Complete your lower division coursework first
Complete preparatory course work first.
If your lower division course work is not completed, start by reviewing the Preparatory Subject Matter list below for more information on specific courses (especially organic chemistry).
Lower division courses at UC Davis are numbered from 1-99.
Select two (2) courses from your major preparatory courses. The remaining units in your fall schedule should consist of electives, general education (GE), English composition courses, research, or internships. See the following pages for more specific suggestions.
Important: Because your first opportunity to register (Pass 1) comes after our continuing students have registered, be aware some courses or sections may already be full (closed). It is essential that you prepare a list of alternate sections and courses before your Pass 1 appointment. The Class Search Tool and your Schedule Builder can show you what is currently available. During Pass 2, a waiting list is generated for most closed courses. Using Schedule Builder, dur your Pass 2 appointment add yourself to the wait list(s) for any closed course(s) in which you hope to enroll. Then, for lab courses, visit as many labs on their first day to ask for a "permission to add" (PTA) number.
Duplication of Credit
Are you signing up for courses that seem familiar? Or, do you need to complete the second half of a course series? You could be duplicating credit already earned at your previous institution. Taking part or all of a course at UCD that you completed elsewhere will result in loss of those UCD course units. Check with an adviser if you have any doubts or questions. Students from a California Community College can also check duplication on Assist .
List of Preparatory Subject Matter Courses for Lower Division Coursework
These are the preparatory courses for all CBS majors. Transfer students should compare their preparation with the list at the website Assist (articulation agreements between California Community College and four-year universities in California) and complete any course or series deficiencies first before starting upper division course work.
MAT 17A+17B+17C or 21A +21B
Transfer students should have already completed the equivalent of one of these series.
Transfer students should have already completed the equivalent of this series.
Biological Sciences (Introductory Biology)
These classes must be taken in order and cannot be taken at the same time as they are prerequisites for the next BIS 2 course. A grade of C- or higher is required to advance to the next course. Transfer students have completed the equivalent to at least one of these courses. If you have not finished the series, you should do so as soon as possible.
CHE 8A+8B or CHE 118A+118B+118C
If your organic chemistry is not completed, you should enroll in 8A or 118A or the appropriate continuing course. The CHE 8 series provides a solid background in organic chemistry for biology students. If you desire a more in-depth coverage of organic chemistry or plan to attend a professional school (such as medical school) that requires a full year of organic chemistry, take the CHE 118ABC series. The Biochemistry and Molecular Biology major requires either the 118 series or the 128ABC and 129ABC series and does not accept the CHE 8AB series.
If you have not started your physics series you should enroll in 7A. If your physics is not completed, you should enroll in the appropriate continuing course. The continuing course is best determined by the UCD Physics Department because our series progression is different than most schools. Neurobiology, Physiology, and Behavior majors should complete their physics series before taking upper division NPB courses.
STA 100 (or 13 or 32 or 102, or 130A+130B)
You need to complete only a course from this list (dependent on your major). You may already have completed the equivalent of our STA 13 course at your community college. Consult Assist. Statistics 100 is specifically designed for life science majors. Evolution, Ecology, and Biodiversity; and Genetics majors require specific upper division statistics course(s). Please see your adviser if you are in one of these majors to determine your Statistics requirement.
Chapter 14 - Mendel and the Gene Idea
Chapter 14 - Mendel and the Gene Idea
Chapter 14 Mendel and the Gene Idea
Overview: Drawing from the Deck of Genes
- Every day we observe heritable variations (such as brown, green, or blue eyes) among individuals in a population.
- These traits are transmitted from parents to offspring.
- One possible explanation for heredity is a “blending” hypothesis.
- This hypothesis proposes that genetic material contributed by each parent mixes in a manner analogous to the way blue and yellow paints blend to make green.
- With blending inheritance, a freely mating population will eventually give rise to a uniform population of individuals.
- Everyday observations and the results of breeding experiments tell us that heritable traits do not blend to become uniform.
- An alternative model, “particulate” inheritance, proposes that parents pass on discrete heritable units, genes, that retain their separate identities in offspring.
- Genes can be sorted and passed on, generation after generation, in undiluted form.
- Modern genetics began in an abbey garden, where a monk named Gregor Mendel documented a particulate mechanism of inheritance.
Concept 14.1 Mendel used the scientific approach to identify two laws of inheritance
- Mendel’s first experiments followed only a single character, such as flower color.
- All F1 progeny produced in these crosses were monohybrids, heterozygous for one character.
- A cross between two heterozygotes is a monohybrid cross.
- Mendel identified the second law of inheritance by following two characters at the same time.
- In one such dihybrid cross, Mendel studied the inheritance of seed color and seed shape.
