BIO 305 1st Edition Lecture 7 Outline of Last Lecture I. Linkage Analysis and Mapping in Haploid EukaryotesII. Mapping the CentromereIII. Exchange frequencies – Parents, Non-parents, Tetra-typesIV. Gene Mapping in HumansV. RFLPs and Assessing Linkage with Likelihood RatiosOutline of Current Lecture I. Sex DeterminationII. Dosage CompensationIII. Sex Determination in Drosophila and HoneybeesIV. Vocab and Sample QuestionsCurrent LectureI. Sex Determination3 Forms of Reproductive Modes:No sexual reproduction (bacteria)Alternation between sexual and non-sexual (yeast)Sexual reproduction as the mechanism to create new individuals (diploid eukaryotes)Example: MaizeUndergoes double fertilization where a triploid (3n) contributes to a nutrient-richendosperm nucleus within a diploid zygote, post-fertilizationAnother Example: C. ElegansThey are hermaphroditic worms that can undergo self and cross fertilization because they contain both female and male reproductive organs.Last example: BirdsFemale birds are the heterogametic species (ZW) and their W chromosome transfers to their daughters. Males are (ZZ)Thus, there is a variety of different means of fertilization and sex determination across species on earth. The general rule for placental mammals: If you have Y, you are a maleSex Determination in humans, briefly – there is a gene on the Y chromosome called the SRY gene that produces testis-determining factor (TDF) whose presence induces testes development, and the testis produce hormones that develop male sexual characteristics/femaleness relies on the absence of thisAn XX male or XY female can survive if the SRY gene is translocated because then the TDF is present, or lacking for females, appropriately. However, both result in infertility.Klinefelter Syndrome (XXY) – sterile male with feminized physique (such as breast development)Individuals with (XYY) are not distinguished enough to be considered a syndrome, but tend to be taller with reduced intelligence and increased risk of behavior issuesTurner Syndrome (XO) – Females with certain secondary sex development issues, such as folds of skin around the neck and poor breast development, sterility and mental retardationII. Dosage CompensationDosage compensation is a regulatory mechanism that works to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they’re ‘equal’ in males and females You can see that barr bodies, inactivated X chromosomes, migrate to the edges of the nuclei of the cells.The Lyon Hypothesis – developed by Mary Lyon & Liane Russell, inactivation of X occurs randomly in somatic cells at a point early in embryonic developmentFemale mammals are mosaic:For example, an X-linked coat color gene encodes for black or yellow for males. But because females have two XX chromosomes (instead of one X, as in XY), they can be heterozygous for this gene and have a calico or tortoiseshell phenotype as a resultWhy doesn’t a female who is heterozygous for hemophilia A have hemophilia in half of her cells?She does, but half of her cells make clotting factor and it diffuses into the serum. That is enough for the blood to clot normally. Thus, the body has ways to compensate for differences in sex genes.X-Chromosome Inactivation – A region of the X p-arm (shorter end of the X chromosome) near the centromere has a X-inactivation-center (XIC), which acts a control centerIt contains a gene for X-inactive specific transcript (XIST) – this is an RNA that coats the X chromosome expressing this gene and locks the chromosome in an active state via DNA methylation.III. Sex Determination in Drosophila and HoneybeesThe ratio of X/A (X-chromosomes to autosome sets) determines the phenotype between females and males. For values of the ratio above 1, the fly is called a metafemale. For below .5, the fly is a metamale. A ratio between .5 and 1 results in an intersex drosophila.Males have white eyes due to X-linked mutations.Drosophila do not have Barr Bodies like human females do. Instead, male X-linked genes are transcribed at twice the level of the comparable genes in females as their form of dosage compensation. ‘Sxl’ also controls dosage compensation in drosophila.Honeybees Sex Determination (haploiddiploid) – a diploid queen (2n) mates with a haploid drone (n). If the fusion of the (n) gametes from each parent successfully fertilize, they become another diploid female, otherwise they become another haploid drone. At a molecular level, temperature sensitive genes involved in steroid hormone metabolism Vocabulary:Barr Bodies – inactivated X chromosomesSRY – a transcription factor that binds DNA and regulates the expression of other genesSexual differentiation – morphological differentiation between sexesPrimary (gonads only) vs. Secondary sexual differentiation (inclusion of overall appearance, suchas external genitalia, mammary glands, etc.)Unisexual, dioecious, gonochoric – individual has only male or female reproductive organsBisexual, monoecious, hermaphroditic – contains both male and female organsIntersex – intermediate sexual differentiation, often sterileProtenor – mode of sex determination (XX/XO), where males have one X and females have XXLygaeus – (our mode of sex determination), females have XX and males have XY chromosomesHeterogametic – when two unlike gametes are produced (like human males, or butterfly females)Homogametic – when two same gametes are produced (like human females)Sex ratio – number of males to femalesPrimary sex ratio – ratio of males to females conceivedSecondary sex ratio – ratio of males to females at birthAnhidrotic ectodermal dysplasia – an X-linked skin condition in which women show random patched of skin lacking sweat glandsBilateral gynandromorph – organisms that have both male and female characteristics, except bilaterally, as in the differences are split down the middle of the organism (Ex: XwmX++, the genotype of a red-eyed + white-eyed drosophila with half its body showing female characteristics and the other male characteristics)Sample Questions:1) Topic from last lecture: In Neurospora, an Irish strain makes big, green colonies (b g). It is crossed with a wild-type strain that makes small, orange colonies, generating the following ordered tetrads (the duplicated mitotic products are not shown here to simplify the problem):tetrad type genotype number I