Patterns of Inheritance

Name: Patterns of Inheritance
Uploaded: 2017-09-10T07:38:37+00:00
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Description: Patterns of Inheritance

Patterns of Inheritance
Chapter 9 Patterns of Inheritance Laura Coronado Bio Chapter 9

Biology And Society: A Matter of Breeding
Genetics is the scientific study of heredity. Genetics explains why the offspring of purebred dogs are like their parents. Inbreeding of dogs makes some genetic disorders common. A dog’s behavior is determined by its Genes Environment Laura Coronado Bio Chapter 9

HERITABLE VARIATION AND PATTERNS OF INHERITANCE
Heredity is the transmission of traits from one generation to the next. Gregor Mendel Worked in the 1860s Was the first person to analyze patterns of inheritance Deduced the fundamental principles of genetics Laura Coronado Bio Chapter 9

Laura Coronado Bio 10 Chapter 9
In an Abbey Garden Mendel studied garden peas because they Are easy to grow Come in many readily distinguishable varieties Are easily manipulated Can self-fertilize Petal Stamen (makes sperm- producing pollen) Carpel (produces eggs) Laura Coronado Bio Chapter 9

Definitions A character is a heritable feature that varies among individuals. Flower color A trait is a variant of a character. Purple or white flower color Each of the characters Mendel studied occurred in two distinct forms. Laura Coronado Bio Chapter 9

Mendel Created true-breeding varieties of plants Purple & white pea flowers Crossed two different true-breeding varieties A monohybrid cross is a cross between parent plants that differ in only one character. Hybrids are the offspring of two different true-breeding varieties. The parental plants are the P generation. Their hybrid offspring are the F1 generation. A cross of the F1 plants forms the F2 generation. Laura Coronado Bio Chapter 9

Removed stamens from purple flower. White Stamens Parents (P) Transferred pollen from stamens of white flower to carpel of purple flower. Carpel Purple Pollinated carpel matured into pod. Figure 9.3 Mendel's technique for cross-fertilizing pea plants. (Step 3) Planted seeds from pod. Offspring (F1) Laura Coronado Bio Chapter 9 Figure 9.3-3

Mendel’s Law of Segregation
Mendel performed many experiments. He tracked the inheritance of characters that occur as two alternative traits. Laura Coronado Bio Chapter 9

Dominant Recessive Dominant Recessive Flower color Pod shape Inflated Constricted Purple White Pod color Flower position Green Yellow Stem length Axial Terminal Seed color Figure 9.4 The seven characters of pea plants studied by Mendel. Yellow Green Tall Dwarf Seed shape Round Wrinkled Laura Coronado Bio Chapter 9 Figure 9.4

P Generation (true-breading parents) Purple flowers White flowers
F1 Generation All plants have purple flowers Fertilization among F1 plants (F1  F1) Figure 9.5 Mendel's cross tracking one character (flower color). (Step 3) F2 Generation 3 4 1 4 of plants have purple flowers of plants have white flowers Figure 9.5-3

Mendel developed four hypotheses from the monohybrid cross:
1. There are alternative versions of genes, called alleles. 2. For each character, an organism inherits two alleles, one from each parent. An organism is homozygous for that gene if both alleles are identical. Called a homozygote. An organism is heterozygous for that gene if the alleles are different. Called a heterozygote. Laura Coronado Bio Chapter 9

Mendel developed four hypotheses from the monohybrid cross
3. If two alleles of an inherited pair differ The allele that determines the organism’s appearance is the dominant allele The other allele, which has no noticeable effect on the appearance, is the recessive allele 4. Gametes carry only one allele for each inherited character. The two members of an allele pair segregate (separate) from each other during the production of gametes. This statement is the law of segregation. Laura Coronado Bio Chapter 9

P Generation Genetic makeup (alleles) Purple flowers White flowers PP pp Alleles carried by parents Gametes All P All p F1 Generation (hybrids) Purple flowers Alleles segregate All Pp 1 2 1 2 Gametes P p F2 Generation (hybrids) Sperm from F1 plant Figure 9.6 The law of segregation. (Step 3) Do Mendel’s hypotheses account for the 3:1 ratio he observed in the F2 generation? A Punnett square highlights the four possible combinations of gametes and offspring that result from each cross. P p P Eggs from F1 plant PP Pp p Pp pp Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Laura Coronado Bio Chapter 9 Figure 9.6-3

