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In a monohybrid cross, F2 refers to __________. - the second filial generation, or the "grandchildren" of the original mating pair The cross-fertilization of two different, but true-breeding, varieties of pea plants will _____. - result in hybrid plants; The offspring of the crossing of different varieties are referred to as hybrids. When applied to Mendel's experiments, the term true-breeding means a self-fertilization of two plants that produces __________. - offspring identical to the parent; True-breeding means that a homozygous dominant individual, when self-fertilized, would produce only homozygous dominant individuals. The same would also be true of a homozygous recessive individual only producing homozygous recessive individuals. Imagine a particular character (such as flower color) 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? - Perform a cross between two true-breeding individuals and observe the trait or traits expressed by the F1 individuals; The F1 plants are hybrids that contain both forms of the gene, so the trait the F1 plants display is the dominant trait. An insect that has the genotype EeGGcc will have the same phenotype as an insect with the genotype __________. - EEGgcc; Individuals with one or two dominant alleles of a gene are indistinguishable, so EE and Ee have the same phenotype, as do GG and Gg. 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; The most common lethal genetic disease in the United States is cystic fibrosis, which is recessively inherited. The parents in this case would be heterozygotes, or carriers for the disease. 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. What is an allele? - an alternative version of a gene
A diploid organism has two alleles for each autosomal gene. The two alleles are found at comparable locations on homologous chromosomes. The alleles may be identical or slightly different, but they affect the same genetic character. 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; Because these traits assort independently, this dihybrid cross can be considered as two separate monohybrid crosses, with the consequence that one-fourth of the offspring of cattle heterozygous for the horn length trait will express the recessive long-horn trait. DdEe pea plants can produce _____ type(s) of gametes, but a ddee plant can produce _____ type(s) of gametes. - four ... one; DdEe individuals can produce the following gametes: DE, De, dE,de. A ddee plant can produce only de gametes. 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 × PpRr are expected to have white flowers and wrinkled seeds? - 1/16; Pp × Pp would produce 3/4 purple and 1/4 white. Rr × Rr would produce 3/4 round and 1/ wrinkled. (1/4)(1/4) = 1/16 would have white flowers and wrinkled seeds. 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. This cross is a testcross (the short-finned fish must be homozygous recessive), and having all offspring show the dominant trait proves that the long-finned parent is homozygous. 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; Trees that produce big yellow apples could be AArr or Aarr. Trees that produce small red apples could be aaRR or aaRr. Because all the offspring are big, the big parent must be homozygous for A. Half the apples are red and half are yellow, so the red parent must be heterozygous for red. The conclusion is that the cross must be AArr × aaRr.
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; Because the woman will transmit a normal allele to the child, the child will not be affected; however, there is a 50% chance that the child will inherit a Tay-Sachs allele from the father and thus be a carrier. Cystic fibrosis is inherited in an autosomal recessive pattern. Males who have cystic fibrosis are usually sterile. Furthermore, the disease is often lethal before the age of reproduction. 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; Cystic fibrosis is inherited as a Mendelian recessive trait, and affected individuals usually have unaffected parents, both of whom are carriers of the recessive allele. 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; Ultrasound testing is the least invasive of the tests, CVS involves sampling fetal tissue, and AFP (alpha-fetoprotein) is obtained from a maternal blood sample. 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; Mendel's laws are general for all sexually reproducing organisms, but they must be extended in ways Mendel never knew in order to account for many 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? - 1/4; This is the probability of giving birth to an HH homozygote, the only genotype conferring normal cholesterol levels. 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 another pink-flowered plant, the expected progeny plants will be __________. - 25% red, 50% pink, and 25% white; Breeding the F1 hybrids of the snapdragon cross produces F2 offspring with a phenotypic ratio of 1 red: 2 pink: 1 white. 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; Because there are two homologous chromosomes, each with one locus for this and every other gene, a person may carry up to two different alleles for this gene. 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; In the case of incomplete dominance of CS and CD, the heterozygotes would have some intermediate phenotype, for example, pigmented areas bigger than dots but smaller than spots. 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? - I^Ai x I^AI^B; Since blood groups show codominance, the child of blood type A must be either IAIA or IAi. The child of blood type B must be either IBIB or IBi. The child of blood type AB can only be IAIB. Because there are more offspring expected with blood type A, we know that an A allele is being contributed from both parents. By reversing the Punnett square, the parents' genotypes are observed to be IAi and IAIB. If one parent is blood type AB and the other is type O, what fraction of their offspring is expected to have blood type A? - 0.5; About half the offspring would be expected to inherit the IA allele and be phenotypically A. Two parents of mixed ethnicity have twins, one of which is born with very light skin and one of which is born with very dark skin. This is because of __________. - the polygenic nature of skin color genes; Skin color is known to be affected by at least three genes, possibly more. 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
Which of the following experiments would provide the best data for determining the effect of genetics? - comparing identical twins raised in the same environment with fraternal twins raised in the same environment In the essay, the point is made that the right questions must be asked when designing an experiment that measures how genetics and environment "shape who we are." Which of the following measurements is probably the least valuable in such an experiment? - the average lifespan 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, blue smooth; 1,436 white wrinkled; 150 blue wrinkled; 145 white smooth. Explain the inheritance of the coloration and shape traits. - The coloration and shape genes are linked on the same chromosome; The fact that there isn't the 1:1:1:1 ratio of phenotypes expected in this cross if the alleles assorted independently and that there is a pair of large phenotypic classes (1,447 blue smooth; 1,436 white wrinkled) and a pair of small phenotypic classes (150 blue wrinkled; 145 white smooth) indicates that the genes are linked. Linked genes are inherited together. This is because linked genes _____. - are on the same chromosome; Genes located on the same chromosome tend to be inherited together and are said to be linked genes. They may or may not be functionally related. 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; This explains why the parental types occur more often than would be expected if the characters assorted at random. 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; If the two characters are located on the same chromosome, they will not segregate independently.
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; If SRY is deleted, the Y chromosome would still be present, but development would occur along the female path. The Y chromosomes of mammals contain genes that code for _____. - "maleness" and a few other characteristics; Researchers have identified a number of extra genes on the Y chromosome that are required for normal testis functioning. A woman and her male partner have normal color vision. However, her father and her first son are colorblind. What is her genotype? Use C as the gene for colorblindness. - XCXc; Because this is an X-linked recessive condition, she must possess at least one allele for color blindness to have an affected son. Because she has normal color vision, she must also have one normal allele. 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; Because sons obtain their single X chromosome from their mother, they have a 50% chance of inheriting the homolog that carries the hemophilia allele. And because daughters inherit their father's only X chromosome and one of the two X chromosomes of their mother, in this cross they have a 50% chance of being homozygous for the hemophilia allele. Hemophilia appears rarely in females. This is because __________. - the female must possess the hemophilia allele on both X chromosomes; Because hemophilia is an X-linked recessive disorder, the female must have the gene on both X chromosomes. For this to occur, one parent must have hemophilia and one must be a carrier. We can better understand our ancestry by analyzing the __________ in males. - Y chromosome; Scientists have been tracking male descendants using the Y chromosome because the Y chromosome passes essentially intact from father to son. Your 28 year old friend is pregnant. Which of the following screenings is she most likely to initially have to test for Down syndrome? - blood test How is the fetal DNA used in this new screening tool unique? - It is not from a cell, but is floating freely in the mother's blood.
