Concept explainers
In general terms, genes found on the same chromosome are linked, and will appear to defy Mendel’s Law of Independent Assortment. This law states that alleles (Links to an external site.) for different traits (Links to an external site.) are transmitted (Links to an external site.) to offspring (Links to an external site.) independently of one another. Functionally, this means that in a dihybrid testcross, in which a heterozygote is crossed to a double homozygous recessive individual, the expected 1:1:1:1 ratio will not be obtained.
Instead, lower than expected numbers of non-parentals will result, because these non-parental flies are the result of recombination during synapsis. Interestingly, and functionally important in this exercise, synapsis only occurs in female fruit flies, requiring that the heterozygote in any study of linkage must be the female.
Determining the relative positions of linked genes on a chromosome can be accomplished by calculating the frequency of crossing-over between two genes of interest. The closer the genes are together, the less frequently crossing-over will occur between them. The probability of crossing-over is expressed as a percentage. Each 1% of crossing-over is also referred to as one map unit (m.u.).
How are cross-over events identified? Consider two flies, a male that is homozygous recessive for the two genes cross-vein wings (cv)and cut wings (ct), so has the genotype cv+ct+/cvct
He is crossed to a double heterozygote female with the genotype cv+ct+/cvct
Question: Assuming that the genes are unlinked, what will be the
Question: If the genes were completely linked (think of them as being at the same spot with no possibility of crossing-over), what phenotypes would you expect? (COMPLETE linkage as described here is not a usual occurrence, but working through this problem helps to visualize the differences)
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Assuming that the genes are unlinked, what will be the
Assuming that the genes are unlinked, what will be the
- The tiny blue-eyed Mary flower is often one of the first flowers seen in the spring in some regions of the United States. The flower is normally blue, but sometimes a white or pink flower variation is found. The following data were obtained after several crosses. A student claims that these flower colors are an inherited trait, and the F1 and F2 phenotypes of the flowers arising from the pink and white cross can best be explained by epistasis, where another gene product influences phenotype expression. This data supports this claim because in order to produce a __________ (pink, white, or blue) flower, a gene for a switch that 'turns on' the color gene would be recessive, meaning it has a mutation that causes it to be 'off'. In order to express a color, this switch needs to be in the dominant form. The color gene is a structural gene that has two variants: a dominant trait, ______ (pink, white, or blue) and a recessive _______ (pink, white, or blue) variety.arrow_forwardIn a certain plant, the orange flower allele (Y) is dominant to the yellow flower allele (y). An allele that prevents flower color pigment formation (W) results in white flowers irrespectively of the Y and y alleles, and is dominant to an allele (w) that allows flower color formation. What is the expected proportion of phenotypes in the offspring of a cross between two dihybrid white-flower plants? 1/3 white flowers, 1/3 orange flowers, 1/3 yellow flowers 9/16 white flowers, 3/16 orange flowers, 3/16 yellow flowers, 1/16 orange-yellowish flowers 12/16 white flowers, 3/16 orange flowers, 1/16 yellow flowers 6/8 white flowers, 1/8 orange flowers, 1/8 yellow flowers 1/2 white flowers, 1/4 orange flowers, 1/4 yellow flowersarrow_forwardIn a certain species of plant, a dihybrid (round, pink seeds) is mated with a test-cross strain (wrinkled, white seeds). The following progeny are obtained: round, pink – 440 round, white – 76 wrinkled, pink - 84 wrinkled, white - 400 What is the distance (in map units) between the two genes in question? Your answer should be a single whole number between 0 and 50. Choose 50 if you think the genes are not linked.arrow_forward
- The phenotypes and genotypes that result from a cross of Aa and aa (A = normal pigmentation and a = albinism) arearrow_forwardA plant having the genotype AABb will produce ______ kinds of gametes. (Note that this question refers to the possible allele combinations for the gametes that can be made given the genotype of the parent that is shown, and is not talking about a cross/offspring) a) 1 b) 8 c) 2arrow_forwardSeed color is controlled by 3 independently assorting bi-allelic genes (P, Q, R), such that homozygote pp exhibits recessive epistasis over the seed color pathway, converting a white pigment into yellow, which then becomes orange in the presence of a Q allele, or red in the presence of R. Individuals with both Q and R alleles show pink seeds. In a cross of PpQqRr individuals with ppqqrr individuals, what is the ratio of white-seeds to red seeds? а. 1:4 b. 2:3 с. 1:1 d. 4:1 e. 1:5arrow_forward
- In Drosophila, white eyes (w) are recessive to red eyes (w+) at one locus and black body (b) is recessive to gray body (b+). A homozygous white eyes, gray bodied female is crossed with a homozygous red eyes, black bodied male to produce the F1 progeny. The F1 progeny are testcrossed and produce the following progeny: White eyes, black body: 212 White eyes, gray body: 288 Red eyes, black body: 308 Red eyes, gray body: 192 Does the evidence indicate that w and b loci are linked? Explain why or why not? If they are linked, what is the map distance between the two loci? If they are not linked, what is the map distance between the two loci? If they are linked, are the allels in the F1 in coupling or repulsion? How do you know? Draw the genotypes of all individuals described in the problem (original parents, F1, testcross, and F2 progeny) using the appropriate notation.arrow_forwardIn a dihybrid cross of two bi-allelic Mendelian genes, A (two alleles – completely dominant A and recessive a), and B (two alleles – completely dominant B and recessive b), such that the parental generation comprises of pure-bred homozygotes (i.e. AABB with aabb), what are the expected genotype and phenotype ratios in the F1 and F2, if the F1 cross is (a) an intercross and if the F1 cross is (b) a testcrossarrow_forwardIn the garden pea, several different genes affect pod characteristics.A gene affecting pod color (green is dominant to yellow) is approximately7 mu away from a gene affecting pod width (wide is dominantto narrow). Both genes are located on chromosome 5. A thirdgene, located on chromosome 4, affects pod length (long is dominantto short). A true-breeding wild-type plant (green, wide, longpods) was crossed to a plant with yellow, narrow, short pods. TheF1 offspring were then testcrossed to plants with yellow, narrow,short pods. If the testcross produced 800 offspring, what are theexpected numbers of the eight possible phenotypic combinations?arrow_forward
- Wild-type mice have brown fur and short tails. Loss of function of a particular gene produces white fur, while loss of function of another gene produces long tails, and loss of function at a third locus produces agitated behavior. Each of these loss of function alleles is recessive. If a wild-type mouse is crossed with a triple mutant, and their F1 progeny is test-crossed, the following recombination frequencies are observed among their progeny. Produce a genetic map for these loci. Brown, short tailed, normal: 955 White, short tailed, normal: 16 Brown, short tailed, agitated: 0 White, short tailed, agitated: 36 Brown, long tailed, normal: White, long tailed, normal: Brown, long tailed, agitated: 46 0 14 White, long tailed, agitated: 933arrow_forwardThree enzymes are part of a sequential pathway that produces a pigment that makes flowers red. Each enzyme is the gene product of a different locus. If a trihybrid, heterozygous (+/-) for each of the three loci, is selfed, what fraction of the offspring will not be able to produce the red pigment (and will thus have white flowers)? Express your answer as a number between 0 and 1, rounding to 2 decimal places. For example, if you think 1/4 of the offspring will have white flowers, then answer 0.25arrow_forwardKernel color in wheat is controlled by 2 pairs of genes (AABB). Determine the color of each offspring with the following genotypes: (Note: 4 alleles – red; 3 – medium red; 2 – intermediate red; 1 – light red; 0 – white). CAPITAL letters only and with spaces when applicable. AABb - AaBb - AABB - aaBb - aabb -arrow_forward
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