Two normal parents have an albino child. Determine the probability that a the next child is albino, 6 the next 2 children are albinos. Short hair is due to a dominant gene L in rabbits, and long hair to its recessive allele 1.
A cross between a short-haired female and a long-haired male produced a litter of 1 long-haired and 7 short-haired bunnies. A dominant gene W produces wire-haired texture in dogs; its recessive allele w produces smooth hair. A group of heterozygous wire-haired individuals are crossed and their F, progeny are then testcrossed.
Determine the expected genotypic and phenotypic ratios among the testcross progeny. Black wool of sheep is due to a recessive allele 6 and white wool to its dominant allele B. A white buck male is crossed to a white ewe female , both animals carrying the allele for black. They produce a white buck lamb that is then backcrossed to the female parent. What is the probability of the backcross offspring being black? In foxes, silver-black coat color is governed by a recessive allele 6 and red color by its dominant allele B.
Determine the genotypic and phenotypic ratios expected from the following matings: a pure red X carrier red, 6 carrier red x silver-black, c pure red X silver-black. In the Holstein-Friesian breed of dairy cattle, a recessive allele r is known to produce red and white; the dominant allele R is known to produce black and white. If a carrier bull is mated to carrier cows, determine the probability a of the first offspring being born red and white, 6 of the first 4 offspring born being black and white.
Consider a cross between two heterozygous black guinea pigs Bb. When chickens with splashed white feathers are crossed with black-feathered birds, their offspring are all slate blue Blue Andalusian. When Blue Andalusians are crossed among themselves, they produce splashed white, blue, and black offspring in the ratio of 1 : 2 : 1, respectively.
Yellow coat color in guinea pigs is produced by the homozygous genotype C y C y ,cream color by the heterozygous genotype C y C w ,and white by the homozygous genotype CwCw. What genotypic and phenotypic ratios are matings between cream-colored individuals likely to produce? If long radishes are crossed to oval radishes and the F, then allowed to cross at random among themselves, what phenotypic ratio is expected in the F2'? The Palomino horse is a hybrid exhibiting a golden color with lighter mane and tail.
A pair of codominant alleles D'and D' is known to be involved in the inheritance of these coat colors. Genotypes homozygous for the D'allele are chestnut-colored reddish , heterozygous genotypes are Palomino-colored, and genotypes homozygous for the D' allele are almost white and called cremeflo. When creepers are mated to creepers they produce 2 creepers to 1 normal.
Crosses between normal birds produce only normal progeny. How can these results be explained'? In the Mexican Hairless breed of dogs, the hairless condition is produced by the heterozygous genotype H h.
Normal dogs are homozygous recessive hh. Puppies homozygous for the H allele are usually born dead with abnormalities of the mouth and absence of external ears. If the average litter size at weaning is 6 in matings between hairless dogs, what would be the average expected number of hairless and normal offspring at weaning from matings between hairless and normal dogs?
A pair of codominant alleles is known to govern cotyledon leaf color in soybeans. The homozygous genotype C';C'; produces dark green, the heterozygous genotype C"C produces light green, and the other homozygous genotype C y C yproduces yellow leaves so deficient in chloroplasts that seedlings do not grow to maturity.
If dark-green plants are pollinated only by light-green plants and the FI crosses are made at random to produce an F7, what phenotypic and genotypic ratios would be expected in the mature F, plants'? Thalassemia is a hereditary disease of the blood of humans resulting in anemia. If all individuals with thalassemia major die before sexual maturity, a what proportion of the adult F, from marriages of thalassemia minors by normals would be expected to be normal, 6 what fraction of the adult FI from marriages of minors by minors would be expected to be anemic?
The Pelger anomaly of rabbits involves abnormal white blood cell nuclear segmentation. Pelgers are heterozygous , normal individuals are homozygous PP. The homozygous recessive genotypes p p have grossly deformed skeletons and usually die before or soon after birth. If Pelgers are mated together, what phenotypic ratio is expected in the adult Fz? List all of the genotypes possible for each of the phenotypes in this series.
