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Chapter 12 Outline 12.1 What is the Physical Basis of Inheritance? p. 222 12.2 How Mendel Laid the Foundations of Modern Genetics, p,222 12.3 Inheritance of Single Traits, p. 223 12.4 Inheritance of Multiple Traits on Different Chromosomes, p. 227 12.5 Inheritance of Genes Located on the Same Chromosome, p. 229

Chapter 12 Outline 12.6 Sex Determination and Sex-Linked Inheritance, p. 231 12.7 Do Mendel s Rules Apply to All Traits? p. 233 12.8 Investigating Human Genetic Disorders, p. 237 12.9 Inheritance of Single Gene Disorders, p. 238 12.10 Errors in Chromosome Number, p. 240

Section 12.1 Outline 12.1 What is the Physical Basis of Inheritance?

Inheritance Inheritance is the process by which the characteristics of individuals are passed to their offspring Genes encode these characteristics

Genes A gene is a unit of heredity that encodes information for the form of a particular characteristic The location of a gene on a chromosome is called its locus

Alleles Homologous chromosomes carry the same kinds of genes for the same characteristics Genes for the same characteristic are found at the same loci on both homologous chromosomes

Alleles Genes for a characteristic found on homologous chromosomes may not be identical Alternate versions or forms of genes found at the same gene locus are called alleles

Alleles Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus

Alleles If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)

Section 12.2 Outline 12.2 How Mendel Laid the Foundations of Modern Genetics Who Was Gregor Mendel? Doing It Right: The Secrets of Mendel s Success

Who Was Gregor Mendel? Mendel was a monk in a monastery in Brno (now in Czech Republic) in late 1800s

Who Was Gregor Mendel? Mendel studied botany and mathematics at the university level before becoming a monk Experimentation with pea plant inheritance took place in the monastery garden Mendel s background allowed him to see patterns in the way plant characteristics were inherited

The Secrets of Mendel s Success Mendel s choice of experimental organism contributed to his success

The Secrets of Mendel s Success Important aspects of pea plants Pea flowers have male structures that produce pollen (male gametes) by meiosis Pea flowers have female structures that produce eggs (female gametes) by meiosis Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant

The Secrets of Mendel s Success Pea flowers can self-fertilize Pollen from male structures transfers to eggs female structures

The Secrets of Mendel s Success Pea plants that are homozygous for a particular characteristic always produce the same physical forms If a plant is homozygous for purple flowers, it will always produce offspring with purple flowers Plants homozygous for a characteristic are true-breeding

The Secrets of Mendel s Success Mendel was able to mate two different plants by hand (cross-fertilization) Female parts (carpels) were dusted with pollen from other selected plants

The Secrets of Mendel s Success Mendel experimental design was simple and methodical He studied characteristics that have unmistakably different forms (like purple versus white) He only studied one trait (characteristic) at a time

Section 12.3 Outline 12.3 Inheritance of Single Traits The Language of a Genetic Cross Mendel s Flower Color Experiments Alleles of a Gene Are Dominant or Recessive How Meiosis Separates Genes: Segregation

Section 12.3 Outline 12.3 Inheritance of Single Traits (continued) Understanding the Results of Mendel s Flower Color Experiments Genetic Bookkeeping Practical Application: The Test Cross

The Language of a Genetic Cross A genetic cross is the mating of pollen and eggs (from same or different parents)

The Language of a Genetic Cross The parents used in a cross are part of the parental generation (known as P) The offspring of the P generation are members of the first filial generation (F 1 ) Offspring of the F 1 generation are members of the F 2 generation, etc.

Mendel s Flower Color Experiments 1. Mendel crossed a true-breeding purple flower plant with a true-breeding whiteflower plant (P generation) 2. The F 1 generation consisted of all purpleflowered plants What had happened to the white flower trait?

Mendel s Flower Color Experiments 3. Mendel allowed the F 1 generation to self fertilize 4. The F 2 were composed of ¾ purple flower plants and ¼ white flower plants

Dominant and Recessive Alleles There are two alleles for a given gene characteristic (such as flower color) Let P stand for the purple flower allele Let p stand for the white flower allele

Dominant and Recessive Alleles Every cell in a pea plant carries two alleles per characteristic (either the same or different)

Dominant and Recessive Alleles The particular combination of the two alleles carried by an individual is called the genotype (PP, Pp, or pp) The physical expression of the genotype is known as the phenotype (e.g. purple or white flowers)

Dominant and Recessive Alleles The phenotype of the homozygous genotype PP is purple flowers The phenotype of the homozygous genotype pp is white flowers

Dominant and Recessive Alleles What is the phenotype of genotype Pp? The phenotype of Pp is white flowers The P allele masks the presence of the p allele P is the dominant allele while p is recessive The dominant allele is always written with a capital letter while the recessive allele is written in lower case

How Meiosis Separates Genes The two alleles for a characteristic separate during gamete formation (meiosis) Homologous chromosomes separate in meiosis anaphase I Each gamete receives one of each pair of homologous chromosomes and thus one of the two alleles per characteristic

How Meiosis Separates Genes The separation of alleles in meiosis is known as Mendel s Law of Segregation

Mendel s Flower Color Experiments The purple-flower, true-breeding parent (PP) produced two kinds of gametes, P and p The white-flower, true-breeding parent (pp) produced two kinds of gametes, p and p.

Mendel s Flower Color Experiments The first filial generation (F1) offspring were produced from the fertilization of pollen and eggs from both parents

Mendel s Flower Color Experiments The F 1 offspring were all heterozygous (Pp) for flower color When the F 1 offspring were allowed to self-fertilize, four types of gametes were produced from the Pp parents Sperm: P p Eggs: P p

Mendel s Flower Color Experiments Combining these four gametes into genotypes in every possible way produces offspring PP, Pp, Pp, and pp Can also be tabulated by the genotypic fraction of total offspring: ¼ PP, ½ Pp, and ¼ pp

Genetic Bookkeeping Punnett Square Method predicts offspring genotypes from combinations of parental gametes 1. First assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive) 2. Determine the gametes and their fractional proportions (out of all the gametes) from both parents

Genetic Bookkeeping 3. Write the gametes from each parent, together with their fractional proportions, along each side of a 2 x 2 grid (Punnett square) 4. Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete (e.g. ¼ P with ½ p = 1/8 Pp)

Genetic Bookkeeping 5. Add together the fractions of any genotypes of the same kind (1/4 Pp + ¼ pp = ½ Pp total) 6. From the sums of all the different kinds of offspring genotypes, create a genotypic ratio 1/4 PP, ½ Pp, ¼ pp is in the ratio 1PP: 2Pp: 1pp

Genetic Bookkeeping 7. Based on dominant and recessive rules, determine the phenotypic ratio A genotypic ratio of 1PP: 2Pp: 1pp yields 3 purple flower plants: 1 white flower plant

Practical Application: The Test Cross A test cross is used to deduce the actual genotype of an organism with a dominant phenotype (i.e., is the organism PP or Pp?) 1. Cross the unknown dominant-phenotype organism (P_) with a homozygous recessive organism (pp)

Practical Application: The Test Cross 2. If the dominant-phenotype organism is homozygous dominant (PP), only dominantphenotype offspring will be produced (Pp) 3. If the dominant-phenotype organism is heterozygous (Pp), approximately half of the offspring will be of recessive phenotype (pp)

Section 12.4 Outline 12.4 Inheritance of Multiple Traits on Different Chromosomes Traits are Inherited Independently Mendel s Genius Went Unrecognized in His Lifetime

Traits Are Inherited Independently Mendel performed genetic crosses in which he followed the inheritance of two traits at the same time

Traits Are Inherited Independently Seed color (yellow vs. green peas) and seed shape (smooth vs. wrinkled peas) were the characteristics studied The allele symbols were assigned: Y = yellow (dominant), y = green (recessive) S = smooth (dominant), s = wrinkled (recessive)

Traits Are Inherited Independently Two trait cross was between two true breeding varieties for each characteristic P: SSYY x ssyy

