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Chapter 14 Human Inheritance
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Variation in Human Skin Color
Fraternal twins Kian and Remee inherited different alleles of genes for skin color from their mixed-race parents, who must be heterozygous for those alleles
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14.1 Shades of Skin Like most human traits, skin color has a genetic basis Minor differences in alleles for melanin synthesis and deposition of melanosomes affect skin color Differences probably evolved as a balance between vitamin D production and protection against harmful UV radiation, which increases risk of birth defects
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Variation in Human Skin Color
More than 100 gene products are involved in melanin synthesis, and melanosome formation and deposition Light-skinned people of European descent carry a mutation in gene SLC24A5 that encodes a transport protein in melanosome membranes Figure 14.1 Variation in human skin color (right) begins with differences in alleles inherited from parents. Opposite, fraternal twin girls Kian and Remee, born in Both of the children’s grandmothers are of European descent, and have pale skin. Both of their grandfathers are of African descent, and have dark skin. The twins inherited different alleles of some of the genes that affect skin color from their mixed-race parents, who, given the appearance of their children, must be heterozygous for those alleles.
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All in the Family: Mixed Race Twins
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14.2 Human Genetic Analysis
Geneticists study inheritance patterns in humans by tracking genetic disorders and abnormalities through families A genetic abnormality is an uncommon version of a heritable trait that does not result in medical problems A genetic disorder is a heritable condition that sooner or later results in mild or severe medical problems
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Pedigrees Charting genetic connections with pedigrees reveals inheritance patterns of certain traits: Dominant and recessive alleles Alleles on autosomes or sex chromosomes Probability that a trait will recur a family or population pedigree Chart showing the pattern of inheritance of a trait through generations in a family
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Charting Pedigrees Standard symbols used in pedigrees
Figure Pedigrees
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Example: Polydactyly Polydactyly is characterized by extra fingers, toes, or both Black numbers on the pedigree = number of fingers on each hand Red numbers = number of toes on each foot
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Pedigree for Polydactyly
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Charting Pedigrees male female marriage/mating offspring
individual showing trait being studied sex not specified generation Figure Pedigrees Gene not expressed in this carrier. Fig. 14.2ab, p. 204
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Animation: Pedigree diagrams
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Pedigree for Huntington’s Disease
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Types of Genetic Variation
Single genes that follow Mendelian inheritance patterns govern more than 6,000 genetic abnormalities and disorders Most human traits are polygenic (influenced by multiple genes) and often have environmental factors as well Alleles that give rise to severe genetic disorders are rare
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Types of Genetic Variation
We will look at 6 main patterns of inheritance for genetic abnormalities and disorders: Autosomal dominant inheritance pattern Autosomal recessive inheritance pattern X-linked recessive inheritance pattern X-linked dominant inheritance pattern Changes in chromosome number Changes in chromosome structure
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Key Concepts Tracking Traits in Humans
Inheritance patterns in humans are determined by following traits through generations of family trees Types of traits followed in such studies include genetic abnormalities or syndromes associated with a genetic disorder
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ANIMATION: Human Sex Determination
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14.3 Autosomal Inheritance Patterns
An allele is inherited in an autosomal dominant pattern if the trait it specifies appears in homozygous and heterozygous people An allele is inherited in an autosomal recessive pattern if the trait it specifies appears only in homozygous people
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The Autosomal Dominant Pattern
An autosomal dominant trait appears in every generation. When one parent is heterozygous, and the other is homozygous recessive, each child has a 50% chance of inheriting the dominant allele and displaying the trait
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Autosomal Dominant Inheritance
A dominant allele (red) is fully expressed in heterozygous people
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Autosomal Dominant Inheritance
normal mother Autosomal Dominant Inheritance affected father meiosis and gamete formation Figure Autosomal dominant inheritance, in which a dominant allele (red) is fully expressed in heterozygous people. affected child normal child disorder-causing allele (dominant) Fig. 14.3, p. 206
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Autosomal Dominant Inheritance
disorder-causing allele (dominant) affected father normal mother Autosomal Dominant Inheritance meiosis and gamete formation affected child normal child Figure Autosomal dominant inheritance, in which a dominant allele (red) is fully expressed in heterozygous people. Stepped Art Fig. 14.3, p. 206
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ANIMATION: Autosomal-recessive inheritance
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Autosomal Dominant Disorders
Disorder Main Symptoms Achondroplasia One form of dwarfism Aniridia Defects of the eyes Camptodactyly Rigid, bent fingers Hypercholesterolemia High cholesterol level Huntington’s disease Degeneration of nervous system Marfan syndrome Abnormal connective tissue Polydactyly Extra fingers, toes, or both Progeria Drastic premature aging Neurofibromatosis Tumors of nervous system, skin
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Achondroplasia Achondroplasia interferes with formation of the embryonic skeleton Figure 14.4 Examples of autosomal dominant disorders. A Achondroplasia affects Ivy Broadhead (left), as well as her brother, father, and grandfather. B Five-year-old Megan is already showing symptoms of Hutchinson–Gilford progeria.