Geneticists distinguish between an organism’s physical traits and its genetic makeup. An organism’s physical traits are its phenotype. An organism’s genetic makeup is its genotype. Homologous chromosomes have Genes at specific loci Alleles of a gene at the same locus Laura Coronado Bio Chapter 9

Gene loci Dominant allele P a B Homologous chromosomes P a b Recessive allele Figure 9.7 The relationship between alleles and homologous chromosomes. Genotype: PP aa Bb Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous Laura Coronado Bio Chapter 9 Figure 9.7

Mendel’s Law of Independent Assortment
A dihybrid cross is the crossing of parental varieties differing in two characters. What would result from a dihybrid cross? Two hypotheses are possible: 1. Dependent assortment 2. Independent assortment Laura Coronado Bio Chapter 9

Law of Independent Assortment
Mendel’s dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formation. Thus, the inheritance of one character has no effect on the inheritance of another. Independent assortment is also seen in two hereditary characters in Labrador retrievers. Laura Coronado Bio Chapter 9

(a) Hypothesis: Dependent assortment (b) Hypothesis: Independent assortment P Generation RRYY rryy RRYY rryy Gametes RY ry Gametes RY ry F1 Generation RrYy RrYy F2 Generation Sperm 1 4 1 4 1 4 1 4 RY rY Ry ry Sperm 1 2 1 2 RY ry 1 4 RY RRYY RrYY RRYy RrYy 1 2 Figure 9.8 Testing alternative hypotheses for gene assortment in a dihybrid cross. RY 1 4 rY 16 9 Yellow round Eggs RrYY rrYY RrYy rrYy Eggs 1 2 ry 1 4 Ry 16 3 Green round RRYy RrYy RRyy Rryy 16 3 Yellow wrinkled 1 4 ry RrYy rrYy Rryy rryy 1 16 Green wrinkled Predicted results (not actually seen) Actual results (support hypothesis) Laura Coronado Bio Chapter 9 Figure 9.8

Mating of double heterozygotes (black coat, normal vision)
Blind dog Blind dog Phenotypes Black coat, normal vision Black coat, blind (PRA) Chocolate coat, normal vision Chocolate coat, blind (PRA) Genotypes B_N_ B_nn bbN_ bbnn (a) Possible phenotypes of Labrador retrievers Mating of double heterozygotes (black coat, normal vision) Figure 9.9 Independent assortment of genes in Labrador retrievers. BbNn BbNn Phenotypic ratio of offspring 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA) (b) A Labrador dihybrid cross Laura Coronado Bio Chapter 9 Figure 9.9

Using a Testcross to Determine an Unknown Genotype
A testcross is a mating between An individual of dominant phenotype (but unknown genotype) A homozygous recessive individual Laura Coronado Bio Chapter 9

Testcross Genotypes B_ bb Two possible genotypes for the black dog: BB or Bb Figure 9.10 A Labrador retriever testcross Gametes B B b b Bb b Bb bb Offspring All black 1 black : 1 chocolate Laura Coronado Bio Chapter 9 Figure 9.10

The Rules of Probability
Mendel’s strong background in mathematics helped him understand patterns of inheritance. The rule of multiplication states that the probability of a compound event is the product of the separate probabilities of the independent events. Laura Coronado Bio Chapter 9

F1 Genotypes Bb female Bb male Formation of eggs Formation of sperm F2 Genotypes Male gametes 1 2 1 2 B b B B B 1 2 b Figure 9.11 Segregation of alleles and fertilization as chance events. B 1 4 1 4 1 2 1 2 (  ) Female gametes 1 2 b b B b b 1 4 1 4 Laura Coronado Bio Chapter 9 Figure 9.11

Mendel’s principles apply to the inheritance of many human traits. A family pedigree Shows the history of a trait in a family Allows geneticists to analyze human traits Dominant traits are not necessarily Normal or More common Wild-type traits are Those seen most often in nature Not necessarily specified by dominant alleles Laura Coronado Bio Chapter 9

DOMINANT TRAITS Freckles Widow’s peak Free earlobe Figure 9.12 Examples of inherited traits thought to be controlled by a single gene. RECESSIVE TRAITS No freckles Straight hairline Attached earlobe Laura Coronado Bio Chapter 9 Figure 9.12

First generation (grandparents) Ff Ff ff Ff Second generation (parents, aunts, and uncles) FF ff ff Ff Ff ff or Ff Third generation (brother and sister) ff FF or Ff Figure 9.13 A family pedigree showing inheritance of free versus attached earlobes. Female Male Attached Free Laura Coronado Bio Chapter 9 Figure 9.13