Plumage color in mallard ducks is dependent upon a set of 3 alleles: M R for restricted mallard pattern, M for mallard, and m for dusky mallard. A number of self-incompatibility alleles is known in clover such that the growth of a pollen tube down the style of a diploid plant is inhibited when the latter contains the same self-incompatibility allele as that in the pollen tube. Given a series of self-incompatibility alleles S ' , S2,S 3 , S4, what genotypic ratios would be expected in embryos and in endosperms of seeds from the following crosses?
The coat colors of many animals exhibit the "agouti" pattern, which is characterized by a yellow band of pigment near the tip of the hair. In rabbits, a multiple allelic series is known where the genotypes E D E D and EDe produce only black nonagouti , but the heterozygous genotype EDE produces black with a trace of agouti. The genotypes EE or Ee produce full color, and the recessive genotype ee produces reddishyellow.
The S allele puts a band of white color around the middle of the animal and is referred to as a Dutch belt; the s" allele produces Hereford-type spotting; solid color is a result of the s ' allele; and Holstein-type spotting is due to the s allele.
Homozygous Dutch-belted males are crossed to Holstein-type spotted females. The Fl females are crossed to a Hereford-type spotted male of genotype shs".
Predict the genotypic and phenotypic frequencies in the progeny. The genetics of the AB0 human blood groups was presented in Example 2. A man of blood group B is being sued by a woman of blood group A for paternity. The woman's child is blood group 0. A fullcolored mouse carrying the lethal is mated to a dilute-colored mouse also carrying the lethal.
The F, is backcrossed to the dilute parent. The phenotypic expression of a dominant gene in Ayrshire cattle is a notch in the tips of the ears. In the pedigree below, where solid symbols represent notched individuals, determine the probability of notched progeny being produced from the matings a x , 6 x , c x , d x , e X I I11 2. Dark hair is largely due to its dominant allele R.
In the family pedigree shown below, unless there is evidence to the contrary, assume that individuals who marry into this family do not carry the r allele.
Solid symbols represent red hair; open symbols represent dark hair. In the following pedigree assume that those individuals brought into the family from outside do not carry the gene for solid color, unless there is evidence to the contrary. Solid symbols represent solidcolored animals, open symbols represent spotted animals. The allele As produces an even distribution of dark pigment over the body; the allele a" reduces the intensity of pigmentation and produces sable or tan-colored dogs; the allele a' produces spotted patterns such as tan and black, tan and brown, etc.
Given the following family pedigree, a determine the genotypes of all the individuals insofar as possible, 6 calculate the probability of spotted offspring being produced by mating by , c find the fraction of the dark-pigmented offspring from I1 x I13 that is expected to be heterozygous. Match the correct answer in the right column with the question in the left column.
Female Male 1. All Bb : t white C. All bb D. All white F. All BB G. All black B. Any measurable or distinctive characteristic or trait possessed by an organism. The genetic endowment of an individual or cell. A cell produced by the union of gametes carrying identical alleles. A cell produced by the union of gametes carrying different alleles. Adjective descriptive of an allele that is not expressed in a heterozygote; also descriptive of the phenotype produced when the allele is homozygous.
Any phenotype that is extremely rare in a natural population. One or two words. Adjective describing any pair of alleles that interact in the heterozygous condition to produce a phenotype different from those of the respective homozygotes. Any gene that when homozygous results in death of the individual prior to sexual maturity.
The proportion of individuals of a specified genotype that shows the expected phenotype. The degree of effect produced by a given genotype under a given set of environmental conditions or over a range of environmental conditions.
A phenotype is either the product of gene or of environmental influences. Bamng mutation, a pure line is expected to breed true to type.
An individual with a hybrid genotype or in a carrier state must also be heterozygous. The simplest multiple allelic system consists of 3 alleles. A backcross is equivalent to a testcross. In pedigrees, circles represent males and squares represent females. The probabilities of independent events are added to find the probability of their joint occurrence [CHAP.
Codominant alleles cannot express lethality when homozygous. Dominant traits are expected to be the most frequent phenotypes in a population. The F7 generation is conventionally produced by random union of the F, gametes. Questions use the following information. In guinea pigs, black coat color governed by gene B is a dominant trait, and white attributed to allele 6 is a recessive trait.
A black female is testcrossed, producing 6 black offspring. When heterozygous black pigs are intercrossed, approximately what fraction of the black progeny are expected to be homozygous? How many genetically different kinds of matings can be made in a population containing these 2 allelcs ignoring reciprocal crosses '? The A B 0 blood groups of humans are determined by 3 alleles Example 2. How many genotypes are U 3 b 4 c 6 d 8 P none of the above possible for these phenotypes'?
A mother of blood group 0 has a group 0 child. The father could be c A o r B d AB only e none of the above 8. When snapdragons with pale-green leaves are intercrossed, their adult progeny consist of approximately 1 dark green: 2 pale green.
What portion of all F, zygotes are expected to reach sexual maturity? Creepers are heterozygous. Normal birds and lethal zygotes are homozygous for alternative alleles.
One of the alleles is dominant with respect to the creeper phenotype; the other allele is dominant with respect to viability. SJ:4S'S'S' 2. In certain cases. C Vocabulary 6. F only if the backcross involves mating an offspring of dominant 4.
F orboth 2. F vice versa 7. F multiplied 8. F see phenotype back to a parent with recessive phenotype 9. F frequencies of genes are a function of evolutionary forces, such as natural selecExample 2.
T Multiple-Choice Questions 1. Example 3. Two different genotypes produce a black, long-haired pig: BBll or Bbll; likewise 2 genotypes for a white, short-haired pig: bbLL or bbLI; and only 1 genotype specifies a white, longhaired pig: A dihybrid genotype is heterozygous at 2 loci. Dihybrids form 4 genetically different gametes with approximately equal frequencies because of the random orientation of nonhomologous chromosome pairs on the first meiotic metaphase plate Chapter 1.
A dihybrid black, short-haired guinea pig BbLI produces 4 types of gametes in equal frequencies. Gametes Freauencv A summary of the gametic output for all 9 genotypes involving two pairs of independently assorting factors is shown below.
The phenotypes of the offspring produced by a testcross reveal the number of different gametes formed by the parental genotype under test. When all of the gametes of an individual are known, the genotype of that individual also becomes known. A monohybrid testcross gives a 1 : 1 phenotypic ratio, indicating that one pair of factors is segregating.
A dihybrid testcross gives a 1 : 1 : 1 : 1 ratio, indicating that two pairs of factors are segregating and assorting independently. Testcrossing a dihybrid yields a 1 : 1 : 1 : 1 genotypic and phenotypic ratio among the progeny. Gametic Checkerboard Method. When 2 dihybrids are crossed, four kinds of gametes are produced in equal frequencies in both the male and the female. A 4 X 4 gametic checkerboard can be used to show all 16 possible combinations of these gametes.
This method is laborious and time-consuming, and offers more opportunities for error than the other methods that follow. Genotypic and Phenotypic Checkerboard Methods. A knowledge of the monohybrid probabilities presented in Chapter 2 may be applied in a simplified genotypic or phenotypic checkerboard. Genotypic checkerboard. Let us place these genotypic probabilities in a checkerboard and combine independent probabilities by multiplication.
Branching Systems. This procedure was introduced in Chapter 1 as a means for determining all possible ways in which any number of chromosome pairs could orient themselves on the first meiotic metaphase plate. It can also be used to find all possible genotypic or phenotypic combinations. It will be the method of choice for solving most examples in this and subsequent chapters. Genotypic trichotomy. A mathematical solution can be readily obtained by combining independent probabilities.
This ratio appears whenever the alleles at both loci display dominant and recessive relationships. The classical dihybrid ratio may be modified if one or both loci have codominant alleles or lethal alleles. A summary of these modified phenotypic ratios in adult progeny is shown below. Black coat color in Cocker Spaniels is governed by a dominant allele B and red coat color by its recessive allele 6 ; solid pattern is governed by the dominant allele of an independently assorting locus S , and spotted pattern by its recessive allele s.
A solid-black male is mated to a solid-red female and produces a litter of 6 pups: 2 solid black, 2 solid red, 1 black and white. Determine the genotypes of the parents. Solution: An unknown portion of a genotype will be indicated by a dash P: solid-black male F, : B-S2 solid black : 2 solid red x : -.
