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Chromosomal Inheritance II

Chromosomal Inheritance II. Outline. Incomplete Dominance, Codominance , and Multiple Allelism Interaction of genes Pedigree Studies Genetics and Ethics. Extending Mendel’s Rules. Incomplete dominance heterozygotes have an intermediate phenotype Codominance

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Chromosomal Inheritance II

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  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. Carbohydrate Allele Phenotype (blood group) Red blood cell appearance Genotype Multiple Alleles and Polymorphism IA A B IB i none (a) The three alleles for the ABO blood groups and their associated carbohydrates • ABO blood group in humans are determined by three alleles : IA, IB, and i. IAIA or IA i A B IBIB or IB i AB IAIB ii O (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 HBB/hbb Malaria protection Mild sickle cell disease Healthy Unhealthy ???

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

  8. Fig. 14-12 Epistasis  BbCc BbCc Sperm • 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) 1/4 1/4 1/4 1/4 BC bC Bc bc Eggs 1/4 BC BBCc BBCC BbCC BbCc 1/4 bC bbCC bbCc BbCC BbCc 1/4 Bc BBcc Bbcc BBCc BbCc 1/4 bc BbCc bbCc Bbcc bbcc : 4 9 : 3

  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 aa bb cc (pure-line white) AA BB CC (pure-line red) F1 generation Aa Bb Cc (medium red) Self-fertilization F2 generation 20 15 15 6 6 1 1

  11. Polygenic Inheritance  AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 • 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 1/8 1/8 1/8 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 1/64 6/64 Number of dark-skin alleles: 2 6 0 3 4 5 1

  12. Relationship among genes Example Description Epistasis One gene affects the expression of another BbCc BbCc BC bC Bc bc BC bC Bc bc : 3 : 4 9 A single phenotypic character is affected by two or more genes Polygenic inheritance AaBbCc 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 dominant 2) Autosomal recessive 3) X-linked recessive 4) X-linked dominant 5) Y-linked • Pedigrees (family trees) • analyze the human crosses that already exist.

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

  15. 1st generation (grandparents) Ww Ww ww ww 2nd generation (parents, aunts, and uncles) Fig. 14-15b Ww Ww ww ww Ww ww 3rd generation (two sisters) WW ww or Ww Widow’s peak No widow’s peak Is a widow’s peak a dominant or recessive trait?

  16. Autosomal Recessive Traits I Carrier male Carrier female Carriers (heterozygotes) are indicated with half-filled symbols • 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. Each row represents a generation II Affected male III Affected female IV

  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. I Prince Albert Queen Victoria Female carrier of hemophilia allele II Affected male III IV

  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_fiyh_E&feature=related

  21. Eugenics • 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

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

  23. Eugenics and the Third Reich • 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

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