Campbell Biology · Chapter 14
Mendel and the Gene Idea
pp. 262–285 · 4 sections
This chapter explains how traits pass from parents to offspring, starting with Mendel's two laws and the probability rules that let you predict a cross of any size. It then looks at traits that don't follow simple dominance and at inheritance in human families. Almost all of it lines up with AP Topics 5.3–5.5, so expect Punnett squares, probability, pedigrees and chi-square questions.
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14.1 Mendel's crosses and his two laws
pp. 262–269
Topic 5.3 covers this: segregation, independent assortment, and monohybrid, dihybrid and test crosses. You won't be asked about Mendel's life or his list of pea traits.
In the course: Topic 5.3 Mendelian Genetics, Topic 5.1 Meiosis, Topic 5.2 Meiosis and Genetic Diversity (notes, videos and more questions)
Key points
- In Mendel's day, many people thought a parent's traits mixed in offspring like two paints stirred together. Mendel's results showed instead that heredity comes in separate units, now called genes, that pass on unchanged. That's why a trait can vanish for a generation and then return.
- Garden peas suited his work: they come in varieties with clear either-or traits, grow quickly and make lots of seeds, and he could choose exactly which plant pollinated which. He started with true-breeding lines and counted thousands of offspring.
- Crossing two true-breeding lines (the P generation) gave F₁ hybrids that all looked like one parent. When F₁ plants self-pollinated, the hidden trait came back in about a quarter of the F₂, a 3:1 ratio.
- In modern terms, a gene can come in different versions called alleles. A diploid organism carries two alleles of each gene, one from each parent, and in a heterozygote the dominant allele decides how it looks.
- The law of segregation says a gene's two alleles part ways when gametes form, so each egg or sperm gets just one. This happens because homologous chromosomes go to different cells in meiosis I.
- Phenotype is what you can observe; genotype is the set of alleles. Because PP and Pp look alike, an F₂ with a 3:1 phenotype ratio has a 1:2:1 genotype ratio. A testcross with a homozygous recessive partner tells them apart: all dominant offspring means homozygous, and about half recessive means heterozygous.
- Mendel's second law, independent assortment: the allele a gamete receives for one gene doesn't affect which allele it receives for another. So a dihybrid (AaBb) makes AB, Ab, aB and ab gametes in equal numbers, and crossing two dihybrids gives a 9:3:3:1 F₂. This holds for genes on different chromosomes; genes close together on one chromosome are linked and don't follow it (Topic 5.4).
Key terms (15)
- character
- A kind of inherited feature, like fur length or seed color, that comes in two or more forms within a species.
- trait
- One particular version of a character, such as yellow seeds or long fur.
- true-breeding
- Describes a line of plants or animals that, bred among themselves, always produce offspring just like the parents for a given trait. They are homozygous for it.
- P, F₁ and F₂ generations
- The parents in a cross (P), their offspring (F₁, the first filial generation) and the offspring of the F₁ (F₂). "Filial" refers to sons and daughters.
- allele
- One version of a gene. Different alleles have slightly different DNA sequences at the same spot on a chromosome.
- dominant allele
- The allele whose effect shows up in a heterozygote. One copy is enough to produce the trait.
- recessive allele
- An allele whose effect is hidden in a heterozygote. You only see its trait when there's no dominant allele present.
- homozygous
- Having two identical alleles of a gene, like AA or aa.
- heterozygous
- Having two different alleles of a gene, like Aa.
- genotype
- The alleles an organism carries for a gene, or for all its genes.
- phenotype
- The traits you can observe or measure in an organism, including body chemistry and behavior, not just looks.
- law of segregation
- The first of Mendel's laws: the two copies of a gene an organism carries split up when it makes eggs or sperm, so every gamete ends up with one copy.
- testcross
- Crossing an individual with the dominant phenotype to a homozygous recessive one. The offspring reveal whether the first parent was homozygous or heterozygous.
- monohybrid and dihybrid
- A monohybrid is heterozygous for one gene you are tracking (Aa); a dihybrid is heterozygous for two (AaBb). Crossing two of them is a monohybrid or dihybrid cross.
- law of independent assortment
- The second of Mendel's laws: which allele a gamete gets for one gene has no bearing on which allele it gets for a different gene. It holds for genes on different chromosomes.
