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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 _____.
Mendel's view of the mechanism of heredity was radically different from the prevailing view of the time because he saw heredity working through _____
unchanging (immutable), heritable factors that were contributed by each parent and never mixed
Two mice are crossed. Matings are carried out between the offspring of these mice to produce "grandchildren" of the original mated pair. In the standard terminology of genetics, the "grandchildren" are the _____.
True-breeding individuals differing in a single character, say a pea plant bearing green seeds and another bearing yellow seeds, are crossed. Assuming that this trait is determined by a single gene that is present in two forms (yellow and green, in this case), how can you tell which allele is dominant and which is recessive?
Observe the trait expressed by the F1 plants.
Mendel was a meticulous experimentalist. One set of crosses he performed to test his idea that a pair of hereditary determinants segregated into gametes was to allow self-fertilization of F2 individuals to produce F3 offspring. What proportion of the purple-flowered F2 individuals did Mendel predict to be true-breeding
An insect that has the genotype EeGGcc will have the same phenotype as an insect with the genotype _____
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.
Mendel's principle of independent assortment applies to the _____
independent assortment of alleles of one gene relative to the alleles of any other gene
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?
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
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 rare 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.
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
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.
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
Imagine you're counseling a couple who have undergone carrier screening for Tay-Sachs disease. The man is a carrier, and the woman does not carry the Tay-Sachs allele. How should you advise them?
They should be informed that if they have a child, the child will not have Tay-Sachs disease but will have a 50% chance of being a carrier of the Tay-Sachs allele.
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
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.
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?
Snapdragons show incomplete dominance in their flowers. A pink snapdragon is crossed with a red snapdragon. What is true about the offspring?
50% red, 50% pink
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 are carried by a person. What is the maximum number of different alleles that any person can carry?
In lentils, the C gene has two different alleles. CSCS homozygotes have spotted seeds, CDCD homozygotes have dotted seeds, and CSCD heterozygotes have seeds with both spots and dots. This indicates that _____.
CS and CD are codominant
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?
IAi x IAIB
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.
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).
Our understanding of the role played by genes in many human characteristics—for example, body size, performance on IQ tests, and personality traits—is advancing rapidly. In this new genetic era, the role of the environment _____.
is to work with genes in complex and often unknown ways in the development of these traits
Which of the following line or lines of evidence support the chromosome theory of inheritance?
Genes segregate; chromosomes come in pairs.
In corn, blue kernels are produced by a dominant allele of a coloration gene, and white kernels are produced in individuals homozygous for a recessive allele of the same coloration gene. Another gene has two alleles for shape, with smooth kernels being dominant to wrinkled. A plant heterozygous for both genes is testcrossed (crossed to a homozygous recessive white, wrinkled strain). The testcross offspring consist of the following types: 1,447 blue smooth; 1,436 white wrinkled; 150 blue wrinkled; 145 yellow smooth. Explain the inheritance of the coloration and shape traits.
The coloration and shape genes are linked on the same chromosome.
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.
Imagine that a mutant strain of Drosophila undergoes crossing over at half the normal rate. How would a genetic map prepared for this mutant differ from a genetic map prepared for a normal (wild type) fly?
The order of genes would be the same in both strains, but the distances measured between genes in the mutant would be half those of the wild type
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.
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?
A woman who is a carrier of hemophilia marries a man affected with hemophilia. What percentage of their sons and daughters are expected to have hemophilia?
50% of sons and 50% of daughters
Two true-breeding plants are cross-fertilized. Their offspring are then cross-fertilized. What is the name given to this last offspring generation?
DdEe pea plants can produce _____ type(s) of gametes, but a ddee plant can produce _____ type(s) of gametes.
four ... one
Three characteristics assort independently in one species of insect: Red eyes (R) are dominant to black eyes (r). Blue wings (B) are dominant to white wings (b). Green bodies (G) are dominant to yellow bodies (g). A cross is made between two heterozygous insects (RrBbGg). What is the probability that the offspring will be homozygous dominant for all three characteristics?
Many genetic disorders can be detected before birth. Procedures include _____, which is noninvasive, or _____, which allows the chromosomes of the fetus to be examined. Alternatively, maternal blood samples can be taken and tested for _____ .
ultrasound imaging ... chorionic villus sampling ... AFP
After crossing a true-breeding red-flowered snapdragon with a true-breeding white-flowered one, you note that all of your F1 population are pink. This is because _____.
red and white show incomplete dominance
Two parents of mixed ethnicity have twins, one of which is born white and one of which is born black. This is because of _____.
the polygenic nature of skin color genes
The frequency at which crossing over occurs between two linked genes depends on _____.
how far apart the genes are on the chromosome
Gene Q is an X-linked dominant allele in crustaceans that controls eye color. A cross is performed between two crustaceans: a green-eyed male (XQY) and a green-eyed female (X QX q). What percentage of the offspring have green eyes?
A woman and her male partner have normal color vision. However, her father and her first son are color-blind. What is her genotype? Use C as the gene for color blindness
Previously, it was thought that the blending hypothesis explained inheritance. The blending hypothesis maintained that _____.
after a mating, genes of the two parents are mixed in the offspring and lose their individual identities
The cross-fertilization of two different, but true-breeding, varieties of pea plants will _____.
result in hybrid plants
When applied to Mendel's experiments, the term true-breeding means a self-fertilization of two plants that produces _____.
offspring identical to the parent
In a certain plant, the alleles A, B, and C are dominant to the alleles a, b, and C. A plant with the genotype AABbcc will have the same phenotype as the plant with the genotype _____
A child with cystic fibrosis can be born to two parents who do not have the disease. This is because the disease ____
is caused by a recessive allele
Assume tall (T) is dominant to dwarf (t). If a homozygous dominant true-breeding individual is crossed with a homozygous true-breeding dwarf, the offspring will _____.
all be tall
Phenylketonuria (PKU) is an autosomal recessive disorder that can lead to mental retardation and seizures. Using P and p to represent the alleles, what is the genotype of a phenylketonuric person?
If a homozygous dominant (AA) is crossed with a homozygous recessive (aa) for a given character, the offspring will be _____.
all of the dominant phenotype
When you cross two heterozygotes (Aa), the F1 generation will be _____.
in the ratio 1:1 homozygotes to heterozygotes
In Mendel's monohybrid cross of true-breeding purple-flowered and white-flowered peas, all members of the F1 generation were of the _____ phenotype because their genotype was _____ for the flower-color gene.
purple-flowered ... heterozygous
Huntington's disease is caused by an autosomal dominant allele. If a heterozygous male were to marry a female not carrying the disease, what percentage of the offspring will have Huntington's?
Achondroplasia is a common cause of dwarfism and is an autosomal dominant condition. If a male individual has achondroplasia, which statement below must be true?
At least one of this man's parents must have shown signs of dwarfism.
An AABb individual is mated with another AABb individual. The possible number of genetically different kinds of offspring is _____.
Pea flowers may be purple (P) or white (p). Pea seeds may be round (R) or wrinkled (r). What proportion of the offspring from the cross PpRr x PpRr will have white flowers and wrinkled seeds?
You have one tree that produces big yellow apples and another tree that produces small red apples. When the two are crossed, you find that half the offspring trees produce big red apples and half produce big yellow apples. What are the genotypes of the parents? A = big apples; a = small apples; R = red apples; r = yellow apples.
AArr and aaRr
A couple who both have the ability to roll their tongues have a son who is also a tongue-roller. Tongue-rolling is a dominant characteristic. The son is curious about whether he is homozygous or heterozygous for tongue-rolling. How could he find out?
Have children with a woman who is a non-roller. If any of their children is a non-roller, then he is heterozygous for the trait.
Pea plants are tall if they have the genotype TT or Tt, and they are short if they have the genotype tt. A tall plant is crossed with a short plant. Half the offspring are tall, and half are short. This allows us to conclude that the tall plant _____.
The results of a testcross (that is, the offspring of this cross) were 50% phenotypically dominant and 50% phenotypically recessive. The genotype of the phenotypically dominant parent in this cross was _____
What is indicated when a single-character testcross yields offspring in a 1:1 phenotypic ratio?
The parent with the dominant phenotype was heterozygous.
Suppose we have a pea plant with purple flowers, determined by the dominant allele P. How might you determine whether the plant is homozygous (PP) or heterozygous (Pp)
Perform a testcross: Cross the plant with a white one, which must be homozygous recessive (pp).
If each parent can produce 100 genetically distinct gametes, how many genetically distinct offspring can two parents produce
A cross is made between two heterozygous pea plants, and observations are made about three characteristics: flower color, seed color, and seed shape. What is the probability that the offspring will be heterozygous for all three characteristics?
Tay-Sachs disease runs in Rebecca's family. On a family pedigree, she saw a shaded circle. This represented _____
a female with Tay-Sachs
John and Jane are planning a family, but since each has a brother who has sickle-cell disease, they are concerned that their children may develop sickle-cell disease. Neither John, Jane, nor their respective parents have the disease. They consult a genetic counselor who tells them that _____.
it's possible that none of their children will have the disease, but blood tests on them both will be required to make sure
When two average-height parents give birth to a child exhibiting achondroplastic dwarfism, it is most likely due to a new mutation. This is because _____.
achondroplasia is caused by an allele that is always expressed, so the parents couldn't have the allele
Cystic fibrosis, which is usually lethal before the age of reproduction, is a homozygous recessive trait. Even though people with the disease rarely reproduce, cases continue to arise because _____.
the harmful allele "hides" inside heterozygous individuals and one-fourth of the offspring of two heterozygotes should be afflicted
It is far more common to find human genetic disease caused by _____ alleles than by _____ alleles because _____.
recessive ... dominant ... harmful recessive alleles can survive in the heterozygote without causing any adverse effects on the phenotype
Michelle and Keith are apparently normal, but their daughter was born with alkaptonuria, an inherited metabolic disorder that causes urine to turn black when exposed to air. If alkaptonuria is like most other human hereditary disorders, the probability of their next child being born with alkaptonuria is _____.
Huntington's disease is an example of a genetic disorder caused by _____
a late-acting lethal dominant allele
Several inherited disorders are much more common in close-knit religious communities, such as the Amish, than in the general population. This is at least partly due to the fact that _____
people in such communities are more likely to marry relatives
The test that is used to determine whether someone possesses a potentially harmful recessive allele is referred to as _____ testing
A red bull is crossed with a white cow and all of the offspring are roan, a shade between red and white. This is an example of genes that are _____
Flower color in snapdragons is an example of incomplete dominance. If a red-flowered plant is crossed with a white-flowered plant, the F1 generation has pink flowers. If a pink-flowered plant is crossed with a pink-flowered plant, the progeny plants will be _____
25% red, 50% pink, and 25% white
A male who is homozygous recessive for hypercholesterolemia marries a woman is homozygous dominant. What percentage of their children will have higher than normal blood cholestero
A man who has type B blood and a woman who has type A blood could have children of which of the following phenotypes?
A, B, AB, or O
Two individuals have children with A and B blood types. What are the blood types of the parents?
O and AB
If one parent is blood type AB and the other is type O, what fraction of their offspring will be blood type A?
A woman with type O blood is expecting a child. Her husband is type A. Both the woman's father and her husband's father had type B blood. What is the probability that the child will have type O blood
In people with sickle-cell disease, the red blood cells break down, clump, and clog the blood vessels. The blood vessels and the broken cells accumulate in the spleen. Among other things, this leads to physical weakness, heart failure, pain, and brain damage. Such a suite of symptoms can be explained by _____.
the pleiotropic effects of the sickle-cell allele
Two identical twins are raised in different environments. They possess _____ genotypes and _____ phenotypes.
identical ... variable
Seeds from a single sexually reproducing plant are harvested and later planted under identical conditions. What results should be expected
The seedlings should differ from each other, depending on their genetic constitution
If hair color, eye color, and the presence or absence of freckles were consistently inherited together, the best explanation would be that ____
the genes for these characters are linked on the same chromosome
If the two characteristics that Mendel looked at in his dihybrid cross of smooth yellow peas with wrinkled green peas had been controlled by genes that were located close together on the same chromosome, then the F2 generation _____.
would have deviated from the 9:3:3:1 phenotypic ratio that is predicted by the law of independent assortment
In a particular species of mammal, black hair (B) is dominant to green hair (b), and red eyes (R) are dominant to white eyes (r). When a BbRr individual is mated with a bbrr individual, offspring are produced in a ratio of 5 black hair and red eyes : 5 green hair and white eyes : 1 black hair and white eyes : 1 green hair and red eyes. Which of these explanations accounts for this ratio?
The genes for hair color and eye color are linked.
The following distribution of offspring was observed:
blue wings, green eyes 1,070
blue wings, black eyes 177
red wings, green eyes 180
red wings, black eyes 1,072
In a particular species of mammal, blue wings (B) are dominant to red wings (b), and green eyes (G) are dominant to black eyes (g). Based on these data, what is the recombination frequency?
Correct. 177 + 180 = 357 recombinants. 357 recombinants/2,499 total offspring x 100 = 14%.
The crossover percentage between two different genes is _____
greater as the distance between the two genes increases
The Y chromosomes of mammals contain genes that code for _____.
"maleness" and a few other characteristics
Gene R is an X-linked recessive allele in fruit flies that controls wing formation. A cross is performed between two fruit flies where the male is XRY and the female is XRXr. What percentage of the offspring are female with abnormal wing development?
0% Correct. In this cross, only the males would be affected because they do not possess a second X chromosome. The females would be XRXRor XRXr.
Hemophilia appears rarely in females. This is because _____
the female must possess the hemophilia gene on both X chromosomes
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