Unit 5.4 - AP Biology

Loci

Loci

plural form of locus (positions)

Morgan’s experiments:

• he noted wild-type (normal) phenotypes that were common in the fly populations

• Traits alternative to the wild type are called mutant phenotypes

Chromosomal Basics of Gender

• humans (and other mammals) have 2 types of sex chromosomes: X and Y (all other chromosomes are autosomes)

  • two X chromosomes associated w/ female anatomy

  • One X and one Y chromosome associated w/ male anatomy

Crossing can happen in males

• ends of the Y chromosome have regions that are homologous w/ corresponding regions of X chromosome so they can pair and behave like homologs during meiosis in males

Sex- linked inheritance

• a gene that is located on either sex chromosome is called a sex-linked gene— either X-linked or Y-linked

  • many y- linked genes help determine male gender

  • X- chromosome have genes for many other traits

• For a recessive X lined trait to be expressed:

  • females need 2 copies of the allele (homozygous)

  • Males need only 1 copy of the allele (hemizygous)

• Ex. Color blindness and hemophilia

X inactivation in female mammals

In mammalian females, one of the two X chromosomes is randomly inactivated during embryonic development which condenses into a Barr body

Abnormal chromosome number

• nondisjunction occurs when pairs of homologous chromosomes (or sister chromatids) do not separate normally during meiosis I or II

• As a result, 1 gamete receives 2 of the same type of chromosome, and another gamete receives no copy

Aneuploidy

Fertilization after no disjunction results in aneuploidy (abnormal number of chromosomes)

Polyploid

• a condition in which an organism has more than two complete sets of chromosomes

  • Triploidy (3n) is three sets of chromosomes

  • Tetraploidy (4n) is four sets of chromosomes

• Only common in plants

Pedigrees

Family tree that contains a family’s history for a particular trait across generations