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Genes, Chromosomes, and Human Genetics. Chapter 13. Why It Matters. Progeria. 13.1 Genetic Linkage and Recombination. The principles of linkage and recombination were determined with Drosophila Recombination frequency can be used to map chromosomes
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Genes, Chromosomes, and Human Genetics Chapter 13
Why It Matters • Progeria
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
Chromosomes • Genes • Sequences of nucleotides in DNA • Arranged linearly in chromosomes
Linked Genes • Genes carried on the same chromosome • Linked during transmission from parent to offspring • Inherited like single genes • Recombination can break linkage
Drosophila melanogaster • Fruit fly • Model organism for animal genetics • Compared to Mendel’s peas • Used to test linkage and recombination
Gene Symbolism • Normal alleles (wild-type) • Usually most common allele • Designated by “+” symbol • Usually dominant Wild-typeMutant + = red eyes pr = purple + = normal wings vg = vestigial wings
Genetic Recombination • Alleles linked on same chromosome exchange segments between homologous chromosomes • Exchanges occur while homologous chromosomes pair during prophase I of meiosis
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
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
Recombination Occurs Often • Widely separated linked genes often recombine • Seem to assort independently • Detected by testing linkage to genes between them
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
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
Sex Chromosomes • Sex chromosomes determine gender • X and Y chromosomes in many species • XX: female • XY: male • Other chromosomes are called autosomes
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
Sex Linkage • Female (XX): 2 copies of X-linked alleles • Male (XY): 1 copy of X-linked alleles • Only males have Y-linked alleles
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
Eye Color Phenotypes in Drosophila • Normal wild-type: red eye color • Mutant: white eye color
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
Inheritance of Hemophilia • In descendents of Queen Victoria of England
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
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
Calico Cats • Heterozygote female (no male calico cats)
Barr Body • Tightly coiled condensed X chromosome • Attached to side of nucleus • Copied during mitosis but always remains inactive
13.3 Chromosomal Alterations That Affect Inheritance • Most common chromosomal alterations: deletions, duplications, translocations, and inversions • Number of entire chromosomes may also change
Chromosomal Alterations (1) • Deletion: broken segment lost from chromosome • Duplication: broken segment inserted into homologous chromosome
Chromosomal Alterations (2) • Translocation: broken segment attached to nonhomologous chromosome • Inversion: broken segment reattached in reversed orientation
Nondisjunction (1) • Failure of homologous pair separation during Meiosis I
Nondisjunction (2) • Failure of chromatid separation during Meiosis II
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
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
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
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
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
Modes of Inheritance • Autosomal recessive inheritance • Autosomal dominant inheritance • X-linked recessive inheritance
Autosomal Recessive Inheritance • Males or females carry a recessive allele on an autosome • Heterozygote • Carrier • No symptoms • Homozygote recessive • Shows symptoms of trait
Autosomal Dominant Inheritance • Dominant gene is carried on an autosome • Homozygote dominant or heterozygote • Show symptoms of the trait • Homozygote recessive • Normal
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
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
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
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
Cytoplasmic Inheritance • Genes carried on DNA in mitochondria or chloroplasts • Cytoplasmic inheritance follows the maternal line • Zygote’s cytoplasm originates from egg cell
Cytoplasmic Inheritance • Mutant alleles in organelle DNA • Mendelian inheritance not followed (no segregation by meiosis) • Uniparental inheritance from female
Cytoplasmic Inheritance • Inheritance of variegation in Mirabalis
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