Chapter 23

No two people are genetically identical, except for identical twins. The main source of genetic variation among humans is \begin{equation}\begin{array}{l}{\text { (A) new mutations that occurred in the preceding generation. }} \\ {\text { (B) genetic drift. }} \\ {\text { (C) the reshuffling of alleles in sexual reproduction. }} \\ {\text { (D) environmental effects. }}\end{array}\end{equation}

If the nucleotide variability of a locus equals \(0 \%,\) what is the gene variability and number of alleles at that locus? \begin{equation} \begin{array}{l}{\text { (A) gene variability }0 \%; \text { number of alleles }=0} \\ {\text { (B) gene variability }=0 \% \text { ; number of alleles }=1}\\\\{\text { (C) gene variability }=0 \% \text { ; number of alleles }=2} \\\ {\text { (D) gene variability }>0 \% \text { ; number of alleles }=2}\end{array}\end{equation}

There are 25 individuals in population \(1,\) all with genotype \(A A\) and there are 40 individuals in population \(2,\) all with genotype aa. Assume that these populations are located far from each other and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation most likely resulted from \begin{equation}\begin{array}{ll}{\text { (A) genetic drift. }} & {\text { (C) nonrandom mating. }} \\ {\text { (B) gene flow. }} & {\text { (D) directional selection. }}\end{array}\end{equation}

A fruit fly population has a gene with two alleles, \(A 1\) and \(A 2\) . Tests show that 70\(\%\) of the gametes produced in the population contain the \(A 1\) allele. If the population is in Hardy-Weinberg equilibrium, what proportion of the flies carry both \(A 1\) and \(A 2 ?\) \begin{equation}{ (A) }0.7 \quad \text { (B) } 0.49 \quad \text { (C) } 0.42 \quad \text { (D) } 0.21 \end{equation}

WRITE ABOUT A THEME: ORGANIZATION Heterozygotes at the sickle-cell locus produce both normal and abnormal (sickle-cell) hemoglobin (see Concept 14.4\() .\) When hemoglobin molecules are packed into a heterozygote's red blood cells, some cells receive relatively large quantities of abnormal hemoglobin, making these cells prone to sickling. In a short essay (approximately \(100-150\) words), explain how these molecular and cellular events lead to emergent properties at the individual and population levels of biological organization.