8 Inheritance Genes and Chromosomes Chapter 8 Inheritance

8 Inheritance, Genes, and Chromosomes

Chapter 8 Inheritance, Genes, and Chromosomes Key Concepts 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws 8. 2 Alleles and Genes Interact to Produce Phenotypes 8. 3 Genes Are Carried on Chromosomes 8. 4 Prokaryotes Can Exchange Genetic Material

Chapter 8 Opening Question How is hemophilia inherited, and why is it most frequent in males?

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Breeding of plants and animals had yielded two hypotheses by the mid-19 th century: • Blending inheritance—gametes contained determinants (genes) that blended when gametes fused during fertilization • Particulate inheritance—each determinant was physically distinct and remained intact during fertilization

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws In the 1860 s, the monk Gregor Mendel used the scientific method in studies of garden peas that clearly supported the particulate hypothesis. Garden peas were easy to grow and manipulate: their flowers have both male and female sex organs (pistils and stamens) that produce gametes.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Peas normally self-fertilize, but male organs can be removed to allow fertilization with pollen from other flowers.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Characters—observable physical features (e. g. , flower color, seed shape) Traits—forms of a character (e. g. , purple flowers, wrinkled seeds) Mendel worked with true-breeding varieties— when plants of the same variety are crossed, all offspring have characters with the same traits.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Parental generation = P Resulting offspring = first filial generation, or F 1 If F 1 plants self-pollinate, they produce the second filial generation, or F 2. “Hybrid” refers to offspring of crosses between organisms differing in one or more characters.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws In the 1 st experiment, Mendel crossed plants differing in just one character, producing monohybrids in the F 1 generation. The monohybrids were then allowed to selfpollinate to form the F 2 generation—a monohybrid cross.

Figure 8. 1 Mendel’s Monohybrid Experiments (Part 1)

Figure 8. 1 Mendel’s Monohybrid Experiments (Part 2)

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws One trait of each pair disappeared in the F 1 generation and reappeared in F 2. Mendel’s findings led to a rejection of the blending hypothesis. • The F 1 offspring were not a blend of the parental traits, but only one trait was present. • The trait did not disappear in the F 2 generation.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws All seven crosses between varieties with contrasting traits gave the same kind of data. Mendel called the trait that appeared in the F 1 and was more abundant in the F 2 the dominant trait, and the other trait recessive. The ratio of dominant to recessive in the F 2 was about 3: 1.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel proposed that the genetic determinants (genes) occur in pairs and are segregated in the gametes. • Each plant has two genes for each character, one from each parent. We now use the term diploid to describe having two copies of a gene, and haploid as having one copy.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel concluded that each gamete contains one copy of each gene (haploid), the resulting zygote contains two copies (diploid), because it is produced by the fusion of two gametes. Different traits arise because there can be different forms of a gene—now called alleles —for a particular character.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Phenotype—the physical appearance of an organism (e. g. , round seeds) Genotype—the genetic makeup (e. g. , Rr) Capital letters designate the dominant allele, lower case designate the recessive. Round seeds can be the result of two different genotypes—RR or Rr.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws True-breeding individuals have two copies of the same allele—they are homozygous for the allele (e. g. , rr). Heterozygous individuals have two different alleles (e. g. , Rr).

Figure 8. 2 Mendel’s Explanation of Inheritance

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s first law: The law of segregation states that the two copies of a gene separate when an individual makes gametes. • Each gamete receives only one copy. Gametes from an RR individual will all be R, gametes from an rr individual will all be r, and their offspring (F 1) will all be Rr.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws In the F 2 generation, gametes will be R or r. Genotypes in the F 2 generation can be predicted using a Punnett square, which considers all possible gamete combinations.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws The Punnett square predicts a 3: 1 ratio of phenotypes in the F 2 generation. Mendel’s experimental values were remarkably close to this for all seven of the character traits he compared.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Genes are now known to be relatively short sequences of DNA found on the much longer DNA molecules that make up chromosomes. Today, we can picture the different alleles of a gene segregating as chromosomes separate during meiosis I.

