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The scientist Jean Baptiste Lamarck proposed that if an individual acquired a particular characteristic – such as strength from lifting weights – this characteristic would be inherited by the organism’s offspring. This same idea is also part of a more ancient idea, the idea of _____. |
pangenesis |
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1. A ______ is an inherited feature that varies from individual to individual. |
1. character 2. trait 3. monohybrid cross 4. alleles 5. dominant 6. recessive 7. phenotype 8. genotype |
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Imagine a particular trait that is determined by a single gene. If this gene is present in two forms, how can you tell which allele is dominant and which is recessive? |
Do a test cross between two true-breeding individuals and observe the trait expressed by the F1 plants. |
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Mendel discovered that the allele for green seed pods (G) is dominant to the allele for yellow seed pods (g). He crossed true-breeding (homozygous) parental plants with green (GG) and yellow (gg) seed pods to produce an F1 generation, all of which were heterozygous (Gg) with green seed pods. He then crossed two of these F1 plants to produce an F2 generation. |
a. GG b. green c. Gg d. green e. Gg f. green g. gg h. yellow |
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An insect that has the genotype EeGGcc will have the same phenotype as an insect with the genotype _____. |
EEGgcc |
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In a test cross, F2 refers to _____. |
the second filial generation, or the "grandchildren" of the original mating pair |
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Learning though Art: Genes and Chromosomes |
a. homologous chromosomes b. gene locus c. recessive allele d. centromere e. sister chromatids f. alleles g. dominant allele |
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In an individual of genotype Aa, where are the A and a alleles physically located? |
One allele is on one chromosome, and the other is in the same position (locus) on the homologous chromosome. |
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Mendel’s principle of independent assortment applies to the _____. |
independent assortment of alleles of one gene relative to the alleles of any other gene |
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Assume that in cattle a spotted coat is dominant to an even coat, short horns are dominant to long horns, and the traits for coat type and horn length assort independently. In a cross between cattle that are each heterozygous for both traits, what proportion of their offspring are expected to have long horns? |
1/4 |
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What is the genotype of the parent with orange eyes and white skin? (Note: orange eyes are recessive.) |
bbgg |
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Black eyes are dominant to orange eyes, and green skin is dominant to white skin. Sam, a MendAlien with black eyes and green skin, has a parent with orange eyes and white skin. Carole is a MendAlien with orange eyes and white skin. If Sam and Carole were to mate, the predicted phenotypic ratio of their offspring would be _____. |
1 black eyes, green skin : 1 black eyes, white skin : 1 orange eyes, green skin : 1 orange eyes, white skin |
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In order to determine the genotype of a MendAlien with black eyes and green skin, you would cross this individual with a(n) _____ individual. |
bbgg |
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A cross between two individuals with black eyes and green skin which results in an individual with orange eyes and white skin is an example of a _____ cross. |
dihybrid |
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A phenotypic ratio of 9:3:3:1 in the offspring of a cross indicates that _____. |
both parents are heterozygous for both genes |
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The observed distribution of alleles into gametes is an illustration of _____. |
Mendel’s laws of segregation and independent assortment |
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An individual heterozygous for eye color, skin color, and number of eyes mates with an individual who is homozygous recessive for all three characters; what would be the expected phenotypic ratio of their offspring? [Hint: B = black eyes, b = orange eyes; G = green skin, g = white skin; C = two eyes, c = one eye] |
1 black eyes, green skin, two eyes : 1 black eyes, green skin, one eye : 1 black eyes, white skin, two eyes : 1 black eyes, white skin, one eye : 1 orange eyes, green skin, two eyes : 1 orange eyes, green skin, one eye : 1 orange eyes, white skin, two eyes : 1 orange eyes, white skin, one eye |
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A BbGg x bbgg cross yields a phenotypic ratio of approximately 5 black eyes, green skin : 5 orange eyes, white skin : 1 black eyes, white skin : 1 orange eyes, green skin. Which of the following best explains these results? |
Mendel’s law of independent assortment is being violated. |
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In the following cross the genotype of the female parent is BbGg. What is the genotype of the male parent? [Hint: B = black eyes, b = orange eyes, G = green skin, g = white skin] |
BBGg |
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In a situation in which genes assort independently, what is the ratio of the gametes produced by an AaBB individual? |
1 AB : 1 aB |
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You cross two fruit trees. One tree produces lemons with spiky leaves. The other produces limes with smooth leaves. Your F1 generation produces lemons with smooth leaves and spiky leaves. What are the genotypes of the parents? L = lemons; l = limes; S = smooth leaf; s = spiky leaf. |
LLss and llSs |
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Imagine that long fins in zebrafish is a dominant trait. A breeder wants to set up a breeding program beginning with homozygous dominant long-finned fish. If she obtains a handful of the long-finned fish, how can she tell which if any of these are homozygous for the trait? |
Cross the long-finned fish with short-finned fish; if the offspring are all long-finned, the long-finned parent is homozygous. |
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Mendel worked _____. |
in a monastery |
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In Labrador dogs, black coat is dominant to chocolate, normal vision is dominant to progressive retinal atrophy (PRA), and normal hip joint is dominant to hip dysplasia. All these genes assort independently. Two dogs that are heterozygous for alleles of all three genes are crossed. Using rules of probability (not a Punnett square), what is the chance that the first pup born to these dogs will be chocolate, have normal vision, and have normal hip joints? |
9/64 |
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The human pedigree shown here is for an inherited form of deafness caused by a recessive allele (d). |
a. Dd b. DD or Dd c. dd d. DD or Dd e. DD or Dd f. dd |
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Two normal parents have three normal children: one son and two daughters. Their son and one of their daughters marry and also have normal children. Their second daughter, Mary, marries a man with a rare, recessive blood disorder. They have two children, and both children develop the blood disorder. What were the genotypes of Mary’s parents? |
Either one of her parents or both of her parents were heterozygous for the trait. |
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Why are lethal dominant alleles so much more rare than lethal recessive alleles? |
Lethal dominant alleles are harmful whether they are carried in homozygous or heterozygous form, so there is always strong selection against these alleles. |
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Human genetic disorders _____. |
are most often recessive |
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A large and increasing number of genetic tests are available to prospective parents and children. Even as these testing methods become more and more sophisticated, what is one thing technology will never solve? |
the ability to prescribe the correct course of action based on test results |
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Hypercholesterolemia is a disorder in which blood cholesterol levels are elevated. The H allele is incompletely dominant to the h allele, with hh homozygotes having extremely high levels of blood cholesterol. A husband and wife are both Hh heterozygotes. What is the chance that their first child will have normal levels of blood cholesterol? |
1/4 |
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Akin to urban legends, there are curious genetics legends – things like eye color being determined by one gene, with a brown eye allele being completely dominant to blue. The problem comes when simple myth meets the complex reality of how eye color and many other traits are transmitted. Why is the inheritance of so many traits difficult to explain using only Mendel’s view of genetics? |
Mendel was correct for the traits he investigated, but his principles must be extended (not discarded) to explain many more complex patterns of inheritance. |
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Snapdragons show incomplete dominance in their flowers. A pink snapdragon is crossed with a red snapdragon. What color(s) are the offspring? |
50% red, 50% pink |
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There are over 100 alleles known for the gene associated with cystic fibrosis. With current technology, it is possible to determine exactly which allele or alleles is/are carried by a person. What is the maximum number of different alleles that any person can carry? |
2 |
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Human ABO blood type is determined by a single gene that comes in 3 distinct alleles: IA , IB , and i. The IA and IB alleles are each dominant to the i allele, but are codominant with each other, meaning that both phenotypes are expressed in the heterozygote. The Punnett square below shows a cross between two parents with different blood types. |
a. IAIB b. Type AB c. IAi d. Type A e. IBi f. Type B g. ii h. Type O |
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In lentils, the C gene has two different alleles. CS CS homozygotes have spotted seeds, CD CD homozygotes have dotted seeds, and CS CD heterozygotes have seeds with both spots and dots. This indicates that _____. |
CS and CD are codominant |
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Two individuals decide to have children. The expected blood group genotypes are 50% of blood type A, and 25% each of blood types AB and B. What genotypes are the parents? |
IA i x IA IB |
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If the gene for seed color that Mendel studied exhibited pleiotropy, how might a green pea be different from a yellow pea? |
With pleiotropy, there would be other differences between the peas – for example, green peas may also be larger, ripen earlier, and be more sensitive to frost. |
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In breeding pure-breeding large and small strains of mice, you cross individuals of each strain and note that their offspring are intermediate in size. Two models (explanations) to account for this result are (1) that body size in these strains is due to one gene with alleles that show incomplete dominance and (2) that body size is a polygenic trait. How could you distinguish between these models? |
Intercross the F1 and see if the F2 contains three size classes (consistent with the incomplete dominance model) or if there is a range of sizes (consistent with the polygenic model). |
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In Morgan’s testcross of a gray-bodied, long-winged heterozygous female Drosophila with a homozygous recessive black-bodied, vestigial-winged male, the following offspring were obtained: 965 gray body, long wing; 944 black body, vestigial wing; 206 gray body, vestigial wing; 185 black body, long wing. Focusing only on the recombinant classes (gray body, vestigial wing and black body, long wing), the numbers of offspring of each type are similar (206 and 185). What accounts for the similar number of offspring of each recombinant phenotype? |
Crossing over between chromosomes is reciprocal, so whenever a recombinant chromosome of one type is produced, there’s a recombinant of the opposite type that is also produced. |
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The existence of rare XY individuals who are phenotypically normal women was instrumental in learning about human sex determination. Maleness is determined by the SRY gene found on the Y chromosome. How is it possible to be an XY woman? |
The SRY locus of the Y chromosome is deleted. |
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The disease hemophilia is caused by a single gene that is located on the X chromosome. Because human females have two X chromosomes (XX), they have two copies of this gene. Since hemophilia is recessive (h), a female must have two copies of the recessive allele to have the disease. A human male (XY), on the other hand, has only one X chromosome, and so needs only one copy of the recessive allele to have the disease. |
a. female b. normal c. male d. normal e. female f. carrier g. male h. hemophiliac |
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Vitamin D – resistant rickets is an X-linked dominant bone disorder. A man with this form of rickets marries a normal woman. What proportion of the couple’s daughters is expected to have vitamin D – resistant rickets? |
100% |
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A woman who is a carrier of hemophilia marries a man affected with hemophilia. What percentage of their sons and daughters is expected to have hemophilia? |
50% of sons and 50% of daughters |
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Because it is passed essentially intact from father to son, the Y chromosome has been particularly useful in research on _____. |
recent human evolution |
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