Mendel and the Gene Idea Chapter 14 Mendel and the Gene Idea

Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring? “Blending” Hypothesis Idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) “Particulate” Hypothesis Idea that parents pass on discrete heritable units (genes) Mendel documented a “particulate” mechanism through his experiments with garden peas Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel discovered the basic principles of heredity by Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by Breeding garden peas in carefully planned experiments Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel’s Experimental, Quantitative Approach Advantages of pea plants for genetic study: Many varieties with distinct heritable features, or characters Ex.) Flower color (white/purple are called traits) Mating can be controlled Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) Cross-pollination (fertilization between different plants) can be achieved Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

TECHNIQUE Parental generation (P) Stamens Carpel RESULTS First filial Fig. 14-2 TECHNIQUE 1 2 Parental generation (P) Stamens Figure14.2 Crossing pea plants Carpel 3 4 RESULTS First filial gener- ation offspring (F1) 5

TECHNIQUE Parental generation (P) Stamens Carpel 1 2 3 4 Fig. 14-2a Figure14.2 Crossing pea plants Parental generation (P) Stamens Carpel 3 4

RESULTS First filial gener- ation offspring (F1) 5 Fig. 14-2b Figure14.2 Crossing pea plants

Mendel’s Experimental, Quantitative Approach Cont’d Tracked only those characters that varied in an either-or manner Used varieties that were true-breeding Plants that produce offspring of the same variety when they self-pollinate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel’s Experimental, Quantitative Approach Cont’d Typical experiment, Hybridization Mating two contrasting, true-breeding varieties, The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate, the F2 generation is produced

The Law of Segregation Mendel’s Results Mendel crossed true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

EXPERIMENT P Generation (true-breeding parents) Purple flowers White Fig. 14-3-1 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation?

F1 Generation (hybrids) Fig. 14-3-2 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation? All plants had purple flowers

EXPERIMENT P Generation (true-breeding parents) Purple flowers White Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation? All plants had purple flowers F2 Generation 705 purple-flowered plants 224 white-flowered plants

The Law of Segregation Mendel’s Results Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids Mendel called the purple flower color a dominant trait and the white flower color a recessive trait Mendel observed the same pattern of inheritance in six other pea plant characters What Mendel called a “heritable factor” is what we now call a gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Table 14-1

Four related concepts make up this model Mendel’s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring Four related concepts make up this model Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

These alternative versions of a gene are now called alleles Mendel’s Model Cont’d First Concept Alternative versions of genes account for variations in inherited characters Ex.) gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Allele for purple flowers Fig. 14-4 Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Figure 14.4 Alleles, alternative versions of a gene Allele for white flowers

Mendel did not know about chromosomes but still made this deduction Mendel’s Model Cont’d Second Concept For each “character” an organism inherits two alleles, one from each parent Mendel did not know about chromosomes but still made this deduction The two alleles at a locus on a chromosome may be identical or different P generation = identical F1 hybrids = differ Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

If the two alleles at a locus differ, then… Mendel’s Model Cont’d Third Concept If the two alleles at a locus differ, then… One (the dominant allele) determines the organism’s appearance The other (the recessive allele) has no noticeable effect on appearance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Mendel’s Model Cont’d Fourth Concept Law of Segregation, The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes An egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Capital letter represents a dominant allele Mendel’s Model Cont’d Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses The possible combinations of sperm and egg can be shown using a Punnett square: Diagram for predicting the results of a genetic cross between individuals of known genetic makeup Capital letter represents a dominant allele Lowercase letter represents a recessive allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

P Generation Appearance: Purple flowers White flowers Genetic makeup: Fig. 14-5-1 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p Figure 14.5 Mendel’s law of segregation

P Generation Appearance: Purple flowers White flowers Genetic makeup: Fig. 14-5-2 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Figure 14.5 Mendel’s law of segregation Genetic makeup: Pp Gametes: 1/2 P 1/2 p

P Generation Appearance: Purple flowers White flowers Genetic makeup: Fig. 14-5-3 P Generation Appearance: Purple flowers White flowers Genetic makeup: PP pp Gametes: P p F1 Generation Appearance: Purple flowers Figure 14.5 Mendel’s law of segregation Genetic makeup: Pp Gametes: 1/2 P 1/2 p Sperm F2 Generation P p P PP Pp Eggs p Pp pp 3 1

