AP ch 11 Mendel KEY - notes answer key PDF

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AP Biology Ch. 11 Mendel and the Gene Idea KEY CONCEPTS 11.1 Mendel used the scientific approach to identify two laws of inheritance 11.2 Probability laws govern Mendelian inheritance 11.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics 11.4 Many human traits follow Mendelian patterns of inheritance

11. 1 Mendel and his Two Laws of Inheritance Mendelian Genetics (Bozeman)

1. One reason that Mendel probably chose to work with peas is that they are available in many varations. A heritable feature that varies among individuals is called a character. Each variant for a characteristic is termed a trait. 2. Other advantages of using peas are their short generation time and the large number of offspring from each mating. 3. Each pea flower has both pollen-producing organs (stamens) and an egg-bearing organ (carpel). 4. In nature, pea plants usually self-fertilize, but Mendel achieved cross-pollination of two plants by removing stamens from one plant and transferring pollen from another plant to the first plant. 5. Mendel chose to track only those characters that occurred in 2 distinct, alternative forms as can be seen in Table 11.1 (next page). 6. Mendel made sure that he started all of his experiments with varieties that, over many generations of self-pollination, had produced only the same variety as the parent plant. Such plants are said to be true-breeding.

7. The mating, or crossing, of two true-breeding varieties is called hybridization. 8. The true-breeding parents are referred to as the P generation (parental) and their hybrid offspring are the F1 generation. Allowing these first generation plants (F1 hybrids) to self- pollinate produces an F2 generation. 9. When Mendel allowed the F1 plants to self-pollinate, the white-flower trait reappeared in the F2 generation. 705 of the F2 plants had purple flowers and 224 had white flowers – this is approximately a 3:1 ratio.

10. In Mendel’s terminology, purple color is a dominant trait and white color is a recessive trait. Mendel observed this pattern in six other characters (shown in Table 11.1 above).

Mendel's Model 11. Alternative versions of genes called alleles account for variations in inherited characters. 12.

For each character, an organism inherits two alleles one from each parent.

13. If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance, while the other, the recessive allele, has no noticeable effect on the organism’s appearance. 14. The fourth and final part of Mendel’s model, the Law of Segregation states that the two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes. 15. A Punnett square is a tool for predicting the allele composition of all offspring resulting from a cross between individuals of a known genetic makeup. 16. An organism that has a pair of identical alleles for a characteristic is said to be homozygous. (TT, tt) 17. An organism that has two different alleles for a gene is said to be heterozygous. (Tt) 18. Phenotype: appearance or observable traits 19. Genotype: inward genetic makeup (TT, Tt, tt)

20. Breeding an unknown genotype (either Homo or Heterozygous) with a homozygous recessive to determine the exact genotype is called a testcross.

21. Dihybrid Cross: a genetic cross involving 2 traits. 22. Mendel tested his seven pea characteristics in various dihybrid combinations and always observed a 9:3:3:1 phenotypic ratio in the F2 generation. 23.

The results of Mendel’s dihybrid experiments are the basis for what we now call the Law of Independent Assortment which states that each pair of alleles segregates independent of each other pair of alleles during gamete formation.

Concept 14.2: Probability laws govern Mendelian inheritance

BOZEMAN VIDEO: PROBABILITY IN GENETICS 24. The multiplication rule states that to determine the probability of multiple events occurring simultaneously, we multiply the probability of one event by the probability of the second event.

25. According to the addition rule, the probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities.

Question 1: In a cross between a trihybrid with purple flowers and yellow round seeds (heterozygous for all three genes) and a plant with purple flowers and green wrinkled seeds (heterozygous for flower color), what fraction of the offspring is predicted to exhibit the recessive phenotype for at least two of the characters? PpYyRr x Ppyyrr Possible genotypes: ppyyRr, ppYyrr, Ppyyrr, PPyyrr, ppyyrr ppyyRr ¼ x ½ x ½ = 1/16 ppYyrr ¼ x ½ x ½ = 1/16 Ppyyrr ½ x ½ x ½ = 2/16 PPyyrr ¼ x ½ x ½ = 1/16 Ppyyrr ¼ x ½ x ½ = 1/16 TOTAL = 6/16 Question 2: Three characters (flower color, seed color, and pod shape) are considered in a cross between two pea plants: PpYyIi x ppYyii. What fraction of offspring is predicted to be homozygous recessive for at least two of the three characters? Possible genotypes: ppyyIi, ppYyii, Ppyyii, ppYYii, ppyyii TOTAL = 6/16 Concept 11.3 Beyond Mendelian Genetics

26.

Incomplete Dominance: The F1 hybrid have a phenotype that is intermediate between the two phenotypes of the parents (neither allele is completely dominant). ex: snapdragons

27. Multiple Alleles: occurs when a gene exists in more than two allelic forms Example: A, B, O blood types.

28. Codominance: the condition of two alleles both affect the phenotype in separate, distinguishable ways Examples: 1. MN blood group 2. AB blood type 3. Roan cattle having both red and white hairs side by side giving it a burnt orange color

29. Pleiotropy: the ability of a single gene to have multiple effects in the body. Examples: 1. multiple symptoms (effects of sickle-cell disease and cystic fibrosis) 2. albino white tigers usually have crossed eyes

30. In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus giving unexpected results Ex: Gene 1: B = black b = brown Gene 2: E = pigment deposited e = no pigment deposited

31. Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single characteristic. Examples: (hair color, skin color, eye color, height)

32. The concept that the environment may have an impact on the phenotypic expression of genes is part of the nature vs. nurture controversy. (EPIGENETICS) 33. Geneticists refer to such characters as multifactorial, meaning that many factors, both genetic and environmental, influence phenotype.

Concept 14.4 Human Traits

34. A pedigree is a family tree describing the traits of parents and children across the generations.

Human Genetic Disorders 35. An allele that causes a genetic disorder codes for either a malfunctioning protein or no protein at all. Give a brief description of each disease including the mode of inheritance. 36. Albinism: An autosomal recessive disorder. Lack of pigmentation leads to increased susceptibility to skin cancers and vision problems.

37. Tay-Sachs disease: An autosomal recessive disorder. Very prevalent among the Ashkenazic Jewish population. One out of 3,600 births have this disease. Inability to produce an enzyme that breaks down certain lipids in the brain causes seizures, blindness and death within a few years. 38. Cystic Fibrosis: An autosomal recessive disorder.

The most common lethal genetic disease in the U.S. affecting 1 in every 2,500 people of European descent. The normal allele codes for a membrane transport protein. One out of 25 are carrier of the recessive allele. Results in the inability to move Cl- across membranes in the lungs, pancreas and digestive track. Very thick mucus then builds up in these organs (pleiotropic effects) 39. Sickle-cell disease: An autosomal recessive disorder. Most prevalent among people of African descent, affecting one in 400 AfricanAmericans. Single amino acid substitution in hemoglobin. Two recessive alleles will cause this disease causing red blood cells to sickle at high altitudes or during periods of exercise. Heterozygotes are said to have Sickle-cell trait and are usually healthy but may suffer some symptoms during prolonged exercise. This condition is advantageous in areas where malaria is prevelant because it helps people better survive malaria.

40. Achondroplasia: An autosomal dominant disorder. Having only one allele will cause dwarfism. 41. Huntington’s disease: An autosomal dominant disorder. This is a degenerative disease of the nervous system. This disease has no phenotypic effects until an individual is 35-45 years old. Once deterioration of the nervous system begins, it is irreversible and fatal. Therefore, any child born to a parent who has the allele will have a 50% chance of inheriting the disorder.

Technology in genetic testing and counseling

1.

Genetic counseling to families with family histories.

2.

Carrier recognition tests to see if you are a carrier for a particular disease.

3.

Fetal testing through amniocentesis or chorionic villus sampling(CVS)

4.

Newborn screening for genetic disorders. Testing for PKU.

Practice Problems:

1. Imagine crossing a pea heterozygous at the loci for flower color (Pp) and seed color (Yy) with a second pea homozygous for flower color (pp) and seed color (yy). What genotypes of gametes will the first pea produce? a. b. c. d.

two gamete types: pp and Pp two gamete types: pY and Py four gamete types: pY, py, PY, and Py four gamete types: Pp, Yy, pp, PP

2. Pea plants were particularly well suited for use in Mendel’s breeding experiments for all of the following reasons except that a. peas show easily observed variations in a number of characters, such as pea shape and flower color. b. it is possible to control matings between different pea plants. c. it is possible to obtain large numbers of progeny from any given cross. d. peas have an unusually long generation time. e. many of the observable characters that vary in pea plants are controlled by single genes. 3. A cross between homozygous purple-flowered and homozygous white-flowered pea plants results in offspring with purple flowers. This demonstrates a. the blending model of genetics. b. true breeding. c. dominance. d. a dihybrid cross. e. the mistakes made by Mendel. 4. Imagine a genetic counselor working with a couple who have just had a child who is suffering from Tay-Sachs disease. Neither parent has Tay-Sachs, nor does anyone in their families. Which of the following statements should this counselor make to this couple? a. “Because no one in either of your families has Tay-Sachs, you are not likely to have another baby with Tay-Sachs. You can safely have another child.” b. “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 50% chance of having the disease.” c. “Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 25% chance of having the disease.” d. “Because you have had one child with Tay-Sachs, you must both carry the allele. However, since the chance of having an affected child is 25%, you may safely have three more children without worrying about having another child with Tay-Sachs.” e. “You must both be tested to see who is a carrier of the Tay-Sachs allele.”

5. Albinism in humans occurs when both alleles at a locus produce defective enzymes in the biochemical pathway leading to melanin. Given that heterozygotes are normally pigmented, which of the following statements is correct? a. One normal allele produces as much melanin as two normal alleles. b. Each defective allele produces a little bit of melanin. c. Two normal alleles are needed for normal melanin production. d. The two alleles are codominant. e. The amount of sunlight will not affect skin color of heterozygotes. 6. In humans, alleles for dark hair are genetically dominant, while alleles for light hair are recessive. Dark hair is also more prevalent in southern Europe than in northern Europe. Which of the following statements about hair color alleles is most likely to be correct? a. Dark hair alleles are more common than light hair alleles in all areas of Europe. b. Dark hair alleles are more common than light hair alleles in southern Europe but not in northern Europe. c. Dark hair alleles are equally common in all parts of Europe. d. Dark hair is dominant to light hair in southern Europe but recessive to light hair in northern Europe. e. Dark hair is dominant to light hair in northern Europe but recessive to light hair in southern Europe. 7. Imagine a locus with four different alleles for fur color in an animal, Da, Db, Dc, and Dd. If you crossed two heterozygotes, DaDb and DcDd, what genotype proportions would you expect in the offspring? a. 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd b. 50% DaDb, 50% DcDd c. 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd d. 50% DaDc, 50% DbDd e. 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd 8. Envision a family in which a man, age 47, has just been diagnosed with Huntington’s disease, which is caused by a dominant allele (and the man is a heterozygote). His daughter, age 25, has a 2-year-old son. No one else in the family has the disease. What is the probability that the daughter will contract the disease? a. 0% b. 25% c. 50% d. 75% e. 100%

9. Review the family described in the previous question. What is the probability that the 2-year-old son will contract the disease?

a. b. c. d. e.

0% 25% 50% 75% 100%

10. When a disease is said to have a multifactorial basis, it means that a. both genetic and environmental factors contribute to the disease. b. it is caused by a gene with a large number of alleles. c. it affects a large number of people. d. it has many different symptoms. e. it tends to skip a generation. 11. Which of the following observations supported Mendel’s hypothesis that inheritance is “particulate” rather than due to blending? a. There are two distinct flower colors in pea plants. b. White-flowered plants are true-breeding. c. Crossing true-breeding purple-flowered and white-flowered plants produced all purple-flowered plants. d. Crossing two purple-flowered heterozygotes produced purple-flowered and white-flowered plants. 12. Manx cats have characteristic stubby tails due to being heterozygous at a single locus. Homozygotes for the Manx allele die before birth with severe spinal deformities. What is the expected phenotypic ratio of live offspring of two Manx cats? a. 3 normal:1 Manx b. 3 Manx:1 normal c. 2 normal:1 Manx d. 2 Manx:1 normal 13. The allele for inflated pods (I) is dominant to that for constricted pods (i), and the allele for green pods (G) is dominant to that for yellow pods (g). A plant of unknown genotype with inflated green pods is crossed with a plant with constricted yellow pods. Among the offspring are 49 plants with inflated green pods, and 53 plants with constricted green pods. What was the previously unknown genotype? a. IiGg b. IiGG c. IIGg d. IIGG 14. Imagine a cross of two triple heterozygous pea plants with purple flowers and yellow round seeds (genotype PpYyRr). If you were to create a Punnett square for this cross

(not recommended!) what would be its dimensions? Recall that all three loci assort independently. a. 3 × 3 b. 4 × 4 c. 6 × 6 d. 8 × 8 15. Imagine the same cross of two triple heterozygous pea plants with purple flowers and yellow round seeds (genotype PpYyRr). Using the rules of probability (and not a Punnett square!), determine what proportion of offspring will have purple flowers and green wrinkled seeds. a. ¼ × ¼ × ¼ = 1/64 b. ¾ + ¼ + ¼ = 5/4 c. ¾ × ¼ × ¼ = 3/64 d. ½ × ½ × ½ = 1/8 16. The Rh blood factor in humans is perhaps the most important after the ABO system. The + phenotype is dominant to the – phenotype, and it is encoded on a separate autosome from the ABO locus (i.e., the two loci assort independently). A child of blood type A+ is born to a mother of blood type O. What can be concluded about the blood type of the biological father? a. A+ or AB+ b. A+ only c. AB+ only d. any possible blood type 17. Assume there are 50 people in the classroom. Theoretically speaking, what is the maximum number of different alleles there could be at a hypothetical autosomal locus? a. 2 b. 3 c. 50 d. 100 18. Consider the case of dominant epistasis in squash, where YY and Yy = yellow and yy = green. At a second, independently assorting locus, WW and ww = white, and ww allows for yellow or green. What is the expected phenotypic ratio among the progeny of the cross YyWw × YyWw? (Note these are white squash.) a. 1:1:1:1 b. 9:3:3:1 c. 9:4:3 d. 12:3:1...