Inheritance, Genes, and Chromosomes 8

Chapter 8 Opening Question How is hemophilia inherited through the mother, and why is it more frequent in males?

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Early experiments with genetics yielded two theories: 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 Mendel used the scientific method and studied garden peas.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Character—observable physical feature (e.g., flower color, seed shape) Trait—form of a character (e.g., purple flowers or white flowers, wrinkled seeds) Mendel worked with true-breeding varieties—when plants of the same variety were crossed, all offspring plants produced the same seeds and flowers.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s crosses: Pollen from one parent was transferred to the stigma of the other parent. Parental generation = P Resulting offspring = first filial generation or F 1 If F 1 plants self-pollinate, they produce second filial generation or F 2.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws In Mendel’s first experiment, he crossed plants differing in just one character (P). This produced monohybrids in the F 1 generation. The monohybrids were then allowed to self- pollinate to form the F 2 generation—a monohybrid cross. Mendel repeated this for seven characters.

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 —these traits are recessive. The trait that appears in the F 1 is the dominant trait. The ratio of dominant traits to recessive traits in the F 2 was about 3:1.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s observations rejected the blending theory of inheritance and supported the particulate theory. He proposed that the determinants occur in pairs and are segregated in the gametes. Each plant has two genes for each character, one from each parent. Diploid—two copies of a gene Haploid—one copy of a gene

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Alleles are different forms of a gene, such as smooth or wrinkled seeds. True-breeding individuals have two copies of the same allele—they are homozygous for the allele (e.g., ss). Heterozygous individuals have two different alleles (e.g., Ss).

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Phenotype—physical appearance of an organism (e.g., spherical seeds) Genotype—the genetic makeup (e.g., Ss) Spherical seeds can be the result of two different genotypes—SS or Ss.

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.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws When the F 1 self- pollinates, there are three ways to get the dominant trait (e.g., spherical), but only one way to get the recessive (wrinkled)— resulting in the 3:1 ratio.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel tested his hypothesis by doing test crosses: He did this to determine whether an individual is homozygous or heterozygous for a trait by crossing it with a homozygous recessive individual. Mendel crossed the F 1 with known homozygotes (e.g., wrinkled or ss).

Figure 8.4 Homozygous or Heterozygous? (Part 1)

Figure 8.4 Homozygous or Heterozygous?

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s next experiment involved: Crossing peas that differed in two characters—seed shape and seed color True-breeding parents: SSYY—spherical yellow seeds ssyy—wrinkled green seeds

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws F 1 generation is SsYy—all spherical yellow. Crossing the F 1 generation (all identical double heterozygotes) is a dihybrid cross. Mendel asked whether, in the gametes produced by SsYy, the traits would be linked, or segregate independently.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Two possibilities included: Alleles could maintain associations seen in parental generation—they could be linked If linked, gametes would be SY or sy; F 2 would have three times more spherical yellow than wrinkled green. If independent, gametes could be SY, sy, Sy, or sY. F 2 would have nine different genotypes; phenotypes would be in 9:3:3:1 ratio.

Figure 8.5 Independent Assortment (Part 1)

Figure 8.5 Independent Assortment (Part 2)

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s second law: The law of independent assortment states that alleles of different genes assort independently during gamete formation. This law doesn’t always apply to genes on the same chromosome, but chromosomes do segregate independently.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws One of Mendel’s contributions to genetics was the use of mathematical analyses— the rules of statistics and probability. His analyses revealed patterns that allowed him to formulate his hypotheses. Probability calculations and Punnett squares give the same results.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Probability If an event is certain to happen, probability = 1 If an event cannot possibly happen, 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 the time. The probability that both will come up heads is: ½ x ½ = ¼ To get the joint probability, multiply the individual probabilities (multiplication rule).

Figure 8.7 Using Probability Calculations in Genetics

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Probability in a monohybrid cross After self-pollination of an F 1 Ss, the probability that the F 2 offspring will have the genotype SS is ½ x ½ = ¼; the same for ss offspring. There are two ways to get a heterozygote Ss; the probability is the sum of the individual probabilities (addition rule): 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. Geneticists use pedigrees 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 for a rare dominant allele: 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 for a rare recessive allele: Affected people often have two unaffected parents. In an affected family, one-fourth of children of unaffected parents are affected.

Figure 8.8 Pedigree Analysis and Inheritance (Part 2)

Concept 8.2 Alleles and Genes Interact to Produce Phenotypes Different alleles arise through mutation— rare, stable, inherited changes in the genetic material. The wild type is the allele present in most of the population. Other alleles are mutant alleles. A gene with a wild-type allele that is present less than 99 percent of the time is called polymorphic.

Concept 8.2 Alleles and Genes Interact to Produce Phenotypes A given gene may have more than two alleles. Multiple alleles increase the number of possible phenotypes and may show a hierarchy of dominance in heterozygotes. One example is the coat color in rabbits.

Concept 8.2 Alleles and Genes Interact to Produce Phenotypes Some alleles are neither dominant nor recessive. A heterozygote has an intermediate phenotype in incomplete dominance. Red + white snapdragons = pink in F 1 Red and white colors reappear in F 2 as well as pink.

