Genetics - Lecture 5

• Barr Bodies - Inactivated X chromosomes seen in female somatic cells

  • The inactive X is highly condensed, can be observed in stained interphase cells
  • Random inactivation of the maternal or paternal chromosome
  • Occurs early in embryonic development and all cellular descendants have the same inactivated chromosome
  • Although apparent inactivation explains dosage compensation, it complicates certain perceptions in this matter. Perhaps inactivation doesn’t take place in early stages of the development of gonadal tissue, not all X chromosome are inactivated
• Lyon hypothesis - Mary Lyon

  • Inactivation of X chromosome is random (Lyonization)
  • Occurs in somatic cells at an early stage of embryonic development and is then passed on to progeny cells by mitosis
• Life cycles depend on sexual differentiation
• In multicellular organisms, it’s important to distinguish between

  • Primary sexual differentiation, which involves only the gonads where gametes are produced
  • Secondary sexual differentiation, which involves the overall appearance of the organism
• Individuals that contain only male or female reproductive organs are unisexual (dioecious or gonochoric)

  Individuals that contain both male and female reproductive organs are bisexual (monoecious or 

  hermaphroditic) and can produce both male and female gametes
• Asexual reproduction in chlamydomonas

  • Some organism (chlamydomonas) spend most of their life cycle in the haploid phase, asexually producing daughter cells by mitotic division
  • Under unfavorable nutrient conditions, certain daughter cells function as gametes
  • In such species, the 2 gametes that fuse together during mating are not usually morphologically distinguishable
  • Such gametes are called isogametes
• Reproduction in Zea mays

  •In maize (Zea mays), the diploid sporophyte stage predominates and both male and female structures are present on the adult plant

  This indicates that sex determination must occur differently in different tissues of the same plant
• •Protenor mode of sex determination

  •The XX/XO (butterfly, Protenor) mode of sex determination depends on the random distribution of the X chromosome into half of the male gametes

  •The presence of two X chromosomes in the zygote results in female offspring

  The presence of only one X chromosome results in male offspring 
• Reproduction in C. elegans

  The nematode worm Caenorhabditis elegans has two sexual phenotypes: Males, which have only testes and Hermaphrodites, which have both testes and ovaries
• As adults the males can mate with the hermaphrodites, producing equal proportions of male and hermaphrodite offspring
• Self-fertilization occurs in the hermaphrodites and produces primarily hermaphrodite offspring, with less than 1 percent male offspring
• Sex determination in C. elegans

  • The ratio of X chromosomes and autosomes determines the male or hermaphrodite
  • Sex determination results from the presence of only one X chromosome in the males and two in the hermaphrodites
  • Absence of Y chromosome
• •Lygaeus mode of sex determination

  •In the XX/XY (Lygaeus) mode of sex determination:

  –female gametes all have an X chromosome

  –male gametes have either an X or a Y chromosome

  •Zygotes with two X chromosomes (homogametous) result in female offspring

  Zygotes with one X and one Y chromosome (heterogametous) result in male offspring
• Females are the heterogametic sex

  In ZZ/ZW sex determination, females are heterogametic (ZW), males are homogametic (ZZ)
• The Y chromosome determines maleness in humans

  The human karyotype revealed that one pair of chromosomes differs in males and females. Females have XX males have XY
• Klinefelter syndrome

  • Tall, long arms and legs
  • Underdeveloped testes and prostate gland, no facial hair
  • Phenotypically male, infertile, sometimes have slight gynecomastia
  • Generally have normal intelligence, but may be slow learners
  • Have more than one X chromosome (XXY, 47, XXY, 48, XXY, 49, XXXXY karyotype)
• Turner syndrome

  • Phenotypically female
  • 1/2000 births
  • Short, may have malformed features: webbed neck, high palate, small jaw, congenital heart and kidney defects, ovarian failure, infertility
  • Generally have normal intelligence, may have learning disabilities
  • 45, XO karyotype
• 47, XXX syndrome

  • The presence of 3 X chromosomes along with a set of autosomes (47, XXX) results in female differentiation
  • Frequently, 47 XXX women are perfectly normal
  • In other cases, underdeveloped secondary sex characteristics, sterility, and mental retardation occur
• 47 XYY condition

  • Only shared characteristic so far is males are over 6 ft tall
  • Subnormal intelligence
• Y chromosome and male development

  • Y chromosome has at least 75 genes
  • The pseudoautosomal regions (PARs) present on both ends of the Y chromosome that share homology with regions on the X chromosome and synapse and recombing with it during meiosis
  • The presence of such a pairing region is critical to segregation of the X and Y chromosomes during male gametogenesis
• Heterochromatin is typically highly condensed, gene-poor, and transcriptionally silent, whereas euchromatin is less condensed, gene-rich, and more accessible to transcription
• Pseudoautosomal region – look similar to the autosomes, but they’re not (pseudo). Autosome chromotypes can’t be find in the sex chromosomes
• At 6-8 weeks of development, the SRY genes become active in XY embryos.

  The testis-determining factor TDF is a protein encoded by a gene in the SRY that triggers testes formation
• MSY consists of 23 million base pairs

  • The MSY can be divided into 3 regions
  • X-transposed regions (15% originally derived from X chromosome
  • X-degenerative region (20%)
  • Ampliconic region (30%, encodes proteins specific to the development and function of the testis)
• Sex ratios

  • The ratio of males to females in humans is NOT 1:1
  • Primary sex ratio reflects the proportion of males to females conceived in a population
  • Secondary sex ratio reflects the proportion of each sex that is born
• Secondary sex ratio of white population in the US is 106 M to 100 F, in the African American populations it’s 1.025. Suggesting more males are conceived than females
• Males produce equal numbers of X and Y bearing sperm

  Each type of sperm has equal viability and motility in a female reproductive tract

  Egg surface is equally receptive to both types of sperm, so why so many males?
• Dosage compensation prevents excessive expression of X-linked genes in humans and other mammals. It balances the dose of X chromosome gene expression in females and males