Sex Determination, Sex Chromosomes, Sex-Linkage Lecture Slides (Ch7&4).pdf

9/15/2024 CHAPTER 7&4 Sex Determination, Sex Chromosomes, and Sex-Linkage Copyright © 2019 Pearson Education, Inc. Al...

9/15/2024 CHAPTER 7&4 Sex Determination, Sex Chromosomes, and Sex-Linkage Copyright © 2019 Pearson Education, Inc. All Rights Reserved 1 Section 7.1 Mode of Sex Determination XX/XO mode of sex determination XX/XY mode of sex determination – Dosage of X Chromosome for Sex – Female gametes have one X Determination chromosome ▪ Depends on random distribution of X – Male gametes have either an X chromosome into half of male or Y chromosome gametes ▪ Presence of two X chromosomes in ZZ/ZW sex determination zygote results in female offspring – The Z chromosome is large, and ▪ Presence of one X chromosome the W chromosome is small results in male offspring – Females are ZW ▪ Examples: butterfly, fruit-fly (most – Males are ZZ insects) – Example: Chickens (most birds) Copyright © 2019 Pearson Education, Inc. All Rights Reserved 2 1  9/15/2024 Section 7.1 Homogametic and Heterogametic Homogametic sex Males XY are the heterogametic – Has two like chromosomes sex in mammals – Producing like gametes – The X chromosome is large and contains almost all genes ▪ Each gamete will carry one of the X chromosomes Females ZW are the heterogametic sex in birds Heterogametic sex – ZZ/ZW sex determination – Has two unlike chromosomes – Producing unlike gametes ▪ The Z chromosome is large and contains almost all the ▪ 50% of gametes carry the X and known sex-linked genes 50% carry the Y chromosome Copyright © 2019 Pearson Education, Inc. All Rights Reserved 3 Section 7.2 The Y Chromosome Determines Maleness in Humans Y chromosome determines maleness – Human karyotype ▪ 22 pairs of autosomal chromosomes ▪ 1 pair of sex chromosomes –differs in males and females – Females: XX – Males: XY Copyright © 2019 Pearson Education, Inc. All Rights Reserved 4 2  9/15/2024 Section 7.2 Klinefelter and Turner Syndromes Klinefelter (47,XXY) and Turner (45,X) syndrome – Two human abnormalities – Characterized by aberrant sexual development – Both syndromes result from nondisjunction ▪ 2 XX's chromosomes don’t separate in meiosis –Review non-disjunction in the Mitosis and Meiosis Chapter 2 Copyright © 2019 Pearson Education, Inc. All Rights Reserved 5 Section 7.2 Triplo-X Syndrome 47,XXX Syndrome: Triplo-X – Three X chromosomes – Normal set of autosomes – It is a female ▪ Sometimes women are perfectly normal ▪ Sometimes underdeveloped secondary sex characteristics occur – Sterility and mental retardation – Results from nondisjunction ▪ X chromosomes don’t separate in meiosis Copyright © 2019 Pearson Education, Inc. All Rights Reserved 6 3  9/15/2024 Section 7.2 XYY Syndrome 47,XYY Syndrome – Only consistently shared characteristic—males are over 6 feet tall ▪ Subnormal intelligence ▪ Personality disorders – results from nondisjunction ▪ Y chromosome 2 sister chromatids do not separate in meiosis Copyright © 2019 Pearson Education, Inc. All Rights Reserved 7 Section 7.2 The Y Chromosome Y chromosome and male development – Y chromosome is very small – Fewer genes than X chromosome PARs: Pseudoautosomal regions – Present on both ends of Y chromosome – Share homology with regions on X chromosome – X and Y PAR regions synapse and recombine with each other during meiosis Prophase I Copyright © 2019 Pearson Education, Inc. All Rights Reserved 8 4  9/15/2024 Section 7.4 Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals Dosage compensation Barr bodies (sex chromatin bodies) – Genetic mechanism – Genetic mechanism compensates – Balances dose of X for X dosage disparities chromosome gene – Inactive X chromosome, highly expression in males condensed and females – Darkly stained body in interphase – Prevents excessive nerve cells observed: Barr bodies expression of X- – Random inactivation linked genes in – Occurs early in embryonic humans and other development mammals Copyright © 2019 Pearson Education, Inc. All Rights Reserved 9 Section 7.4 Lyon Hypothesis Inactivation of X chromosome is random Lyon Hypothesis Example – Occurs in somatic cells at early stage – Calico cats and fur color/patterns of embryonic development – All descendant cells have same X- inactivation – Example: Fur color gene present on the X-chromosome (X-linked) Copyright © 2019 Pearson Education, Inc. All Rights Reserved 10 5  9/15/2024 Section 7.4 X-Inactivation X-inactivation – Explains dosage compensation – Follows N − 1 rule (N = total number of X chromosomes) Copyright © 2019 Pearson Education, Inc. All Rights Reserved 11 Section 4.11 X-Linkage Describes Genes on the X Chromosome X-Linkage – A trait controlled by gene(s) on sex chromosome (X) ▪ Different from autosomal genes – XX vs XY – Females have 2 dosages of each gene; they are homozygous or heterozygous – The male has only 1 dosage of each gene, they are hemizygous Y Chromosome – Relatively inert genetically – Male-specific genes on the human Y chromosome – Lacks copies of genes found on the X chromosome Copyright © 2019 Pearson Education, Inc. All Rights Reserved 12 6  9/15/2024 Section 4.11 Color Blindness: X Chromosome-Linked Red/green color blindness in the USA is Heterozygous XGXg female, even present in