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Do Now Problem Would it be possible for a mother with blood type A and a father with a blood type B to have a baby with blood type O? blood type A? Support your answers with a punnett square (for each one) and probability. Marked out of 8 (1 mark for yes/no for each scenario, 1 mark for each probability and 2 marks for each punnett) Answer Scenario 1 Yes! If both parents were heterozygous for the dominant traits (IAi and IBi respectively) it would be possible. IA Scenario 2 Yes! Mother would have to be either IAIA or IAi and the father would HAVE to be IBi. i IA i IB IAIB IBi IB IAIB IBi i IAi i IAi ii ii Sex-linked Chapter 19.1 Pages 628-631 Chromosomal Theory Walter Sutton and Theodor Boveri (1902) observed chromosomes came in pairs and segregated during meiosis. • chromosomes formed new pairs when the egg and sperm united. • this supported Mendel’s observations on inheritance and his “factors” became alleles of a gene. Chromosomal theory humans have 44 autosomal chromosomes and 2 sex chromosomes. • humans have thousands of different traits. • Sutton hypothesized that each chromosome carries multiple genes • genes that are located on the same chromosome are said to be linked genes. Chromosomal Theory the Chromosomal theory of inheritance: • chromosomes carry genes, the units of heredity • paired chromosomes segregate during meiosis. Each sex cell or gamete has half the number of chromosomes found in the somatic cells. This explains why each gamete has one of each of the paired alleles. Morgan’s Experiment studied the principles of inheritance using Drosophila melanogaster, fruit flies fruit flies a great animals to study because: • they reproduce rapidly (in 10 to 15 days) • offspring can mate shortly after leaving the egg • females produce over 100 eggs • they are small and easy to take care of. • males can be easily distinguished from females. • males have smaller-rounded abdomen, females have a pointed abdomen. Morgan’s Experiment Morgan’s Experiment Morgan observed a white-eyed phenotype and after a number of test crosses figured out it only occurred in males. • a sex-linked trait is one that is determined by genes located on the sex chromosomes Morgan first crossed a white eyed male with a red eyed female (red eyed being dominant) • all members of the F1 generation had red eyes Morgan then crossed to members of the F1 generation • he observed ¾ red eyes and ¼ whites eyes. Morgan’s Experiment • he noticed all the females had red eyes and the white eyed phenotype only appeared in the males. because the sex chromosomes in males are not homologous they contain different genes. Morgan concluded that the Y chromosome does not carry the gene to determine eye colour. • we now know the gene for eye colour in fruit flies is on the X chromosome. Morgan’s Experiment Punnett Squares for Sex Linked Inheritance F1 Generation ♂ Xr Y ♀ XR XR XRXr XRXr XRY XRY • 4/4 red eyed fruit flies F2 Generation ♂ XR Y ♀ XR Xr XRXR XRXr XRY XrY • 3/4 red eyed fruit flies (2 female and 1 male) •¼ white eyed, 1 male Morgan’s Experiment in humans it is estimated that • the X chromosome carries between 100 and 200 genes • the Y chromosome carries less than 100 genes disorders that require two recessive alleles, one on each X chromosome only need to be present once in males. • this is why some sex linked disorders occur more frequently in males. • examples: colour blindness, hemophilia, near-sightedness (myopia), night-blindness. • recessive lethal X-linked disorders also occurs more frequently in males. • example: infantile spinal muscular atrophy Dr.Murray Barr (Uni. Of Western Ontario recognized dark spots in some somatic cells of female mammals • the spot turned out to be sex chromatin. • it comes about when one of the X chromosomes becomes inactivated randomly. • this spot is now called a Barr Body • this means not all female somatic cells are identical, some have two active X chromosomes and some have one. • homozygous recessive X disorders can appear in some cells that are heterozygous but have lost the dominant allele. Homework Workbook “6.Sex linked”