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Hutchinson–Gilford Progeria
A mutation causes defects in transcription, mitosis, and division Symptoms of premature aging begin before age two Figure 14.4 Examples of autosomal dominant disorders. A Achondroplasia affects Ivy Broadhead (left), as well as her brother, father, and grandfather. B Five-year-old Megan is already showing symptoms of Hutchinson–Gilford progeria.
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The Autosomal Recessive Pattern
An autosomal allele is inherited in a recessive pattern if it is expressed only in homozygous people, so recessive traits may skip generations People heterozygous for the allele are carriers – they have the allele but not the trait Each child of two carriers has a 25% chance of being homozygous and having the trait
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Autosomal Recessive Inheritance
Two parents who are carriers of a recessive autosomal allele (red) Each child has a 25% chance of being homozygous for the trait
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Autosomal Recessive Inheritance
carrier mother carrier father meiosis and gamete formation Figure Autosomal recessive inheritance. Only homozygous people show the trait associated with a recessive allele on an autosome. A In this example, both parents are carriers of a recessive autosomal allele (red). Each of their children has a 25 percent chance of being homozygous for it. B The albino phenotype is associated with recessive alleles that cause a deficiency in a melanin-producing enzyme. C Conner Hopf was diagnosed with Tay–Sachs disease, an autosomal recessive disorder, at age 7–1/2 months. He died before his second birthday. affected child carrier child normal child disorder-causing allele (recessive) A Fig. 14.5a, p. 207
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Autosomal Recessive Inheritance
disorder-causing allele (recessive) carrier father carrier mother meiosis and gamete formation Figure Autosomal recessive inheritance. Only homozygous people show the trait associated with a recessive allele on an autosome. A In this example, both parents are carriers of a recessive autosomal allele (red). Each of their children has a 25 percent chance of being homozygous for it. B The albino phenotype is associated with recessive alleles that cause a deficiency in a melanin-producing enzyme. C Conner Hopf was diagnosed with Tay–Sachs disease, an autosomal recessive disorder, at age 7–1/2 months. He died before his second birthday. normal child affected child carrier child Stepped Art Fig. 14.5a, p. 207
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Animation: Autosomal-dominant inheritance
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Autosomal Recessive Disorders
Disorder Main Symptoms Albinism Absence of pigmentation Methemoglobinemia Blue skin coloration Cystic fibrosis Abnormal glandular secretions leading to tissue and organ damage Ellis–van Creveld Dwarfism, heart defects, polydactyly Fanconi anemia Abnormalities, bone marrow failure Galactosemia Brain, liver, eye damage Hemochromatosis Iron overload , joint & organ damage Phenylketonuria (PKU) Mental impairment Sickle-cell anemia Adverse pleiotropic effects Tay–Sachs disease Deterioration of mental and physical abilities; early death
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Albinism Albinism, a lack of melanin, occurs in people homozygous for recessive alleles that code for a defective form of the enzyme tyrosinase Figure Autosomal recessive inheritance. Only homozygous people show the trait associated with a recessive allele on an autosome. A In this example, both parents are carriers of a recessive autosomal allele (red). Each of their children has a 25 percent chance of being homozygous for it. B The albino phenotype is associated with recessive alleles that cause a deficiency in a melanin-producing enzyme. C Conner Hopf was diagnosed with Tay–Sachs disease, an autosomal recessive disorder, at age 7–1/2 months. He died before his second birthday.
