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Chromosomal Inheritance II
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Outline Incomplete Dominance, Codominance, and Multiple Allelism
Interaction of genes Pedigree Studies Genetics and Ethics
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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
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Multiple Alleles and Polymorphism
IA IB 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) IAIA or IA i IBIB or IB i IAIB AB ii O (b) Blood group genotypes and phenotypes Multiple Alleles and Polymorphism ABO blood group in humans are determined by three alleles : IA, IB, and i.
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Pleiotropy A gene that influences many traits rather than just one is pleiotropic. Marfan Syndrome (FBN1): defective fibrillinlimbs, spinal chord, heart Cystic fibrosis (CFTR): defective salt transportlungs, pancreas, sebacious glands, etc. Lung(s) pancreas healthy CF
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Antagonistic pleiotropy
Some effects are good; some are bad Sickle cell anemia (hemoglobin B) Codominant trait HBB/HBB; HBB/hbb; hbb/hbb HBB/hbb Mild sickle cell disease Malaria protection Healthy ??? Unhealthy
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Heterozygous phenotype same as that of homo- zygous dominant PP Pp
Fig. 14-UN2 Degree of dominance Description Example Complete dominance of one allele Heterozygous phenotype same as that of homo- zygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes CRCR CRCW 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 Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease
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Fig Epistasis 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) BbCc BbCc Sperm 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 Figure An example of epistasis BBCc BbCc BBcc Bbcc 1/4 bc BbCc bbCc Bbcc bbcc 9 : 3 : 4
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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
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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 F1 F2 1 6 15 20 aa bb cc (pure-line white) AA BB CC (pure-line red) Aa Bb Cc (medium red) Self-fertilization
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Polygenic Inheritance
Eggs Sperm Phenotypes: Number of dark-skin alleles: 1 2 3 4 5 6 1/64 6/64 15/64 20/64 1/8 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
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Relationship among genes Description Example Epistasis One gene affects the expression of another BbCc BbCc BC bC Bc bc BC bC Bc bc 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc
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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.
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Human Pedigree Reports
Key Male Affected male Mating Offspring, in birth order (first-born on left) Female Affected female
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Is a widow’s peak a dominant or recessive trait?
Fig b 1st generation (grandparents) Ww ww ww Ww 2nd generation (parents, aunts, and uncles) 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?
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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 male Carrier female Affected male female I II III IV Each row represents a generation Carriers (heterozygotes) are indicated with half-filled symbols
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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
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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
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What are the Societal Implications of this Knowledge?
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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
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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
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US Propaganda and Policy
The Immigration Act of 1924 quota for different nationalities perceived tendencies towards crime etc. Forced Sterilization
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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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