The height of the seed head in wheat at maturity is determined by several genes. In one variety, the head is just $9$ inches above the ground; in another, it is $33$ inches above the ground. Plants from the $9$-inch variety were crossed to plants from the $33$-inch variety. Among the F$_{1}$, the seed head was 21 inches above the ground. After self-fertilization, the F$_{1}$ plants produced an F$_{2}$ population in which $9$-inch and $33$-inch plants each appeared with a frequency of $1$/$256$.

(a) How many genes are involved in the determination of seed head height in these strains of wheat?

(b) How much does each allele of these genes contribute to seed head height?

(c) If a 21-inch F$_{1}$ plant were crossed to a $9$-inch plant, how often would you expect $18$-inch wheat to occur in the progeny?

In Drosophila, genes a and b are located at positions $22.0$ and $42.0$ on chromosome $2$, and genes c and d are located at positions 10.0 and 25.0 on chromosome $3$. A fly homozygous for the wild-type alleles of these four genes was crossed with a fly homozygous for the recessive alleles, and the F$_{1}$ daughters were backcrossed to their quadruply recessive fathers. What offspring would you expect from this backcross, and in what proportions?

For alcoholism, the concordance rate for monozygotic twins is 55 percent, whereas for dizygotic twins, it is 28 percent. Do these data suggest that alcoholism has a genetic basis?

The specification of the anterior-posterior axis in Drosophila embryos is initially controlled by various gene products that are synthesized and stored in the mature egg following oogenesis. Mutations in these genes result in abnormalities of the axis during embryogenesis. These mutations illustrate maternal effect. How do such mutations vary from those produced by organelle heredity? Devise a set of parallel crosses and expected outcomes involving mutant genes that contrast maternal effect and organelle heredity.

One form of male sterility in corn is maternally transmitted. Plants of a male-sterile line crossed with normal pollen give male-sterile plants. In addition, some lines of corn are known to carry a dominant nuclear restorer allele (Rf) that restores pollen fertility in male-sterile lines. a. Research shows that the introduction of restorer alleles into male-sterile lines does not alter or affect the maintenance of the cytoplasmic factors for male sterility. What kind of research results would lead to such a conclusion? b. A male-sterile plant is crossed with pollen from a plant homozygous for Rf. What is the genotype of the

$$F_1?$$

The phenotype? c. The

$$F_1$$

plants from part b are used as females in a testcross with pollen from a normal plant (rf/rf). What are the results of this testcross? Give genotypes and phenotypes, and designate the kind of cytoplasm. d. The restorer allele already described can be called Rf-1. Another dominant restorer, Rf-2, has been found. Rf-1 and Rf-2 are located on different chromosomes. Either or both of the restorer alleles will give pollen fertility. With the use of a male-sterile plant as a tester, what will be the result of a cross in which the male parent is (1) heterozygous at both restorer loci? (2) homozygous dominant at one restorer locus and homozygous recessive at the other? (3) heterozygous at one restorer locus and homozygous recessive at the other? (4) heterozygous at one restorer locus and homozygous dominant at the other?

Correlations between relatives provide estimates of the broad and narrow-sense heritabilties on the assumption that the genetic and environmental factors infl uencing quantitative traits are independent of each other and that they do not interact in some peculiar way. Prevoiously, we considered epigenetic modifications of chromatin that regulate genes and noted the possibility that some of these modifications might be induced by environmental factors. How could epigenetic influences on complex traits be incorporated into the basic theory of quantitative genetics?