Human Disorders Controlled by a Single Gene
Many human traits Show simple inheritance patterns Are controlled by single genes on autosomes Recessive Disorders Most human genetic disorders are recessive. Individuals who have the recessive allele but appear normal are carriers of the disorder. Laura Coronado Bio Chapter 9

Table 9.1 Some Autosomal Disorders in Humans Laura Coronado Bio Chapter 9 Table 9.1

Parents Hearing Hearing Dd Dd Offspring D d Sperm Dd DD D Hearing (carrier) Hearing Figure 9.14 Predicted offspring when both parents are carriers for a recessive disorder. Eggs Dd dd d Hearing (carrier) Deaf Laura Coronado Bio Chapter 9 Figure 9.14

Cystic fibrosis Is the most common lethal genetic disease in the United States Is caused by a recessive allele carried by about one in 25 people of European ancestry Prolonged geographic isolation of certain populations can lead to inbreeding, the mating of close relatives. Inbreeding increases the chance of offspring that are homozygous for a harmful recessive trait. Laura Coronado Bio Chapter 9

Dominant Disorders Some human genetic disorders are dominant. Huntington’s disease, which leads to degeneration of the nervous system, does not begin until middle age. Achondroplasia is a form of dwarfism. The homozygous dominant genotype causes death of the embryo. Thus, only heterozygotes have this disorder. Laura Coronado Bio Chapter 9

Parents Normal (no achondroplasia) Dwarf (achondroplasia) dd Dd d d Sperm Dd Dd D Dwarf Dwarf Eggs dd dd d Normal Normal Figure 9.16 A family with and without achondroplasia. Molly Jo Matt Amy Zachary Jake Jeremy Laura Coronado Bio Chapter 9 Figure 9.16

The Process of Science: What Is the Genetic Basis of Hairless Dogs?
Observation: Dogs come in a wide variety of physical types. Question: What is the genetic basis for the hairless phenotype? Hypothesis: A comparison of genes of coated and hairless dogs would identify the gene or genes responsible. Prediction: A mutation in a single gene accounts for the hairless appearance. Experiment: Compared DNA sequences of 140 hairless dogs from 3 breeds with 87 coated dogs from 22 breeds. Results: Every hairless dog, but no coated dogs, had a single change in a single gene. Laura Coronado Bio Chapter 9

Figure 9.17 Hairless versus coated Chinese crested dogs. Laura Coronado Bio Chapter 9 Figure 9.17

Genetic Testing Today many tests can detect the presence of disease-causing alleles. Most genetic testing is performed during pregnancy. Amniocentesis collects cells from amniotic fluid. Chorionic villus sampling removes cells from placental tissue. Genetic counseling helps patients understand the results and implications of genetic testing. Laura Coronado Bio Chapter 9

VARIATIONS ON MENDEL’S LAWS
Some patterns of genetic inheritance are not explained by Mendel’s laws. In incomplete dominance in plants and people, F1 hybrids have an appearance in between the phenotypes of the two parents. Laura Coronado Bio Chapter 9

P Generation Red White RR rr Gametes R r F1 Generation Pink Rr 1 2 1 2 Gametes R r Figure 9.18 Incomplete dominance in snapdragons. (Step 3) F2 Generation Sperm 1 2 1 2 R r 1 2 R RR Rr Eggs 1 2 r Rr rr Laura Coronado Bio Chapter 9 Figure

Hypercholesterolemia
Is characterized by dangerously high levels of cholesterol in the blood. Is a human trait that is incompletely dominant. Heterozygotes have blood cholesterol levels about twice normal. Homozygotes have blood cholesterol levels about five times normal. Laura Coronado Bio Chapter 9

HH Hh hh Homozygous for ability to make LDL receptors Heterozygous Homozygous for inability to make LDL receptors GENOTYPE LDL LDL receptor PHENOTYPE Figure 9.19 Incomplete dominance in human hypercholesterolemia. Cell Normal Mild disease Severe disease Laura Coronado Bio Chapter 9 Figure 9.19

ABO Blood Groups: An Example of Multiple Alleles and Codominance
The ABO blood groups in humans are an example of multiple alleles. The human blood type alleles IA and IB exhibit codominance: Both alleles are expressed in the phenotype. The immune system produces blood proteins called antibodies that can bind specifically to blood cell carbohydrates. Blood cells may clump together if blood cells of a different type enter the body. The clumping reaction is the basis of a blood-typing lab test. Laura Coronado Bio Chapter 9

