YLinked Traits Only males have Y chromosomes Passed
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Y-Linked Traits • Only males have Y chromosomes • Passed from father to sons • All Y-linked traits are expressed • Approximately three dozen Ylinked traits have been discovered Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Pedigree for Y-Linked Trait Fig. 4. 24 Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Y-Linked Genes Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Mitochondrial Inheritance • Mitochondria are cytoplasmic organelles important in cellular respiration • Have their own DNA • Carry 37 genes • Transmitted from mother to ALL of her offspring • No recombination • Males and females equally affected • High mutation rate Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Pedigree of Mitochondrial Inheritance Fig. 4. 25 Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Mitochondrial Genes Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Complications in genetic analyses Pleiotropy One gene influences more than one phenotype/trait Mouse agouti gene Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Mouse agouti gene Genotype Phenotype AA agouti (dark gray) wild-type allele A YA yellow dominant coat color A YA Y lethal recessive lethal Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
The Manx cat tail viability Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Pleiotropy: Single gene can cause more than one phenotype Phenotype: red hair, pale skin, and freckles. A variant of a gene that encodes a protein that controls balance of pigments in the skin (melanocortin-1 -receptor) Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Pleiotropy: One Gene-More Than One Phenotype • Huntington’s: progressive dementia onset around 40 -50 years • involuntary movements • death in about 5 years after onset of disease Phenotype 1 Phenotype 2 Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Extension Mendel Each gene determines a single trait Pleiotropy: one gene influences more than one trait Different ratios depending on dominance/recessive relation for each trait Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Genetic heterogeneity: one phenotype: many genes ØAutosomal recessive heterogenetic traits : ØHearing Loss : 132 genes ØAlbinism: several different genes ØBlood clotting disorders Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Complications in genetic analyses The same genotype does not always produce the same phenotype • Phenotype depends on penetrance • Phenotype depends on expressivity • Phenotype can be affected by other modifier genes • Environment can affect phenotype Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Penetrance Not all individuals of a given genotype show the phenotype Penetrance = # showing phenotype -----------------Total of that genotype Neurofibromatosis: Autosomal Dominant • neurofibromas all over body Penetrance: 50 to 80% Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning %
Complications in genetic analyses The same genotype does not always produce the same phenotype • Phenotype depends on penetrance • Phenotype depends on expressivity • Phenotype can be affected by other modifier genes • Environment can affect phenotype Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Expressivity refers to the degree to which an individual expresses the trait • The severity of phenotype can vary (extreme to mild) in individuals of the same genotype Retinoblastoma: An example of variable expressivity • 25% are unaffected • Among the 75% affected individuals, some are affected only in one eye while, others are affected in both eyes • Affecting both penetrance and expressivity Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Penetrance Expressivity Penetrance and Expressivity Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Complications in genetic analyses The same genotype does not always produce the same phenotype • Phenotype depends on penetrance • Phenotype depends on expressivity • Phenotype can be affected by other modifier genes • Environment can affect phenotype Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Phenotype can be affected by other modifier genes • Modifier genes alter the phenotype produced by other genes. • Modifier genes by themselves may have little or no effect on the phenotype. Bb. Dd cross Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Complications in genetic analyses The same genotype does not always produce the same phenotype • Phenotype depends on penetrance • Phenotype depends on expressivity • Phenotype can be affected by other modifier genes • Environment can affect phenotype Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
Phenylketonurea (PKU) Caused by a mutation in an enzyme: Phenylalanine Hydroxylase Phenylalanine Tyrosine reduced accumulates Melanin Adrenaline Phenylpyruvic acid Affects the Central Nervous System Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning reduced
Phenylketoneurea (PKU) • 1/12, 000 live births • Affected individual is retarded, has very fair skin and blue eyes (melanin reduction), also has less adrenaline. • In US every newborn is screened for PKU • Affected individual is put on a diet limited in phenylalanine • Allows nearly normal development • Environmental change- restricted diet can result in a profound change in phenotype Chapter 4 Human Heredity by Michael Cummings © 2006 Brooks/Cole-Thomson Learning
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