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Mendel and the Gene Idea

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1 Mendel and the Gene Idea
CHAPTER 14

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

3 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

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

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

6 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

7 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

8 CROSSING PEA PLANTS Removed stamens from purple flower
1 5 4 3 2 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 offspring (F1) 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. RESULTS

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

10 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

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

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

13 Mendel also made sure that
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”

14 In a typical breeding experiment
Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents Are called the P1 generation Cross RR x rr yielded all Rr (hybrids)

15 The hybrid offspring of the P1 generation
Are called the F1 generation When F1 individuals self-pollinate The F2 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

16 The Law of Segregation Mendel derived the law of segregation
By following a single trait The F1 offspring produced in this cross Were monohybrids, heterozygous for one character

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

18 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

19 For each character An organism inherits two alleles, one from each parent A genetic locus is the location where it is at one a chromosome

20 The Testcross or backcross
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

21 then 1⁄2 offspring purple
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 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

22 The Law of Independent Assortment
Mendel derived the law of independent assortment By following two traits The F1 offspring produced in this cross Were dihybrids, heterozygous for both characters

23 Phenotypic ratio approximately 9:3:3:1
A dihybrid cross Produces four phenotypes in the F2 generation YYRR P Generation Gametes YR yr yyrr YyRr Hypothesis of dependent assortment independent F2 Generation (predicted offspring) 1⁄2 1 ⁄2 3 ⁄4 1 ⁄4 Sperm Eggs Phenotypic ratio 3:1 Yr yR 9 ⁄16 3 ⁄16 1 ⁄16 YYRr YyRR Yyrr YYrr yyRR yyRr Phenotypic ratio 9:3:3:1 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 F1 Generation 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 F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.

24 Using the information from a dihybrid cross, Mendel developed the law of independent assortment
Each pair of alleles lines up independently during meiosis.

25 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

26 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

27 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? Hornless and fancy tail? 0 x ½ = 0 Hornless and plain tail?

28 OTHER FACTORS AFFECTING INHERITANCE

29 Inheritance patterns are often more complex than predicted by simple Mendelian genetics
The inheritance of characters by a single gene may deviate from simple Mendelian patterns

30 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 Both alleles are expressed Human ABO blood type is an example of codominance

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

32 Incomplete Dominance The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties (blending). P Generation F1 Generation F2 Generation Red CRCR Gametes CR CW White CWCW Pink CRCW Sperm Cw 1⁄2 Eggs CR CR CR CW CW CW

33 Pleiotropy In Pleiotropy 1 gene has multiple phenotypic effects
Frizzle gene in chickens causes feathers to curl outward, abnormal body temperature, and greater digestive capacity, lay fewer eggs

34 Extending Mendelian Genetics for Two or More Genes
Polygenic inheritance May be determined by two or more genes (polygenic) Many human traits are polygenic In Epistasis A gene at one locus alters the phenotypic expression of a gene at a second locus

35 Example of Epistasis - Fruit Color in Squash
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

36 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

37 Another Example Of Epistasis
BC bC Bc bc 1⁄4 BBCc BbCc BBcc Bbcc bbcc bbCc BbCC bbCC BBCC 9⁄16 3⁄16 4⁄16 Sperm Eggs

38 Quantitative variation usually indicates Polygenic Inheritance
An additive effect of two or more genes on a single phenotype Skin color Eye Color Hair color AaBbCc aabbcc Aabbcc AaBbcc AABbCc AABBCc AABBCC 20⁄64 15⁄64 6⁄64 1⁄64 Fraction of progeny

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40 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 Simply put, you are a product of both your genes and your environment!

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

42 GENETIC PATTERNS IN HUMANS

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

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

45 Recessively inherited disorders
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

46 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 internal organs Abnormal absorption of nutrients in the small intestine

47 Sickle-Cell Disease Sickle-cell disease 1/400 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 *Heterozygous Advantage-higher frequency in malaria regions.

48 Mating of Close Relatives(Inbreeding)
Matings between genetically similar individuals 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

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

50 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)

51 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

52 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)

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

54 (b) Chorionic villus sampling (CVS)
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 hours 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 Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping

55 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)

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