Campbell Biology · Chapter 15
The Chromosomal Basis of Inheritance
pp. 286–304 · 5 sections
Mendel worked out the rules of heredity without knowing where genes were. This chapter puts genes on chromosomes: it shows how chromosome movements in meiosis produce Mendel's ratios, then covers genes on the sex chromosomes, genes linked on the same chromosome, errors that change chromosome number or structure, and genes that don't follow Mendel's rules. Nearly all of it lines up with AP Topics 5.2 to 5.4, with the effects of chromosome changes in Topic 6.7.
Independent review — not affiliated with or endorsed by the publisher. You'll need your own copy of the book.
15.1 Chromosomes carry Mendel's genes
pp. 286–289
Topic 5.3 expects you to link Mendel's two laws to what chromosomes do in meiosis (Topics 5.1 and 5.2). You won't be asked who proposed the theory or about the history of Morgan's lab.
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
- Mendel described inherited "factors" without knowing where they were. Around 1900, biologists noticed that chromosomes act just like those factors: a body cell has two of each, a gamete gets only one, and fertilization restores the pair.
- That match became the chromosome theory of inheritance. Each gene sits at a fixed spot, its locus, on a particular chromosome, and the way chromosomes move during meiosis is what produces Mendel's ratios.
- Segregation has a physical cause: the two alleles of a gene sit on the two members of a homologous pair, and those homologs are pulled to opposite poles in anaphase I. So every gamete ends up with one allele.
- Independent assortment has one too: at metaphase I, each homologous pair lines up facing either pole at random, regardless of how the other pairs face. Genes on different chromosomes therefore get sorted into gametes separately.
- Fruit flies became the classic test organism for these ideas: they're cheap, reproduce quickly in large numbers, and have only a handful of chromosomes, which makes them easy to track.
- Geneticists call the usual form of a trait in wild populations the wild type, and any rarer form caused by a changed allele a mutant phenotype. In fly notation, a gene is named for its first known mutant, and a + marks the wild-type allele, as in w⁺.
- Early in the 1900s, a fly eye-color mutation that showed up almost only in males was traced to the X chromosome. It was the first time a specific gene was pinned to a specific chromosome.
Key terms (12)
- chromosome theory of inheritance
- The idea that genes sit on chromosomes, and that chromosome movements during meiosis explain Mendel's laws.
- locus
- The exact place on a chromosome where a particular gene is found. The plural is loci.
- homologous chromosomes
- A matching pair of chromosomes, one from each parent. They carry the same genes at the same loci, though the alleles can differ.
- law of segregation
- Mendel's rule that the two alleles of a gene separate when gametes form, so each gamete gets only one.
- law of independent assortment
- Mendel's rule that alleles of genes on different chromosomes are sorted into gametes independently of one another.
- metaphase I
- The stage of meiosis when homologous pairs line up in the middle of the cell. How each pair faces is random, which is the root of independent assortment.
- anaphase I
- The stage of meiosis when homologous chromosomes are pulled apart to opposite ends of the cell, separating the alleles they carry.
- wild type
- The form of a trait you see most often in a natural population. Rarer forms are called mutant phenotypes.
- mutant phenotype
- Any version of a trait that differs from the wild type, caused by an allele that arose by mutation.
- autosome
- Any chromosome that is not a sex chromosome. Humans have 22 pairs.
- sex chromosomes
- The chromosomes, such as X and Y in mammals, whose combination usually decides an individual's sex.
- model organism
- A species that is easy to raise and study, so scientists use it to learn principles that apply to many living things. Fruit flies, mice and yeast are examples.
Check yourself: 15.1 Chromosomes carry Mendel's genes
4 questions on 15.1 Chromosomes carry Mendel's genes. Pick an answer to see if you got it, and why.
A tomato plant is heterozygous (Hh) for a gene that controls hairy stems. During which event of meiosis do the H and h alleles normally get sent toward different cells, as the law of segregation describes?
A corn plant is heterozygous for gene A, found on chromosome 3, and gene B, found on chromosome 8 (genotype AaBb). It makes AB, Ab, aB and ab gametes in about equal numbers. Which feature of meiosis best explains this?
A frog species has a diploid number of 2n = 12. Counting only the random lineup of homologous pairs at metaphase I, and ignoring crossing over, how many different combinations of maternal and paternal chromosomes are possible in its gametes?
Geneticists have mapped thousands of corn genes using crosses. The genes fall into 10 groups: genes in the same group tend to be inherited together, while genes in different groups assort independently. Corn's body cells have 2n = 20 chromosomes. How does this finding support the chromosome theory of inheritance?