Figure 8. 3 Meiosis Accounts for the Segregation of Alleles (Part 1)

Figure 8. 3 Meiosis Accounts for the Segregation of Alleles (Part 2)

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Genes determine phenotypes mostly by producing proteins with particular functions. In many cases a dominant gene is expressed, and the recessive gene is mutated so that it is no longer expressed or produces a nonfunctional protein. The wrinkled pea phenotype is due to a lack of starch branching enzyme 1 (SBE 1), essential for starch synthesis.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel verified his hypotheses by doing test crosses: • Cross F 1 with known homozygote which has the recessive genotype (e. g. , rr). • If the F 1 individual was homozygous, all offspring will show the dominant trait. If the F 1 was heterozygous, half of the offspring will have the recessive trait.

Figure 8. 4 Homozygous or Heterozygous? (Part 1)

Figure 8. 4 Homozygous or Heterozygous? (Part 2)

Figure 8. 4 Homozygous or Heterozygous? (Part 3)

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s next experiments tested whether genes are distributed independently in the offspring. • He crossed peas that differed in two characters—seed shape and color • True-breeding parents: § RRYY—round yellow seeds § rryy—wrinkled green seeds

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws F 1 generation is Rr. Yy—all round yellow. Crossing the F 1 generation (all identical double heterozygotes) is a dihybrid cross. Mendel asked whether, in the gametes produced by Rr. Yy, the traits would be linked or segregate independently.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Two possible outcomes of the dihybrid cross: • If the alleles were linked, gametes would be RY or ry; F 2 would have three times more round yellow than wrinkled green. § The ratio would be 3: 1 • If independent, gametes could be RY, ry, Ry, or r. Y. § F 2 generation would have nine different genotypes, and the four phenotypes would be in a 9: 3: 3: 1 ratio.

Figure 8. 5 Independent Assortment

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s dihybrid crosses supported the second prediction. Mendel’s second law is the law of independent assortment: • Alleles of different genes assort independently during gamete formation. This law does not always apply to genes near each other on the same chromosome, but chromosomes do segregate independently.

Figure 8. 6 Meiosis Accounts for Independent Assortment of Alleles (Part 1)

Figure 8. 6 Meiosis Accounts for Independent Assortment of Alleles (Part 2)

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws One reason Mendel was successful was his use of large sample sizes. By counting many progeny from each cross, he was able to see clear patterns. After his work became recognized, geneticists began using probability calculations to predict expected ratios of genotypes and phenotypes and statistics to determine whether actual results matched the predictions.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Probability • If an event is certain to happen, its probability = 1 • If an event cannot possibly happen, its probability = 0 • All other events have a probability between 0 and 1

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Two coin tosses are independent events; each will come up heads ½ of the time. The probability that both coins will come up heads is: ½×½=¼ Multiplication rule: the probability of two independent outcomes occurring together is found by multiplying the individual probabilities.

Figure 8. 7 Using Probability Calculations in Genetics

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws In a monohybrid cross: After self-pollination of an F 1 Rr, the probability that the F 2 offspring will have the genotype RR is ½×½=¼ Probability that offspring will have the rr genotype is the same.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Probability that offspring will be heterozygous: There are two ways to get a heterozygote— Rr or r. R. Addition rule: probability of an event that can occur in two or more different ways is the sum of the individual probabilities of those ways: ¼+¼=½

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Human pedigrees can show Mendel’s laws. Humans have few offspring; pedigrees do not show the clear proportions that the pea plants showed. But pedigrees can show patterns and can be used to determine whether a rare allele is dominant or recessive.

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Pattern of inheritance if a rare allele is dominant: • Every person with the abnormal phenotype has an affected parent. • Either all (if homozygous parent) or half (if heterozygous parent) of offspring in an affected family are affected.

Figure 8. 8 Pedigree Analysis and Inheritance (Part 1)

Concept 8. 1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Pattern of inheritance if the rare allele is recessive: • Affected people often have two unaffected parents. • In an affected family, one-fourth of children of unaffected parents are affected. The parents are heterozygous carriers.

Figure 8. 8 Pedigree Analysis and Inheritance (Part 2)

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes New alleles arise by mutations—rare, stable, inherited changes in the genetic material. Wild type is the allele present in most of the population. Other alleles of that gene are mutant alleles. A gene with a wild-type allele that is present less than 99% of the time is called polymorphic.

