Meiosis, Monohybrid and Dihybrid Crosses

Meiosis produces variation in three ways

Meiosis halves the chromosome number and produces four genetically different cells. Variation comes from three sources. In prophase I, homologous chromosomes pair up and non-sister chromatids exchange sections at points called chiasmata — crossing over, which produces new combinations of alleles on the same chromosome. In metaphase I, each bivalent lines up on the equator independently of every other bivalent, so which member of a pair goes to which pole is random; with $n$ pairs there are $2^n$ possible combinations, and in humans that is $2^{23}$. Finally, random fertilisation multiplies the variation again. Note that crossing over happens in prophase I and independent assortment in metaphase I; putting them in the wrong stage loses marks.

The vocabulary that questions are built on

A gene is a length of DNA coding for a polypeptide; an allele is one version of a gene; a locus is its position on the chromosome. Genotype is the alleles present, phenotype is the observable characteristic. An organism is homozygous if both alleles are the same and heterozygous if they differ. Alleles may be dominant, recessive, or codominant when both are expressed in the heterozygote, as in the $AB$ blood group. Human blood groups also illustrate multiple alleles: three alleles exist in the population, though any one person carries only two.

Monohybrid and dihybrid crosses

A cross between two heterozygotes for one gene gives a $3:1$ phenotypic ratio; a cross between two double heterozygotes for two unlinked genes gives $9:3:3:1$. A test cross — crossing an individual showing the dominant phenotype with a homozygous recessive — reveals the unknown genotype: all offspring showing the dominant phenotype means the parent was homozygous, while a $1:1$ ratio means it was heterozygous. Always set out crosses formally: parental phenotypes, parental genotypes, gametes in circles, a Punnett square, then offspring genotypes and phenotypes with the ratio.