1 Chromosomes, Mapping, and the Meiosis-Inheritance Connection Chapter 13

2 Chromosome Theory Chromosomal theory of inheritance - developed in 1902 by Walter Sutton - proposed that genes are present on chromosomes - based on observations that homologous chromosomes pair with each other during meiosis - supporting evidence was provided by work with fruit flies

3 Chromosome Theory T.H. Morgan isolated a mutant white-eyed Drosophila red-eyed female X white-eyed male gave a F 1 generation of all red eyes Morgan concluded that red eyes are dominant Parental generation male Parental generation female F 1 progeny all had red eyes

4 Chromosome Theory Morgan crossed F 1 females X F 1 males F 2 generation contained red and white- eyed flies but all white-eyed flies were male testcross of a F 1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene is linked to the X chromosome Testcross F 1 generation female F 2 female progeny had red eyes, only males had white eyes The testcross revealed that white-eyed females are viable. Therefore eye color is linked to the X chromosome and absent from the Y chromosome F 1 generation male Parental generation male F 1 generation female

5 Sex Chromosomes Sex determination in Drosophila is based on the number of X chromosomes 2 X chromosomes = female 1 X and 1 Y chromosome = male Sex determination in humans is based on the presence of a Y chromosome 2 X chromosomes = female having a Y chromosome (XY) = male

6 Sex Chromosomes In many organisms, the Y chromosome is greatly reduced or inactive. genes on the X chromosome are present in only 1 copy in males sex-linked traits: controlled by genes present on the X chromosome Sex-linked traits show inheritance patterns different than those of genes on autosomes.

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8 Sex Chromosomes Dosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1. In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body. Females heterozygous for genes on the X chromosome are genetic mosaics.

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10 Chromosome Theory Exceptions Mitochondria and chloroplasts contain genes. Traits controlled by these genes do not follow the chromosomal theory of inheritance Genes from mitochondria and chloroplasts are often passed to the offspring by only one parent

11 Chromosome Theory Exceptions Maternal inheritance: uniparental (one- parent) inheritance from the mother The mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilization (Leber’s hereditary optic neuropathy) In plants, the chloroplasts are often inherited from the mother, although this is species dependent

12 Genetic Mapping Early geneticists realized that they could obtain information about the distance between genes on a chromosome. - this is genetic mapping This type of mapping is based on genetic recombination (crossing over) between genes.

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14 Genetic Mapping To determine the distance between genes: - dihybrid organisms are testcrossed - offspring resembling the dihybrid parent result from homologues that were not involved in the crossover - offspring resulting from a crossover are called recombinant progeny

15 Genetic Mapping The distance between genes is proportional to the frequency of recombination events. recombination recombinant progeny frequency total progeny 1% recombination = 1 map unit (m.u.) 1 map unit = 1 centimorgan (cM) =

16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parental male gametes 415 parental wild type (gray body normal wing) F 1 generation female possible gametes b + vg + b + b vg + vg 92 recombinant (gray body vestigial wing) b + b vgvg 88 recombinant (black body normal wing) bb vg + vg 405 parental mutant type (black body vestigial wing) 180 ÷ 1000 = 0.18 total recomnbinant offspring 18% recombinant frequency 18 cM between the two loci bb vgvg b + vg b vg + b vg b + b vg + vg F 1 generation Parental generation Cross-fertilization Testcross b + b + vg + vg + bb vgvg recessive allele (vestigial wings) vg recessive allele (black body) b dominant allele (gray body) b+b+ dominant allele (normal wings) vg +

17 Genetic Mapping Multiple crossovers between 2 genes can reduce the perceived genetic distance Progeny resulting from an even number of crossovers look like parental offspring

18 Genetic Mapping Determining the order of genes can be done with a three- point testcross The frequency of double crossovers is the product of the probabilities of each individual crossover Therefore, the classes of offspring with the lowest numbers represent the double crossovers and allow the gene order to be determined

19 Genetic Mapping Mapping genes in humans involves determining the recombination frequency between a gene and an anonymous marker Anonymous markers such as single nucleotide polymorphisms (SNPs) can be detected by molecular techniques.

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21 Human Genetic Disorders Some human genetic disorders are caused by altered proteins. The altered protein is encoded by a mutated DNA sequence The altered protein does not function correctly, causing a change to the phenotype The protein can be altered at only a single amino acid (e.g. sickle cell anemia)

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23 Human Genetic Disorders Some genetic disorders are caused by a change in the number of chromosomes. nondisjunction during meiosis can create gametes having one too many or one too few chromosomes fertilization of these gametes creates trisomic or monosomic individuals Down syndrome is trisomy of chromosome 21

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25 Human Genetic Disorders Nondisjunction of sex chromosomes can result in: XXX triple-X females may be taller but normal XXY males (Klinefelter syndrome) male with female body characteristics and may have mental development issues XO females (Turner syndrome) short stature sterile female, webbed neck, low-normal mental abilities, the sex organs do not mature during puberty OY nonviable zygotes XYY males (Jacob syndrome) normal

26 Human Genetic Disorders Genomic imprinting occurs when the phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring A specific partial deletion of chromosome 15 results in: Prader-Willi syndrome if the chromosome is from the father (obesity, short stature, respiratory distress, mild retardation….) Angelman syndrome if it’s from the mother (severe mental retardation, aggressive, hyperactivity, inappropriate laughter…)

27 Human Genetic Disorders Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring. Some genetic disorders can be diagnosed during pregnancy. amniocentesis collects fetal cells from the amniotic fluid for examination chorionic villi sampling collects cells from the placenta for examination

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