homologous chromosomes divide.
- Independent Assortment
- Independent orientation of different homologous pairs, creating varied combinations in gametes.
- Test Cross
- A cross with a homozygous recessive individual used to infer an unknown dominant genotype.
- Nondisjunction
- Failure of homologues or sister chromatids to separate normally during cell division.
Meiosis and Mendelian Inheritance FAQ
How does meiosis create genetic variation?
Homologous chromosomes segregate, different chromosome pairs assort independently, and crossing over recombines DNA between homologues. Random fertilisation then joins independently produced gametes. Together these processes create allele combinations that differ from either parent's complete chromosome set.
Why does a test cross reveal genotype?
The homozygous recessive tester contributes only recessive alleles, so each offspring phenotype exposes which allele came from the dominant-phenotype parent. Recessive offspring demonstrate that the unknown parent carries and transmits a recessive allele, proving heterozygosity.
Why are triploid plants often sterile?
Three homologues cannot be divided into two balanced sets consistently during meiosis. Gametes therefore receive unequal chromosome complements, reducing viability or successful fertilisation. The plant can grow normally by mitosis while sexual reproduction is disrupted, which supports seedless fruit production.
How does crossing over generate variation?
Homologous chromosomes pair during meiosis and can exchange corresponding DNA segments between non-sister chromatids. The resulting chromatids carry new combinations of alleles along the same chromosome. Crossing over reshuffles existing variation; it is different from mutation, which creates a new DNA sequence change.
Why does independent assortment multiply gamete types?
Each homologous chromosome pair can orient independently at meiosis I, so maternal and paternal versions enter gametes in different combinations. With independently assorting heterozygous loci, the choices multiply across pairs. Linkage changes this expectation because loci on the same chromosome do not assort independently in every meiosis.
How does a test cross reveal genotype?
The unknown individual is crossed with a homozygous recessive tester, which contributes only recessive alleles. Offspring phenotypes therefore expose which alleles came from the unknown parent. A dominant phenotype in the original individual can then be distinguished as homozygous or heterozygous from the progeny pattern.
How does meiosis I nondisjunction affect gametes?
If homologous chromosomes fail to separate in the first division, both homologues enter one daughter cell and neither enters the other. After meiosis II, all resulting gametes have an abnormal copy number for that chromosome. This differs from a second-division failure, which can leave some gametes normal.
Why are Mendelian ratios probabilities rather than guarantees?
Segregation and fertilisation create expected proportions over many independent offspring, but a small family is a limited random sample. Observed counts can therefore differ from the textbook ratio without changing the underlying inheritance mechanism. State the predicted probability first, then judge whether the available sample is large enough to support a stronger conclusion.
Exam move
Use chromosome drawings before Punnett squares. Colour maternal and paternal homologues, show their orientation at meiosis I and follow alleles into gametes. For an independently assorting heterozygote, list the choices at each locus and multiply them to count gamete classes. In test crosses, write the recessive tester's single gamete type before considering the unknown parent.
For nondisjunction, specify whether homologues fail in the first division or sister chromatids fail in the second, then list every resulting gamete. Treat ratios as long-run probabilities and avoid overinterpreting small offspring samples. Finish by contrasting clonal propagation with self-fertilisation. Use the chromosome sketch to justify each probability before multiplying independent events or filling a cross table.
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