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Tay–Sachs Disease Mutations cause gangliosides to accumulate to toxic levels in nerve cells Affected children, such as Conner Hopf, die before age five Figure Autosomal recessive inheritance. Only homozygous people show the trait associated with a recessive allele on an autosome. A In this example, both parents are carriers of a recessive autosomal allele (red). Each of their children has a 25 percent chance of being homozygous for it. B The albino phenotype is associated with recessive alleles that cause a deficiency in a melanin-producing enzyme. C Conner Hopf was diagnosed with Tay–Sachs disease, an autosomal recessive disorder, at age 7–1/2 months. He died before his second birthday.
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Key Concepts Autosomal Inheritance
Many human traits can be traced to dominant or recessive alleles on autosomes These alleles are inherited in characteristic patterns: dominant alleles tend to appear in every generation; recessive ones can skip generations
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14.4 X-Linked Inheritance Patterns
An allele is inherited in an X-linked pattern when it occurs on the X chromosome Most X-linked inheritance disorders are recessive, because X- linked dominant alleles tend to be lethal in male embryos
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XX and XY X-linked recessive disorders tend to appear in men more often than in women Men (XY) have only one X chromosome Women have two X chromosomes (XX), so they can be heterozygous for a recessive allele Men can transmit an X-linked allele to daughters, but not to sons – only a woman can pass an X-linked allele to a son
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X-Linked Recessive Inheritance
In this case, the mother carries a recessive allele on one of her two X chromosomes (red)
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X-Linked Recessive Inheritance
carrier mother normal father meiosis and gamete formation Figure X-linked recessive inheritance. In this case, the mother carries a recessive allele on one of her two X chromosomes (red). normal daughter or son carrier daughter affected son recessive allele on X chromosome Fig. 14.6, p. 208
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X-Linked Recessive Inheritance
recessive allele on X chromosome normal father carrier mother meiosis and gamete formation Figure X-linked recessive inheritance. In this case, the mother carries a recessive allele on one of her two X chromosomes (red). affected son normal daughter or son carrier daughter Stepped Art Fig. 14.6, p. 208
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Animation: X-linked inheritance
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X-Linked Recessive Disorders
Disorder Main Symptoms Androgen insensitivity XY individual but having some syndrome female traits; sterility Red-green color Inability to distinguish red from blindness green Hemophilia Impaired blood clotting ability Muscular dystrophies Progressive loss of muscle function X-linked anhidrotic Mosaic skin (patches with or without dysplasia sweat glands); other effects
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Red–Green Color Blindness
Most genes involved in proper function of pigment-containing receptors in the eyes are on the X chromosome Color blindness includes a range of conditions in which an individual cannot distinguish among some or all colors Some types of color blindness confuse red and green colors, others see green as shades of gray, but perceive blues and yellows quite well
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Red–Green Color Blindness
Figure 14.7 Color blindness. A View with red–green color blindness. The perception of blues and yellows is normal, but red and green appear similar. B Compare what a person with normal vision sees. Two Ishihara plates, which are standardized tests for color blindness. C You may have one form of red–green color blindness if you see the number 7 instead of 29 in this circle. D You may have another form if you see a 3 instead of an 8.
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Tests for Color Blindness
Figure 14.7 Color blindness. A View with red–green color blindness. The perception of blues and yellows is normal, but red and green appear similar. B Compare what a person with normal vision sees. Two Ishihara plates, which are standardized tests for color blindness. C You may have one form of red–green color blindness if you see the number 7 instead of 29 in this circle. D You may have another form if you see a 3 instead of an 8.
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Hemophilia A Hemophilia A, an X-linked recessive disorder that interferes with blood clotting, involves factor VIII, a protein product of a gene on the X chromosome In the 19th century, hemophilia was relatively common in royal families of Europe and Russia, probably because of the common practice of inbreeding
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Hemophilia in Descendants of Queen Victoria
Figure 14.8 A classic case of X-linked recessive inheritance: a partial pedigree of the descendants of Queen Victoria of England. At one time, the recessive X-linked allele that resulted in hemophilia was present in eighteen of Victoria’s sixty-nine descendants, who sometimes intermarried. Of the Russian royal family members shown, the mother (Alexandra Czarina Nicolas II) was a carrier.
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Duchenne Muscular Dystrophy
A gene on the X chromosome encodes dystrophin, a protein essential in muscle and nerve cells Boys with DMD are in a wheelchair by age 12, and die from a heart disorder or respiratory failure before age 30
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Key Concepts Sex-Linked Inheritance
The X chromosome holds about 10 percent of all human genes, so many traits are affected by alleles on this chromosome Inheritance patterns of such X-linked alleles tend to differ between males and females
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