Blood Group (Phenotype) Antibodies Present in Blood Reactions When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Genotypes Red Blood Cells O A B AB Carbohydrate A IAIA or IAi A Anti-B Carbohydrate B IBIB or IBi B Anti-A AB IAIB — Figure 9.20 Multiple alleles for the ABO blood groups. Anti-A Anti-B O ii Laura Coronado Bio Chapter 9 Figure 9.20

Pleiotropy and Sickle-Cell Disease
Pleiotropy is the impact of a single gene on more than one character. Sickle-cell disease Exhibits pleiotropy Results in abnormal hemoglobin production Causes disk-shaped red blood cells to deform into a sickle shape with jagged edges Laura Coronado Bio Chapter 9

Individual homozygous, for sickle-cell allele
Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes into long flexible chains, causing red blood cells to become sickle-shaped. Figure 9.21 Sickle-cell disease, multiple effects of a single human gene. Colorized SEM Sickled cells can lead to a cascade of symptoms, such as weakness, pain, organ damage, and paralysis. Figure 9.21

Polygenic Inheritance
Polygenic inheritance is the additive effects of two or more genes on a single phenotype. Laura Coronado Bio Chapter 9

P Generation aabbcc (very light) AABBCC (very dark) F1 Generation AaBbCc AaBbCc F2 Generation Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 64 20 1 8 1 8 64 15 Eggs 1 8 Figure 9.22 A model for polygenic inheritance of skin color. Fraction of population 1 8 64 6 1 8 1 8 1 64 Skin pigmentation 1 64 64 6 64 15 64 20 64 15 64 6 1 64 Figure 9.22 Laura Coronado Bio Chapter 9

The Role of Environment
Many human characters result from a combination of heredity and environment. Only genetic influences are inherited. Figure 9.23 As a result of environmental influences, even identical twins can look different. Laura Coronado Bio Chapter 9

THE CHROMOSOMAL BASIS OF INHERITANCE
The chromosome theory of inheritance states that Genes are located at specific positions on chromosomes The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns It is chromosomes that undergo segregation and independent assortment during meiosis and thus account for Mendel’s laws. Laura Coronado Bio Chapter 9

All round-yellow seeds
P Generation Round-yellow seeds (RRYY) Wrinkled-green seeds (rryy) Y r y R Y R r y MEIOSIS FERTILIZATION Gametes R Y y r F1 Generation All round-yellow seeds (RrYy) R r y Law of Segregation: Follow the long chromosomes (carrying R and r) taking either the left or right branch. Law of Independent Assortment: Follow both the long and the short chromosomes. Y MEIOSIS R r Metaphase I (alternative arrangements) r R They are arranged in either of two equally likely ways at metaphase I. The R and r alleles segregate in anaphase I of meiosis. Y y Y y Metaphase II R r r R Only one long chromosome ends up in each gamete. Figure 9.24 The chromosomal basis of Mendel's laws. (Step 4) They sort independently, giving four gamete types. Y y Y y Y Y y y Y Y y y Gametes R R r r r r R R 4 1 4 1 4 1 4 1 RY ry rY Ry Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. Fertilization recombines the r and R alleles at random. FERTILIZATION AMONG THE F1 PLANTS F2 Generation 9 : 3 : 3 : 1 Figure

Linked Genes Linked genes Are located close together on a chromosome May be inherited together Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined that some genes were linked based on the inheritance patterns of their traits. Laura Coronado Bio Chapter 9

Dihybrid testcross Gray body, long wings (wild-type) Black body, short wings (mutant) GgLl ggll Female Male Results Offspring Gray-long GgLl Black-short ggll Gray-short Ggll Black-long ggLl Figure 9.25 Thomas Morgan's experiment and results. 965 944 206 185 Parental phenotypes 83% Recombinant phenotypes 17% Laura Coronado Bio Chapter 9 Figure

Genetic Recombination: Crossing Over
Crossing over can Separate linked alleles Produce gametes with recombinant chromosomes Produce offspring with recombinant phenotypes The percentage of recombinant offspring among the total is called the recombination frequency. Laura Coronado Bio Chapter 9

Parental gametes a b A b a B Pair of homologous chromosomes Crossing over Figure 9.26 Review: Crossing over can produce recombinant gametes. Recombinant gametes Laura Coronado Bio Chapter 9 Figure 9.26

FERTILIZATION G L g l GgLl (female) ggll (male) g l g l Crossing over
Sperm Parental gametes Recombinant gametes Eggs FERTILIZATION Figure 9.27 Explaining the unexpected results from the dihybrid testcross in Figure 9.25. Offspring G L g l G l g L g l g l g l g l Parental Recombinant Figure 9.27

Linkage Maps Early studies of crossing over were performed using the fruit fly Drosophila melanogaster. Alfred H. Sturtevant, a student of Morgan, developed a method for mapping gene loci, which resulted in the creation of linkage maps. He reasoned that the greater the distance between 2 genes, the more points there are between them where crossing over can occur A diagram of relative gene locations on a chromosome is a linkage map. Laura Coronado Bio Chapter 9

Chromosome g c l 17% 9% 9.5% Figure 9.28 Using crossover data to map genes. Recombination frequencies Laura Coronado Bio Chapter 9 Figure 9.28

SEX CHROMOSOMES AND SEX-LINKED GENES
Sex chromosomes influence the inheritance of certain traits. Nearly all mammals have a pair of sex chromosomes designated X and Y. Males have an X and Y. Females have XX. Laura Coronado Bio Chapter 9

Male Female 44  XY 44  XX Somatic cells 22  X 22  Y 22  X Sperm Egg Figure 9.29 The chromosomal basis of sex determination. 44  XX 44  XY Offspring Female Male Laura Coronado Bio Chapter 9 Figure 9.29

Sex-Linked Genes Any gene located on a sex chromosome is called a sex-linked gene. Most sex-linked genes are found on the X chromosome. Red-green color blindness is a common human sex-linked disorder. The gene involved is located on the X chromosome The allege is recessive Laura Coronado Bio Chapter 9

Figure 9.30 A test for red-green colorblindness. Laura Coronado Bio Chapter 9 Figure 9.30

XNXN XnY XNXn XNY XNXn XnY Sperm Sperm Sperm Xn Y XN Y Xn Y XNXn XNY XN XNXN XNY XN XN XNXn XNY Eggs Eggs Eggs XN XNXn XNY Xn XNXn XnY Xn XnXn XnY (a) (b) (c) Figure 9.31 Inheritance of colorblindness, a sex-linked recessive trait. Normal female  colorblind male Carrier female  normal male Carrier female  colorblind male Key Unaffected individual Carrier Colorblind individual Laura Coronado Bio Chapter 9 Figure 9.31

Hemophilia Is a sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises Has plagued royal families of Europe Laura Coronado Bio Chapter 9

Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Figure 9.32 Hemophilia in the royal family of Russia. Alexis Laura Coronado Bio Chapter 9 Figure 9.32

Evolution Connection: Barking Up the Evolutionary Tree
About 15,000 years ago in East Asia, humans began to cohabit with ancestral canines that were predecessors of modern wolves and dogs. As people settled into geographically distinct populations, different canines became separated and inbred. In 2005 researchers sequenced the complete genome of a dog. An evolutionary tree of dog breeds was created. Student Misconceptions and Concerns 1. The prior discussion of “linked” genes addresses a different relationship than the use of the similar termed “sex-linked genes”. The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 2. The likelihood that at least some students are color-blind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy, and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a backup copy that can function if one of the sex-linked genes is damaged. 2. For additional information about hemophilia, consider visiting the Website of the National Hemophilia Foundation at Laura Coronado Bio Chapter 9

Wolf Chinese shar-pei Ancestral canine Siberian husky Akita Alaskan malamute Basenji Afghan hound Saluki Figure 9.33 An evolutionary tree of dog breeds. Rottweiler Sheepdog Retriever Laura Coronado Bio Chapter 9 Figure 9.33

Fertilization Alleles Meiosis Gamete from other parent Diploid zygote (contains paired alleles) Diploid cell (contains paired alleles, alternate forms of a gene) Figure 9.UN1 Summary: Law of Segregation Haploid gametes (allele pairs separate) Laura Coronado Bio Chapter 9 Figure 9.UN1

Pleiotropy Multiple traits (e.g., sickle-cell disease) Single gene Figure 9.UN4 Summary: Pleiotropy Laura Coronado Bio Chapter 9 Figure 9.UN4

Polygenic inheritance Single trait (e.g., skin color) Figure 9.UN5 Summary:Polygenic Inheritance Multiple genes Laura Coronado Bio Chapter 9 Figure 9.UN5

Figure 9.UN7 Summary: Sex Linked Traits Laura Coronado Bio Chapter 9 Figure 9.UN7

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