genetics 4-8

Many of the color varieties of summer squash are determined by several interacting loci: AA or Aa gives white, aaBB or aaBb gives yellow, and aabb produces green. Crosses among heterozygotes give a 12:3:1 ratio. What type of gene interaction would account for these results?

epistasis

The white-eye gene in Drosophila is recessive and sex-linked. Assume that a white-eyed female is mated to a wild-type male. What would be the phenotypes of the offspring?

females wild type, males white eyed

The enzyme glucose-6-phosphate dehydrogenase deficiency (G6PD) is inherited as a recessive gene on the X chromosome in humans. A phenotypically normal woman (whose father had G6PD) is married to a normal man. (a) What fraction of their sons would be expected to have G6PD? (b) If the husband had G6PD, would it make a difference in your answer in part (a)?

(a) 1/2 (b) no

In the mouse, gene A allows pigmentation to be deposited in the individual coat hairs; its allele a prevents such deposition of pigment, resulting in an albino. Gene B gives agouti (wild-type fur); its allele b gives black fur. (a) Diagram the cross between a doubly heterozygous agouti mouse mated with a doubly homozygous recessive white mouse. (b) What would be the expected phenotypic ratio in the progeny?

(a) AaBb × aabb (b) 1 (agouti):1 (black):2 (albino)

Assume that the gene trpA in an auxotrophic strain of E. coli is located at 27 minutes, whereas the gene pyrE is located at 81 minutes. (a) How are minutes arrived at in this context? (b) Present an experimental scheme that would allow you to convert one of the auxotrophic strains to a prototrophic strain.

(a) Minutes are arrived at by a Hfr × F- mating as in the Klug and Cummings text. (b) Hfr (wild type) × F- (auxotroph)

A mutant gene that produces brown eyes (bw) is located on chromosome #2 of Drosophila melanogaster, whereas a mutant gene producing bright red eyes, scarlet (st), is located on chromosome #3. Phenotypically, wild-type flies (with dull red eyes), whose mothers had brown eyes and whose fathers had scarlet eyes, were mated. The 800 offspring possessed the following phenotypes: wild type (dull red), white, scarlet (bright red), and brown. Most of the 800 offspring had wild-type eyes, whereas those with white eyes were the least frequent. (a) Using standard symbolism, diagram the cross from the P generation (brown-eyed mothers scarlet-eyed fathers) and the F1 generation. Be certain to provide the alleles of the mutant genes. (b) From the information presented above, how many white-eyed flies would you expect in the F2 generation?

(a) P bw/bw; st+/st+ × bw+/bw+; st/st F1: bw+/bw; st+/st × bw+/bw; st+/st (b) 50

Regarding the ABO blood group system in humans, if an individual is genetically IBi and yet expresses the O blood type, which genotype is it likely to have?

hh (Bombay)

Provide an example of sex-influenced inheritance.

pattern baldness in humans, horn formation in sheep, certain coat patterns in sheep

When a bacteriophage genome incorporates itself into the chromosome of the host, that phage genome is referred to as a(n) ________.

prophage

Bacteriophages that cannot undergo lysogeny but can infect bacteria are called ________.

virulent

A cross was made between homozygous wild-type female Drosophila and yellow-bodied male Drosophila. All of the resulting offspring were phenotypically wild type. Offspring of the F2 generation had the following phenotypes: Sex Phenotype Number male wild 96 male yellow 99 female wild 197 Based on this information: (a) Is the mutant gene for yellow body behaving as a recessive or dominant? (b) Is the yellow locus on an autosome or on the X-chromosome?

(a) recessive (b) X-chromosome

In mice, there is a set of multiple alleles of a gene for coat color. Four of those alleles are as follows: C = full color (wild) cch = chinchilla cd = dilution c = albino Given that the gene locus is not sex-linked and that each allele is dominant to those lower in the list, diagram the crosses indicated below and give the phenotypic ratios expected from each.

(a) wild (heterozygous for dilution) × chinchilla (heterozygous for albino) (b) chinchilla (heterozygous for albino) × albino Answer: (a) Ccd × cchc ⇒ 2 full color:1 chinchilla:1 dilution (b) cchc × cc ⇒ 1 chinchilla:1 albino

The genes for zeste eyes and forked bristles are located on the X chromosome in Drosophila melanogaster. Both genes are recessive. A cross is made between a zeste-eyed female and a forked-bristled male. (a) If 200 offspring from this cross were obtained, present the expected number, sex, genotype, and phenotype in each class of the F1. (b) If the F1 offspring were crossed to produce 800 flies of an F2 generation, present the expected number, sex, and phenotype in each class. Assume no crossing over.

(a) z+f/z f+ = wild female (100) z f+/Y = zeste male (100) (b) z+f/z f+ = wild female (200) z f+/z f+ = zeste female (200) z+f/Y = forked male (200) z f+/Y = zeste male (200)

Comb shape in chickens represents one of the classic examples of gene interaction. Two gene pairs interact to influence the shape of the comb. The genes for rose comb (R) and pea comb (P) together produce walnut comb. The fully homozygous recessive condition (rrpp) produces the single comb. Assume that a rose-comb chicken is crossed with a walnut-comb chicken and the following offspring are produced: 17 walnut, 16 rose, 7 pea, 6 single. (a) What are the probable genotypes of the parents? (b) Give the genotypes of each of the offspring classes.

(a) Rrpp × RrPp (b) R_Pp (walnut) R_pp (rose) rrPp (pea) rrpp (single)

Two forms of hemophilia are determined by genes on the X chromosome in humans. Assume that a phenotypically normal woman whose father had hemophilia is married to a normal man. What is the probability that their first daughter will have hemophilia?

0

Two forms of hemophilia are determined by genes on the X chromosome in humans. Assume that a phenotypically normal woman whose father had hemophilia is married to a normal man. What is the probability that their first son will have hemophilia?

1/2

A cross is made between a female calico cat and a male cat having the gene for black fur on his X chromosome. What fraction of the offspring would one expect to be calico?

1/4

Many of the color varieties of summer squash are determined by several interacting loci: AA or Aa gives white, aaBB or aaBb gives yellow, and aabb produces green. Assume that two fully heterozygous plants are crossed. Give the phenotypes (with frequencies) of the offspring.

12 (white):3 (yellow):1 (green)

Assume that a dihybrid F2 ratio, resulting from epistasis, was 9:3:4. If a double heterozygote was crossed with the fully recessive type, what phenotypic ratio is expected among the offspring?

1:1:2

Assume that a cross is made between two organisms that are both heterozygous for a gene that shows incomplete dominance. What phenotypic and genotypic ratios are expected in the offspring?

1:2:1

Assume that a dihybrid cross (AaBb × AaBb) is made in which the gene loci are autosomal, independently assorting, and incompletely dominant. What phenotypic ratio would you expect from such a cross? Just provide the ratio, not the phenotypes.

1:2:1:2:4:2:1:2:1

A particular cross gives a modified dihybrid ratio of 9:7. What phenotypic ratio would you expect in a testcross of the fully heterozygous F1 crossed with the fully recessive type? Diagram the testcross using A,a,B,b as symbol sets.

1:3 AaBb × aabb > AaBb, Aabb, aaBb, aabb

The following F2 results occur from a typical dihybrid cross: purple: A_B_ 9/16 white: aaB_ 3/16 white: A_bb 3/16 white: aabb 1/16 If a double heterozygote (AaBb) is crossed with a fully recessive organism (aabb), what phenotypic ratio is expected in the offspring?

3 (white):1 (purple)

Assume that a dihybrid F2 ratio, resulting from epistasis, was 15:1. If a double heterozygote was crossed with the fully recessive type, what phenotypic ratio is expected among the offspring?

3:1

In a mating between individuals with the genotypes IAi × ii, what percentage of the offspring would be expected to have the O blood type?

50%

Assume that one counted 67 plaques on a bacterial plate where 0.1ml of a 10-5 dilution of phage was added to bacterial culture. What is the initial concentration of the undiluted phage?

67 × 105 × 10 = 6.7 × 107 pfu/ml (pfu = plaque-forming units)

Assume that a dihybrid cross is made in which the genes' loci are autosomal, independently assorting, and incompletely dominant. How many different phenotypes are expected in the offspring?