- The allele for yellow seeds (Y) is dominant to the allele for green seeds (y).
- The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r).
- Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr).
- One possibility is that the two characters are transmitted from parents to offspring as a package.
- The Y and R alleles and y and r alleles stay together.
- If this were the case, the F1 offspring would produce yellow, round seeds.
- The F2 offspring would produce two phenotypes (yellow + round; green + wrinkled) in a 3:1 ratio, just like a monohybrid cross.
- This was not consistent with Mendel’s results.
- An alternative hypothesis is that the two pairs of alleles segregate independently of each other.
- The presence of a specific allele for one trait in a gamete has no impact on the presence of a specific allele for the second trait.
- In our example, the F1 offspring would still produce yellow, round seeds.
- However, when the F1s produced gametes, genes would be packaged into gametes with all possible allelic combinations.
- Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts.
- When sperm with four classes of alleles and ova with four classes of alleles combined, there would be 16 equally probable ways in which the alleles can combine in the F2 generation.
- These combinations produce four distinct phenotypes in a 9:3:3:1 ratio.
- This was consistent with Mendel’s results.
- Mendel repeated the dihybrid cross experiment for other pairs of characters and always observed a 9:3:3:1 phenotypic ratio in the F2 generation.
- Each character appeared to be inherited independently.
- If you follow just one character in these crosses, you will observe a 3:1 F2 ratio, just as if this were a monohybrid cross.
- The independent assortment of each pair of alleles during gamete formation is now called Mendel’s law of independent assortment.
- Mendel’s law of independent assortment states that each pair of alleles segregates independently during gamete formation.
- Strictly speaking, this law applies only to genes located on different, nonhomologous chromosomes.
- Genes located near each other on the same chromosome tend to be inherited together and have more complex inheritance patterns than those predicted for the law of independent assortment.
Concept 14.2 The laws of probability govern Mendelian inheritance
- While we cannot predict with certainty the genotype or phenotype of any particular seed from the F2 generation of a dihybrid cross, we can predict the probability that it will have a specific genotype or phenotype.
- Mendel’s experiments succeeded because he counted so many offspring, was able to discern the statistical nature of inheritance, and had a keen sense of the rules of chance.
- Mendel’s laws of independent assortment and segregation explain heritable variation in terms of alternative forms of genes that are passed along according to simple rules of probability.
- These laws apply not just to garden peas, but to all diploid organisms that reproduce by sexual reproduction.
- Mendel’s studies of pea inheritance endure not only in genetics, but as a case study of the power of scientific reasoning using the hypothetico-deductive approach.
Concept 14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics
- In the 20th century, geneticists have extended Mendelian principles not only to diverse organisms, but also to patterns of inheritance more complex than Mendel described.
- In fact, Mendel had the good fortune to choose a system that was relatively simple genetically.
- Each character that Mendel studied is controlled by a single gene.
- Each gene has only two alleles, one of which is completely dominant to the other.
- The heterozygous F1 offspring of Mendel’s crosses always looked like one of the parental varieties because one allele was dominant to the other.
- The relationship between genotype and phenotype is rarely so simple.
- The inheritance of characters determined by a single gene deviates from simple Mendelian patterns when alleles are not completely dominant or recessive, when a gene has more than two alleles, or when a gene produces multiple phenotypes.
- We will consider examples of each of these situations.
- Alleles show different degrees of dominance and recessiveness in relation to each other.
- One extreme is the complete dominance characteristic of Mendel’s crosses.
- At the other extreme from complete dominance is codominance, in which two alleles affect the phenotype in separate, distinguishable ways.
- For example, the M, N, and MN blood groups of humans are due to the presence of two specific molecules on the surface of red blood cells.
- People of group M (genotype MM) have one type of molecule on their red blood cells, people of group N (genotype NN) have the other type, and people of group MN (genotype MN) have both molecules present.
- The MN phenotype is not intermediate between M and N phenotypes but rather exhibits both the M and the N phenotype.
- Some alleles show incomplete dominance, in which heterozygotes show a distinct intermediate phenotype not seen in homozygotes.
- This is not blending inheritance because the traits are separable (particulate), as shown in further crosses.
- Offspring of a cross between heterozygotes show three phenotypes: each parental and the heterozygote.
- The phenotypic and genotypic ratios are identical: 1:2:1.
- A clear example of incomplete dominance is seen in flower color of snapdragons.
- A cross between a white-flowered plant and a red-flowered plant will produce all pink F1 offspring.
- Self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink F2 offspring.
- The relative effects of two alleles range from complete dominance of one allele, through incomplete dominance of either allele, to codominance of both alleles.
- It is important to recognize that a dominant allele does not somehow subdue a recessive allele.