Useful Genetic Vocabulary Homozygous Organism with two identical alleles for a character Heterozygous Organism that has two different alleles for a gene Unlike homozygotes, heterozygotes are not true-breeding Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Useful Genetic Vocabulary Cont’d Different effects of dominant and recessive alleles, do not always reveal the genetic composition Thus we must use other terms to describe… Phenotype, or physical appearance Genotype, or genetic makeup Ex.) Flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous) Fig. 14-6 Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous) Figure 14.6 Phenotype versus genotype 2 Purple Pp (heterozygous) pp 1 White 1 (homozygous) Ratio 3:1 Ratio 1:2:1

The Testcross How can we tell the genotype of an individual with the dominant phenotype? Homozygous or Heterozygous? Individual must have one dominant allele… …but the other could be either dominant or recessive Carry out a Testcross Breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp? Fig. 14-7 TECHNIQUE  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If PP If Pp or Sperm Sperm p p p p Figure 14.7 The testcross P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in this cross were monohybrids Individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Law of Independent Assortment Cont’d Mendel’s second law of inheritance followed two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, Heterozygous for both characters Dihybrid Cross Cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

EXPERIMENT RESULTS Fig. 14-8 P Generation F1 Generation Hypothesis of YYRR yyrr Gametes YR  yr F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1

The Law of Independent Assortment Cont’d Using a dihybrid cross, Mendel developed the law of independent assortment States that each pair of alleles segregates independently of each other pair of alleles during gamete formation Law applies only to genes on different, non-homologous chromosomes Genes located near each other on the same chromosome tend to be inherited together

Concept 14.2: The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortment reflect the rules of probability Ex.) When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss Alleles of one gene segregate into gametes independently of another gene’s alleles Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Multiplication and Addition Rules Applied to Monohybrid Crosses How do we figure out the probability that two or more independent events will occur together By using the Multiplication Rule Multiply probability of 1 event by the probability of the other ( ½ x ½ = ¼) Is used in an F1 monohybrid cross Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 50% chance of carrying either the dominant or recessive allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Rr Rr  Segregation of Segregation of Fig. 14-9 Rr Rr  Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1/2 R 1/2 r Figure 14.9 Segregation of alleles and fertilization as chance events R R 1/2 R R r 1/4 1/4 Eggs r r 1/2 R r r 1/4 1/4

The Multiplication and Addition Rules Applied to Monohybrid Crosses Cont’d Rule of Addition Probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities Used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 When a gene has more than two alleles When a gene produces multiple phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Degrees of Dominance Complete Dominance Occurs when phenotypes of the heterozygote and dominant homozygote are identical Incomplete Dominance Phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties Codominance Two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

P Generation Red White CRCR CWCW Gametes CR CW Fig. 14-10-1 Figure 14.10 Incomplete dominance in snapdragon color

P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Fig. 14-10-2 P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Figure 14.10 Incomplete dominance in snapdragon color Gametes 1/2 CR 1/2 CW

P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Fig. 14-10-3 P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW Figure 14.10 Incomplete dominance in snapdragon color Gametes 1/2 CR 1/2 CW Sperm 1/2 CR 1/2 CW F2 Generation 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele Alleles don’t interact 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Relation Between Dominance and Phenotype Cont’d Example of Dominance and Phenotype: Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain Organismal level, the allele is recessive Biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant Molecular level, the alleles are codominant Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Most genes exist in populations in more than two allelic forms Multiple Alleles Most genes exist in populations in more than two allelic forms Ex.) 4 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Red blood cell appearance Phenotype (blood group) Fig. 14-11 Allele Carbohydrate IA A IB B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Red blood cell appearance Phenotype (blood group) Genotype IAIA or IA i A Figure 14.11 Multiple alleles for the ABO blood groups IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes

Genes have multiple phenotypic effects Ex.) Pleiotropy Pleiotropy Genes have multiple phenotypic effects Ex.) Alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Flowers: gene that determines flower color also affects the color of the seed coat Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Ex.) In mice and many other mammals, coat color depends on two genes Epistasis Epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus Ex.) In mice 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 C for color and c for no color) determines whether the pigment will be deposited in the hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc Fig. 14-12  BbCc BbCc Sperm 1/4 1/4 1/4 1/4 BC bC Bc bc Eggs 1/4 BC BBCC BbCC BBCc BbCc 1/4 Figure 14.12 An example of epistasis bC BbCC bbCC BbCc bbCc 1/4 Bc BBCc BbCc BBcc Bbcc 1/4 bc BbCc bbCc Bbcc bbcc 9 : 3 : 4

Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Not an either-or classification Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color and height in humans is an example of polygenic inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-13  AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Figure 14.13 A simplified model for polygenic inheritance of skin color Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 6/64 1/64 Number of dark-skin alleles: 1 2 3 4 5 6

Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics: Phenotype for a character depends on environment as well as genotype Norm of Reaction Phenotypic range of a genotype is influenced by the environment Ex.)Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-14 Figure 14.14 The effect of environment on phenotype

Humans are not good subjects for genetic research Why??? Concept 14.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research Why??? Generation time is too long Parents produce relatively few offspring Breeding experiments are unacceptable However, basic Mendelian genetics is the foundation of human genetics Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Pedigree Analysis Pedigree Family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

(a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Figure 14.15a Pedigree analysis Widow’s peak No widow’s peak (a) Is a widow’s peak a dominant or recessive trait?

