The Chromosomal Behavior of Inheritance Chapter 15 The Chromosomal Behavior of Inheritance

Relating Mendelaism to Chromosomes Relating Mendelasim to Chromosomes Chromosome theory of Inheritance Based on observations from Sutton, Boveri, de Vries, and von Seysenegg between mendel’s principles and chromosome behavior Mendelian factors or genes are located on chromosomes It is chromosomes that segregate and assort independently

Morgan’s Experimental Design Thomas Hunt Morgan performed experiments at Columbia University on Drosphila Melanogaster in the 1900’s which provided evidence that the inherited factors are located on chromosomes Wild type – normal or most frequently observed phenotype (Cy+= allele for straight wings) Mutant phenotypes – alternative phenotypes to the wild-type due to mutation (recessive= w); (dominant= Cy)

Discovery of Sex Linkage Found single white eye male instead of wild type red eye; all females where red eyed Morgan deduced that eye color is located on the X chromosome because females where able to mask the mutant allele. Sex-linked genes – genes located on sex chromosomes

Genetic Recombination Genetic Recombination – production of offspring with new combinations of traits different from those found in the parents; results from independent assortment of chromosomes during meiosis Parental types – progeny whose phenotypes have the same as on or the other of the parents Recombinants – progeny whose phenotypes differ from either parent

Crossing Over Crossing Over – the appearance of combinations that should not appear during normal independent assortment Morgan’s results did not produce linked or unlinked phenotypic ratios

Recombination Data Map Recombination Data Map – sequence of genes related to there frequency of occurrence The farther apart genes are on a chromosome, the higher the probability a crossover will be between them increasing the recombination frequency (Sturtevant) Centimorgan – a map unit defined as 1% recombinant frequency Linkage Map – genetic map based on genetic frequencies

Sex Chromosomes Chromosomal basis of sex in humans Mammals have the X-Y mechanism (male = XY) (female = XX) Heterogametic – produces two kinds of gametes; male Homogametic – sex that produces one kind of gamete; females SRY – sex-determining region of Y chromosome that triggers testicular development; in the absence of SRY, ovaries form

Sex-Linked Genes In humans, usually refers to X-liked traits; since the X chromosome is larger than the Y-chromosome. More traits are carried on the X chromosome Y chromosome traits code only for traits found in males Fathers pass X-linked alleles to daughters only Males receive X-alleles from their mother Fathers cannot pass X-linked traits to their sons Mothers pass sex-linked alleles to both sons and daughters Any male receiving a mutant allele from his mother will express the trait

X-inactivation in Female Mammals In females, most diploid cells have only one fully functioning X chromosome Barr body – located inside the nuclear envelope. A densely staining object that is an inactivated X- chromosome in female mammalian cells Chromosome in activation is determined randomly

Errors and Exceptions in Chromosomal Inheritance Nondisjunction – failure of homologues to separate during meiosis or mitosis Meiosis errors: Occur during meiosis I when pairs do not separate Occur during meiosis II when chromatids do not separate Meiotic errors: Occur during embryonic divisions; will pass on to all successive cells Aneuploidy – having an abnormal number of chromosomes Polyploidy – a chromosome number that is more than two complete sets Triploidy – 3n Tetraploidy – 4n

Errors and Exceptions in Chromosomal Inheritance (continued) Alteration of chromosome structure Deletion – loss of a fragment or centromere Translocation – joining of a fragment chromosome to another nonhomologous chromosome Inversion – reattach fragment to original homologue except in reverse position

Errors and Exceptions in Chromosomal Inheritance (continued) Human chromosome disorders due to alteration Down syndrome – Trisomy 21 Patau syndrome – Trisomy 13 Edwards syndrome – Trisomy 18 Klinefelter syndrome – XXY Male sex organs with small testes, sterile, feminine body contours, breast enlargement. Klinefelter syndrome – XYY Normal small, above average height Triple X syndrome – XXX Fertile, normal phenotype Turner syndrome – XO Short, secondary sex characteristics do not develop, sterile, internal sex organs immature Cri Du Chat syndrome – deletion on chromosome number 5 Mental retardation