Chapter 13 Observing Patterns in Inherited Traits Cengage

Chapter 13 Observing Patterns in Inherited Traits © Cengage Learning 2016

13. 1 Menacing Mucus • Cystic fibrosis (CF) is the most common fatal genetic disorder in the United States • The CFTR gene encodes CFTR protein – Transport protein that moves chloride ions out of epithelial cells; water follows these ions by osmosis – Maintains a thin film of water on the surface of the epithelial sheets © Cengage Learning 2016

Menacing Mucus • A deletion of three base pairs (ΔF 508, deletion) prevents proper membrane trafficking of CFTR – Cannot properly transport chloride ions – Results in epithelial cells sheets that are too dry • Mucus obstructs the airways and bacteria infect the intestine and lungs – most CF patients live no longer than thirty years © Cengage Learning 2016

CFTR protein ATP ΔF 508 © Cengage Learning 2016

13. 2 Mendel, Pea Plants, and Inheritance Patterns • In the 19 th century, it was believed that hereditary material was a fluid that blended at fertilization – Inherited traits were thought to result from a blend of parental characters carried in this fluid – However, a cross between a black horse and a white one does not produce a grey offspring • Gregor Mendel studied inherited traits for many years and is the ‘Father of Genetics’ © Cengage Learning 2016

Mendel’s Experimental Approach • Mendel was a monk with training in plant breeding and mathematics – Studied the garden pea (Pisum sativum), which breeds true for a number of traits • True breeding traits appear the same in all offspring as in the parents – Discovered that traits of offspring of crossfertilized pea plants often appear in predictable patterns • Concluded that hereditary information passes from one generation to the next in discrete units © Cengage Learning 2016

Breeding Garden Pea Plants carpel anther 1 In the flowers of garden pea plants, pollen grains that form in anthers produce male gametes; female gam- etes form in carpels. 2 Experimenters control the trans- fer of hereditary material from one pea plant to another by snipping off a flower’s pollenproducing anthers (to prevent it from selffertilizing), then brushing pollen from another flower onto its egg-producing carpel. In this example, pollen from a plant with purple flowers is brushed onto the carpel of a white-flowered plant. 3 Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant. 4 Every plant that arises from the cross has purple flowers. Predictable patterns such as this are evidence of how inheritance works. © Cengage Learning 2016 #1 right: Jean M. Labat/ardea. com; #2–#4: © Cengage Learning.

Inheritance in Modern Terms • Each gene has a specific locus on a chromosome • The particular set of alleles that an individual carries is the individual’s genotype • An individual with two identical alleles of a gene is homozygous for that gene • An individual with nonidentical alleles of a gene is heterozygous for that gene © Cengage Learning 2016

Genetic Loci cytochrome c ribosomal RNA elastin DLX 5/6 homeotic genes skin pigmentation fibrillin 1 (Marfan syndrome) CFTR (cystic fibrosis) leptin (obesity) (blue-deficient colorblind) 7 TCR β subunit © Cengage Learning 2016 (Tay–Sachs disease) 15

Inheritance in Modern Terms • A hybrid is the heterozygote offspring of a cross between two individuals that breed true for different forms of a trait • An individual’s genotype determines its phenotype, which refers to an individual’s observable traits • Any mutated gene is a new allele, whether or not it affects phenotype © Cengage Learning 2016

Inheritance in Modern Terms • An allele is dominant if its effect masks the effect of a recessive allele paired with it – Capital letters (P) signify dominant alleles; lowercase letters (p) signify recessive alleles – Homozygous dominant (PP) – Homozygous recessive (pp) – Heterozygous (Pp) © Cengage Learning 2016

Genetic Loci genotype phenotype PP (homozygous for dominant allele P) pp (homozygous for recessive allele p) Pp (heterozygous for alleles P and p) © Cengage Learning 2016 (left) © Cengage Learning; (right) Tamara Kulikova/Shutterstock. com.

13. 3 Mendel’s Law of Segregation • Homologous chromosomes (and all the alleles they carry) segregate into separate gametes during meiosis – Plants homozygous for the dominant allele (PP) can only make gametes that carry the allele P – Plants homozygous for the recessive allele (pp) can only make gametes that carry the allele p – Heterozygous plants produce both type of gametes © Cengage Learning 2016

Gene Segregation © Cengage Learning 2016

DNA replication meiosis I 2 1 meiosis II 3 gametes (P) gametes (p) zygote (Pp) Stepped Art © Cengage Learning 2016

Calculating Probabilities • Probability – A measure of the chance that a particular outcome will occur • Punnett square – A grid used to calculate the probability of genotypes and phenotypes in offspring © Cengage Learning 2016

Punnett Square Predictions male gametes female gametes P p P P Pp p Pp P Pp p Pp Pp Pp © Cengage Learning 2016 Pp