- Alleles are simply variations in a gene’s nucleotide sequence.
- When a dominant allele coexists with a recessive allele in a heterozygote, they do not interact at all.
- To illustrate the relationship between dominance and phenotype, let us consider Mendel’s character of round versus wrinkled pea seed shape.
- Pea plants with wrinkled seeds have two copies of the recessive allele.
- The seeds are wrinkled due to the accumulation of monosaccharides because of the lack of a key enzyme that converts them to starch.
- Excess water enters the seed due to the accumulation of monosaccharides.
- The seeds wrinkle when the excess water dries.
- Both homozygous dominants and heterozygotes produce enough enzymes to convert all the monosaccharides into starch.
- As a result, they do not fill with excess water and form smooth seeds as they dry.
- For any character, dominance/recessiveness relationships depend on the level at which we examine the phenotype.
- For example, humans with Tay-Sachs disease lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in the brain, harming brain cells, and ultimately leading to death.
- Children with two Tay-Sachs alleles (homozygotes) have the disease.
- Both heterozygotes with one working allele and homozygotes with two working alleles are healthy and normal at the organismal level.
- The activity level of the lipid-metabolizing enzyme is reduced in heterozygotes. At the biochemical level, the alleles show incomplete dominance.
- Heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. At the molecular level, the Tay-Sachs and functional alleles are codominant.
- A dominant allele is not necessarily more common in a population than the recessive allele.
- For example, one baby in 400 is born with polydactyly, a condition in which individuals are born with extra fingers or toes.
- Polydactyly is due to a dominant allele.
- However, the recessive allele is far more prevalent than the dominant allele.
- 399 individuals out of 400 have five digits per appendage.
- Many genes exist in populations in more than two allelic forms.
- The ABO blood groups in humans are determined by three alleles, IA, IB, and i.
- Both the IA and IB alleles are dominant to the i allele.
- The IA and IB alleles are codominant to each other.
- Because each individual carries two alleles, there are six possible genotypes and four possible blood types.
- Individuals that are IAIA or IAi are type A and have type A carbohydrates on the surface of their red blood cells.
- Individuals that are IBIB or IBi are type B and have type B carbohydrates on the surface of their red blood cells.
- Individuals that are IAIB are type AB and have both type A and type B carbohydrates on the surface of their red blood cells.
- Individuals that are ii are type O and have neither carbohydrate on the surface of their red blood cells.
- Matching compatible blood groups is critical for blood transfusions because a person produces antibodies against foreign blood factors.
- If the donor’s blood has an A or B carbohydrate that is foreign to the recipient, antibodies in the recipient’s blood will bind to the foreign molecules, cause the donated blood cells to clump together, and can kill the recipient.
- The genes that we have covered so far affect only one phenotypic character.
- However, most genes are pleiotropic, affecting more than one phenotypic character.
- For example, the wide-ranging symptoms of sickle-cell disease are due to a single gene.
- Considering the intricate molecular and cellular interactions responsible for an organism’s development, it is not surprising that a gene can affect a number of characteristics.
- In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus.
- For example, in mice and many other mammals, coat color depends on two genes.
- One, the epistatic gene, determines whether pigment will be deposited in hair or not.
- Presence (C) is dominant to absence (c) of pigment.
- The second gene determines whether the pigment to be deposited is black (B) or brown (b).
- The black allele is dominant to the brown allele.
- An individual that is cc has a white (albino) coat regardless of the genotype of the second gene.
- A cross between two black mice that are heterozygous (BbCc) will follow the law of independent assortment.
- However, unlike the 9:3:3:1 offspring ratio of a normal Mendelian experiment, the offspring ratio is nine black, three brown, and four white.
- All cc mice will be albino, regardless of the alleles they inherit at the B gene.
- Some characters cannot be classified as either-or, as Mendel’s genes were.
- Quantitative characters vary in a population along a continuum.
- These are usually due to polygenic inheritance, the additive effects of two or more genes on a single phenotypic character.
- For example, skin color in humans is controlled by at least three independent genes.
- Imagine that each gene has two alleles, one light and one dark, which demonstrate incomplete dominance.
- An AABBCC individual is very dark; an aabbcc individual is very light.
- A cross between two AaBbCc individuals (with intermediate skin shade) will produce offspring covering a wide range of shades.
- Individuals with intermediate skin shades will be most common, but some very light and very dark individuals could be produced as well.
- The range of phenotypes will form a normal distribution, if the number of offspring is great enough.
- Phenotype depends on environment and genes.
- A person becomes darker if they tan, despite their inherited skin color.
- A single tree may have leaves that vary in size, shape, and greenness, depending on exposure to wind and sun.