Check yourself: 14.1 Mendel's crosses and his two laws
4 questions on 14.1 Mendel's crosses and his two laws. Pick an answer to see if you got it, and why.
A breeder crosses true-breeding long-tailed lizards with true-breeding short-tailed lizards of the same species. Every offspring has a long tail. When those offspring are mated with each other, about a quarter of the next generation has tails just as short as the original short-tailed parents. Which observation most directly shows that the short-tail factor passed through the first hybrid generation unchanged?
A plant is heterozygous for a gene and shows the dominant trait. Which statement about this plant must be true?
In a beetle species, green shells (G) are dominant to brown shells (g). A green beetle of unknown genotype is mated with a brown beetle, and their offspring are 22 green and 19 brown. What is the green parent's genotype?
A squash plant has the genotype AaBb, and the two genes are on different chromosomes. Which list gives the kinds of gametes it makes, in the expected proportions?
0 of 4 answered
14.2 Using probability to predict crosses
pp. 269–271
Topic 5.3 puts both rules on your formula sheet: multiply for "and" with independent events, add for "or" with mutually exclusive ones. Expect to use them for crosses too big for a Punnett square.
In the course: Topic 5.3 Mendelian Genetics (notes, videos and more questions)
Key points
- A probability runs from 0 (it can't happen) to 1 (it's certain). The probabilities of every possible outcome add up to 1, so the chance something doesn't happen is 1 minus the chance it does.
- A heterozygote passes on each of its two alleles with a 1/2 chance, like a fair coin. Each fertilization is a separate chance event: what earlier offspring got has no effect on the next one.
- Multiplication rule: for independent events, the chance both happen is the product of their chances. From Aa × Aa, an offspring needs a from the egg and a from the sperm, so P(aa) = 1/2 × 1/2 = 1/4.
- Addition rule: when outcomes are mutually exclusive, the chance that one or the other happens is the sum. An Aa offspring from Aa × Aa can get A from the egg and a from the sperm, or the reverse: 1/4 + 1/4 = 1/2.
- For genes that assort independently, a big cross is just several one-gene crosses at once. Work out each gene separately, then multiply. For example, AaBbCc × AaBbCc gives aabbcc with probability (1/4)³ = 1/64.
- For questions like "at least one" or "exactly one," list every outcome that counts, find each probability, and add them, or subtract the chance of the opposite outcome from 1.
- Ratios are predictions, not promises. Small samples often stray far from them by chance, and big samples land closer. In AP Biology you use a chi-square test to decide whether a gap between observed and expected numbers is bigger than chance alone would explain.
Key terms (7)
- probability
- How likely an event is, written as a number from 0 (never) to 1 (certain). A 1/4 chance means about 1 time in 4 over many tries.
- independent events
- Events where one outcome doesn't change the chance of the other, like two coin flips or two separate fertilizations.
- mutually exclusive events
- Outcomes that can't both happen at once. An offspring's A allele came from either the egg or the sperm, not both.
- multiplication rule
- To find the chance that two independent events both happen, multiply their probabilities: P(A and B) = P(A) × P(B).
- addition rule
- To find the chance that one of several mutually exclusive outcomes happens, add their probabilities: P(A or B) = P(A) + P(B).
- Punnett square
- A grid that pairs every possible gamete from one parent with every possible gamete from the other, showing the offspring's possible genotypes and how likely each is.
- sample size
- How many individuals are counted. The more offspring you count, the closer the results usually come to the expected ratio.
Check yourself: 14.2 Using probability to predict crosses
4 questions on 14.2 Using probability to predict crosses. Pick an answer to see if you got it, and why.
Two rabbits are both heterozygous (Ee) for a gene that affects ear shape. How likely is it that their next offspring is heterozygous?
In a pepper plant, three genes assort independently. For each gene, the dominant allele (A, B or D) gives one trait and the recessive allele gives another. A plant with genotype AaBbDd is crossed with a plant with genotype AabbDd. What fraction of the offspring is expected to show the dominant trait for gene A but the recessive trait for both gene B and gene D?
In corn, a recessive allele (a) blocks chlorophyll production, so aa seedlings are white and die soon after sprouting. A gardener plants kernels from a self-pollinated Aa plant. The first five seedlings to come up are all green. How likely is the sixth seedling to be white?
Two goldfish are both heterozygous for an allele that, in this example, is recessive and causes bubble-shaped sacs under the eyes. If they have exactly two offspring, what is the probability that exactly one of the two has the bubble-eye trait?