In E.coli, the ability to utilize lactose as a carbon source requires the presence of the enzymes $\beta$-galactosidase and $\beta$-galactoside permease. These enzymes are encoded by two closely linked genes, lacZ and lacY, respectively. Another gene, proC, controls, in part, the ability of E.coli cells to synthesize the amino acid proline. The alleles str$^{r}$ and str$^{s}$, respectively, control resistance and sensitivity to streptomycin. Hfr H is known to transfer the two lac genes, proC, and str, in that order, during conjugation. A cross was made between Hfr H of genotype lacZ$^{-}$ lacY$^{+}$ proC$^{+}$ str$^{s}$ and an F$^{-}$ strain of genotype lacZ$^{+}$ lacY$^{-}$ proC$^{-}$ str$^{r}$.After about $2$ hours, the mixture was diluted and plated out on medium containing streptomycin but no proline. When the resulting proC$^{+}$ str$^{r}$ recombinant colonies were checked for their ability to grow on medium containing lactose as the sole carbon source, very few of them were capable of fermenting lactose. When the reciprocal cross (Hfr H lacZ$^{+}$ lacY$^{-}$ proC$^{+}$ str$^{s}$ X F$^{-}$ lacZ$^{-}$ lacY$^{+}$ proC$^{-}$ str$^{r}$) was done, many of the proC$^{+}$ str$^{r}$ recombinants were able to grow on medium containing lactose as the sole carbon source. What is the order of the lacZ and lacY genes relative to proC?

On the basis of the observed correlations for personality traits learnt, what can you say about the value of the environmentality ?

In maize, the genes Tu, j$2$, and gl$3$ are located on chromosome $4$ at map positions $101$, $106$, and $112$, respectively. If plants homozygous for the recessive alleles of these genes are crossed with plants homozygous for the dominant alleles, and the F$_{1}$ plants are testcrossed to triply recessive plants, what genotypes would you expect, and in what proportions? Assume that interference is complete over this map interval.

A selection differential of 40 $\mu$g per generation was used in an experiment to select for increased pupa weight in Tribolium. The narrow-sense heritability for pupa weight was estimated to be $0.3$. If the mean pupa weight was initially $2000$ g and selection was practiced for $10$ generations, what was the mean pupa weight expected to become?

Plant breeders have long appreciated the phenomenon called hybrid vigor or heterosis, in which hybrids formed between two inbred strains have increased vigor and crop yield relative to the two parental strains. Starting in the 1930s, seed companies exploited cytoplasmic male sterility (CMS) in corn so that they could cheaply produce hybrid corn seed to sell to farmers. This type of CMS is caused by mutant mitochondrial genomes that prevent pollen formation. a. How would CMS aid seed companies in producing hybrid corn seed? Dominant R alleles of a nuclear gene called Restorer suppress the CMS phenotype, so that Rf-containing plants with mutant mitochondrial genomes are male fertile. b. Describe a cross generating hybrid corn seed that would grow into fertile (self-fertilizing) plants. (Farmers planting hybrid seed-want fertile plants because corn kernels result from fertilized ovules.) c. One of the historical challenges in the commercialization of hybrid corn produced through CMS was the maintenance of strains with CMS mitochondria: How could the seed companies keep producing male sterile corn plants if these plants never themselves produced pollen? Suggest a strategy by which seed companies could continue to obtain male sterile plants every breeding season. d. Are there any potential disadvantages to the use of hybrid corn? If so, what issues might arise?

The fl ower colors of plants in a particular population may be blue, purple, turquoise, light-blue, or white. A series of crosses between different members of the population produced the following results:

How many genes and alleles are involved in the inheritance of flower color? Indicate all possible genotypes for the following phenotypes: (a) purple; (b) blue; (c) turquoise; (d) light-blue; (e) white.

A DNA library is a. an orderly array of all the genes within an organism. b. a collection of vectors. c. the collection of plasmids found within a single E. coli. d. a collection of DNA fragments representing the entire genome of an organism.

One way to estimate a maximum value for the narrowsense heritability is to calculate the correlation between half-siblings that have been reared apart and divide it by the fraction of genes that half-siblings share by virtue of common ancestry. A study of human half-siblings found that the correlation coefficient for height was $0.14$. From this result, what is the maximum value of the narrowsense heritability for height in this population?