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes One gene may have more than two alleles. Multiple alleles increase the number of possible phenotypes and may show a hierarchy of dominance in heterozygotes. • Example: coat color in rabbits

Figure 8. 9 Multiple Alleles for Coat Color in Rabbits

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Incomplete dominance Some alleles are neither dominant nor recessive, and heterozygotes have an intermediate phenotype. • Example: flower color in snapdragons

Figure 8. 10 Incomplete Dominance Follows Mendel’s Laws

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Codominance Two alleles of a gene produce both phenotypes in the heterozygote. • Example: § § ABO blood group system has three alleles of the gene: IA, IB, and IO. IA and IB are codominant.

Figure 8. 11 ABO Blood Groups Are Important in Transfusions

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Epistasis Phenotypic expression of one gene is influenced by another gene. • Example: Coat color in Labrador retrievers § Allele B (black) is dominant to b (brown) § Allele E (pigment deposition) is dominant to e (no pigment deposition— hair is yellow) § Gene E determines the phenotypic expression of gene B.

Figure 8. 12 Genes Interact Epistatically

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Hybrid vigor (heterosis): hybrid offspring grow larger, produce more seeds, etc, than the parental varieties.

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes “Vigor” is a complex trait; most complex phenotypes are determined by multiple genes. Most are quantitative traits: they must be measured, rather than assessed qualitatively (e. g. , grain yields).

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Genotype and environment often interact to determine phenotype. • Example: point restriction coat patterns

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes These rabbits and cats have a mutant allele for the coat color gene. The enzyme encoded by the gene is inactive at temperatures above about 35°C. The extremities are cooler than the main body (around 25°C), so the fur on these regions is dark.

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Two parameters describe the effects of genes and environment on phenotype: • Penetrance: proportion of individuals with a certain genotype that show the expected phenotype • Expressivity: degree to which genotype is expressed in an individual

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Many people, but not all, who inherit a mutant allele of the gene BRCA 1 develop breast cancer. • The mutation is said to be incompletely penetrant. A woman with the mutant allele may get both breast and ovarian cancer, but another woman may only get breast cancer. • The mutation is said to have variable expressivity.

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Heritability: relative contribution of genetic versus environmental factors to variation in a character in a population • Example: human height

Concept 8. 2 Alleles and Genes Interact to Produce Phenotypes Heritability varies from 0 to 1. For human height, heritability varies from about 0. 65 to 0. 8. If heritability is 0. 65, 65% of the variation in height is due to genetic factors, 35% is due to environmental effects. Heritability estimates apply only to variation within populations, not individuals.

Concept 8. 3 Genes Are Carried on Chromosomes A gene is a sequence of DNA that resides at a particular site on a chromosome—the locus (plural loci). Genetic linkage of genes on a single chromosome can alter their pattern of inheritance from those described by Mendel’s law.

Concept 8. 3 Genes Are Carried on Chromosomes Genetic linkage was discovered by Thomas Hunt Morgan, using the fruit fly Drosophila melanogaster. Much genetic research has been done with Drosophila, which is considered a model organism because of its small size, ease of breeding, and short generation time.

Concept 8. 3 Genes Are Carried on Chromosomes Some crosses performed with Drosophila did not yield expected ratios according to the law of independent assortment. Instead, some alleles for body color and wing shape were inherited together. Morgan hypothesized that the two loci were linked on the same chromosome and could not assort independently.

Figure 8. 13 Some Alleles Do Not Assort Independently (Part 1)

Figure 8. 13 Some Alleles Do Not Assort Independently (Part 2)

Concept 8. 3 Genes Are Carried on Chromosomes But if linkage were absolute, the F 1 would only have the two parental phenotypes in a 1: 1 ratio. Some of the offspring had recombinant phenotypes, resulting from crossing over during prophase I in meiosis. • Two of the four chromatids in the tetrad exchange chromosome segments.

Figure 8. 14 Crossing Over Results in Genetic Recombination

Concept 8. 3 Genes Are Carried on Chromosomes Recombinant offspring generally appear in proportions related to the recombination frequency between the two genes: • Calculated by dividing the number of recombinant progeny by the total number of progeny. Recombinant frequencies are higher for loci that are farther apart on the chromosome.

Figure 8. 15 Recombination Frequencies

Concept 8. 3 Genes Are Carried on Chromosomes Recombinant frequencies can be used to infer the locations of genes along a chromosome and make a genetic map.

Concept 8. 3 Genes Are Carried on Chromosomes Fruit flies have one pair of sex chromosomes (involved in sex determination) and 3 pairs of autosomes (all the other chromosomes).

Concept 8. 3 Genes Are Carried on Chromosomes Males have X and Y sex chromosomes that are different in size, and many genes on the X chromosome are not present on the Y. Thus, males have only one copy of some genes. A gene that is present as a single copy in a diploid organism is called hemizygous.

Concept 8. 3 Genes Are Carried on Chromosomes Morgan’s experiments showed that the gene for eye color was carried on the X chromosome. The wild-type is red (R), the mutant allele is white (r).

Figure 8. 16 A Gene for Eye Color Is Carried on the Drosophila X Chromosome

Concept 8. 3 Genes Are Carried on Chromosomes The term sex-linked inheritance refers to inheritance of a gene that is carried on a sex chromosome. In mammals, the X chromosome is larger and carries more genes that the Y; most sexlinked inheritance involves genes that are carried on the X chromosome.

Figure 8. 17 Red–Green Color Blindness Is Carried on the Human X Chromosome

Concept 8. 3 Genes Are Carried on Chromosomes Patterns in X-linked recessive phenotypes: • They appear much more often in males than females. • A male with the mutation can pass it only to daughters. • Daughters who receive one X-linked mutation are heterozygous carriers. • Mutant phenotype can skip a generation if it passes from a male to his daughter (normal phenotype) and then to her son.

Concept 8. 3 Genes Are Carried on Chromosomes Mitochondria and plastids also have a small chromosome. Egg cells have abundant cytoplasm and organelles, but the only part of the sperm that takes part in fertilization is the nucleus. So, mitochondria and plastids are inherited only from the mother. Inheritance of organelles and their genes is thus non-Mendelian and is called maternal, or cytoplasmic, inheritance.

Concept 8. 3 Genes Are Carried on Chromosomes Some organelle genes are important for organelle assembly and function, and mutations can have large effects. In plants, a mutation that affects proteins that assemble chlorophyll molecules into photosystems results in all white plants.

Figure 8. 18 Cytoplasmic Inheritance

Concept 8. 4 Prokaryotes Can Exchange Genetic Material Prokaryotes reproduce asexually by binary fission, which gives rise to genetically identical progeny. But they can also exchange genetic material. Transfer of genes from one individual to another without sexual reproduction is called horizontal or lateral gene transfer. Along with mutation, this process generates genetic diversity among prokaryotes.

Concept 8. 4 Prokaryotes Can Exchange Genetic Material Bacteria exchange genes by bacterial conjugation. A projection called the sex pilus initiates contact between cells. Genetic material then passes from the donor to the recipient through a conjugation tube. There is no reciprocal transfer of DNA.

Figure 8. 19 Bacterial Conjugation and Recombination (Part 1)

Figure 8. 19 Bacterial Conjugation and Recombination (Part 2)

Concept 8. 4 Prokaryotes Can Exchange Genetic Material In the recipient cell, the donor DNA lines up with the recipient’s DNA and crossing over occurs. Genes from the donor are incorporated into the recipient’s genome and are passed on to its progeny.

Concept 8. 4 Prokaryotes Can Exchange Genetic Material Many bacteria also have plasmids—small circular DNA molecules that replicate independently. Plasmids often have: • Genes for unusual metabolic tasks, such as breaking down hydrocarbons • Genes for antibiotic resistance Plasmids can move between cells during conjugation.

Figure 8. 20 Gene Transfer by Plasmids

Concept 8. 4 Prokaryotes Can Exchange Genetic Material Discovery of antibiotics such as penicillin greatly reduced many lethal infections. But over time, some bacteria acquired mutations that made them resistant. With a selective advantage, as penicillin use spread, so did the resistant bacteria. Scientists designed variants of penicillin; but new strains of resistant bacteria also develop. This “arms race” still continues.

Answer to Opening Question In hemophilia, the mutant gene for the clotting factor is recessive, and is carried on the X chromosome. Affected males inherit their single X chromosome from their mothers. If they receive their mother’s recessive mutant chromosome, they cannot produce the clotting factor and have hemophilia.

Figure 8. 21 Sex Linkage in Royal Families of Europe