Testcrosses • A testcross is a method of determining if an individual is heterozygous or homozygous dominant – An individual with unknown genotype is crossed with one that is homozygous recessive (PP x pp) or (Pp x pp) – If all the offspring have the dominant trait, then the unknown parent is homozygous dominant – If some of the offspring have the recessive trait, the parent is heterozygous © Cengage Learning 2016

Monohybrid Crosses • A monohybrid cross is a testcross that checks for a dominance relationship between two alleles at a single locus • May be a cross between true breeding (homozygous) individuals (PP x pp), or between identical heterozygotes (Pp x Pp) © Cengage Learning 2016

Generations in a Genetic Cross • • P stands for parents; F for filial (offspring) F 1: First generation offspring of parents F 2: Second generation offspring of parents In a monohybrid test cross, true breeding parents of each phenotype are crossed to yield the F 1 generation • A cross of F 1 hybrids is the actual test cross – The F 2 generation offspring offer information about dominance between the alleles © Cengage Learning 2016

Possible Monohybrid Results Possible Event Outcome Sperm P meets egg P Sperm P meets egg p zygote genotype is PP zygote genotype is Pp Sperm p meets egg P Sperm p meets egg p zygote genotype is Pp zygote genotype is pp © Cengage Learning 2016

Mendel’s Monohybrid Crosses • Mendel used monohybrid crosses to find dominance relationships among pea plant traits • When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F 1 plants had purple flowers © Cengage Learning 2016

Mendel’s Monohybrid Pea Tests parent plant homozygous for purple white flowers pp PP Pp hybrid p two types of gametes A All of the F 1 offspring of a cross between two plants that breed true for different forms of a trait are identically heterozygous ( Pp). These offspring make two types of gametes: P and p. P © Cengage Learning 2016 P p P PP Pp pp A monohybrid cross is a cross between these F 1 offspring. n this example, the phe- notype ratio in F 2 offspring is 3: 1 (3 purple to 1 white). B

Mendel’s Law of Segregation • Mendel observed a phenotype ratio of 3: 1 in the F 2 offspring of his monohybrid crosses – Matches the probability of the pp genotype in the offspring of a heterozygous cross (Pp x Pp) • This is the basis of Mendel’s law of segregation – Diploid cells have pairs of genes on pairs of homologous chromosomes – The two genes of each pair separate during meiosis, and end up in different gametes © Cengage Learning 2016

13. 4 Mendel’s Law of Independent Assortment • Dihybrid crosses test for dominance relationships between alleles at two loci • Individuals that breed true for two different traits are crossed (PPTT x pptt) • During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs © Cengage Learning 2016

Mendel’s Dihybrid Pea Tests parent plant homozygous for purple flowers and long for white flowers and short stems PPT T pptt PT pt 2 A cross between the two homozygous individuals yields offspring heterozygous for two genes (dihybrids). Pp. Tt dihybrid four types of gametes PT PT © Cengage Learning 2016 Pt p. T 1 In each individual that is homozygous for two genes, meiosis results in only one type of gamete. pt Pt 3 Meiosis in dihybrid individuals results in four kinds of gametes. p. T pt

Mendel’s Dihybrid Pea Tests PT p. T Pt Pp. TT Pp. T PPt Pp. Tt Pptt Pp. TT Pp. T pp. Tt Pptt pp. Tt pptt PT Pt PPTt p. T pt PT pt 4 If two of the dihybrid individuals are crossed, the four types of gametes can meet up in 16 possible ways. Of 16 possible offspring genotypes, 9 will result in plants that are purple-flowered and tall; 3, purpleflowered and short; 3, white-flowered and tall; and 1, white-flowered and short. Thus, the ratio of phenotypes is 9: 3: 3: 1. © Cengage Learning 2016

Mendel’s Law of Independent Assortment • F 2 phenotype ratio is 9: 3: 3: 1 (four phenotypes) – Individually, each dominant trait has an F 2 ratio of 3: 1 • Inheritance of one trait does not affect inheritance of the other = law of independent assortment © Cengage Learning 2016

The Contribution of Crossovers • Independent assortment also occurs when the genes are on the same chromosome, but far enough apart that crossing over occurs between them very frequently • All genes on one chromosome are called a linkage group • Genes that have loci very close to one another on a chromosome tend to be inherited together during meiosis © Cengage Learning 2016

Independent Assortment on Different Chromosomes A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2 ) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated. B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles. or meiosis I C Two nuclei form with each scenario, so there a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis II D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations of maternal and paternal chromosomes. gamete genotype: © Cengage Learning 2016 pt PT p. T Pt

Linkage Groups • The farther apart two genes are on a chromosome, the more often crossing over occurs between them • Linked genes are very close together; crossing over rarely occurs between them • The probability that a crossover will separate alleles of two genes is proportional to the distance between those genes © Cengage Learning 2016

13. 5 Beyond Simple Dominance • Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward © Cengage Learning 2016

Codominance • Codominance – Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive – May occur in multiple allele systems • Multiple allele systems – Genes with three or more alleles in a population – Example: ABO blood types © Cengage Learning 2016

Codominance in ABO Blood Types Genotype: Phenotype: © Cengage Learning 2016 AA or AB AO BB or OO B O BO

Incomplete Dominance • Incomplete dominance – One allele is not fully dominant over its partner – The heterozygote’s phenotype is somewhere between the two homozygotes, resulting in a 1: 2: 1 phenotype ratio in F 2 offspring – Example: Snapdragon color • RR is red • Rr is pink • rr is white © Cengage Learning 2016

Incomplete Dominance in Snapdragons homozygous parent (RR) × homozygous parent (rr) heterozygous offspring (Rr) A Cross a red-flowered with a white-flowered snapdragon plant, and all of the offspring will have pink flowers. R r B If two of the pink- R RR Rr Rr rr r © Cengage Learning 2016 flowered snapdragons are crossed, the phenotypes of their offspring will occur in a 1: 2: 1 ratio. (A) © Jupiter. Images Corporation; (B) © Cengage Learning.

Epistasis • Epistasis – Two or more gene products influence a trait – Typically, one gene product suppresses the effect of another – Example: Coat color in dogs • Alleles B and b designate coat colors (black or brown) • Alleles E and e designate melanin colors (brown or red) © Cengage Learning 2016

Epistatis in Dogs EB Eb e. B eb EB EEBb Ee. BB Ee. Bb Eb EEBb EEbb Ee. Bb Eebb e. B Ee. Bb ee. BB ee. Bb eb Ee. Bb Eebb ee. Bb eebb © Cengage Learning 2016 © 2016 Cengage Learning; Photo: Susan Schmitz/Shutterstock

Pleiotropy • A pleiotropic gene influences multiple traits – Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder © Cengage Learning 2016

13. 6 Nature and Nurture • The environment affects the expression of many genes, which in turn affects phenotype – including behavioral traits • We can summarize this relationship as: genotype + environment → phenotype © Cengage Learning 2016

Environment and Epigenetics • Environmentally driven changes in gene expression patterns can be permanent and heritable • Such changes are implemented by gene controls, such as chromatin modifications and RNA interference that act on DNA itself – Example: Many environmental factors affect DNA methylation patterns, enhancing or suppressing gene expression © Cengage Learning 2016

Some Environmental Effects • Alternative phenotypes in water fleas – Individual water fleas adapt to environmental differences in standing bodies of water by adjusting their gene expression • Seasonal changes in coat color – Temperature changes and the length of day affect melanin production and other pigments that color skin and fur © Cengage Learning 2016

Water Fleas and Water Conditions © Cengage Learning 2016

Coat Color and Seasons A The color of the snowshoe hare’s fur varies by season. In summer, the fur is brown (left); in winter, white (right). Both forms offer seasonally appropriate camouflage from predators. © Cengage Learning 2016

Some Environmental Effects • Effect of altitude on yarrow – Genetically identical yarrow plants grow to different heights at different altitudes • Psychiatric disorders – Environment is a factor in schizophrenia, bipolar disorder, depression, and other mood disorders – Future treatments for many disorders may involve deliberate modification of epigenetic marks in one’s DNA © Cengage Learning 2016

13. 7 Complex Variations in Traits • Many traits do not appear in distinct forms – Often the result of complex genetic interactions with added environmental influences • Continuous variation – Traits with a range of small differences – The more factors that influence a trait, the more continuous the distribution of phenotype – Short tandem repeats expand or contract quickly compared to the rate of mutation • Resulting DNA changes may be preserved as alleles © Cengage Learning 2016

Variations in Eye Color © Cengage Learning 2016

Continuous Variations • Bell curve – When continuous phenotypes are divided into measurable categories and plotted as a bar chart, they form a bell-shaped curve – This reveals the relative frequencies of phenotypes across the range of values and within the population © Cengage Learning 2016

Bell Curves of Phenotypes Number of individuals 20 15 10 5 63 64 68 73 65 69 74 75 66 70 76 77 67 71 72 A To see if human height varies continuously, male biology stu- dents at the University of Florida were divided into categories of one-inch increments in height and counted. © Cengage Learning 2016 63 66 67 68 69 70 71 72 Measured values 74 64 73 75 76 from the experiment 77 B Graphing the data that resulted in (A) produces a 65 bell-shaped curve, which is an indication that height does vary continuously in humans. (A) Courtesy of Ray Carson, University of Florida News and Public Affairs; (B) © Cengage Learning.

Points to Ponder • What conclusions might Mendel have made if he had chosen snapdragons instead of peas for his study material? • There are four possible blood types in the ABO system. How many different alleles are in the human population for this marker? © Cengage Learning 2016