0 of 4 answered
15.2 Sex chromosomes and sex-linked genes
pp. 289–292
Topic 5.4 tests X-linked inheritance and other systems like ZW birds and haplodiploid bees. X inactivation, Barr bodies and SRY aren't named in the course, but X inactivation is a good example of epigenetic gene control (Topic 6.5).
In the course: Topic 5.4 Non-Mendelian Genetics, Topic 5.5 Environmental Effects on Phenotype, Topic 6.5 Regulation of Gene Expression (notes, videos and more questions)
Key points
- In mammals, an XX embryo usually develops as female and an XY embryo as male. The Y is far smaller than the X and carries only a few dozen genes; short matching stretches at its tips let it pair with the X in meiosis.
- A Y-linked gene called SRY acts as the switch: its protein turns on other genes that make the early gonads become testes. Without SRY, ovaries form. Every egg carries an X, so the sperm, X-bearing or Y-bearing, decides the chromosomal sex.
- Other animals use other systems. Birds use Z and W: a ZW bird is female and a ZZ bird is male, so the egg decides. Bees and ants are haplodiploid: fertilized eggs become females and unfertilized ones become males. In some reptiles, nest temperature decides (Topic 5.5).
- Genes carried on the X or the Y are called sex-linked. The X carries hundreds of genes, most of which have nothing to do with sex. An XY individual has just one copy of each X-linked gene, so a single recessive allele shows up in its phenotype.
- That's why X-linked recessive conditions are much more common in XY individuals. An XX individual needs two copies of the allele to show the trait; with one, she's a carrier.
- Follow the X: a father gives his X to every daughter and his Y to every son, so he never passes an X-linked allele to a son. A mother gives one of her two X's to each child, so an affected son usually has a carrier mother.
- In XX mammals, one X in each cell of the early embryo is randomly shut down and packed into a dense Barr body, and every cell descended from it keeps the same X off. So a heterozygous female is a mosaic of patches using different alleles. An RNA called XIST coats the X being shut down, and DNA methylation helps keep it silent, though some genes escape.
Key terms (13)
- SRY
- A Y-chromosome gene whose protein switches on the genes that turn early gonads into testes. Without it, ovaries develop.
- sex-linked gene
- Any gene located on a sex chromosome, X or Y. Note that this is different from "linked genes."
- X-linked gene
- A gene on the X chromosome. Most have nothing to do with sex; they affect ordinary traits, such as how certain enzymes or immune cells work.
- Y-linked gene
- A gene carried on the Y. It passes from father to son and never to a daughter.
- hemizygous
- Having only one copy of a gene instead of two. XY individuals are hemizygous for X-linked genes, so whatever allele they have shows.
- carrier
- Someone with one copy of a recessive allele who doesn't show the trait but can pass the allele on.
- X inactivation
- Early in an XX mammal embryo, every cell silences one of its two X's, so XX and XY cells alike rely on a single active X.
- Barr body
- The tightly packed, inactive X chromosome you can see near the edge of the nucleus in XX mammal cells.
- mosaic
- An individual made of patches of cells that differ genetically or in which allele is switched on, like a female with random X inactivation.
- XIST
- A gene whose RNA product coats the X chromosome that will be shut down and starts the inactivation process. The RNA is never translated into protein.
- Z-W system
- The sex-determining system of birds and some fish and insects, in which ZW individuals are female and ZZ individuals are male.
- haplodiploidy
- A system, found in bees and ants, in which females develop from fertilized diploid eggs and males from unfertilized haploid eggs.
- X-O system
- A system found in grasshoppers and some other insects in which females are XX and males have a single X and no partner sex chromosome.
Check yourself: 15.2 Sex chromosomes and sex-linked genes
4 questions on 15.2 Sex chromosomes and sex-linked genes. Pick an answer to see if you got it, and why.
In chickens, hens are ZW and roosters are ZZ. A dominant allele on the Z chromosome, B, gives feathers a striped (barred) pattern, while b gives solid-colored feathers. A farmer crosses a barred hen with a solid-colored rooster. How will the chicks look once their feathers grow in?
A rare trait is being studied in a large family pedigree. Which single observation would rule out X-linked recessive inheritance?
A woman is heterozygous for an X-linked gene: one allele makes a working enzyme and the other makes none. A thin slice of her liver tissue is stained so that cells containing the enzyme turn dark. Under the microscope, dark patches sit next to unstained patches. What best explains this pattern?
Cells taken from a person show two Barr bodies in every nucleus. Which sex chromosomes does the person most likely have?