Traits Are Inherited Independently Genes of pea color and pea shape (S, s and Y, y) separate independently during meiosis (Mendel s Law of Independent Assortment) Possible gametes of parent SSYY are SY, SY, SY, and SY (each S can combine with each Y) Possible gametes of parent ssyy are sy, sy, sy, and sy (each s and combine with each y)

Traits Are Inherited Independently Punnett Square from SSYY x ssyy cross Gametes ¼sy ¼sy ¼sy ¼sy 1 16 1 16 1 16 ¼SY SsYy SsYy SsYy SsYy F 1 : All SsYy 1 16 1 16 1 16 ¼SY SsYy SsYy SsYy SsYy 1 16 1 16 ¼SY SsYy SsYy SsYy SsYy 1 16 1 16 1 16 1 16 1 16 1 16 1 16 1 16 ¼SY SsYy SsYy SsYy SsYy Smooth yellow peas

Traits Are Inherited Independently Mendel then allowed the F1 offspring to self fertilize: SsYy x SsYy Gametes are ¼SY, ¼Sy, ¼sY, ¼sy from each parent

Traits Are Inherited Independently 4 x 4 Punnett square yields: 9/16 smooth yellow peas 3/16 smooth green peas 3/16 wrinkled yellow peas 1/16 wrinkled green peas

Mendel s Genius Went Unrecognized Mendel s work was published in 1865 but went unnoticed Three biologists independently rediscovered Mendel s principles of inheritance in 1900 Mendel was credited in new papers as laying the groundwork of genetics 30 years previously

Section 12.5 Outline 12.5 Inheritance of Genes Located on the Same Chromosome Genes on the Same Chromosome Tend to Be Inherited Together Recombination Can Create New Combinations of Linked Alleles

Genes on the Same Chromosome Mendel s Law of Independent Assortment only works for genes whose loci are on different chromosomes

Genes on the Same Chromosome Different gene loci located on the same chromosome tend to be inherited together Characteristics whose genes tend to assort together are said to be linked

Genes on the Same Chromosome Example of genetic linkage Flower color and pollen shape are on the same chromosome in peas Gene assignments Let P = purple flowers and p = red flowers Let L = long pollen shape and l = round shape

Genes on the Same Chromosome Example of genetic linkage What are the expected gametes from parent PpLl, where P is linked with L and p is linked with l? Independent assortment would yield: ¼PL, ¼Pl, ¼ pl, ¼pl Instead, the gametes are mostly PL and pl

Recombination Genes on the same chromosome do not always sort together Crossing over in Prophase I of meiosis creates new gene combinations Crossing over involves the exchange of DNA between chromatids of paired homologous chromosomes in synapsis

Recombination Example of crossing over for flower color and pollen shape with parent PpLl Assume P is linked with L and p is linked with l Crossover between chromatids of different homologous replicated chromosomes yields some pl and Pl gametes P L P L P L P L p L p L p l P l P l p l p l p l

Recombination Crossing over juxtaposes alleles carried on one homologue with alleles on the other Cross of P L with p l produces: P L p l 1. Mostly PL and pl gametes (parental types) 2. A few pl and Pl gametes (recombined chromosomes) Crossing over occurs more frequently between loci that are far apart on the chromosome

Section 12.6 Outline 12.6 Sex Determination and Sex-Linked Inheritance Sex Chromosomes and Autosomes Sex-Linked Genes Are on the X or the Y How Sex-Linkage Affects Inheritance

Sex Chromosomes and Autosomes Mammals and many insect species have a set of sex chromosomes that dictate gender Females have two X chromosomes Males have an X chromosome and a Y chromosome Sex chromosomes segregate during meiosis

Sex Chromosomes and Autosomes The rest of the (non-sex) chromosomes are called autosomes

Sex-Linked Genes Are on the X or the Y Genes carried on one sex chromosome are sex-linked X chromosome is much larger than the Y and carries over 1000 genes Y chromosome is smaller and carries only 78 genes

Sex-Linked Genes Are on the X or the Y The X and the Y have very few genes in common Females (XX) can be homozygous or heterozygous for a characteristic Males (XY) have only one copy of the genes on the X or the Y