(b) Is an attached earlobe a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff Ff ff Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff 3rd generation (two sisters) Figure 14.15b Pedigree analysis ff FF or Ff Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait?

Pedigree Analysis Cont’d Also used to make predictions about future offspring Can use the multiplication and addition rules to predict the probability of specific phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

The Behavior of Recessive Alleles Many genetic disorders are inherited in a recessive manner 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 (i.e., pigmented) Ex.) Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Parents Normal Normal Aa Sperm A a Eggs Aa AA A Normal (carrier) Fig. 14-16 Parents Normal Normal Aa  Aa Sperm A a Eggs Figure 14.16 Albinism: a recessive trait Aa AA A Normal (carrier) Normal Aa aa a Normal (carrier) Albino

Cystic Fibrosis Cystic Fibrosis Most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent Allele results in defective or absent chloride transport channels in plasma membranes Symptoms: mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Affects one out of 400 African-Americans Sickle-Cell Disease Sickle-cell disease Affects one out of 400 African-Americans Caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms: Physical weakness, pain, organ damage, and even paralysis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Do you think that these diseases are common in populations? No. 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd Fig. 14-17 Parents Dwarf Normal Dd  dd Sperm D d Figure 14.17 Achondroplasia: a dominant trait Eggs Dd dd d Dwarf Normal Dd dd d Dwarf Normal

Degenerative disease of the nervous system Huntington’s Disease Huntington’s disease Degenerative disease of the nervous system Disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Multifactorial Disorders Many diseases have both genetic and environmental components Ex.) heart disease and cancer Little is understood about the genetic contribution to most multifactorial diseases Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Would you want to know what your genes are? Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Video: Ultrasound of Human Fetus I Fetal Testing Amniocentesis Liquid that bathes the fetus is removed and tested Chorionic Villus Sampling (CVS) Sample of the placenta is removed and tested Other techniques: ultrasound and fetoscopy, allow fetal health to be assessed visually in utero Video: Ultrasound of Human Fetus I Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Amniotic fluid withdrawn Centrifugation Fetus Placenta Uterus Cervix Fig. 14-18a Amniotic fluid withdrawn Centrifugation Fetus Placenta Uterus Cervix Fluid Bio- chemical tests Figure 14.18 Testing a fetus for genetic disorders Fetal cells Several hours Several weeks Several weeks Karyotyping (a) Amniocentesis

Placenta Bio- chemical tests Fig. 14-18b Fetus Suction tube inserted through cervix Placenta Chorionic villi Bio- chemical tests Figure 14.18 Testing a fetus for genetic disorders Fetal cells Several hours Several hours Karyotyping (b) Chorionic villus sampling (CVS)

THE END

Heterozygous phenotype same as that of homo- zygous dominant PP Pp Fig. 14-UN2 Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homo- zygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes CRCR CRCW CWCW Codominance Heterozygotes: Both phenotypes expressed IAIB Multiple alleles In the whole population, some genes have more than two alleles ABO blood group alleles IA , IB , i Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease

Relationship among genes Description Example Epistasis Fig. 14-UN3 Relationship among genes Description Example Epistasis One gene affects the expression of another BbCc BbCc BC bC Bc bc BC bC Bc bc 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc

Fig. 14-UN4

George Arlene Sandra Tom Sam Wilma Ann Michael Carla Daniel Alan Tina Fig. 14-UN5 George Arlene Sandra Tom Sam Wilma Ann Michael Carla Daniel Alan Tina Christopher

Fig. 14-UN6

Fig. 14-UN7

Fig. 14-UN8

Fig. 14-UN9

Fig. 14-UN10

Fig. 14-UN11

You should now be able to: Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F1 generation, F2 generation Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F2 generation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Define and give examples of pleiotropy and epistasis Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance Define and give examples of pleiotropy and epistasis Explain why lethal dominant genes are much rarer than lethal recessive genes Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings