In the left-hand part of Figure 2-4, the red arrows show selfing as pollination within single flowers of one F₁ plant. Would the same F₂ results be produced by cross-pollinating two different F₁ plants?

In the right-hand part of Figure 2-4, in the plant showing an 11:11 ratio, do you think it would be possible to find a pod with all yellow peas? All green? Explain.

In Table 2-1, state the recessive phenotype in each of the seven cases.

Considering Figure 2-8, is the sequence “pairing → replication → segregation → segregation” a good shorthand description of meiosis?

Point to all cases of bivalents, dyads, and tetrads in Figure 2-11.

In Figure 2-11, assume (as in corn plants) that allele A encodes an allele that produces starch in pollen and allele a does not. Iodine solution stains starch black. How would you demonstrate Mendel’s first law directly with such a system?

Considering Figure 2-13, if you had a homozygous double mutant m3/m3 m5/m5, would you expect it to be mutant in phenotype? (Note: This line would have two mutant sites in the same coding sequence.)

In which of the stages of the Drosophila life cycle would you find the products of meiosis?

If you assume Figure 2-15 also applies to mice and you irradiate male sperm with X rays (known to inactivate genes), what phenotype would you look for in progeny in order to find cases of individuals with an inactivated SRY gene?

In Figure 2-17, how does the 3:1 ratio in the bottom-left-hand grid differ from the 3:1 ratios obtained by Mendel?

In Figure 2-19, assume that the pedigree is for mice, in which any chosen cross can be made. If you bred IV-1 with IV-3, what is the probability that the first baby will show the recessive phenotype?

Which part of the pedigree in Figure 2-23 in your opinion best demonstrates Mendel’s first law?

Could the pedigree in Figure 2-31 be explained as an autosomal dominant disorder? Explain.

Make up a sentence including the words chromosome, genes, and genome.

Peas (Pisum sativum) are diploid and 2n = 14. In Neurospora, the haploid fungus, n = 7. If you were to isolate genomic DNA from both species and use electrophoresis to separate DNA molecules by size, how many distinct DNA bands would be visible in each species?

The broad bean (Viciafaba) is diploid and 2n = 18. Each haploid chromosome set contains approximately 4 m of DNA. The average size of each chromosome during metaphase of mitosis is 13 μm. What is the average packing ratio of DNA at metaphase? (Packing ratio = length of chromosome/length of DNA molecule therein.) How is this packing achieved?

If we call the amount of DNA per genome “x,” name a situation or situations in diploid organisms in which the amount of DNA per cell is

Name the key function of mitosis.

Name two key functions of meiosis.

Design a different nuclear-division system that would achieve the same outcome as that of meiosis.

In a possible future scenario, male fertility drops to zero, but, luckily, scientists develop a way for women to produce babies by virgin birth. Meiocytes are converted directly (without undergoing meiosis) into zygotes, which implant in the usual way. What would be the short- and long-term effects in such a society?

In what ways does the second division of meiosis differ from mitosis?

Make up mnemonics for remembering the five stages of prophase I of meiosis and the four stages of mitosis.

In an attempt to simplify meiosis for the benefit of students, mad scientists develop a way of preventing premeiotic S phase and making do with having just one division, including pairing, crossing over, and segregation. Would this system work, and would the products of such a system differ from those of the present system?

Theodor Boveri said, “The nucleus doesn’t divide; it is divided.” What was he getting at?

Francis Galton, a geneticist of the pre-Mendelian era, devised the principle that half of our genetic makeup is derived from each parent, one-quarter from each grandparent, one-eighth from each great-grandparent, and so forth. Was he right? Explain.

If children obtain half their genes from one parent and half from the other parent, why aren’t siblings identical?

State where cells divide mitotically and where they divide meiotically in a fern, a moss, a flowering plant, a pine tree, a mushroom, a frog, a butterfly, and a snail.

Human cells normally have 46 chromosomes. For each of the following stages, state the number of nuclear DNA molecules present in a human cell:

Four of the following events are part of both meiosis and mitosis, but only one is meiotic. Which one? (1) Chromatid formation, (2) spindle formation, (3) chromosome condensation, (4) chromosome movement to poles, (5) synapsis.

In corn, the allele f′ causes floury endosperm and the allele f″ causes flinty endosperm. In the cross f ′/f′ ♀ × f″/f″ ♂, all the progeny endosperms are floury, but, in the reciprocal cross, all the progeny endosperms are flinty. What is a possible explanation? (Check the legend for Figure 2-7.)

What is Mendel’s first law?

If you had a fruit fly (Drosophila melanogaster) that was of phenotype A, what cross would you make to determine if the fly’s genotype was A/A or A/a?

In examining a large sample of yeast colonies on a petri dish, a geneticist finds an abnormal-looking colony that is very small. This small colony was crossed with wild type, and products of meiosis (ascospores) were spread on a plate to produce colonies. In total, there were 188 wild-type (normal-size) colonies and 180 small ones.