How Sex-Linkage Affects Inheritance Patterns of sex-linked inheritance were first discovered in fruit flies (Drosophila) in early 1900s Eye color genes were found to be carried by the X chromosome R = red eyes (dominant) r = white eyes (recessive)

How Sex-Linkage Affects Inheritance Sex-linked (specifically X-linked) recessive alleles displayed their phenotype more often in males Males showed recessive white-eyed phenotype more often than females in an X R X r x X r Y cross

How Sex-Linkage Affects Inheritance Males do not have a second X-linked gene (as do females) which can mask a recessive gene if dominant

Section 12.7 Outline 12.7 Do Mendel s Rules Apply to All Traits? Inheritance Patterns That Depart from Mendel s Rules Incomplete Dominance Multiple Alleles Codominance Polygenic Inheritance Environmental Influence

Departure from Mendel s Rules Assumptions drawn from Mendel s Rules All genes are governed by alleles found at a single locus on a pair of homologous chromosomes There are two alleles (gene forms) for each characteristic or gene type One allele is dominant over the other, which is recessive

Departure from Mendel s Rules A number of traits in humans show non- Mendelian inheritance patterns

Incomplete Dominance Dominance of one allele over another breaks down in incompletely dominant characteristics When the heterozygous phenotype is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance

Incomplete Dominance Human hair texture is influenced by a gene with two incompletely dominant alleles, C1 and C2 A person with two copies of the C1 allele has curly hair Someone with two copies of the C2 allele has straight hair Heterozygotes (C1C2 genotype) have wavy hair

Incomplete Dominance If two wavy-haired people marry, their children could have any of the three hair types: curly (C1C1), wavy (C1C2), or straight (C2C2)

Multiple Alleles A species may have more than two alleles for a given characteristic Each individual still carries two alleles for this characteristic

Multiple Alleles Examples of multiple allelism Thousands of alleles for eye color in fruit flies, producing white, yellow, orange, pink, brown, or red eyes Human blood group genes producing blood types A, B, AB, and O Three alleles in this system: A, B, and O

Codominance Some alleles are always expressed even in combination with other alleles Heterozygotes display phenotypes of both the homozygote phenotypes in codominance

Codominance Example: Human blood group alleles Alleles A and B are codominant Type AB blood is seen where individual has the genotype AB

Polygenic Inheritance Some characteristics show a range of continuous phenotypes instead of discrete, defined phenotypes Examples include human height, skin color, and body build, and grain color in wheat

Polygenic Inheritance Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci Human skin color is controlled by at least 3 genes, each with pairs of incompletely dominant alleles

Environmental Influence The environment can module how genes are expressed Example: Himalayan rabbit Himalayan rabbits have the genotype for black fur all over the body Black pigment is only produced in colder areas of the body: the nose, ears, and paws

Environmental Influence Both heredity and environment play major roles in the development of some characteristics Identical twin studies in humans reveal different IQ scores between twins

Section 12.8 Outline 12.8 Investigating Human Genetic Disorders Pedigree Analysis

Pedigree Analysis Records of gene expression over several generations of a family can be diagrammed Careful analysis of this diagram (a pedigree) can reveal inheritance pattern of a trait

Pedigree Analysis Pedigree analysis is often combined with molecular genetics technology to elucidate gene action and expression

Section 12.9 Outline 12.9 Inheritance of Single Gene Disorders Recessive Alleles Cause Some Human Genetic Disorders Albinism Results From a Defect in Melanin Production Sickle-Cell Anemia Is Caused by a Defective Allele for Hemoglobin Synthesis

Section 12.9 Outline 12.9 Inheritance of Single Gene Disorders (continued) Some Human Genetic Disorders Are Caused by Dominant Alleles Some Human Genetic Disorders Are Sex- Linked

Recessive Genetic Disorders New alleles produced by mutation usually code for non-functional proteins Alleles coding for non-functional proteins are recessive to those coding for functional ones

Recessive Genetic Disorders Heterozygous individuals are carriers of a recessive genetic trait (but otherwise have a normal phenotype) Recessive genes are more likely to occur in a homozygous combination (expressing the defective phenotype) when related individuals have children

Albinism Melanin is the dark pigment that colors skin cells Melanin is produced by the enzyme tyrosinase An allele known as TYR (for tyrosinase) encodes a defective tyrosinase protein in skin cells, producing no melanin

Albinism Humans and other mammals who are homozygous for TYR have no skin, fur, or eye coloring (skin and hair appear white, eyes are pink)

Sickle-Cell Anemia Hemoglobin is an oxygen-transporting protein found in red blood cells

Sickle-Cell Anemia A mutant hemoglobin gene causes hemoglobin molecules in blood cells to clump together Red blood cells take on a sickle (crescent) shape and easily break Blood clots can form, leading to oxygen starvation of tissues and paralysis Condition is known as sickle-cell anemia

Sickle-Cell Anemia About 8% of the African population is heterozygous for sickle-cell anemia Heterozygous individuals have some resistance to malaria

Sickle-Cell Anemia The presence of the mutant allele can be detected by a blood test Results of blood testing can help couples understand odds of giving birth to a child with sickle-cell anemia

Dominant Genetic Disorders Many serious genetic disorders, e.g. Marfan syndrome, are caused by dominant alleles

Dominant Genetic Disorders A dominant disease can be transmitted to offspring if at least one parent suffers from the disease and lives long enough to reproduce Dominant disease alleles also arise due to new mutations in the DNA of eggs or sperm of healthy parents

Dominant Genetic Disorders Dominant disease alleles disrupt normal cell function in a variety of ways Produce an abnormal protein that interferes with the function of the normal one Encode toxic proteins Encode a protein that is overactive or active at inappropriate times and places

Sex-Linked Genetic Disorders Several defective alleles for characteristics encoded on the X chromosome are known Sex-linked disorders appear more frequently in males and often skip generations

Sex-Linked Genetic Disorders Examples of sex-linked (X-linked) disorders Red-green color blindness

Sex-Linked Genetic Disorders Examples of sex-linked (X-linked) disorders Hemophilia (deficiency in blood clotting protein) Hemophilia gene in Queen Victoria of England was passed among the royal families of Europe

Section 12.10 Outline 12.10 Errors in Chromosome Number Chromosomal Errors in Meiosis: Non- Disjunction Genetic Disorders Caused by Abnormal Numbers of Sex Chromosomes Genetic Disorders Caused by Abnormal Numbers of Autosomes

Non-Disjunction Incorrect separation of chromosomes or chromatids in meiosis known as nondisjunction

Non-Disjunction Most embryos arising from gametes with abnormal chromosome numbers abort spontaneously (are miscarried) Some combinations of abnormal chromosome number survive to birth or beyond

Abnormal Sex Chromosome Number Non-disjunction of sex chromosomes in males or females produce abnormal numbers of X and Y chromosomes

Abnormal Sex Chromosome Number Sex chromosome disorders that survive beyond birth Turner Syndrome (XO): an underdeveloped, infertile woman with only one X chromosome

Abnormal Sex Chromosome Number Sex chromosome disorders that survive beyond birth Trisomy X (XXX): a fertile, normal woman with an extra X chromosome

Abnormal Sex Chromosome Number Sex chromosome disorders that survive beyond birth Kleinfelter Syndrome (XXY): an infertile man with an extra X chromosome, having partial breast development and small testes

Abnormal Sex Chromosome Number Sex chromosome disorders that survive beyond birth XYY Male: a tall man with an extra Y that produces high levels of testosterone and may score lower on IQ tests

Abnormal Autosome Number Non-disjunction of autosomes can occur during meiosis in the father or mother Frequency of non-disjunction increases with the age of the parents

Abnormal Autosome Number Fertilized egg has either one or three copies of an autosomal chromosome Single copy autosome embryos usually abort very early in development

Abnormal Autosome Number Trisomy 21 (Down Syndrome) is an example of an abnormal autosomal number Down syndrome individuals have three copies of chromosome 21 Down syndrome characterized by distinctively shaped eyelids, among other physical features