Mendel and the Gene Idea CHAPTER 14

What genetic principles account for the transmission of traits from parents to offspring?

One possible explanation of heredity is a “blending” hypothesis –The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green

An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea –Parents pass on discrete heritable units, genes

Gregor Mendel –Documented a particulate mechanism of inheritance through his experiments with garden peas –DNA wasn’t known yet

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

Mendel’s Experimental, Quantitative Approach Mendel chose to work with peas because –They are available in many varieties –He could strictly control which plants mated with which –Self pollinating (able to start with pure plants) –Lots of offspring

CROSSING PEA PLANTS Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F 1 ) APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. TECHNIQUE When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. TECHNIQUERESULTS

SOME GENETIC VOCABULARY Character: a heritable feature, such as flower color Trait: a variant of a character, such as purple or white flowers

More Genetic Vocabulary An organism that is homozygous for a particular gene –Has a pair of identical alleles for that gene (RR or rr) –Exhibits true-breeding (pure) An organism that is heterozygous for a particular gene –Has a pair of alleles that are different for that gene (Rr) –Hybrid

An organism’s phenotype –Is its physical appearance An organism’s genotype –Is its genetic makeup

Phenotype Versus Genotype Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1

Mendel chose to track –Only those characters that varied in an “either-or” manner Mendel also made sure that –He started his experiments with varieties that were “true-breeding”

In a typical breeding experiment –Mendel mated two contrasting, true- breeding varieties, a process called hybridization The true-breeding parents –Are called the P 1 generation –Cross RR x rr yielded all Rr (hybrids)

The hybrid offspring of the P 1 generation –Are called the F 1 generation When F 1 individuals self-pollinate –The F 2 generation is produced –Hybrid x Hybrid Rr x Rr –Offspring 25% RR, 50% Rr, 25% rr –3:1 phenotypic ratio –1:2:1 genotypic ratio

The Law of Segregation Mendel derived the law of segregation –By following a single trait The F 1 offspring produced in this cross –Were monohybrids, heterozygous for one character

The Law of Segregation The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

Alternative Versions Of Genes –Account for variations in inherited characters, which are now called alleles –Capital letter (dominant allele) –Lowercase (recessive) Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

For each character –An organism inherits two alleles, one from each parent –A genetic locus is actually represented twice

The Testcross or F 2 Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait RR x Rr or rr x Rr 1:1 ratio

THE TESTCROSS  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1 ⁄ 2 offspring purple and 1 ⁄ 2 offspring white: p p P P Pp pp Pp P p pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. RESULTS

The Law of Independent Assortment Mendel derived the law of independent assortment –By following a two traits The F 1 offspring produced in this cross –Were dihybrids, heterozygous for both characters

YYRR P Generation GametesYR yr  yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1⁄21⁄2 YR yr 1 ⁄ 2 1⁄21⁄2 yr YYRRYyRr yyrr YyRr 3 ⁄ 4 1 ⁄ 4 Sperm Eggs Phenotypic ratio 3:1 YR 1 ⁄ 4 Yr 1 ⁄ 4 yR 1 ⁄ 4 yr 1 ⁄ 4 9 ⁄ 16 3 ⁄ 16 1 ⁄ 16 YYRR YYRr YyRR YyRr YyrrYyRr YYrr YyRR YyRr yyRR yyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yR yr 1 ⁄ 4 Sperm RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. A dihybrid cross –Illustrates the inheritance of two characters Produces four phenotypes in the F 2 generation

Using the information from a dihybrid cross, Mendel developed the law of independent assortment –Each pair of alleles segregates independently during gamete formation

LAWS OF PROBABILITY

The Laws Of Probability Govern Mendelian Inheritance Mendel’s laws of segregation and independent assortment –Reflect the rules of probability

The Multiplication and Addition Rules Applied to Monohybrid Crosses The Multiplication Rule –States that the probability that two or more independent events will occur together is the product of their individual probabilities Probability of two independent events A,B: P(A and B) = P(A)*P(B)

Probability in a monohybrid cross –Can be determined using this rule  Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄21⁄2 r r R r r Sperm  Eggs

The Rule Of Addition –States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

Solving Complex Genetics Problems with the Rules of Probability We can apply the rules of probability –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 from such crosses –Each character first is considered separately and then the individual probabilities are multiplied together

Probability Problem If H horns is dominant to h hornless, and T fancy tail in dragons is dominant to t plain tail, what is the probability that results from a cross of Dragon #1 HHtt x HhTt Dragon #2: –Dragon #1 always contributes H & Dragon #2 contributes H half the time and h the other half SO the probability of having horns (HH x Hh) is equal to 1 and no horns is equal to 0

Probability Problem cont.) Dragon #1 HHtt x HhTt Dragon #2 Dragon #1 always contributes a t while Dragon #2 contributes a T half the time and a t half the time (tt x Tt) SO the probability of having a fancy tail is ½ and the probability of having a plain tail is ½

Probability Problem cont.) SO what’s the probability of having a baby dragon with: –Horns and fancy tail? 1 x ½ = ½ –Horns and a plain tail? 1 x ½ = ½ –Hornless and fancy tail? 0 x ½ = 0 –Hornless and plain tail? 0 x ½ = 0

