gen bio - lecture 13: chromosomes

chromosome theory of inheritance

fits idea of mendel’s “heritable factors”

1. genes have specific loci on chromosomes

2. chromosomes undergo segregation + independent assortment

morgan’s experiment - organism + it’s advantages

• fruit fly (d. melanogaster)

• advantages:

  • lots off offspring

  • short generations of about 2 weeks

  • 4 pairs of chromosomes (3 pairs of autosomes + 1 pair of sex chromosomes)

morgan’s experiment - wild + mutant type

• wild type: means the phenotype is the most common in the population (ex. red eyes)

• mutant type: alternative to wild type (ex. white eyes)

morgan’s experiment - crossing red eyed female and white eyed male (dictate generations + findings)

• P: red eyed female x white eyed male

• F1: all offspring have red eyes

  • cross red eyed F1 female and red eyed F1 male

• F2: 3:1 phenotype ratio (3 red, 1 white)

why no white eyed f2 female? eye color gene = located on x chromosome w/no corresponding locus on Y chromosome → x linked

• findings:

  • specific gene = carried on specific chromosome

  • unique inheritance patterns for genes on sex chromosomes

sex chromosomes + sex linked genes

• humans have 1 pair

• act homologous during meiosis

• contain sex-determining genes + genes unrelated to sex determination

• sex linked genes: genes on sex chromosome

  • can be X or Y linked

heterogametic

• describes males, XY

  • ½ sperm receive an X, ½ receive a Y

  • Y chromosome has genes for male development

homogametic

• describes females, XX

  • all eggs have an X

  • female phenotype = due to absence of Y chromosome

males + x-linked genes (define hemizygocity)

• have just one x chromosome = every allele expressed

• hemizygous: having just ONE ALLELE for a gene

  • recessive alleles are NEVER MASKED

for an x-linked recessive trait, a male can be…

• hemizygous dominant (XY) = not affected

• hemizygous recessive (X*Y) = affected

** no situation where they are a carrier!

for an x-linked recessive trait, a female can be…

• homozygous dominant (XX) = not affected

• heterozygous (XX*) = not affected BUT carrier

• homozygous recessive (X*X*) = affected

barr bodies + purpose

• one X chromosome in females is INACTIVATE DURING DEVELOPMENT (barr body)

  • very dense structure

  • this inactivation is RANDOM

• purpose: since every allele is expressed on the x chromosome, and females have two, the purpose of a barr body is to equalize gene expression w/males who only have one x chromosome

  • (aka. explains why females don’t make 2x the proteins from x-linked genes)