0 of 4 answered
14.3 Beyond simple dominance
pp. 271–275
Topic 5.4 covers incomplete dominance, codominance and pleiotropy, and Topic 5.5 covers how the environment shapes phenotype. Epistasis, polygenic traits and multiple alleles aren't named in the current course, so treat them as background that helps you explain ratios that don't fit.
In the course: Topic 5.4 Non-Mendelian Genetics, Topic 5.5 Environmental Effects on Phenotype (notes, videos and more questions)
Key points
- Mendel's pea traits were simple: one gene, two alleles, one fully dominant. Many traits are more complicated, but segregation and independent assortment are still at work underneath.
- In complete dominance, the heterozygote looks just like the dominant homozygote. In incomplete dominance, it falls in between, so the F₂ phenotypes come out 1:2:1, matching the genotypes. The pure parental types still come back in the F₂, which shows the alleles never blended. In codominance, the heterozygote shows both alleles' effects fully and separately.
- "Dominant" describes what shows up in the phenotype; it doesn't mean one allele attacks or switches off the other. Often one working copy just makes enough protein. Which label fits can depend on what you measure: an allele can look dominant for health but incompletely dominant for enzyme activity.
- A dominant allele isn't necessarily the common one. How common an allele is depends on things like selection and chance, not on dominance.
- Many genes have more than two alleles in a population, though each diploid individual carries only two. Many genes also affect several traits at once, which is called pleiotropy.
- Genes can also interact. In epistasis, one gene masks or changes the effect of another, turning 9:3:3:1 into ratios like 9:3:4 or 9:7. In polygenic inheritance, several genes each add a little to one trait, so the trait varies smoothly along a bell-shaped range instead of falling into a few classes.
- The environment matters too. A genotype can produce a range of phenotypes depending on conditions, called its norm of reaction; the AP course calls this flexibility phenotypic plasticity. The range is usually widest for polygenic traits, and traits shaped by many genes plus the environment are called multifactorial.
Key terms (11)
- complete dominance
- When the heterozygote looks exactly like the dominant homozygote, so one allele's effect fully hides the other's.
- incomplete dominance
- When the heterozygote has an in-between phenotype, like a medium shade between two parental colors.
- codominance
- When the heterozygote shows both alleles' effects fully and side by side, not a mix of the two.
- multiple alleles
- A gene that has three or more different alleles in a population. Each individual still carries only two of them.
- pleiotropy
- When one gene affects several different traits, because its protein is used in many places or processes.
- epistasis
- When one gene's alleles hide or change how another gene's alleles show up, as when a gene that blocks all pigment hides a gene for pigment color.
- quantitative character
- A trait that varies smoothly over a range, like height or mass, instead of falling into a few clear categories.
- polygenic inheritance
- Inheritance of a trait controlled by several genes whose effects add up. It produces quantitative characters.
- norm of reaction
- For one genotype, the whole spread of phenotypes it can show as conditions change.
- phenotypic plasticity
- An individual's ability to develop different phenotypes depending on its environment, without any change in its genes. The AP course uses this term.
- multifactorial
- Describes a trait or disease shaped by many genes together with the environment, such as height or heart disease.
Check yourself: 14.3 Beyond simple dominance
4 questions on 14.3 Beyond simple dominance. Pick an answer to see if you got it, and why.
In a species of tetra (a small aquarium fish), true-breeding gold tetras crossed with true-breeding silver tetras give only bronze offspring, and two bronze tetras produce gold, bronze and silver young in about a 1:2:1 ratio. Which cross would produce only bronze offspring?
In one breed of cattle, RR animals have red coats, WW animals have white coats, and RW animals have coats in which separate red hairs and white hairs are mixed together. What does the RW phenotype show?
A gene codes for a liver enzyme that breaks down a sugar in the diet. People with two working alleles have full enzyme activity, people with one working and one nonworking allele have about half as much, and people with two nonworking alleles have almost none and become ill. People with one working allele stay healthy. Which statement best describes the working allele?
In a flowering plant, purple pigment is made in two enzyme steps. Two true-breeding white-flowered lines are crossed, and every F₁ plant has purple flowers. Self-pollinating the F₁ gives 287 purple and 221 white plants (invented data). Which explanation best fits these results?