Two black guinea pigs were mated and over several years produced 29 black and 9 white offspring. Explain these results, giving the genotypes of parents and progeny.

In a fungus with four ascospores, a mutant allele lys-5 causes the ascospores bearing that allele to be white, whereas the wild-type allele lys-5⁺ results in black ascospores. (Ascospores are the spores that constitute the four products of meiosis.) Draw an ascus from each of the following crosses:

For a certain gene in a diploid organism, eight units of protein product are needed for normal function. Each wild-type allele produces five units.

A Neurospora colony at the edge of a plate seemed to be sparse (low density) in comparison with the other colonies on the plate. This colony was thought to be a possible mutant, and so it was removed and crossed with a wild type of the opposite mating type. From this cross, 100 ascospore progeny were obtained. None of the colonies from these ascospores was sparse, all appearing to be normal. What is the simplest explanation of this result? How would you test your explanation? (Note: Neurospora is haploid.)

From a large-scale screen of many plants of Collinsia grandiflora, a plant with three cotyledons was discovered (normally, there are two cotyledons). This plant was crossed with a normal pure-breeding wild-type plant, and 600 seeds from this cross were planted. There were 298 plants with two cotyledons and 302 with three cotyledons. What can be deduced about the inheritance of three cotyledons? Invent gene symbols as part of your explanation.

In the plant Arabidopsis thaliana, a geneticist is interested in the development of trichomes (small projections). A large screen turns up two mutant plants (A and B) that have no trichomes, and these mutants seem to be potentially useful in studying trichome development. (If they were determined by single-gene mutations, then finding the normal and abnormal functions of these genes would be instructive.) Each plant is crossed with wild type; in both cases, the next generation (F₁) had normal trichomes. When F₁ plants were selfed, the resulting F₂’s were as follows:

F₂ from mutant A: 602 normal; 198 no trichomes

F₂ from mutant B: 267 normal; 93 no trichomes

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You have three dice: one red (R), one green (G), and one blue (B). When all three dice are rolled at the same time, calculate the probability of the following outcomes:

In the pedigree below, the black symbols represent individuals with a very rare blood disease.

If you had no other information to go on, would you think it more likely that the disease was dominant or recessive? Give your reasons.

Holstein cattle are normally black and white. A superb black-and-white bull, Charlie, was purchased by a farmer for $100,000. All the progeny sired by Charlie were normal in appearance. However, certain pairs of his progeny, when interbred, produced red-and-white progeny at a frequency of about 25 percent. Charlie was soon removed from the stud lists of the Holstein breeders. Use symbols to explain precisely why.

Suppose that a husband and wife are both heterozygous for a recessive allele for albinism. If they have dizygotic (two-egg) twins, what is the probability that both the twins will have the same phenotype for pigmentation?

The plant blue-eyed Mary grows on Vancouver Island and on the lower mainland of British Columbia. The populations are dimorphic for purple blotches on the leaves—some plants have blotches and others don’t. Near Nanaimo, one plant in nature had blotched leaves. This plant, which had not yet flowered, was dug up and taken to a laboratory, where it was allowed to self. Seeds were collected and grown into progeny. One randomly selected (but typical) leaf from each of the progeny is shown in the accompanying illustration.

Can it ever be proved that an animal is not a carrier of a recessive allele (that is, not a heterozygote for a given gene)? Explain.

In nature, the plant Plectritis congesta is dimorphic for fruit shape; that is, individual plants bear either wingless or winged fruits, as shown in the illustration.

Plants were collected from nature before flowering and were crossed or selfed with the following results:

*Phenotype probably has a nongenetic explanation.

Interpret these results, and derive the mode of inheritance of these fruit-shaped phenotypes. Use symbols. What do you think is the nongenetic explanation for the phenotypes marked by asterisks in the table?

The accompanying pedigree is for a rare, but relatively mild, hereditary disorder of the skin.

Four human pedigrees are shown in the accompanying illustration. The black symbols represent an abnormal phenotype inherited in a simple Mendelian manner.

Tay-Sachs disease is a rare human disease in which toxic substances accumulate in nerve cells. The recessive allele responsible for the disease is inherited in a simple Mendelian manner. For unknown reasons, the allele is more common in populations of Ashkenazi Jews of eastern Europe. A woman is planning to marry her first cousin, but the couple discovers that their shared grandfather’s sister died in infancy of Tay-Sachs disease.

The pedigree below was obtained for a rare kidney disease.

This pedigree is for Huntington disease, a late-onset disorder of the nervous system. The slashes indicate deceased family members.

Consider the accompanying pedigree of a rare autosomal recessive disease, PKU.

(Assume that all people marrying into the pedigree lack the abnormal allele.)

A man has attached earlobes, whereas his wife has free earlobes. Their first child, a boy, has attached earlobes.

A rare recessive allele inherited in a Mendelian manner causes the disease cystic fibrosis. A phenotypically normal man whose father had cystic fibrosis marries a phenotypically normal woman from outside the family, and the couple consider having a child.

The allele c causes albinism in mice (C causes mice to be black). The cross C/c × c/c produces 10 progeny. What is the probability of all of them being black?

The recessive allele s causes Drosophila to have small wings, and the s⁺ allele causes normal wings. This gene is known to be X linked. If a small-winged male is crossed with a homozygous wild-type female, what ratio of normal to small-winged flies can be expected in each sex in the F₁? If F₁ flies are intercrossed, what F₂ progeny ratios are expected? What progeny ratios are predicted if F₁ females are backcrossed with their father?

An X-linked dominant allele causes hypophosphatemia in humans. A man with hypophosphatemia marries a normal woman. What proportion of their sons will have hypophosphatemia?

Duchenne muscular dystrophy is sex linked and usually affects only males. Victims of the disease become progressively weaker, starting early in life.

A recently married man and woman discover that each had an uncle with alkaptonuria (black urine disease), a rare disease caused by an autosomal recessive allele of a single gene. They are about to have their first baby. What is the probability that their child will have alkaptonuria?

The accompanying pedigree concerns a rare inherited dental abnormality, amelogenesis imperfecta.

A couple who are about to get married learn from studying their family histories that, in both their families, their unaffected grandparents had siblings with cystic fibrosis (a rare autosomal recessive disease).

A sex-linked recessive allele c produces a red–green color blindness in humans. A normal woman whose father was color blind marries a color-blind man.

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Male house cats are either black or orange; females are black, orange, or calico.

The pedigree below concerns a certain rare disease that is incapacitating but not fatal.

In corn, the allele s causes sugary endosperm, whereas S causes starchy. What endosperm genotypes result from each of the following crosses?

A plant geneticist has two pure lines, one with purple petals and one with blue. She hypothesizes that the phenotypic difference is due to two alleles of one gene. To test this idea, she aims to look for a 3:1 ratio in the F₂. She crosses the lines and finds that all the F₁ progeny are purple. The F₁ plants are selfed, and 400 F₂ plants are obtained. Of these F₂ plants, 320 are purple and 80 are blue. Do these results fit her hypothesis well? If not, suggest why.

A geneticist working on peas has a single plant monohybrid Y/y (yellow) plant and, from a self of this plant, wants to produce a plant of genotype y/y to use as a tester. How many progeny plants need to be grown to be 95 percent sure of obtaining at least one in the sample?

A curious polymorphism in human populations has to do with the ability to curl up the sides of the tongue to make a trough (“tongue rolling”). Some people can do this trick, and others simply cannot. Hence, it is an example of a dimorphism. Its significance is a complete mystery. In one family, a boy was unable to roll his tongue but, to his great chagrin, his sister could. Furthermore, both his parents were rollers, and so were both grandfathers, one paternal uncle, and one paternal aunt. One paternal aunt, one paternal uncle, and one maternal uncle could not roll their tongues.

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Red hair runs in families, as the pedigree above shows.

(Pedigree data from W. R. Singleton and B. Ellis, Journal of Heredity 55, 1964, 261.)

When many families were tested for the ability to taste the chemical phenylthiocarbamide, the matings were grouped into three types and the progeny were totaled, with the results shown below:

With the assumption that PTC tasting is dominant (P) and nontasting is recessive (p), how can the progeny ratios in each of the three types of mating be accounted for?

A condition known as icthyosis hystrix gravior appeared in a boy in the early eighteenth century. His skin became very thick and formed loose spines that were sloughed off at intervals. When he grew up, this “porcupine man” married and had six sons, all of whom had this condition, and several daughters, all of whom were normal. For four generations, this condition was passed from father to son. From this evidence, what can you postulate about the location of the gene?

The wild-type (W) Abraxas moth has large spots on its wings, but the lacticolor (L) form of this species has very small spots. Crosses were made between strains differing in this character, with the following results:

Provide a clear genetic explanation of the results in these two crosses, showing the genotypes of all individual moths.

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The pedigree above shows the inheritance of a rare human disease. Is the pattern best explained as being caused by an X-linked recessive allele or by an autosomal dominant allele with expression limited to males? (Pedigree data from J. F. Crow, Genetics Notes, 6th ed. Copyright 1967 by Burgess Publishing Co., Minneapolis.)

A certain type of deafness in humans is inherited as an X-linked recessive trait. A man with this type of deafness married a normal woman, and they are expecting a child. They find out that they are distantly related. Part of the family tree is shown here.

How would you advise the parents about the probability of their child being a deaf boy, a deaf girl, a normal boy, or a normal girl? Be sure to state any assumptions that you make.

The accompanying pedigree shows a very unusual inheritance pattern that actually did exist. All progeny are shown, but the fathers in each mating have been omitted to draw attention to the remarkable pattern.