OTHER FACTORS AFFECTING INHERITANCE

Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely simple The inheritance of characters by a single gene may deviate from simple Mendelian patterns

The Spectrum of Dominance Complete dominance –Occurs when the phenotypes of the heterozygote and dominant homozygote are identical Codominance –Two dominant alleles affect the phenotype in separate, distinguishable ways –Human ABO blood type is an example of codominance

The ABO blood group in humans –Is determined by multiple alleles –A and B are dominant to 0

Incomplete Dominance –The phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties P Generation F 1 Generation F 2 Generation Red C R Gametes CRCR CWCW  White C W Pink C R C W Sperm CRCR CRCR CRCR CwCw CRCR CRCR Gametes 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 Eggs 1⁄21⁄2 C R C R C W C W C R C W

Pleiotropy In Pleiotropy –A gene has multiple phenotypic effects –Frizzle gene in chickens causes feathers to curl outward, abnormal body temperature, and greater digestive capacity

Extending Mendelian Genetics for Two or More Genes Some traits –May be determined by two or more genes In Epistasis –A gene at one locus alters the phenotypic expression of a gene at a second locus

Color is recessive to no color at one allelic pair This recessive allele must be expressed before the specific color allele at a second locus is expressed. At the first gene, white colored squash is dominant to colored squash, and the gene symbols are W=white and w=colored Example of Epistasis - Fruit Color in Squash

Fruit Color in Squash cont.) At the second gene, yellow is dominant to green, and the symbols used are Y=yellow, y=green The presence of the dominant W allele masks the effect of either the Y or y alleles W_Y_ & W_yy give white (12) wwY_ is yellow (3) wwyy is green (1) 12:3:1 ratio

Another Example Of Epistasis BC bCBc bc 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 BC bC Bc bc 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 BBCcBbCc BBcc Bbcc bbcc bbCc BbCc BbCC bbCC BbCc bbCc BBCCBbCC BBCc BbCc 9 ⁄ 16 3 ⁄ 16 4 ⁄ 16 BbCc  Sperm Eggs

 AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCcAABBCC 20 ⁄ ⁄ 64 6 ⁄ 64 1 ⁄ 64 Fraction of progeny Quantitative variation usually indicates Polygenic Inheritance –An additive effect of two or more genes on a single phenotype –Skin color –Eye Color –Hair color

Nature and Nurture: The Environmental Impact on Phenotype Another departure from simple Mendelian genetics arises –When the phenotype for a character depends on environment as well as on genotype –May inherit genes for height, but not receive the needed nutrition to grow tall –Heart disease & cancer

The Norm Of Reaction –Is the phenotypic range of a particular genotype that is influenced by the environment (iron in soil affects color)

GENETIC PATTERNS IN HUMANS

Pedigree Analysis A pedigree –Is a family tree that describes the interrelationships of parents and children across generations –Male –Female

Inheritance patterns of particular traits Can be traced and described using pedigrees Ww ww Ww ww Ww ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ffFf ff Ff ff Ff FF or Ff ff FF or Ff Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

Pedigrees –Can also be used to make predictions about future offspring Recessively inherited disorders –Show up only in individuals homozygous for the allele (albino) Carriers –Are heterozygous individuals who carry the recessive allele but are phenotypically normal –Indicated by ½ shaded circle

Cystic Fibrosis Cystic fibrosis (CF) –Recessively inherited, defective gene –Carried by millions of people –Must inherit 2 genes (one from each parent) Symptoms of cystic fibrosis include –Mucus buildup in the some internal organs –Abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease Sickle-cell disease –Affects one out of 400 African-Americans –Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include –Physical weakness, pain, organ damage, and even paralysis

Mating of Close Relatives Matings between relatives –Can increase the probability of the appearance of a genetic disease –Hemophilia (free bleeders) once found in royal families that intermarried (female carriers) –Are called consanguineous matings –Prohibited in the United States

Dominantly Inherited Genetic Disorders One example is achondroplasia –A form of dwarfism that is lethal when homozygous for the dominant allele

Dominantly Inherited Genetic Disorders Huntington’s disease –Is a degenerative disease of the nervous system –Has no obvious phenotypic effects until about 35 to 40 years of age (adult onset)

Genetic Testing and Counseling Genetic counselors –Can provide information to prospective parents concerned about a family history for a specific disease –Genetic counselors help couples determine the odds that their children will have genetic disorders

Tests for Identifying Carriers For a growing number of diseases –Tests (1000’s) are available that identify carriers and help define the odds more accurately –Newborn screening (PKU) –Carrier screening (Tay-Sach) –Adult onset screeening (Huntington’s) –Estimating risk screening (Alzheimer’s)

Fetal Testing In amniocentesis –The liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS) –A sample of the placenta is removed and tested

Fetal Testing (a) Amniocentesis Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. (b) Chorionic villus sampling (CVS) Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Several weeks Biochemical tests Several hours Fetal cells Placenta Chorionic viIIi A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Suction tube Inserted through cervix Fetus Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping

Newborn Screening Some genetic disorders can be detected at birth –By simple tests that are now routinely performed in most hospitals in the United States –Phenylketonuria (PKU)