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 How the inheritance of a trait is correlated with sex chromosomes? T.H. Morgan isolated a mutant white-eyed Drosophila Normal red-eyed female X Mutant white- eyed male gave a F 1 generation of all red eyes Morgan concluded that red eyes are dominant

4 How the inheritance of a trait is correlated with sex chromosomes? 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

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, for example grasshoppers etc. 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 (XX and XY are called as allosomes) show inheritance patterns different than those of genes on autosomes (22 pairs or 44 chromosomes).

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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 1X Females heterozygous for genes on the X chromosome (sex linked trait) are genetic mosaics In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body

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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 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.

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

14 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) =

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16 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

17 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

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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.

20 Human Genetic Disorder – Sickle cell anemia 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)

21 Nondisjunction Some genetic disorders are caused by a change in the number of chromosomes. Nondisjunction – Failure of homologues or sister chromatids to separate properly during meiosis. Aneuploidy - Creation of gametes having many (gain) or few (loss) chromosomes

22 Trisomic- (gain of one copy of autosome) Monosomic - (loss of one copy of autosome) Down syndrome is trisomy of chromosome 21

23 Nondisjunction of sex chromosomes Nondisjunction of sex chromosomes can result in: XXX triple-X females XXY males (Klinefelter syndrome) XO females (Turner syndrome) OY nonviable zygotes XYY males (Jacob syndrome)

24 Genomic Imprinting 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 Angelman syndrome if it’s from the mother

25 Genetic Counseling Genetic counseling is the process which identifies parents at risk for having children with genetic defects -Can assess genetic state of early embryos -Uses pedigree analysis to determine probability of genetic disorders in offspring. - Some genetic disorders can be diagnosed during pregnancy via amniocentesis and chorionic villi sampling

26 Aminocentesis Amniocentesis - collects fetal cells from the amniotic fluid for examination

27 Chorionic villi sampling Chorionic villi sampling collects cells from the placenta for examination

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