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THE CHROMOSOMAL BASIS OF INHERITANCE 9.17 Chromosome behavior accounts for Mendel’s principles • Genes are located on chromosomes – Their behavior during meiosis accounts for inheritance patterns Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 9.4 Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes GENE LOCI P P a a B DOMINANT allele b RECESSIVE allele GENOTYPE: PP aa HOMOZYGOUS for the dominant allele HOMOZYGOUS for the recessive allele Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Bb HETEROZYGOUS Figure 9.4 • The chromosomal basis of Mendel’s principles Figure 9.17 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 9.18 Genes on the same chromosome tend to be inherited together • Certain genes are linked – They tend to be inherited together because they reside close together on the same chromosome Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.18 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 9.19 Crossing over produces new combinations of alleles • This produces gametes with recombinant chromosomes • The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A B a b a B A B a b Tetrad A b Crossing over Gametes Figure 9.19A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.19C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 9.20 Geneticists use crossover data to map genes • Crossing over is more likely to occur between genes that are farther apart – Recombination frequencies can be used to map the relative positions of genes on chromosomes Chromosome g c l 17% 9% Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 9.5% Figure 9.20B SEX CHROMOSOMES AND SEX-LINKED GENES 9.21 Chromosomes determine sex in many species • A human male has one X chromosome and one Y chromosome • A human female has two X chromosomes • Whether a sperm cell has an X or Y chromosome determines the sex of the offspring Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings (male) (female) Parents’ diploid cells X Y Male Sperm Egg Offspring (diploid) Figure 9.21A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Other systems of sex determination exist in other animals and plants – The X-O system – The Z-W system – Chromosome number Figure 9.21B-D Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 9.22 Sex-linked genes exhibit a unique pattern of inheritance • All genes on the sex chromosomes are said to be sex-linked – In many organisms, the X chromosome carries many genes unrelated to sex – Fruit fly eye color is a sex-linked characteristic Figure 9.22A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings – Their inheritance pattern reflects the fact that males have one X chromosome and females have two – These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait Female XRXR Male Xr Y XR Female XRXr Xr XRXr Male XRY XRY Xr XRXR XrXR XRY XrY R = red-eye allele r = white-eye allele Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Male XRXr XR XR Y Female XrY Xr XR Y Xr XRXr Xr Xr Y XRY XrY Figure 9.22B-D 9.23 Connection: Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles – Examples: hemophilia, red-green color blindness – These are mostly seen in males Figure 9.23A – A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • A high incidence of hemophilia has plagued the royal families of Europe Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.23B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings