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Independent Assortment. Mendel’s Law of Independent Assortment only works for genes whose loci are on different chromosomes. Genes on Same Chromosome. Linked genes. Genes on Same Chromosome. Example of genetic linkage
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Independent Assortment • Mendel’s Law of Independent Assortment only works for genes whose loci are on different chromosomes
Genes on Same Chromosome • Linked genes
Genes on Same Chromosome • Example of genetic linkage • Flower color and pollen shape are on same chromosome in sweet peas • Gene assignments • Let P = purple flowers and p = red flowers • Let L = long pollen shape and l = round shape
Recombination • Genes on same chromosome do not always sort together • Crossing over in Prophase I of meiosis creates new gene combinations • Crossing over involves the exchange of DNA between chromatids of paired homologous chromosomes
Sex Chromosomes and Autosomes • Autosomes • Sex Chromosomes • XX (x) XY
Sex-Linked Genes • Genes carried on one sex chromosome are sex-linked • X and Y chromosomes have very few genes in common
How Sex-Linkage Affects Inheritance • Eye color genes were found to be carried by the X chromosome • R = red eyes (dominant) • r = white eyes (recessive) http://www.taxodros.uzh.ch/
How Sex-Linkage Affects Inheritance • Sex-linked (specifically X-linked) recessive alleles displayed their phenotype more often in males • Males showed recessive white-eyed phenotype more often than females in an XRXrxXRY cross
Incomplete Dominance • Incomplete Dominance • When the heterozygous phenotype is intermediate between the two homozygous phenotypes
Multiple Alleles • A species may have more than 2alleles for a given characteristic • Each individual still carries 2alleles for this characteristic • Ex. Fruit fly eye color • Ex. A, B, O blood type
Codominance • Heterozygotes display phenotypes of both the homozygote phenotypes in codominance • Example: Human blood group alleles
Polygenic Inheritance • Are governed by the interaction of more than 2 genes at multiple loci • Examples include human height, skin color, and body build, and grain color in wheat
Epigenetics • Environment can influence how genes are expressed • Example: Himalayan rabbit
Recessive Genetic Disorders • New alleles produced by mutation usually code for non-functional proteins • Alleles coding for non-functional proteins are recessive to those coding for functional ones
Recessive Genetic Disorders • Heterozygous individuals are carriersof a recessive genetic trait • (but otherwise have a normal phenotype) • Homozygous recessive genes express the defective phenotype
Albinism • Melanin is the dark pigment that colors skin cells • Melanin is produced by the enzyme tyrosinase (TYR) • Mutant TYR allele encodes a defective tyrosinase protein in skin cells, producing no melanin
Albinism • Humans and other mammals who are homozygous for the mutant TYRhave no skin, fur, or eye coloring (skin and hair appear white, eyes are pink)
Sickle-Cell Anemia • Hemoglobin- oxygen-transporting protein found in red blood cells • Mutant hemoglobin gene causes hemoglobin molecules in blood cells to clump together • Red blood cells take on a sickle (crescent) shape and easily break
Dominant Genetic Disorders • Dominant disease alleles disrupt normal cell function in a variety of ways • Produce an abnormal protein that interferes with the function of the normal one • Encode toxic proteins • Encode a protein that is overactive or active at inappropriate times and places
Sex-Linked Genetic Disorders • Several defective alleles for characteristics encoded on X chromosome • Sex-linked disorders appear more frequently in males and often skip generations • Ex. Red-green color blindness
Sex-Linked Genetic Disorders • Hemophilia (deficiency in blood clotting protein) • Hemophilia gene in Queen Victoria of England was passed among the royal families of Europe
Non-Disjunction • Incorrect separation of chromosomes or chromatids in meiosis known as non-disjunction
Abnormal Sex Chromosome Number • Non-disjunction of sex chromosomes in males or females produce abnormal numbers of X and Y chromosomes
Abnormal Sex Chromosome Number • Turner Syndrome (XO): an underdeveloped, infertile woman with only one X chromosome lucinafoundation.org
Abnormal Sex Chromosome Number • Trisomy X (XXX): a fertile, “normal” woman with an extra X chromosome
Abnormal Sex Chromosome Number • Kleinfelter Syndrome (XXY): an infertile man with an extra X chromosome, having partial breast development and small testes beliefnet.com
Abnormal Sex Chromosome Number • Jacob Syndrome, • XYY Male: a tall man with an extra Y that produces high levels of testosterone and may score lower on IQ tests biology.iupui.edu
Abnormal Autosome Number • Non-disjunction of autosomes can occur during meiosis in the father or mother • Frequency increases with age of parents
Abnormal Autosome Number • Trisomy 21 (Down Syndrome) • Individuals have 3copies of chromosome 21 • Characterized by distinctively shaped eyelids, among other physical features