9

Which types of ratios are likely to occur in crosses (F2) when one is dealing with two interacting, epistatic gene pairs?

9:7, 9:3:4, 12:3:1, 15:1

What is a bacteriophage?

A bacteriophage is a virus that has a bacterium as its host.

What is a merozygote?

A merozygote is a partially diploid cell that results when, through recombination, a portion of a donor chromosome is introduced into a bacterium.

The trait for medium-sized leaves in iris is determined by the genetic condition PP'. Plants with large leaves are PP, whereas plants with small leaves are P'P'. The trait for red flowers is controlled by the genes RR, pink by RR', and white by R'R'. A cross is made between two plants each with medium-sized leaves and pink flowers. If they produce 320 seedlings, what would be the expected phenotypes, and in what numbers would they be expected? Assume no linkage.

Answer: 20 large, red 40 medium, red 20 small, red 40 large, pink 80 medium, pink 40 small, pink 20 large, white 40 medium, white 20 small, white

Describe how different strains of E. coli can reveal different linkage arrangements of genes in Hfr crosses.

Different strains may have different F factors and therefore different initiation points for chromosome transfer.

How does an auxotroph differ from a prototroph?

Auxotrophs have lost, through mutation, the ability to grow on minimal medium. Prototrophs are capable of growth on minimal medium.

Compare and contrast bacteriophage lysis and lysogeny.

Both lysis and lysogeny involve bacterial/phage interactions related to the production of phage progeny. Lysis occurs when progeny phage burst from the bacterial cell; lysogeny involves the incorporation of the phage chromosome into the bacterial chromosome.

The following coat colors are known to be determined by alleles at one locus in horses: palomino = golden coat with lighter mane and tail cremello = almost white chestnut = brown The following table gives ratios obtained in matings of the above varieties: Cross Parents Offspring 1 cremello × cremello all cremello 2 chestnut × chestnut all chestnut 3 cremello × chestnut all palomino 4 palomino × palomino 1/4 = chestnut 1/2 = palomino 1/4 = cremello Assign gene symbols for the genetic control of coat color on the basis of these data. Diagram the last two matings.

C1C1 = cremello, C2C2 = chestnut, C1C2 = palomino (3) C1C1 × C2C2 (4) C1C2 × C1C2

Jacob, Wollman, and others developed a linkage map of E. coli that is based on time. What form of recombination is involved in generating a linkage map based on time?

Conjugation: An Hfr bacterium conjugating with an F- strain allows one to generate a map that is dependent on the passage of the donor chromosome across a conjugation tube.

What is meant by the term cotransformation?

Cotransformation occurs when several linked genes are transformed simultaneously.

What is meant by the term epistasis? Distinguish between epistasis and dominance. Do not use examples in answering this question.

Epistasis refers to a gene (or genes) of one pair masking the expression of a gene (or genes) at a different locus. Dominance refers to the form of expression of a gene in relation to its allele (or alleles). When an allele is dominant, the heterozygous combination is the same phenotypically as one of the homozygotes. Epistasis is a nonallelic interaction; dominance is an allelic interaction.

Distinguish between F+ and F- bacteria.

F+ bacteria contain an F factor or plasmid that is capable of initiating conjugation. F- bacteria contain an F plasmid that possess a portion of the bacterial chromosome.

A 9:7 ratio indicates incomplete dominance.

FALSE

A plaque is a substance that causes mutation in bacteria

FALSE

Expressivity is the term used to describe the balanced genetic output from a hemizygous condition.

FALSE

Genomic imprinting occurs when one allele converts another.

FALSE

In a bacterial cross in which the donor (Hfr) is a+b+ and the recipient strain (F-) is a-b-, it is expected that recombinant bacteria will all be a+b+.

FALSE

Lysogeny is a process that occurs during transformation and conjugation.

FALSE

One result of X-linkage is a crisscross pattern of inheritance in which sons express recessive genes of their fathers and daughters express recessive genes of their mothers.

FALSE

Pattern baldness and hen/cock feathering in fowl are examples of X-linked inheritance.

FALSE

Penetrance specifically refers to the expression of lethal genes in heterozygotes.

FALSE

R plasmids often contain genes for antibody production.

FALSE

Sex-limited inheritance is the same as sex-linked inheritance.

FALSE

The "interrupted mating technique" provides a genetic map in Drosophila.

FALSE

The ABO blood group locus in humans provides an example of epistasis.

FALSE

The term expressivity defines the percentage of individuals who show at least some degree of expression of a mutant genotype.

FALSE

To produce recombinants in bacteria, one crossover is better than two.

FALSE

With both incomplete dominance and codominance, one expects heterozygous and homozygous classes to be phenotypically identical.

FALSE

Explain what is meant by the term heteroduplex in the context of bacterial transformation.

If transforming DNA is derived from a genetically distinct bacterium, incoming DNA may be different from the host DNA. During initial stages of integration into the bacterial chromosome, the recombinant region contains one strand of DNA that may have a different base sequence than the other strand. Because these strands are not genetically identical (complementary), this double-stranded region is called a heteroduplex.

State a significant difference between X-linked and sex-influenced inheritance.

In X-linked inheritance, the gene in question is on the X chromosome; in sex-influenced inheritance, the gene is autosomal.

What is a significant difference between a lytic and a lysogenic cycle?

In a lytic cycle, the phage genome does not insert into the bacterial chromosome. In the lysogenic cycle, the phage genome inserts into the bacterial chromosome.

What distinguishes sex-limited from sex-influenced inheritance?

In sex-limited inheritance, expression is limited to one sex; in sex-influenced inheritance, expression differs between the sexes.

Lysogeny is an important phenomenon in bacteria and phages. Briefly describe lysogeny (using labeled diagrams if helpful).

Lysogeny is the process in which a temperate bacteriophage infects a bacterial cell and subsequently integrates its chromosome into the bacterial chromosome.

Explain the composition and use of minimal medium in the study of bacterial genetics.

Minimal medium consists of an organic carbon source such as glucose or lactose and a variety of inorganic ions: Na+, K+, Mg++, Ca++ and NH4+. It is useful in isolating bacterial strains (auxotrophs) that are incapable of synthesizing more complex nutritional requirements.

What experimental observation demonstrated that genes are involved in the integration of foreign DNA into the bacterial chromosome?

Mutations allowed identification of rec genes. When such genes were mutated, integration does not occur.

In Drosophila, an individual female fly was observed to be of the XXY chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's father had red eyes, and the mother had white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, white-eyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality.

Nondisjunction could have occurred either at meiosis I or meiosis II in the mother, thus giving the XwXwY complement in the offspring.

In Drosophila, an individual female fly was observed to be of the XXY chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's mother and father had red eyes. The mother, however, was heterozygous for the gene for white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, white-eyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality.

Nondisjunction would have occurred at meiosis II in the mother, thus giving the XwXwY complement in the offspring.

What are prototrophs?

Prototrophs are bacteria that can grow on minimal medium and are assumed to be wild type.

Give the sex of the following organisms assuming that the autosomes are present in the normal number. Sex Chromosome Organism Complement Humans Drosophila XX _____ _____ XY _____ _____ XO _____ _____ XXX _____ _____ XXY _____ _____

Sex Chromosome Organism Complement Humans Drosophila XX female female XY male male XO female male XXX female female XXY male female

"Gain of function" mutations are generally dominant since one copy in a diploid organism is sufficient to alter the normal phenotype.

TRUE

A conditional mutant is one whose expression is influenced by some environmental condition.

TRUE

A symbiotic relationship between a phage and a bacterium apparently occurs in the process of lysogeny.

TRUE

A typical epistatic ratio is 9:3:4.

TRUE

Alleles that are masked by an epistatic locus are said to be hypostatic to the genes at that locus

TRUE

An Hfr cell can initiate chromosome transfer from one E. coli to another.

TRUE

Assume that a mutation occurs in the gene responsible for the production of hexosaminidase A, such that only about 50% of the enzyme activity is found in the heterozygote compared with a homozygous normal individual. If heterozygotes are phenotypically normal, we would say that the mutant allele is recessive to its normal allele.

TRUE

Cotransduction of genes is an indication that the genes are linked.

TRUE

Genomic anticipation refers to observations that a genetic disorder occurs at an earlier age in successive generations, whereas genetic imprinting occurs when gene expression varies depending on parental origin.

TRUE

Hemizygosity is the term one uses to describe the state of a gene that has no allele on the opposing chromosome.

TRUE

Lysogeny is most likely associated with transduction.

TRUE

Viral mutations and variants are often categorized by changes in host range and/or plaque morphology.

TRUE

With multiple alleles, there can be more than two genetic alternatives for a given locus.

TRUE

What is the role of the F factor in bacterial recombination?

The F factor confers the capacity to produce a conjugation tube, which functions in conjugation by generating the Hfr and F' recombinant possibilities.

In what way was the interrupted mating technique used to generate a genetic map in E. coli based on time?

The interrupted mating technique showed that genes were passed in a linear fashion from the Hfr bacterial strain to an F- strain. By interrupting the mating tube, it could be determined when (in minutes) genes were transferred.

A color-blind, chromatin-positive male child (one Barr body) has a maternal grandfather who was color blind. The boy's mother and father are phenotypically normal. Construct and support (using appropriately labeled diagrams) a rationale whereby the chromosomal and genetic attributes of the chromatin-positive male are fully explained.

The female (mother) must be heterozygous and undergo nondisjunction at meiosis II to produce the XrgXrgY boy.

If an X-linked disorder is lethal to the affected individual prior to the age at which one reaches reproductive maturation, the lethality will be expressed only in males. Why is this so?

The only sources of the lethal allele in the population are heterozygous females who are "carriers" and do not express the disorder.

What is the consequence of a mutation in the recA gene in bacteria?

The wild-type product of recA is required for bacterial recombination to occur at an appreciable level. Absence of the recA protein diminishes recombination by about 1000-fold.

A color-blind woman with Turner syndrome (XO) has a father who is color blind. Given that the gene for the color-blind condition is recessive and X-linked, provide a likely explanation for the origin of the color-blind and cytogenetic conditions in the woman.

The woman inherited an Xrg chromosome from the father. Nondisjunction in the female (either at meiosis I or II) produced an egg with no X chromosome, which, when fertilized by the Xrg-bearing sperm, produced the Turner syndrome condition.

How many different alleles can a gene have?

Theoretically, an extremely large number of possibilities exist. Various bases could change, giving a variety of alleles, and combinations of those changed bases could provide additional variety.

What is the role of the recBCD genes?

They function in bacterial recombination.

Pattern baldness is determined by a single autosomal gene pair. When females are homozygous for this gene pair, can they show pattern baldness?

Yes, but the phenotype is less pronounced and is expressed later in life.

Can females display pattern baldness?

When females inherit the BB genotype, they can definitely express hair thinning; however, it is less pronounced than in males and occurs later in life.

Name four modes of inheritance that are influenced by the sex of individuals.

X-linked, sex-influenced, sex-limited, Y-linked

Present the general structural features of a plasmid and give an example.

circular double-stranded DNA existing autonomously in the bacterial cytoplasm; F factor

Bacteria that are in a particular physiological state to become transformed are called ________.

competent

What is a form of recombination in bacteria that involves the F plasmid?

conjugation

Name three forms of recombination in bacteria.

conjugation, transformation, transduction

Name the typical phases of the bacterial growth cycle in liquid culture medium.

lag, log (exponential), and stationary

In general, what two methods are used to grow bacteria in the laboratory?

liquid and semisolid (agar) media

Name the term used to describe the phenomenon in which a bacteriophage genome incorporates its genome into the chromosome of the host.

lysogeny

If two different auxotrophic strains are placed in a liquid medium culture tube, prototrophic strains can sometimes be subsequently recovered. Name several mechanisms by which this is possible.

reverse mutation, genetic suppression, genetic recombination (conjugation, transformation)

A form of bacterial recombination that involves a viral intermediate is called ________.

transduction

In a mating between individuals with the genotypes IAIB × ii, what percentage of the offspring would be expected to have the O blood type?

zero