Chapter 11 CHROMOSOMES A. What Is a Chromosome? A long, continuous strand of DNA, plus several types of associated proteins, and RNA. Chromosome pairs are distinguished by: size, banding pattern & centromere position. B. Linked Genes Genes carried on the same chromosome. Linked genes violate Mendels law of independent assortment because they may not separate during crossing-over of meiosis I. What types of gametes are expected from this individual? PL pl Pl pL What types of gametes are expected from this individual? PL pl The further apart two linked genes are, the more likely they will separate during gamete formation. Parental gametes retain the gene combinations from the parents. Recombinant gametes result from the mixing of maternal & paternal alleles during crossing- over. Closely linked genes yield few recombinant chromosomes - will NOT obtain expected 9:3:3:1 phenotypic ratio. Knowing allele arrangement is important in predicting trait transmission. Ex. Two allele combinations are possible for a pea plant with genotype PpLl. ] Alleles in coupling tend to be transmitted together. ] Alleles in repulsion separate with each generation. C. Sex Determination Mechanism by which an individual develops as a male or a female. 1. Total chromosome number is diploid (develops from a fertilized ovum) is haploid (develops from an unfertilized ovum) Ex. bees 2. X-O System (number of X chromosomes determines sex) is XX is XO Ex. grasshoppers, crickets & roaches 3. X-Y System (presence of Y chromosome determines sex) SRY gene is XX is XY Ex. all mammals In X-Y system, determines sex of offspring. D. Inheritance of Sex-Linked Traits Most sex-linked traits are carried on the X chromosome (X-linked) & are recessive. Ex. colorblindness, hemophilia ] more common in ] cannot be a carrier ( is hemizygous) ] inherits condition from his mother, NOT his father Hemophilia: recessive X-linked trait GenotypePhenotype X H X H non-carrier X H X h carrier X h X h with hemophilia X H Ynormal X h Y with hemophilia What is the probability that a carrier and a normal will have a son with hemophilia? or 25% What is the probability that a non- carrier and a hemophiliac will have a son with hemophilia? ZERO E. X Inactivation Female mammals have 2 alleles for every gene on the X chromosome, while males have only 1. This inequality is balanced by turning off one X chromosome in each cell of a 3 week old embryo. F some cells turn off paternal X F some cells turn off maternal X Inactivated X appears as a dark- staining structure called a Barr body. How many Barr bodies would cells of a male possess? X inactivation is responsible for the appearance of calico cats. The earlier X inactivation occurs, the larger the patches. F. Chromosome Abnormalities 1. Polyploidy - extra full sets of chromosomes. F animal polyploids spontaneously abort or die shortly after birth F plant polyploids are relatively common (wheat, lilies) 2. Aneuploidy - an extra (trisomy) or missing (monosomy) chromosome. Aneuploidy is usually due to a meiotic error called nondisjunction. ] Autosomal aneuploids F trisomy 13 F trisomy 18 F trisomy 21 (Down syndrome) ] Sex chromosome aneuploids F Turner syndromeXO F Triplo-X XXX F Klinefelter syndromeXXY F Jacobs syndromeXYY 3. Deletion - part of a chromosome is missing. 4. Duplication - part of a chromosome is present twice. 5. Inversion - part of a chromosome is reversed. 6. Translocation - nonhomologous chromosomes exchange parts (reciprocal translocation) or combine (Robertsonian translocation).