The black-and-white pup also indicates that both parents were heterozygous at the S locus. The solid-red pups likewise indicate that the male parent must have been heterozygous at the B locus.
The solid-black pups fail to be of any help in determining the genotypes of these parents. Complete genotypes may now be written for both parents and for 2 of the pups. How many different crosses may be made of factors, and X from any given number from a single pair of factors. The symmetry of matings above and below the squares on the diagonal becomes obvious. The number of different crosses may be counted as follows: 3 in the first column. If a 9 x 9 checkerboard were constructed, the same symmetry would exist above and below the squares on the diagonal as was shown in part a.
Thus at each of the individual loci a 3 : 1 Fz phenotypic ratio is observed, just as would be expected for a monohybrid cross. Tall tomato plants are produced by the action of a dominant allele D ,and dwarf plants by its recessive allele d.
Hairy stems are produced by a dominant gene H, and hairless stems by its recessive allele h. A dihybrid tall, hairy plant is testcrossed. The F, progeny were observed to be 1 18 tall, hairy : dwarf, hairless : tall, hairless : dwarf, hairy. Thus the testcross results for each locus individually approximate a 1 : 1 phenotypic ratio.
Whenever the results of a testcross approximate a 1 : 1 : 1 : 1 ratio, it indicates that the two gene loci are assorting independently of each other in the formation of gametes. That is to say, all four types CHAP.
A dominant allele L governs short hair in guinea pigs and its recessive allele 1 governs long hair. From matings between dihybrid short, cream pigs LlC'C w , predict the phenotypic ratio expected in the progeny.
Solution: O. We may therefore accept the data as being in conformity with a 9 : 3 : 3 : 1 ratio, indicating that the gene for cotyledon color assorts independently of the gene for pod form. Pure red-fleshed tomatoes crossed with yellow-fleshed tomatoes produced an all red F1. Among F2 plants, 90 were yellow. Test this hypothesis by use of formula 7. A total of families with 4 children each were surveyed with the following results: Girls Families 7 55 50 16 32 Is the family distribution consistent with the hypothesis of equal numbers of boys and girls?
We may therefore accept the hypothesis, but the test would be more definitive if it could be run on a larger sample. It is a well-known fact that a greater mortality occurs in males than in females and therefore an attempt should be made to ascertain family composition on the basis of sex of all children at birrh including prematures, aborted fetuses, etc. Correction for Small Samples 7.
In Problem 7. Apply the Yates correction for continuity and retest the data. This is not a significant chi-square value. Because the data are discrete jumping from unit to unit there is a tendency to underestimate the probability, causing too many rejections of the null hypothesis.
The Yates correction removes this bias and produces a more accurate test near the critical values column headed by a probability of 0. Black hair in the guinea pig is dominant to white hair. In families of 5 offspring where both parents are heterozygous black, with what frequency would we expect to find a 3 whites and 2 blacks, 6 2 whites and 3 blacks, c 1 white and 4 blacks, d all whites?
In families of five children, what is the probability of finding a 3 or more boys, b 3 or more boys or 3 or more girls? A dozen strains of corn are.
How many different ways can these strains be paired? Five coat colors in mice are agouti, cinnamon, black, chocolate, and albino. In mice litters of size 8 , determine a the most frequently expected number of males and females, b the term of the binomial part a represents, c the percentage of all litters of size 8 expected to have 4 males and 4 females. Find the number of possible a genotypes, 6 types of matings. White plumage in chickens can result from the action of a recessive genotype CT or from the action of an independently assorting dominant gene 1.
The hybrid bird produced by crossing these two breeds is also white. How frequently in clutches a nest of eggs; a brood of chicks of 10 chicks produced by hybrid birds would you expect to find 5 colored and 5 white chicks? A case of dominant interaction among coat colors has been discovered in the dog: B- results in black, bb in brown; I - inhibits color development, ii allows color to be produced. In the FZ,determine the fraction of all litters of size 6 that is expected to contain 3 white, 1 black, and 2 brown puppies.
A pair of alleles in the rat, C and c , act on coat color in such a way that the genotypes CC and Cc allow pigment to be produced, but the genotype cc prevents any pigment from being produced albinos. Black rats possess the dominant gene R of an independently assorting locus. Cream rats are produced by the recessive genotype rr. Determine the percentage of F2 litters of size 7 which is expected to have 4 black, 2 cream, and 1 albino.
Two independently assorting loci, each with codominant allelic pairs, are involved in the shape and color characteristics of radishes. The shape may be long or round, due to different homozygous genotypes, or oval due to the heterozygous genotype.
The color may be red or white due to different homozygous genotypes, or purple due to the heterozygous genotype. A long white variety is crossed to a round red variety. The F, is all oval purple. A dozen seeds are saved from each self-pollinated FI plant and grown out the next season in sibling groups.
A bacterial suspension contains 5 million cells per milliliter. This culture is serially diluted tenfold in six successive tubes by adding 1 milliliter from the previous tube into 9 milliliters of diluent fluid. The variance is the square of the standard deviation. Radioactive elements are extensively used in molecular genetic research. Radioactive disintegration of these atoms follow a Poisson distribution. What is the error of estimating the mean number of radioactive disintegrations per minute dpm if dpm are actually detected when counted for a 1 minute?
If two linked genes produce Find the number of degrees of freedom in applying a chi-square test to the results from e testcrossing a dihybrid, f testcrossing a trihybrid, g trihybrid X trihybrid cross, h mating repulsion dihybrid Drosophila males and females. Two phenotypes appear in an experiment in the numbers 4 : Would a sample with the same proportional deviation fit a 3 : 1 ratio if it were 6 10 times larger than a , c 20 times larger than a?
Reds crossed to whites produced only pink offspring. When pink flowered plants were crossed they produced red, white, and pink. It is hypothesized that these colors are produced by a single-gene locus with codominant alleles. Is this hypothesis acceptable on the basis of a chi-square test? Matings between creepers produced creeper : normal progeny. Genetically pure white dogs, when testcrossed to brown dogs, produce an all-white F,. Data on FZ progeny: white, 41 black, 13 brown.
These coat colors are postulated to be under the genetic control of two loci exhibiting dominant epistasis 12 : 3 : 1 ratio expected. Are these results consistent with the hypothesis?
Pure black rats, when testcrossed to albinos, produce only black F, offspring. The F2 in one experiment was found to consist of 43 black, 14 cream, and 22 albino. The genetic control of these coat colors is postulated to involve two gene loci with recessive epistasis 9 : 3 : 4 ratio expected.
Is the genetic hypothesis consistent with the data? Colored aleurone in corn is hypothesized to be produced by the interaction of 2 dominant genes in the genotype A-C-; all other genotypes at these two loci produce colorless aleurone. A homozygous colored strain is testcrossed to a pure colorless strain. The FI exhibits only kernels with colored aleurone.
The F2 exhibits colored : colorless. Analyze the data by chi-square test. The results of phenotypic analysis of 96 F2 progeny in two replicate experiments is shown below. A total of families with six children each were surveyed with the results shown below. Does this distribution indicate that boys and girls are occurring with equal frequency? A virescent-4 strain is crossed to a strain homogyzous for chocolate pericarp. The FI is then testcrossed. The resulting progeny are scored for phenotype with the following results: green seedling, light pericarp : green seedling, chocolate pericarp : virescent seedling, light pericarp : virescent seedling, chocolate pericarp.
Purple anthocyanin pigment in tomato stems is governed by a dominant gene A , and its recessive allele a produces green stem. Hairy stem is governed by a dominant gene HI, and hairless stem by its recessive allele hl. A dihybrid purple, hairy plant is testcrossed and produces 73 purple, hairy : 12 purple, hairless : 75 green, hairless : 9 green, hairy.
What conclusion do you reach? In guinea pigs, it is hypothesized that a dominant allele L governs short hair and its recessive allele 1 governs long hair. Codominant alleles at an independently assorting locus are assumed to govern hair color. Are the data consistent with the genetic hypothesis? Observations of 30 : 3 in a genetic experiment are postulated to be in conformity with a 3 : 1 ratio.
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