Figure 8.10 Incomplete Dominance Follows Mendel’s Laws (Part 1)

Figure 8.10 Incomplete Dominance Follows Mendel’s Laws (Part 2)

Concept 8.2 Alleles and Genes Interact to Produce Phenotypes Codominance—two alleles of a gene produce phenotypes that are both present in the heterozygote. Example: ABO blood group system has three alleles of the gene: I A, I B, and I O.

Figure 8.11 ABO Blood Reactions 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) dominant to b (brown) Allele E (pigment deposition) is dominant to e (no pigment deposition—yellow)

Figure 8.12 Genes Interact Epistatically

Concept 8.2 Alleles and Genes Interact to Produce Phenotypes Hybrid vigor, or heterosis, is a cross between two different true-breeding homozygotes. It can result in offspring with stronger, larger phenotypes. Most complex phenotypes are determined by multiple genes. Quantitative traits conferred by multiple genes are measured, rather than assessed qualitatively.

In-Text Art, Ch. 8, p. 154

Concept 8.2 Alleles and Genes Interact to Produce Phenotypes Genotype and environment interact to determine the phenotype of an organism. Two parameters describe the effects: Penetrance is the proportion of individuals with a certain genotype that show the phenotype. Expressivity is the degree to which genotype is expressed in an individual.

Concept 8.3 Genes Are Carried on Chromosomes Genes are sequences of DNA that reside at a particular site on a chromosome—a locus (plural loci). The genetic linkage of genes on a single chromosome can alter their patterns of inheritance.

Concept 8.3 Genes Are Carried on Chromosomes Genetic linkage was discovered by Thomas Hunt Morgan and students at Columbia University using the fruit fly Drosophila melanogaster. Much genetic research has been done with Drosophila, which is considered a model organism because of its 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 genes for body color and wing shape were inherited together. Morgan theorized 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 Recombinant offspring phenotypes (non- parental) appear in recombinant frequencies. To determine the recombinant frequencies, divide the number of recombinant offspring by the total number of offspring. Recombinant frequencies are greater for loci that are farther apart on the chromosome.

Figure 8.15 Recombination Frequencies (Part 1)

Figure 8.15 Recombination Frequencies (Part 2)

Concept 8.3 Genes Are Carried on Chromosomes Recombinant frequencies can be used to make genetic maps showing the arrangement of genes along a chromosome. Recombinant frequencies are converted to map units corresponding to distances between genes.

Concept 8.3 Genes Are Carried on Chromosomes The fruit fly genome has four pairs of chromosomes—three pairs are similar in size, called autosomes. The fourth pair are of different size, the sex chromosomes. Many genes on the X chromosome are not present on the Y chromosome.

Concept 8.3 Genes Are Carried on Chromosomes Genes on sex chromosomes don’t follow Mendelian patterns. The Y chromosome carries few genes; the X chromosome carries many. Thus, males have only one copy of these genes—hemizygous.

Concept 8.3 Genes Are Carried on Chromosomes Sex-linked inheritance—inheritance of a gene that is carried on a sex chromosome One example is the eye color in Drosophila.

Figure 8.16 A Gene for Eye Color Is Carried on the Drosophila X Chromosome (Part 1)

Concept 8.3 Genes Are Carried on Chromosomes X-linked recessive phenotypes: They appear much more often in males than females. A male with the mutation can only pass it on 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) and then to her son.

Figure 8.17 Red–Green Color Blindness Is Carried on the Human X Chromosome

Concept 8.3 Genes Are Carried on Chromosomes Besides the genes in the nucleus, mitochondria and plastids contain small numbers of genes. Mitochondria and plastids are inherited only from the mother. The inheritance of organelles and their genes is non-Mendelian and is called maternal or cytoplasmic inheritance.

Figure 8.18 Cytoplasmic Inheritance

Concept 8.4 Prokaryotes Can Exchange Genetic Material Bacteria exchange genes by bacterial conjugation. Sex pilus is a projection that initiates contact between bacterial cells. Conjugation tube is a cytoplasmic bridge that forms between cells. The donor chromosome fragments and some material enters the recipient cell.

Concept 8.4 Prokaryotes Can Exchange Genetic Material Bacteria have plasmids—small circular DNA molecules—besides the main chromosome. Genes on the plasmids are in categories: Metabolic tasks, breaking down hydrocarbons Involved in conjugation Antibiotic resistance

Concept 8.4 Prokaryotes Can Exchange Genetic Material Plasmids can move between the cells during conjugation. They can: Replicate independently of the main chromosome Add their genes to the recipient cell’s genome

Figure 8.20 Gene Transfer by Plasmids

Answer to Opening Question In hemophilia, the mutant gene for factor VIII, the clotting factor, is carried on the X chromosome. The affected males inherited their single X chromosome from their mothers—if the mutated form of the gene was present, they would develop the disease. Daughters would inherit a normal X chromosome as well and would not express the recessive trait, though could be carriers.

Figure 8.21 Sex Linkage in Royal Families of Europe