Quick questions on Mendelian inheritance, Punnett squares and test crosses (QCE Biology Unit 4)

Law of segregation. Each individual has two alleles for each gene; the two alleles separate during meiosis so that each gamete carries only one. This is the chromosomal behaviour of homologous chromosomes separating in anaphase I.

A monohybrid cross follows one gene with two alleles.

A dihybrid cross follows two genes simultaneously. The dihybrid ratio is the product of the two monohybrid ratios.

When an organism shows the dominant phenotype, its genotype could be either homozygous dominant (TT) or heterozygous (Tt). A test cross distinguishes them.

A Punnett square gives probabilities for each offspring. For two independent events use the product rule (multiply probabilities); for either of two mutually exclusive outcomes use the sum rule (add probabilities). For example, the probability that a TtYy x TtYy cross gives an offspring that is both homozygous recessive at both loci is 1/4 x 1/4 = 1/16. The probability that it shows at least one recessive trait is 1 minus the probability of showing both dominants = 1 minus 9/16 = 7/16.

Each individual has two alleles for each gene; the two alleles separate during meiosis so that each gamete carries only one. This is the chromosomal behaviour of homologous chromosomes separating in anaphase I.

Alleles for different genes segregate into gametes independently of one another, provided the genes are on different chromosomes (or far enough apart on the same chromosome to recombine freely). This is the chromosomal behaviour of independent metaphase I alignment.

Cross the unknown with a homozygous recessive individual (tt). The recessive partner contributes only t gametes, so any t in the offspring must come from the unknown parent.

A TtYy parent produces four gamete types (TY, Ty, tY, ty), not the four alleles directly. Gametes are haploid.

Tt x Tt gives a 1:2:1 genotype ratio but a 3:1 phenotype ratio.

Genes on the same chromosome do not always assort independently; they may be linked and produce ratios that deviate from 9:3:3:1.

A test cross uses a homozygous recessive, not another heterozygote.

Real offspring numbers vary around the predicted ratio. A 3:1 ratio in 4 offspring might appear as 4:0 or 2:2 by chance.