BIO 110 - Lecture 16: Chromosomal Basis of Inheritance PDF

Dr. Wyatt BIO 110 – Lecture 16 2/17/25  Announcements Quiz 4 next Wednesday Module 1 Presentations: Thursday (2/20) Module 1 Written report: Monday (2/24) Data Club meeting (Ras-eligible event!): 1:15 PM, Tuesday (2/18) in Cousins 335  C...

Dr. Wyatt BIO 110 – Lecture 16 2/17/25  Announcements Quiz 4 next Wednesday Module 1 Presentations: Thursday (2/20) Module 1 Written report: Monday (2/24) Data Club meeting (Ras-eligible event!): 1:15 PM, Tuesday (2/18) in Cousins 335  Chromosomal Basis of Inheritance allele = “form” of a gene Pair of chromosomes  Evidence for Chromosomal Inheritance Thomas Morgan (1866 – 1945) showed that a specific gene is carried on a specific chromosome genes on sex chromosomes show unique inheritance patterns Drosophila melanogaster – another model system!  Inheritance patterns associated with sex chromosomes is dependent on how sex is determined  Parental generation: Wild type female (w+w+) and mutant male (w) for white eyes All F1 offspring have red eyes The only offspring with white eyes in the F2 generation are males! The eye color locus is on the x chromosome, not on the y  Sex Gender https://www.nature.com/news/sex-redefined-1.16943 Sexuality https://blogs.scientificamerican.com/sa-visual/visualizing-sex-as-a-spectrum/  Chromosomal Basis for Sex People were aware of this before understanding that genes are found at specific locations on chromosomes (loci) Cytologists simply observed the size and shape of chromosomes of individuals of a known sex (males have x & y chromosomes, females two x chromosomes). Several chromosomal sex-determining systems have been classified in animals…  Chromosomal Basis for Sex in Animals X-Y system (sperm determines sex) X-0 system (sperm determines sex) Z-W system (egg determines sex) Haploid-diploid system (or Haplodiploidy) Males (haploid) develop from unfertilized eggs Females (diploid) develop from fertilized eggs Chromosomal Basis for Sex in Animals Homo = same ; Hetero = different ; Gametic = having to do with sex cells (sperm and egg) System Males Females X-Y Heterogametic (X-Y) Homogametic (X-X) X-0 Monogametic (X-0) Homogametic (X-X) Z-W Homogametic (Z-Z) Heterogametic (Z-W) Homogametic = possessing only one type of sex chromosome (e.g. XX) Heterogametic = possessing different types of sex chromosomes (e.g. XY) Monogametic = possessing only one sex chromosome (e.g. X-0) Haploid-diploid system (don’t memorize)  X Inactivation in female mammals Mammals, having the X-Y system, often have inactivation of an X chromosome in females (since they have two) – forming a Barr body – a condensed X chromosome with little to no genes being expressed Which X chromosome gets ‘muted’?... Female mammals are a mosaic of paternal X and maternal X chromosome gene expression This female tortoiseshell cat (XX) is heterozygous for black fur and orange fur  Evolution of Sex Chromosomes in Amniotes The different systems probably evolved from a temperature dependent sex determination system Lineages co-opted different chromosomes for sex determination systems Note heteromorphism of sex determining chromosomes (i.e. sex chromosomes of different sizes in modern birds, snakes, and mammals) Vallender & Lahn 2006  Chromosomal Basis for Sex in Plants Like animals, has evolved in multiple, independent flowering plant lineages (dioecy = separate male/female individuals, 5 – 10% of plant spp.) Some heteromorphic, some homomorphic X-Y system, along with a variety of others Cannabis sativa Some organisms (e.g. sea turtles) have sex determination that is not determined by the number or type of chromosomes at all! http://savetheseaturtle.org/Why-sexing-a-sea-turtle-hatchling-is-important.html At your tables: Create a Punnett Square for each of the three scenarios to predict the probability of male and female offspring being (a) unaffected (white), (b) carriers (light orange), or (c) color-blind (dark orange) for the X-linked disorder of color-blindness. N denotes the wild type allele; n denotes the mutant allele Scenario 1 Scenario 2 Scenario 3 In your groups: Create a Punnett Square for each of the three scenarios to predict the probability of male and female offspring being (a) unaffected (white), (b) carriers (light orange), or (c) color-blind (dark orange) for the X-linked disorder of color-blindness Scenario 1 Scenario 2 Scenario 3 Male humans are more likely to express a recessive, sex-linked trait (in nearly all cases an X-linked trait) than female humans because they only have one copy of the X chromosome. What is a male’s susceptibility to recessive, sex-linked genetic diseases in other systems of sex determination?  Reminders Quiz 4 next Wednesday Module 1 Presentations: Thursday (2/20) Module 1 Written report: Monday (2/24) Data Club meeting (Ras-eligible event!): 1:15 PM, Tuesday (2/18) in Cousins 335