0 of 4 answered
15.3 Linked genes, crossing over and gene maps
pp. 292–297
Topic 5.4 tests this directly: be ready to pick out recombinants, calculate a recombination frequency, turn it into map units and put genes in order. You won't be asked about the original fly crosses or who made the first map.
In the course: Topic 5.4 Non-Mendelian Genetics, Topic 5.2 Meiosis and Genetic Diversity, Topic 5.3 Mendelian Genetics (notes, videos and more questions)
Key points
- Every chromosome carries hundreds or thousands of genes. Genes close together on one chromosome are linked: they tend to be passed on as a unit, so they don't give Mendel's independent-assortment ratios. (Don't mix this up with a sex-linked gene, which is any gene on a sex chromosome.)
- A testcross shows linkage clearly. Cross a double heterozygote with a double homozygous recessive, and each offspring's phenotype reveals exactly which alleles were in the gamete from the heterozygous parent.
- Offspring with the same allele combinations the heterozygous parent inherited are parental types; offspring with new combinations are recombinants. For genes on different chromosomes, about half the offspring are recombinant, so 50% recombination means the genes are unlinked.
- Linked genes still produce some recombinants, always fewer than 50%. They come from crossing over: in prophase I, nonsister chromatids of a homologous pair swap matching segments, so alleles that started on one chromosome can end up apart.
- Recombination frequency = (recombinant offspring ÷ total offspring) × 100. One percent recombination is one map unit, also called a centimorgan. Widely spaced genes leave more room for a crossover between them, so the higher the frequency.
- A linkage map puts genes in order using these frequencies. Genes very far apart look unlinked, because some crossover between them happens nearly every time, so long distances are found by adding up shorter intervals. Two crossovers between distant genes can cancel each other out, which makes the measured value too low.
- Map units show the right order of genes, but not exact physical distances, because crossovers happen more often in some parts of a chromosome than others. New allele combinations made by crossing over add to the variation that natural selection works on.
Key terms (12)
- linked genes
- Genes close together on the same chromosome, which tend to be inherited together instead of assorting independently.
- genetic recombination
- Producing offspring with combinations of alleles that differ from the combinations either parent inherited.
- testcross
- Crossing an individual with a homozygous recessive one. The offspring's phenotypes show which alleles the first individual's gametes carried.
- parental type
- An offspring whose allele combination matches one of the chromosomes the heterozygous parent inherited.
- recombinant
- An offspring with a new combination of alleles that neither of its parent's chromosomes had, made by independent assortment or crossing over.
- crossing over
- The exchange of matching segments between nonsister chromatids of homologous chromosomes during prophase I of meiosis.
- nonsister chromatids
- Chromatids that belong to different members of a homologous pair. Crossing over happens between them.
- recombination frequency
- The percentage of offspring that are recombinants. For linked genes it is always under 50%.
- map unit
- The unit of distance on a linkage map: one map unit equals a 1% recombination frequency. Also called a centimorgan.
- linkage map
- A diagram that lines genes up in order along a chromosome, spaced according to their recombination frequencies.
- double crossover
- Two crossovers between the same pair of genes. The second undoes the first for those two genes, so it hides a recombination.
- linkage group
- A set of genes that are all inherited together because they share one chromosome.
Check yourself: 15.3 Linked genes, crossing over and gene maps
4 questions on 15.3 Linked genes, crossing over and gene maps. Pick an answer to see if you got it, and why.
In a flowering plant, a plant heterozygous for petal color (purple P or white p) and leaf edge (smooth S or jagged s) is crossed with a ppss plant. Offspring phenotype | Number Purple, smooth | 436 White, jagged | 421 Purple, jagged | 72 White, smooth | 71 About how far apart are the two genes?
A plant heterozygous for height (tall T or short t) and fruit color (red R or yellow r) is crossed with a ttrr plant. Offspring phenotype | Number Tall, red | 48 Tall, yellow | 205 Short, red | 197 Short, yellow | 50 How were the alleles most likely arranged on the heterozygous plant's chromosomes?
Three genes, P, Q and R, are on the same chromosome. Testcrosses give these recombination frequencies: P–Q, 6%; P–R, 15%; Q–R, 20%. What is the most likely order of the genes?
DNA sequencing shows that two genes in a mouse are on the same chromosome. Yet when a mouse heterozygous for both is testcrossed, the four phenotype classes appear in nearly equal numbers. What best explains this?
0 of 4 answered
15.4 Changes in chromosome number and structure
pp. 297–300
Nondisjunction is in Topic 5.2, and what extra, missing or rearranged chromosomes do to the phenotype is in Topic 6.7. You don't need to know specific syndromes, like Turner or cri du chat, by name.
In the course: Topic 5.2 Meiosis and Genetic Diversity, Topic 6.7 Mutations (notes, videos and more questions)
Key points
- Nondisjunction is a mistake in which chromosomes don't separate: homologs stay together in meiosis I, or sister chromatids stay together in meiosis II. It produces gametes with an extra copy of a chromosome (n + 1) or none (n − 1). An error in meiosis I affects all four gametes; one in meiosis II affects two.
- If an abnormal gamete joins a normal one, the zygote is aneuploid: trisomic (2n + 1) with three copies of a chromosome, or monosomic (2n − 1) with one. Mitosis copies the error into every cell. If nondisjunction happens in mitosis in an early embryo, only some cells are affected, making a mosaic.
- Aneuploidy throws off the balance of gene products, so most human embryos with an extra or missing autosome don't survive. Trisomy 21, which causes Down syndrome, is the most common one that does, and its risk climbs with the mother's age.
- Extra or missing sex chromosomes are usually milder, because extra X's are inactivated and the Y carries few genes. People who are XXY, XYY or XXX usually live healthy lives, often with mild effects (XXY usually causes infertility), and a single X with no partner (X0) is the only full monosomy humans survive.
- Polyploidy means having more than two full sets of chromosomes, like 3n or 4n. It is common in plants, including many crops, and rare in animals. A whole extra set keeps the genes in balance, so polyploids are usually healthier than aneuploids, though odd-numbered sets make meiosis go wrong and cause sterility.
- Broken chromosomes can be rejoined wrongly. A deletion loses a segment, a duplication repeats one, an inversion flips one end to end, and a translocation shifts a segment onto a nonhomologous chromosome; if two chromosomes trade segments, the swap is reciprocal. Unequal crossing over can create a deletion and a duplication at once.
- Inversions and balanced translocations keep every gene but can still matter: a break can cut a gene or move it next to different control regions, and carriers may make gametes with unbalanced sets. In body cells, translocations can cause cancer. In one leukemia, a swap between chromosomes 9 and 22 fuses two genes into one that makes an always-on kinase, and drugs that block it now control the disease for many patients.
Key terms (13)
- nondisjunction
- A mistake in cell division in which homologs or sister chromatids don't come apart, so daughter cells get the wrong number of chromosomes.
- aneuploidy
- Having an abnormal number of one or a few chromosomes, rather than whole extra or missing sets.
- trisomic
- Having three copies of one chromosome instead of two (2n + 1).
- monosomic
- Having just one copy of a chromosome instead of two (2n − 1).
- polyploidy
- Having three or more full sets of chromosomes in every cell.
- triploid
- Having three full sets of chromosomes (3n). Triploids are usually sterile because three sets can't pair up evenly.
- tetraploid
- Having four full sets of chromosomes (4n).
- deletion
- A chromosome change in which a segment is lost, along with the genes on it.
- duplication
- A chromosome change in which a segment appears twice, giving extra copies of its genes.
- inversion
- A chromosome change in which a segment breaks out, flips around and reattaches in reverse order.
- translocation
- A chromosome change in which a segment ends up attached to a different, nonhomologous chromosome. When two chromosomes trade pieces, it is called reciprocal.
- karyotype
- A picture of a cell's chromosomes arranged in pairs by size and shape, used to spot extra, missing or rearranged chromosomes.
- Down syndrome
- A condition usually caused by an extra copy of chromosome 21 (trisomy 21). Its features include developmental delay and some heart defects.
Check yourself: 15.4 Changes in chromosome number and structure
4 questions on 15.4 Changes in chromosome number and structure. Pick an answer to see if you got it, and why.
In a cell undergoing meiosis to make sperm, meiosis I is normal. In one of the two meiosis II divisions, the sister chromatids of chromosome 7 fail to separate; the other meiosis II division is normal. What fraction of the four sperm will have the normal number of chromosome 7?
A plant species has a diploid number of 2n = 24. One plant's cells each contain 25 chromosomes, with three copies of chromosome 4. Which term best describes this plant?
Growers produce seedless watermelons by crossing a tetraploid (4n) watermelon plant with a diploid (2n) one. What is the ploidy of the embryos in the seeds this cross produces?
Researchers compared the same chromosome in two populations of a grasshopper species. Reading from the same end, the genes (invented names) are in this order, with • marking the centromere: Population 1: kel mup dsa • vor ost nim bek Population 2: kel mup dsa • nim ost vor bek Every gene appears exactly once in both, and no genes from other chromosomes have been added. Which change most likely produced the difference?
0 of 4 answered
15.5 Genomic imprinting and organelle genes
pp. 300–302
Topic 5.4 tests inheritance of mitochondrial and chloroplast genes, which usually come from the mother. Genomic imprinting isn't named in the course, but it is an example of epigenetic gene control (Topic 6.5).
In the course: Topic 5.4 Non-Mendelian Genetics, Topic 6.5 Regulation of Gene Expression, Topic 3.5 Cellular Respiration (notes, videos and more questions)
Key points
- Usually it doesn't matter which parent an allele came from. For a small set of mammal genes it does: only the copy from one particular parent is used. This is genomic imprinting, and the genes involved are mostly autosomal, not sex-linked.
- Imprints are set while eggs and sperm form, mostly by adding methyl groups to DNA and changing nearby histones. The DNA sequence doesn't change. In each new generation the old imprints are wiped clean in the cells that make gametes and reset to match the sex of the parent making them.
- Because of imprinting, the effect of a mutant allele can depend on which parent passed it on. Losing the same small region of chromosome 15, for example, causes one disorder when the faulty copy comes from the father and a different one when it comes from the mother.
- Many imprinted genes help control how much an embryo grows before birth. Development goes wrong when both copies are switched on or both are off, which is one reason a mammal embryo needs a set of chromosomes from each parent.
- Mitochondria and chloroplasts carry their own small, circular DNA. These organelles divide on their own and are handed out randomly when cells divide, so their genes don't follow Mendel's rules.
- In most animals and plants, nearly all of a zygote's organelles come from the egg, so organelle genes are inherited from the mother. An affected mother can pass a mitochondrial trait to all her children; an affected father passes it to none. Patches of white and green on a leaf can come from a mix of normal and mutant plastids.
- Most proteins encoded by mitochondrial DNA are parts of the cellular respiration machinery (Topic 3.5). A faulty one lowers the ATP supply, so energy-hungry tissues such as brain and muscle usually suffer first. A person can carry a mix of normal and mutant mitochondrial DNA, and the share passed to each egg varies, so siblings can be affected very differently.
Key terms (11)
- genomic imprinting
- When a gene is expressed only from the copy inherited from one particular parent, while the other copy is kept silent.
- imprinted gene
- A gene whose activity depends on which parent passed it on.
- DNA methylation
- Attaching methyl groups (–CH₃) to DNA bases, usually cytosine. It often helps switch genes off without changing the sequence.
- epigenetic
- Describes a change in how genes are used that can be passed to daughter cells without any change to the DNA sequence.
- gene dosage
- How many active copies of a gene a cell has. For some genes, too many or too few active copies cause problems.
- non-nuclear inheritance
- Passing on genes that sit outside the nucleus, in mitochondria or chloroplasts. These genes don't follow Mendel's laws.
- maternal inheritance
- Inheritance through the mother only. It is typical of organelle genes, because the egg supplies nearly all of a zygote's cytoplasm.
- mitochondrial DNA
- The small, circular chromosome inside each mitochondrion. Many of its genes code for parts of the machinery that makes ATP.
- plastid
- A plant organelle with its own DNA. Chloroplasts are the best-known kind.
- variegation
- A patchy pattern of color, like white streaks on a green leaf. It can come from a mix of normal and mutant plastids.
- heteroplasmy
- Having a mix of normal and mutant mitochondrial DNA in the same cell or body. The mix can differ from one tissue or one child to the next.
Check yourself: 15.5 Genomic imprinting and organelle genes
4 questions on 15.5 Genomic imprinting and organelle genes. Pick an answer to see if you got it, and why.
A rare disorder runs in a family. An affected woman and an unaffected man have four children, and all four are affected. Their affected son has three children (two daughters and a son) with an unaffected woman, and none of them is affected. Their affected daughter has two children with an unaffected man, and both are affected. Which inheritance pattern fits best?
In a canola-like crop, resistance to a certain weed killer comes from a mutation in a chloroplast gene, and chloroplasts pass only through the egg, never the pollen. A breeder has a resistant plant and a high-yielding plant that is not resistant. Which cross will produce resistant offspring?
In a hypothetical mouse gene G, only the copy inherited from the mother is expressed; the father's copy is silenced. A mutant allele, g, makes no protein, and mice lacking the protein have kinked tails. Male mouse M is Gg: he got g from his mother and G from his father, and he has a kinked tail. M mates with a GG female. What fraction of their pups will have kinked tails?
Which observation best shows that a genomic imprint is not a change in the DNA sequence?
0 of 4 answered