genes, chromosomes, and human genetics chapter 13
TRANSCRIPT
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Genes, Chromosomes, and Human Genetics
Chapter 13
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Why It Matters
Progeria
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13.1 Genetic Linkage and Recombination
The principles of linkage and recombination were determined with Drosophila
Recombination frequency can be used to map chromosomes
Widely separated linked genes assort independently
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Chromosomes
Genes• Sequences of nucleotides in DNA• Arranged linearly in chromosomes
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Linked Genes
Genes carried on the same chromosome• Linked during transmission from parent to
offspring• Inherited like single genes• Recombination can break linkage
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Drosophila melanogaster
Fruit fly• Model organism for animal genetics• Compared to Mendel’s peas• Used to test linkage and recombination
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Gene Symbolism
Normal alleles (wild-type)• Usually most common allele• Designated by “+” symbol• Usually dominant
Wild-type Mutant+ = red eyes pr = purple
+ = normal wings vg = vestigial wings
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Genetic Recombination
Alleles linked on same chromosome exchange segments between homologous chromosomes
Exchanges occur while homologous chromosomes pair during prophase I of meiosis
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Recombination Frequency
Amount of recombination between two genes reflects the distance between them
The greater the distance, the greater the recombination frequency • Greater chance of crossover between genes
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Chromosome Maps
Recombination frequencies used to determine relative locations on a chromosome
Linkage map for genes a, b, and c:
1 map unit = 1% recombination = 1 centimorgan
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Recombination Occurs Often
Widely separated linked genes often recombine• Seem to assort
independently• Detected by testing
linkage to genes between them
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13.2 Sex-Linked Genes
In both humans and fruit flies, females are XX, males are XY
Human sex determination depends on the Y chromosome
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13.2 (cont.)
Sex-linked genes were first discovered in Drosophila
Sex-linked genes in humans are inherited as they are in Drosophila
Inactivation of one X chromosome evens out gene effects in mammalian females
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Sex Chromosomes
Sex chromosomes determine gender• X and Y chromosomes in many species• XX: female• XY: male
Other chromosomes are called autosomes
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Human Sex Chromosomes
Human X chromosome• Large (2,350 genes)• Many X-linked genes are nonsexual traits
Human Y chromosome• Small (few genes)• Very few match genes on X chromosome• Contains SRY gene• Regulates expression of genes that trigger male
development
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Sex Linkage
Female (XX): 2 copies of X-linked alleles
Male (XY): 1 copy of X-linked alleles
Only males have Y-linked alleles
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Sex Linkage
Males have only one X chromosome• One copy of a recessive allele results in
expression of the trait
Females have two X chromosomes• Heterozygote: recessive allele hidden (carrier)• Homozygote recessive: trait expressed
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Eye Color Phenotypes in Drosophila
Normal wild-type: red eye color Mutant: white eye color
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Human Sex-Linked Genes
Pedigree chart show genotypes and phenotypes in a family’s past generations
X-linked recessive traits more common in males• Red-green color blindness• Hemophilia: defective blood clotting protein
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Inheritance of Hemophilia
In descendents of Queen Victoria of England
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X Inactivation (1)
Dosage compensation • In female mammals, inactivation of one X
chromosome makes the dosage of X-linked genes the same as males
Occurs during embryonic development
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X Inactivation (2)
Random inactivation of either X chromosome
Same X chromosome inactivated in all descendents of a cell
Results in patches of cells with different active X chromosomes
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Calico Cats
Heterozygote female (no male calico cats)
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Barr Body
Tightly coiled condensed X chromosome
Attached to side of nucleus
Copied during mitosis but always remains inactive
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13.3 Chromosomal Alterations That Affect Inheritance
Most common chromosomal alterations: deletions, duplications, translocations, and inversions
Number of entire chromosomes may also change
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Chromosomal Alterations (1)
Deletion: broken segment lost from chromosome
Duplication: broken segment inserted into homologous chromosome
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Chromosomal Alterations (2)
Translocation: broken segment attached to nonhomologous chromosome
Inversion: broken segment reattached in reversed orientation
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Nondisjunction (1)
Failure of homologous pair separation during Meiosis I
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Nondisjunction (2)
Failure of chromatid separation during Meiosis II
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Changes in Chromosome Number
Euploids • Normal number of chromosomes
Aneuploids• Extra or missing chromosomes
Polyploids • Extra sets of chromosomes (triploids, tetraploids)• Spindle fails during mitosis
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Aneuploids
Abnormalities usually prevent embryo development
Exception in humans is Down syndrome• Three copies of chromosome 21 (trisomy 21)• Physical and learning difficulties• Frequency of nondisjunction increases as women
age
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Polyploids
Common in plants• Polyploids often hardier and more successful• Source of variability in plant evolution
Uncommon in animals• Usually has lethal effects during embryonic
development
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13.4 Human Genetics and Genetic Counseling
In autosomal recessive inheritance, heterozygotes are carriers and homozygous recessives are affected by the trait
In autosomal dominant inheritance, only homozygous recessives are unaffected
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13.4 (cont.)
Males are more likely to be affected by X-linked recessive traits
Human genetic disorders can be predicted, and many can be treated
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Modes of Inheritance
Autosomal recessive inheritance
Autosomal dominant inheritance
X-linked recessive inheritance
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Autosomal Recessive Inheritance
Males or females carry a recessive allele on an autosome
Heterozygote• Carrier• No symptoms
Homozygote recessive• Shows symptoms of trait
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Autosomal Dominant Inheritance
Dominant gene is carried on an autosome
Homozygote dominant or heterozygote• Show symptoms of the trait
Homozygote recessive• Normal
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X-Linked Recessive Inheritance
Recessive allele carried on X chromosome
Males• Recessive allele on X chromosome• Show symptoms
Females • Heterozygous carriers, no symptoms• Homozygous, show symptoms
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Genetic Counseling
Identification of parental genotypes• Construction of family pedigrees• Prenatal diagnosis
Allows prospective parents to reach an informed decision about having a child or continuing a pregnancy
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Genetic Counseling Techniques
Prenatal diagnosis tests cells for mutant alleles or chromosomal alterations
Cells obtained from:• Embryo• Amniotic fluid around embryo (amniocentesis)• Placenta (chorionic villus sampling)
Postnatal genetic screening• Biochemical and molecular tests
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13.5 Nontraditional Patterns of Inheritance
Cytoplasmic inheritance follows the pattern of inheritance of mitochondria or chloroplasts
In genomic imprinting, the allele inherited from one of the parents is expressed while the other allele is silent
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Cytoplasmic Inheritance
Genes carried on DNA in mitochondria or chloroplasts
Cytoplasmic inheritance follows the maternal line • Zygote’s cytoplasm originates from egg cell
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Cytoplasmic Inheritance
Mutant alleles in organelle DNA• Mendelian inheritance not followed (no
segregation by meiosis)• Uniparental inheritance from female
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Cytoplasmic Inheritance
Inheritance of variegation in Mirabalis
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Genomic Imprinting
Expression of an allele is determined by the parent that contributed it • Only one allele (from either father or mother) is
expressed
Other allele is turned off (silenced)• Often, result of methylation of region adjacent to
gene responsible for trait