0 of 4 answered
14.4 Inheritance in human families
pp. 275–281
Topic 5.3 expects you to read pedigrees and work out carrier chances. Sickle-cell disease and cystic fibrosis come back in Topics 6.7 and 7.2. Fetal tests and genetic counseling are background, and you won't need disease rates.
In the course: Topic 5.3 Mendelian Genetics, Topic 6.7 Mutations, Topic 7.2 Natural Selection (notes, videos and more questions)
Key points
- Geneticists can't arrange human matings, and people have few children and long generations. So they study pedigrees: family trees built from matings that already happened, with circles for females, squares for males and shading for affected people.
- Pedigree clues: unaffected parents with an affected child means the trait is recessive and both parents are carriers. Two affected parents with an unaffected child means it is dominant. Dominant traits usually show in every generation, while recessive ones can skip generations.
- A recessive disorder allele usually makes a faulty protein or none. Heterozygous carriers are normally healthy because one working copy makes enough. Most affected children have two carrier parents, and an unaffected child of two carriers has a 2/3 chance of being a carrier.
- Close relatives are more likely than strangers to carry the same rare allele, so children of related parents have a higher chance of being homozygous for a harmful recessive allele.
- Cystic fibrosis is recessive: a faulty chloride channel leads to thick mucus with effects in many organs. Sickle-cell disease comes from a one-amino-acid change in hemoglobin. Carriers are partly protected from malaria, which keeps the allele common where malaria is common.
- A dominant allele that kills before people can have children is rarely passed on. One whose symptoms start in middle age, like Huntington's disease, can be passed on before anyone knows it's there, and each child of a heterozygous parent has a 1/2 chance of inheriting it.
- Many common conditions, such as heart disease and type 2 diabetes, are multifactorial: many genes plus lifestyle and surroundings. Carrier tests, fetal tests (amniocentesis and chorionic villus sampling) and newborn screening all rest on Mendelian rules, and each child is an independent event.
Key terms (12)
- pedigree
- A family tree showing who in several generations has a trait. It's used to work out how the trait is inherited and who might carry it.
- carrier
- A heterozygous person who has one copy of a recessive disease allele but no symptoms, and can pass the allele on.
- consanguineous mating
- A mating between close relatives, such as first cousins. It raises the chance that a child gets two copies of the same rare recessive allele.
- cystic fibrosis
- A recessive disorder in which a faulty chloride channel protein (CFTR) leads to thick, sticky mucus in the lungs, pancreas and other organs.
- sickle-cell disease
- A recessive disorder caused by a single amino acid change in hemoglobin. Red blood cells can bend into a sickle shape and block small blood vessels.
- heterozygote advantage
- When heterozygotes survive or reproduce better than either homozygote, as sickle-cell carriers do where malaria is common. It keeps a harmful allele in the population.
- Huntington's disease
- A fatal nervous-system disease caused by a dominant allele. Symptoms usually start in middle age, often after the person has had children.
- multifactorial disorder
- A disease whose risk depends on many genes plus environment and lifestyle, such as heart disease.
- amniocentesis
- A prenatal test in which a small amount of the fluid around the fetus is drawn out with a needle, so fetal cells and chemicals can be tested.
- chorionic villus sampling (CVS)
- A prenatal test that takes a tiny piece of placenta tissue, which has the fetus's genotype. It can be done earlier in pregnancy than amniocentesis.
- newborn screening
- Routine tests on a baby's blood soon after birth that catch treatable inherited conditions before they cause harm.
- genetic counselor
- A health professional who uses family history and test results to explain the chance of passing on an inherited condition.
Check yourself: 14.4 Inheritance in human families
4 questions on 14.4 Inheritance in human families. Pick an answer to see if you got it, and why.
In a family pedigree, a man and a woman who are both unaffected have four children, and one of their daughters is affected by a rare trait. Which mode of inheritance is most consistent with this pedigree?
Which single observation in a pedigree rules out recessive inheritance of a trait?
Lena is unaffected, but her brother has a recessive disorder; neither of their parents is affected. Lena's partner, Omar, has no family history of the disorder, and a genetic test shows he is not a carrier. What is the probability that a child of theirs will be a carrier of the disorder allele?
A dominant allele that causes death in early childhood is found in a population only rarely, while recessive alleles that are just as deadly to homozygotes are far more common. What best explains the difference?
0 of 4 answered