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

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Presentation on theme: "Fig. 14-1. Fig. 14-2a Stamens Carpel Parental generation (P) TECHNIQUE 1 2 3 4."— Presentation transcript:

1 Fig. 14-1

2 Fig. 14-2a Stamens Carpel Parental generation (P) TECHNIQUE

3 Monohybrid Cross for Flower Color Parentals X Purple White 100% Purple F 1 Generation Purple and White Flowered Plants F 2 Generation X 705 purple and 224 white 3:1 Ratio 1.Parents differ for one trait 2.Parents are true breeding 3.Analyze parents for two generations, F1 and F2 4.Examine large number of progeny

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5 Mendel’s Postulates (based on monohybrid crosses) Alternative versions of genes (alleles) account for variation in characteristics Each individual has two alleles for each gene, one inherited from each parent If the individual has two different alleles, one allele is dominant to the other which is recessive The two alleles randomly segregate during gamete formation with 50% of the gametes having one allele and 50% having the other allele.

6 Important Terminology Genotype – the list of alleles an individual possesses. Phenotype – a measurable characteristic displayed by an individual Homozygote – an individual possessing two identical alleles for a gene Heterozygote – an individual possessing two different alleles for a gene

7 Fig Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers

8 Example testcross

9 Independent Assortment vs Linkage P1 F1F1 Genotype of F 1 Gametes YRYryRyr YRyr Parental Combination of Alleles Parental Only 50% Parental 50% Recombinant

10 Law of Independent Assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation Linked Independent Assort YRYryRyrYRyr

11 Dihybrid Testcross to Detect Independent Assort YYRR X yyrr YrRr X yyrr YRYryRyr yyRf Yyrr yyrr YyRr Eggs Sperm yr YR yr Yr yR Phenotypic ratio 1:1:1:1 Ratio of parental:Recombinant 1:1 Dihybrid

12 Independent Assortment and Meiosis

13 What is Mendel’s Law of independent assortment? A gene is distributed into gametes independently of how other genes are distributed. Take home message:

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

15 Variations on Mendel Allelic Interactions Sex Linkage Complete/Partial Linkage Organellular Genes Mendel focused on traits based clearly on dominant and recessive alleles. However, the expression pattern of genes for some traits are not so straight forward.

16 Allelic Interactions Complete Dominance Partial Dominance Codominance Multiple Alleles Pleiotropy Multigenic traits Environmental Interactions

17 Complete Dominance – the heterozygote is indistinguishable from one of the two homozygotes. Rh factor – a protein found on the surface of the blood cells in some people. Two Alleles – Rh + - possess the Rh factor – Rh - - lacks the Rh factor Heterozygote Rh +,Rh - - possess the Rh factor and has positive type blood.

18 Partial Dominance Hypercholesterolemia – defect in LDL receptor mediated endocytosis. Two alleles Hc+ - functional LDL receptor Hc- defective LDL receptor Homozygotes Hc-,Hc- super high serum cholesterol early onset of heart disease. Hc+,Hc+ normal serum cholesterol- typical risk of heart disease. Hc+,Hc- high serum cholesterol – high risks of heart disease.

19 Multiple Alleles – More than two alleles in the population, although any individual still has only two alleles. ABO Blood type – carbohydrate attached to surface protein of blood cells. Three Alleles – I A – A type blood – I B – B type blood – I O – O type blood Six genotypes HomozygotesHeterozygotes I A,I A I A,I B I B,I B I A,I O I O,I O I B,I O A type B type AB type O type Codominance – Heterozygote expresses both alleles (with no blending as in partial dominance) Two non-identical alleles of a gene are both expressed in heterozygotes, so neither is dominant or recessive. Co-dominance may occur in multiple allele systems.

20 Pleiotropy – one gene may influence multiple traits. Cystic Fibrosis Two Alleles CF – healthy allele –functional Cl transporter (dominant) cf – cystic fibrosis allele – non functional Cl transporter (recessive) Homozygotes for cf exhibit several symptoms - mucus filled lungs - kidney failure - digestive problems - salty sweat

21 Genes encoding products used throughout the body are the ones most likely to be pleiotropic. Mutations in the fibrillin gene cause a disorder called Marfan syndrome. Fibrillin (long fibers) impart elasticity to the heart skin, blood vessels, tendons and other parts of the body. In Marfan syndrome the fibrillin are defective or not present. The aorta is particularly affected, can rupture during exercise. Hard to diagnose, affected people are typically, tall, thin, loose jointed. 1 in 5000 Sergei Rachmaninoff

22 Marfan Syndrome. Become Aware Haris Charalambous, a 6’10’’ Centre, died during a light training session at Toledo University on 9 October He was 21 years of age. He began his basketball career at Manchester Magic at age 13 and left the club at 18 to take up his scholarship at Toledo. He was later diagnosed with Marfan Syndrome, which is a genetic disease associated with abnormally tall people. At the time, neither the club nor Toledo knew of this situation Syndrome are a tall thin physique, disproportionately long limbs, fingers and toes, flat feet, spinal curvature and abnormally shaped narrow chest.

23 Multigenic Traits – traits influenced by more than one gene. Epistasis – Alleles at one gene mask the expression of alleles at a second gene. Example – black/ brown gene of mice- albino gene B – black is dominantC – allows pigment (dominant) b – brown is recessivec – prevents pigment (recessive)

24 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 

25 Polygenic Inheritance – many genes influencing a single trait Results in a continuum of variation (height in humans, skin pigmentation in humans, Fat content of milk cows, yield of kernels in corn) # dark alleles /64 6/64 15/64 30/64 15/64 6/64 1/64

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27 Multifactoral traits – traits influenced by both the genetic and environment. Examples Flower color in hydrangea Skin pigmentation in humans Milk yield in cows

28 Sex-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed – A female needs two copies of the allele – A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

29 Sex-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed – A female needs two copies of the allele – A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

30 Fig P Generation F1F1 F2F2 All offspring had red eyes Sperm Eggs F1F1 F2F2 P Sperm Eggs   X X X Y CONCLUSION EXPERIMENT RESULTS w w w w w w w w w w ww w w w w w

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32 Reciprocal Crosses Fruit flies use XY sex chromosomes X-linked white eye gene R – normal red eyes (dominant) r - white eyes (recessive) White eye X Red eyeRed eye X White eye ♂ ♀ ♂ ♀ Assume flies came from true breeding cultures

33 White eye X Red eyeRed eye X White eye ♂ ♀ ♂ ♀ rY RRRYrr RrRY RrRY Sperm Eggs r Y RRRR 50% Red female 50% Red male RrrY RrrY Sperm Eggs R Y rrrr 50% Red female 50% White male

34 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 Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left)

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

36 Pedigrees can also be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

37 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

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

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

40 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 Symptoms include 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

41 Sickle-Cell Disease 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 Symptoms include physical weakness, pain, organ damage, and even paralysis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

43 Fig Eggs Parents Dwarf Normal Dwarf Sperm Dd  dd d D Dd dd Dd d d

44 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 Huntington’s Disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

45 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

46 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 For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Tests for Identifying Carriers

47 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

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


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