1. The Chromosomal Behavior of Inheritance
Chapter 15
The Chromosomal Behavior of Inheritance

2. 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

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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