Unit 5 practice test: Heredity

The F1 kernels were heterozygous for both genes and showed purple and smooth, so those alleles are dominant.

• Correct: The purple allele and the smooth allele: A heterozygote shows the dominant phenotype: the F1 were purple and smooth.
• The yellow allele and the wrinkled allele: Yellow and wrinkled disappeared in the F1 and returned in the F2: they are recessive.
• The purple allele and the wrinkled allele: Wrinkled was hidden in the heterozygous F1, so it is recessive.
• The yellow allele and the smooth allele: Yellow was hidden in the heterozygous F1, so it is recessive.

Each F1 is PpSs. Segregation gives each gamete one allele of each gene, and because the two pairs assort independently, the four combinations are equally likely (1/4 each). Random fertilization of 4 × 4 gamete types gives 16 equally likely boxes: 9 P_S_, 3 P_ss, 3 ppS_, 1 ppss.

• Correct: The two genes' chromosome pairs line up independently at metaphase I, so PS, Ps, pS and ps gametes form in equal numbers and combine at random.: Independent assortment makes four equal gamete types; 4 × 4 random combinations give 9:3:3:1.
• Homologs pair and swap pieces in each meiosis, which makes the four kernel types appear in a 9:3:3:1 ratio.: The ratio comes from independent assortment of separate chromosome pairs; crossing over affects genes on the same chromosome.
• The purple and smooth alleles are dominant, so they go into 9 out of 16 gametes and the recessive alleles into the rest.: Dominance decides what the offspring look like, not which alleles enter gametes; each allele goes into half the gametes.
• Each F1 plant makes two kinds of gamete, PS and ps, and their random combination at fertilization gives 9:3:3:1.: Only two gamete types would give a 3:1 pattern of purple smooth to yellow wrinkled, with no purple wrinkled or yellow smooth kernels.

A testcross to ppss means every offspring gets p and s from that parent, so the offspring show directly which alleles the tested plant's gametes carried. PpSs makes PS, Ps, pS and ps equally, giving a 1:1:1:1 ratio.

• Correct: Cross it with a plant from a yellow, wrinkled kernel; find the four phenotypes in about equal numbers.: In a testcross the offspring phenotypes mirror the tested plant's gametes: four equal classes mean PpSs.
• Cross it with a plant from a purple, smooth kernel; find purple, smooth offspring and no other kind.: Another purple, smooth parent can hide recessive alleles, so the result would not reveal the genotype.
• Cross it with a plant from a yellow, wrinkled kernel; find purple, smooth offspring and no others.: Offspring that are purple and smooth every time would mean the plant is PPSS, not PpSs.
• Self-pollinate it; find purple, smooth and yellow, wrinkled kernels in a 3:1 ratio.: Selfing PpSs gives four phenotypes in a 9:3:3:1 ratio, not 3:1.

Reading the graph: 18% female at 28 °C, 86% at 30 °C. A 2 °C rise near the pivotal temperature shifts the sex ratio strongly toward females.

• Correct: The share of females rises from about 18% to about 86%.: 28 °C gives about 18% females and 30 °C about 86%: a large shift toward females.
• The share of females falls from about 86% to about 18%.: This reverses the direction: warmer eggs give more females in this species.
• The sex ratio stays the same, because sex is set by the hatchlings' genes.: In this species sex is set by temperature, so the ratio changes with warming.
• Each hatchling becomes female, because warming above 28 °C gives females.: At 30 °C the mean is 86% female, not 100%; some males still hatch.

If each nest went to one incubator, a nest whose eggs were, for genetic reasons, more likely to become female would be confused with the effect of its temperature. Splitting every nest across all temperatures controls for that.

• Correct: So nest-to-nest genetic differences were spread across temperatures, leaving temperature as the one difference.: Spreading each nest's eggs across every temperature stops nest differences from being mistaken for temperature effects.
• So that each incubator held the same number of eggs, which made the incubators warm up evenly.: Equal numbers help, but the purpose of splitting nests is to control for differences between nests.
• So that eggs from the same mother would hatch as the same sex in each incubator they were placed in.: Sex is expected to differ with temperature, even among eggs from one mother.
• So that the researchers could later tell which of the ten nests each hatchling came from just by looking at its sex.: Sex cannot identify a nest; the nests' eggs would be labeled directly.

At 31 °C the interval is 94% to 100%, which includes the 100% seen at 32 °C. The difference is not shown to be real.

• Correct: No; 97 ± 3 reaches 100, so the two intervals overlap.: The 31 °C interval reaches 100%, so it overlaps the 32 °C value.
• Yes; 100% is higher than 97%, so the difference is real.: Means can differ by chance; overlapping ± 2 SE intervals mean the difference is not shown to be real.
• Yes; the error bars at 32 °C are zero, so any difference is real.: A zero-width bar at 32 °C does not stop it overlapping the 31 °C interval.
• No; percentages are not compared using error bars.: Means of percentages from several nests can be compared with ± 2 SE intervals like any other mean.