In 1866, Gregor Mendel, a friar in Brno, published the results of crossing thousands of pea plants in his monastery garden. Cross purple-flowered and white-flowered plants and every offspring is purple. Let those offspring self-pollinate, and white flowers come back, in about one plant in four. His work was largely overlooked until 1900, when other botanists rediscovered it. Let's look closer at why that 3:1 ratio appears, and when it doesn't.
Meiosis
Meiosis makes haploid gametes from a diploid cell. It has two divisions.
- Meiosis I separates homologous chromosomes. In prophase I, homologs pair up (synapsis) and chiasmata may form. In metaphase I, the pairs line up along the metaphase plate. In anaphase I, the homologs move to opposite poles while sister chromatids stay attached. Telophase I and cytokinesis leave two haploid cells.
- Meiosis II separates sister chromatids, much like mitosis. The result is four haploid cells, each with unduplicated chromosomes.
Mitosis and meiosis both use a spindle to move chromosomes. They differ in the number of cells made and their genetic content: mitosis gives two identical diploid cells, meiosis four genetically different haploid ones.
Meiosis and genetic diversity
Three things make offspring different from each other:
- Crossing over in prophase I swaps segments between non-sister chromatids.
- Random assortment of homologous pairs in metaphase I. Humans have 23 pairs, so one person can make 2^23 (about 8.4 million) combinations of chromosomes, before counting crossing over.
- Random fertilization: any sperm can fuse with any egg.
When chromosomes don't separate properly, that's nondisjunction, and some or all of the gametes are no longer haploid. A familiar example is trisomy 21, an extra copy of chromosome 21, which causes Down syndrome.
Mendelian genetics
Mendel's law of segregation says the two alleles of a gene separate into different gametes. His law of independent assortment says genes on different chromosomes sort independently. Both follow from what chromosomes do in meiosis.
Use probability to predict crosses. For independent events, multiply: in AaBb x AaBb, the chance of aabb is 1/4 x 1/4 = 1/16. For mutually exclusive outcomes, add. A Punnett square shows the same thing visually. A test cross with a homozygous recessive partner reveals an unknown genotype.
To check whether data fit a predicted ratio, use a chi-square test. State the null hypothesis (no difference between observed and expected), calculate the chi-square value, and compare it with the critical value for your degrees of freedom (number of categories minus one).
Non-Mendelian genetics
Many traits don't follow Mendel's ratios:
- Linked genes sit close together on one chromosome and are usually inherited together. The percentage of recombinant offspring gives the map distance: 1% equals 1 map unit.
- Codominance: both alleles show, as in type AB blood.
- Incomplete dominance: the heterozygote is a blend, as in pink snapdragons from red and white parents.
- Sex-linked traits come from genes on the X or Y chromosome. An X-linked recessive allele shows up more often in XY individuals, who have only one X. Some species use other systems, such as ZW in birds and haplodiploidy in bees.
- Pleiotropy: one gene affects several traits.
- Non-nuclear inheritance: mitochondria (and chloroplasts in plants) usually come only from the egg or ovule, so their traits are typically inherited from the mother.
Environmental effects on phenotype
The same genotype can produce different phenotypes in different conditions. That's phenotypic plasticity. Bigleaf hydrangeas often flower blue in acidic soil and pink in alkaline soil. Arctic foxes change fur color with the seasons, and in many turtles the temperature of the eggs decides the offspring's sex.
Common mistakes students make
- Mixing up genotypic and phenotypic ratios. A monohybrid cross gives 1:2:1 genotypes but 3:1 phenotypes.
- Saying homologous chromosomes are sister chromatids. Homologs come from different parents; sister chromatids are copies made in S phase.
- Stating the alternative as the null hypothesis. The null hypothesis is that there is no statistically significant difference between observed and expected results.
- Treating linked genes as independent. Multiply probabilities only for genes that assort independently.
Frequently asked questions
What is the difference between meiosis I and meiosis II?
Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids. Only meiosis I reduces the chromosome number from diploid to haploid.
How do you calculate map units?
Divide the number of recombinant offspring by the total offspring and multiply by 100. That percentage is the distance in map units.
Why are X-linked recessive traits more common in males?
An XY individual has only one X, so a single recessive allele on it is expressed. An XX individual needs two copies.
What does it mean to reject the null hypothesis in chi-square?
Your chi-square value is larger than the critical value, so the difference between observed and expected is unlikely to be chance alone.
Try it
Work through each card and say the answer before you flip it. Then predict: what would you expect to see if you crossed two pink snapdragons?
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