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Mendel and Genes Chapter 14. Fig. 14-1 The particulate hypothesis is the idea that parents pass on discrete heritable units (genes) The blending hypothesis.

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Presentation on theme: "Mendel and Genes Chapter 14. Fig. 14-1 The particulate hypothesis is the idea that parents pass on discrete heritable units (genes) The blending hypothesis."— Presentation transcript:

1 Mendel and Genes Chapter 14

2 Fig The particulate hypothesis is the idea that parents pass on discrete heritable units (genes) The blending hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)

3 How can we tell the genotype of an individual with the dominant phenotype? –The answer is to carry out a testcross:

4 Fig TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp? Predictions Recessive phenotype, known genotype: pp If PPIf Pp or Sperm ppp p P P P p Eggs Pp pp or All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white

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

6 The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities –Probability in an F 1 monohybrid cross can be determined using the multiplication rule Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele The Multiplication and Addition Rules Applied to Monohybrid Crosses Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

7 Fig Rr Segregation of alleles into eggs Sperm R R R R R R r r r r r r 1/21/2 1/21/2 1/21/2 1/21/2 Segregation of alleles into sperm Eggs 1/41/4 1/41/4 1/41/4 1/41/4

8 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 The rule of addition can be used to figure out the probability that an F 2 plant from a monohybrid cross will be heterozygous rather than homozygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

9 Fig Rr Segregation of alleles into eggs Sperm R R R R R R r r r r r r 1/21/2 1/21/2 1/21/2 1/21/2 Segregation of alleles into sperm Eggs 1/41/4 1/41/4 1/41/4 1/41/4

10 Summary of Mendels Principles The inheritance of biological characteristics is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring. In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. In most sexually reproducing organisms, each adult has two copies of each geneone from each parent. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another.

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

12 Beyond dominant/recessive incomplete dominance - situation in which one allele is not completely dominant over another There is no white no red: new phenotype pink Fig Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2

13 Beyond dominant and recessive alleles Codominance - situation in which both alleles of a gene contribute to the phenotype of the organism

14 Beyond dominant and recessive alleles multiple alleles - three or more alleles of the same gene

15 Fig IAIA IBIB i A B none (a) The three alleles for the ABO blood groups and their associated carbohydrates Allele Carbohydrate Genotype Red blood cell appearance Phenotype (blood group) I A I A or I A i A B I B I B or I B i IAIBIAIB AB iiO (b) Blood group genotypes and phenotypes

16 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 Height and Skin color in humans is an example of polygenic inheritance

17 Fig Eggs Sperm Phenotypes: Number of dark-skin alleles: / 64 6 / / / / 64 6 / 64 1 / 64 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 1/81/8 AaBbCc

18 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 Frequency of Dominant Alleles

19 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 –PKU –Sickle-cell disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20 Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus –For example, 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

21 Fig BbCc Sperm Eggs BCbC Bcbc BC bC Bc bc BBCC 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 BbCC BBCc BbCc BbCC bbCC BbCc bbCc BBCcBbCc bbCc BBccBbcc bbcc 9: 3 : 4

22 Nature and Nurture: The Environmental Impact on Phenotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity multifactorial Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig

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

24 Fig b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Widows peakNo widows peak (a) Is a widows peak a dominant or recessive trait? Wwww Ww ww Ww wwWW Ww or

25 Fig c Attached earlobe 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Free earlobe (b) Is an attached earlobe a dominant or recessive trait? Ff ffFf ff FFor FF Ff

26 (Start Next)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 (i.e., pigmented) 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

27 Fig Parents Normal Sperm Eggs Normal (carrier) Normal (carrier) Albino Aa A A AA Aa a aa a

28 Autosomal Disorders Genes for these disorders are located on autosomes –Recessive disorder –Dominant disorders –Codominant disorders

29 Cystic Fibrosis

30 Fig Eggs Parents Dwarf Normal Dwarf Sperm Dd dd d D Dd dd Dd d d

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

33 Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

34 Fig Amniotic fluid withdrawn Fetus Placenta Uterus Cervix Centrifugation Fluid Fetal cells Several hours Several weeks Several weeks (a) Amniocentesis (b) Chorionic villus sampling (CVS) Several hours Several hours Fetal cells Bio- chemical tests Karyotyping Placenta Chorionic villi Fetus Suction tube inserted through cervix

35 Karyotype Making a Karyotype –Photograph chromosom es during mitosis –Cut chromosom es out of photograph –Group them in order, in pairs –Male 46XY –Female 46XX

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37 Sex-Linked Genes Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.

38 Some disorders caused by recessive alleles on the X chromosome in humans: –Color blindness –Duchenne muscular dystrophy –Hemophilia Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

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


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