- For humans, nutrition influences height, exercise alters build, sun-tanning darkens skin, and experience improves performance on intelligence tests.
- Even identical twins, who are genetically identical, accumulate phenotypic differences as a result of their unique experiences.
- The relative importance of genes and the environment in influencing human characteristics is a very old and hotly contested debate.
- The product of a genotype is generally not a rigidly defined phenotype, but a range of phenotypic possibilities, the norm of reaction, that are determined by the environment.
- In some cases, the norm of reaction has no breadth, and a given genotype specifies a particular phenotype (for example, blood type).
- In contrast, a person’s red and white blood cell count varies with factors such as altitude, customary exercise level, and presence of infection.
- Norms of reaction are broadest for polygenic characters.
- For these multifactorial characters, environment contributes to their quantitative nature.
- A reductionist emphasis on single genes and single phenotypic characters presents an inadequate perspective on heredity and variation.
- A more comprehensive theory of Mendelian genetics must view organisms as a whole.
- The term phenotype can refer not only to specific characters such as flower color or blood group, but also to an organism in its entirety, including all aspects of its physical appearance.
- Genotype can refer not just to a single genetic locus, but also to an organism’s entire genetic makeup.
- An organism’s phenotype reflects its overall genotype and its unique environmental history.
Concept 14.4 Many human traits follow Mendelian patterns of inheritance
- A preventive approach to simple Mendelian disorders is sometimes possible.
- The risk that a particular genetic disorder will occur can sometimes be assessed before a child is conceived or early in pregnancy.
- Many hospitals have genetic counselors to provide information to prospective parents who are concerned about a family history of a specific disease.
- Consider a hypothetical couple, John and Carol, who are planning to have their first child.
- In both of their families’ histories, a recessive lethal disorder is present. Both John and Carol had brothers who died of the disease.
- While not one of John, Carol, or their parents have the disease, their parents must have been carriers (Aa × Aa).
- John and Carol each have a 2/3 chance of being carriers and a 1/3 chance of being homozygous dominant.
- The probability that their first child will have the disease is 2/3 (chance that John is a carrier) × 2/3 (chance that Carol is a carrier) × 1/4 (chance that the offspring of two carriers is homozygous recessive) = 1/9.
- If their first child is born with the disease, we know that John and Carol’s genotype must be Aa and they are both carriers.
- In that case, the chance that their next child will also have the disease is 1/4.
- Mendel’s laws are simply the rules of probability applied to heredity.
- Because chance has no memory, the genotype of each child is unaffected by the genotypes of older siblings.
- The chance that John and Carol’s first three children will have the disorder is 1/4 × 1/4 × 1/4 = 1/64. Should that outcome happen, the likelihood that a fourth child will also have the disorder is still 1/4.
- Because most children with recessive disorders are born to parents with a normal phenotype, the key to assessing risk is identifying whether prospective parents are carriers of the recessive trait.
- Recently developed tests for several disorders can distinguish normal phenotypes in heterozygotes from homozygous dominants.
- These results allow individuals with a family history of a genetic disorder to make informed decisions about having children.
- However, issues of confidentiality, discrimination, and counseling may arise.
- Tests are also available to determine in utero if a child has a particular disorder.
- One technique, amniocentesis, can be used from the 14th to 16th week of pregnancy to assess whether the fetus has a specific disease.
- Fetal cells extracted from amniotic fluid are cultured and karyotyped to identify some disorders.
- Other disorders can be identified from chemicals in the amniotic fluids.
- A second technique, chorionic villus sampling (CVS) allows faster karyotyping and can be performed as early as the eighth to tenth week of pregnancy.
- This technique extracts a sample of fetal tissue from the chorionic villi of the placenta.
- This technique is not suitable for tests requiring amniotic fluid.
- Other techniques, ultrasound and fetoscopy, allow fetal health to be assessed visually in utero.
- Both fetoscopy and amniocentesis cause complications such as maternal bleeding or fetal death in about 1% of cases.
- Therefore, these techniques are usually reserved for cases in which the risk of a genetic disorder or other type of birth defect is relatively great.
- If fetal tests reveal a serious disorder, the parents face the difficult choice of terminating the pregnancy or preparing to care for a child with a genetic disorder.
- Some genetic traits can be detected at birth by simple tests that are now routinely performed in hospitals.
- One test can detect the presence of a recessively inherited disorder, phenylketonuria (PKU).
- This disorder occurs in one in 10,000 to 15,000 births.
- Individuals with this disorder accumulate the amino acid phenylalanine and its derivative phenylpyruvate in the blood to toxic levels.
- This leads to mental retardation.
- If the disorder is detected, a special diet low in phenylalanine usually promotes normal development.
- Unfortunately, few other genetic diseases are so treatable.
Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 14-1