Mendel and the Gene Idea: Mendelian Genetics

Chapter 14: Mendel and the Gene Idea Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritabl...

Chapter 14: Mendel and the Gene Idea Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive Incomplete dominance Codominance When a gene has more than two alleles Multiple alleles When one gene produces multiple phenotypes Pleiotropy Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties (Intermediate effect) In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways (BOTH alleles are expressed) Figure 14.10-1 P Generation Red Whit CRCR e C C W W Gametes C R C W © 2014 Pearson Education, Inc. Figure 14.10-2 P Generation Red Whit CRCR e C C W W Gametes C R C W F1 Generation Pink CRC W Gametes 1 2 C 1 2 C R W © 2014 Pearson Education, Inc. Figure 14.10-3 P Generation Red Whit CRCR e C C W W Gametes C R C W F1 Generation Pink CRC W Gametes 1 2 C 1 2 C R W F2 Generation Sperm 1 2 CR 1 2 C W 1 2 CR Egg CRCR CRCW s 1 C 2 W CRCW CWC W © 2014 Pearson Education, Inc. The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don’t interact that way Alleles are simply variations in a gene’s nucleotide sequence For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes Polydactyly: many fingers The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage In this example, the recessive allele is far more prevalent than the population’s dominant allele Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Figure 14.11 (a The three alleles for the ABO blood groups and their ) carbohydrates Allele IA IB i Carbohydrate A B none (b Blood group genotypes and phenotypes ) Genotype IAIA or IAi IBIB or IBi IA IB ii Red blood cell appearance Phenotype A B A O (blood group) B © 2014 Pearson Education, Inc. Practice punnet square with blood types Parent AB and O blood type Out come and ratio Parent B and O Parent AB and A Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Symptoms of cystic fibrosis include: Coughing up thick mucus Wheezing or shortness of breath Getting sinus infections, bronchitis, or pneumonia often Growths, called polyps, in the nose Bulky, oily, or foul-smelling stool Too much gas, constipation, or stomach pain Weight loss or failure to gain weight Low bone density Wide, rounded fingertips and toes, called clubbing (Source: WebMD) HOWEVER: All of the symptoms are traceable to a SINGLE allele of the CFTR gene. Many phenotypic effects of a single genotype =PLEIOTROPY Fig 15.3 and Fig 15.7 Multiple alleles Three alleles for wing shape in fruit fly +, vga, vg + is dominant to vga, vg vga, vg Incomplete or codominant? Codominance What do you get when you cross two speckled chickens? What do you get when you cross two speckled chickens?  BW X BW B W B BB BW W BW WW What do you get when you cross two speckled chickens?  BW X BW B W ¼ Black ¼ White ½ Speckled B BB BW W BW WW Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes Epistasis “Standing upon” In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus Labrador retrievers and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair >1 GENES ONE PHENOTYPE Figure 14.12 BbEe BbEe Sperm 1 4 B 1 4 b 1 4 B 1 4 be Egg E E e s 1 4 B E BBEE BbEE BBEe BbEe 1 4 b E BbEE bbEE BbEe bbEe 1 4 B e BBEe BbEe BBee Bbee 1 4 be BbEe bbEe Bbee bbee 9 : 3 : 4 © 2014 Pearson Education, Inc. Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color in humans is an example of polygenic inheritance Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The phenotypic range is broadest for polygenic characters Traits that depend on multiple genes combined with environmental influences are called multifactorial Figure 14.14 Hydrangeas grown in acidic and basic soils © 2014 Pearson Education, Inc. A Mendelian View of Heredity and Variation An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior An organism’s phenotype reflects its overall genotype and unique environmental history Concept 14.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research Generation time is too long Parents produce relatively few offspring Breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics New Combinations of Alleles: Variation for Normal Selection Recombinant chromosomes  new alleles combinations in gametes Random fertilization increases number of variant combinations produced Abundance of genetic variation is the raw material upon which natural selection works Alterations of chromosome number or structure cause some genetic disorders Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Plants tolerate such genetic changes better than animals do Abnormal Chromosome Number In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy Figure 15.13-1 Meiosis I Nondisjunction Figure 15.13-2 Meiosis I Nondisjunction Meiosis II Non- disjunction Figure 15.13-3 Meiosis I Nondisjunction Meiosis II Non- disjunction Gametes n1 n1 n1 n1 n1 n1 n n Number of chromosomes (a)Nondisjunction of homo- (b)Nondisjunction of sister logous chromosomes in chromatids in meiosis II meiosis I Aneuploidy results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome Polyploidy is a condition in which an organism has more than two complete sets of chromosomes Triploidy (3n) is three sets of chromosomes Tetraploidy (4n) is four sets of chromosomes Polyploidy is very