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Chromosomal Inheritance II. Outline Incomplete Dominance, Codominance, and Multiple Allelism Interaction of genes Pedigree Studies Genetics and Ethics.

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Presentation on theme: "Chromosomal Inheritance II. Outline Incomplete Dominance, Codominance, and Multiple Allelism Interaction of genes Pedigree Studies Genetics and Ethics."— Presentation transcript:

1 Chromosomal Inheritance II

2 Outline Incomplete Dominance, Codominance, and Multiple Allelism Interaction of genes Pedigree Studies Genetics and Ethics

3 Extending Mendel’s Rules Incomplete dominance – heterozygotes have an intermediate phenotype Codominance – Heterozygotes displays the phenotype of both alleles multiple allelism – Multiple distinct genes versions (i.e., alleles) are present in the population polymorphism – Multiple distinct phenotypes are present in a population

4 Multiple Alleles and Polymorphism ABO blood group in humans are determined by three alleles : I A, I B, and i. 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

5 Pleiotropy A gene that influences many traits rather than just one is pleiotropic. – Marfan Syndrome (FBN1): defective fibrillin  limbs, spinal chord, heart – Cystic fibrosis (CFTR): defective salt transport  lungs, pancreas, sebacious glands, etc. Lung(s) pancreas healthy CF

6 Antagonistic pleiotropy Some effects are good; some are bad Sickle cell anemia (hemoglobin B) – Codominant trait – HBB/HBB; HBB/hbb; hbb/hbb HealthyUnhealthy ??? HBB/hbb Mild sickle cell disease Malaria protection

7 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

8 Fig. 14-12 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  A gene at one locus alters the phenotypic expression of a gene at a second locus Coat color in mice – pigment color (B for black; b for brown) – Pigment deposit (C for color; c for no color) Epistasis

9 Discrete vs. Quantitative Traits Discrete traits. – seed color in peas—no intermediate phenotypes Quantitative traits – Traits that fall into a continuum Frequencies – form a bell-shaped curve (normal distribution) for a population. A phenotype distribution that forms a bell-shaped curve.Normal distribution—bell-shaped curve

10 Quantitative Traits Result from the Action of Many Genes Wheat kernel color is a quantitative trait.Hypothesis to explain inheritance of kernel color Parental generation F 1 generation F 2 generation 11 66 15 20 aa bb cc (pure-line white) AA BB CC (pure-line red) Aa Bb Cc (medium red) Self-fertilization

11 Eggs Sperm Phenotypes: Number of dark-skin alleles: 0 1 2 345 6 1 / 64 6 / 64 15 / 64 20 / 64 15 / 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  Polygenic Inheritance Traits that vary in the population along a continuum Additive effect of 2+ genes on a single phenotype Skin color in humans is an example of polygenic inheritance

12 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

13 Applying Mendel’s Rules to Humans Humans  terrible genetic models –Generation time is too long –Parents produce relatively few offspring –Breeding experiments are frowned upon Human disorders follow 5 patterns 1) Autosomal dominant2) Autosomal recessive 3) X-linked recessive4) X-linked dominant 5) Y-linked Pedigrees (family trees) – analyze the human crosses that already exist.

14 Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left) Human Pedigree Reports

15 Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) 3rd generation (two sisters) Widow’s peakNo widow’s peak Is a widow’s peak a dominant or recessive trait? Wwww Ww ww Ww wwWW Ww or

16 Autosomal Recessive Traits If a phenotype is due to an autosomal recessive allele – trait = homozygous – parents (w/o trait) = heterozygous carriers. Carriers carry the allele and transmit it even though they do not exhibit the phenotype. Carrier maleCarrier female Affected male Affected female I II III IV Each row represents a generation Carriers (heterozygotes) are indicated with half-filled symbols

17 Autosomal or Sex-Linked trait? Equally often in males and females – likely to be autosomal. Males more likely to have the trait – usually X-linked. Hemophilia is an example of an X-linked trait resulting from a recessive allele. Queen Victoria Prince Albert Female carrier of hemophilia allele I II III IV Affected male

18 Frequency of Dominant Alleles Not necessarily more common (NOT always “WT”) one baby out of 400 in the United States is born with extra fingers or toes Dominant allele; uncommon occurrence In this example, the recessive allele is far more prevalent than the population’s dominant allele

19 What are the Societal Implications of this Knowledge?

20 Fetal Testing Tests to determine in utero if a child has a disorder. 14th to 16th week of pregnancy Blood or amniocentesis Fetal tests can reveal a serious disorder Trisomy 21, 18, etc. Some testing after birth eg Type I diabetes http://www.youtube.com/watch?v=qA25_fiy h_E&feature=related

21 Science of “improving the genetic stock” of humans Old Testament Plato’s Republic ( description of the ideal society ) Francis Galton “National Eugenics Laboratory” Experimental studies of heredity Twins Karl Pearson The higher birth rate of the poor Supplant by "higher" races Eugenics

22 US Propaganda and Policy The Immigration Act of 1924 – quota for different nationalities – perceived tendencies towards crime etc. Forced Sterilization

23 Nazi Germany The Aryan Nation and the Holocaust Human races Ill-founded concept Populations with overlapping gene pools. No major difference in the genome sequence Eugenics and the Third Reich


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