common in plants, but not animals Polyploids are more normal in appearance than aneuploids Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion removes a chromosomal segment Duplication repeats a segment Inversion reverses orientation of a segment within a chromosome Translocation moves a segment from one chromosome to another Study Guide for Chapter 15 T H Morgan`s experiments with D. melanogaster** know the wild types and mutant and outcomes for F1 and F2 (15.3) Demonstrated the chromosomal basis of inheritance Supported the laws of segregation and independent assortment Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with some serious disorders Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy The fact that males and females inherit a different number of X chromosomes leads to a pattern of inheritance different from that produced by genes located on autosomes. While there are very few Y-linked genes, many of which help determine sex, the X chromosomes have numerous genes for characters unrelated to sex. X-linked genes in humans follow the same pattern of inheritance that Morgan observed for the eye color locus he studied in Drosophila (see Figure 15.3). Fathers pass X-linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass X-linked alleles to both sons and daughters, as shown in Figure 15.7 for the inheritance of a mild X-linked disorder, red-green color blindness. Down Syndrome (Trisomy 21) Down syndrome an aneuploid condition that results from three copies of chromosome 21 It affects about one out of every 700 children born in the United States The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained Disorders Caused by Structurally Altered Chromosomes The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5 Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from relatively mild to life-threatening The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Figure 14.16 Parents Normal Normal Aa Aa Sperm A a Eggs Aa A Normal A A Normal (carrier) Aa Normal aa a Albino (carrier) © 2014 Pearson Education, Inc. If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives Hemophilia is an X-linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting. When a person with hemophilia is injured, bleeding is prolonged because a firm clot is slow to form. Small cuts in the skin are usually not a problem, but bleeding in the muscles or joints can be painful and can lead to serious damage. In the 1800s, hemophilia was widespread among the royal families of Europe. Queen Victoria of England is known to have passed the allele to several of her descendants. Subsequent intermarriage with royal family members of other nations, such as Spain and Russia, further spread this X-linked trait, and its incidence is well documented in royal pedigrees. Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications Sickle-cell disease affects one out of 400 African- Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells In homozygous individuals, all hemoglobin is abnormal (sickle-cell) Symptoms include physical weakness, pain, organ damage, and even paralysis Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms About one out of ten African Americans has sickle-cell trait, an unusually high frequency Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common Figure 14.17 Sickle-cell alleles Low O2 Sickle- cell disease Sickle-cell Part of a fiber of Sickled red hemoglobin sickle-cell hemo- blood cells proteins globin proteins (a) Homozygote with sickle-cell disease: Weakness, anemia, pain and fever, organ damage Sickle-cell allele Normal allele Very low O2 Sickle- cell trait Sickle-cell and Part of a sickle-cell Sickled normal hemo- fiber and normal and globin proteins hemoglobin proteins normal red blood cells (b) Heterozygote with sickle-cell trait: Some symptoms when blood oxygen is very low; reduction of malaria symptoms © 2014 Pearson Education, Inc. Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia is a form of dwarfism caused by a rare dominant allele Figure 14.18 Parents Dwarf Norma Dd l dd Sperm D d Eggs Dd dd d Dwarf Norma l Dd dd d Dwarf Norma l © 2014 Pearson Education, Inc. Huntington’s Disease: A Late-Onset Lethal Disease Huntington’s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Once the deterioration of the nervous system begins the condition is irreversible and fatal Woody Guthrie: “This Land Is Your Land” Marfan Syndrome Another dominantly- inherited disease Affects connective tissue formation Affected individuals typically are tall and lanky Isaiah Austin http://www.isaiahaustinfoundation.org/ Multifactorial Disorders Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and some forms of cancer have both genetic and environmental components No matter what our genotype, our lifestyle has a tremendous effect on phenotype Study Guide for Chapter 14 Gregor Mendel and garden peas What are the laws of segregation and independent assortment? How did Mendel deduce these laws? What is a Punnett square? How can Punnett squares be used to make predictions about progeny from a cross? Deviations from Mendelian patterns Non-Mendelian inheritance patterns due to: Codominance Incomplete dominance Multiple alleles Pleiotropy Epistasis Polygenic inheritance Multifactorial conditions