1. Chromosomal Inheritance
Chapter 15

2. Morgan and Fruit Flies Thomas Hunt Morgan – experimental embryologist
Evaluated Mendel’s findings using fruit flies (Drosphila melongaster)
Fast reproduction
Large offspring production
4 pairs of chromosomes (1 sex, 3 autosomes)
Wild type – phenotype found most commonly observed in natural populations (w+)
Mutant type – versions due to assumed allele changes due to mutations (w)

3. Morgan and Sex-linked Traits
Morgan mated white eyed (w) males with red eyed (w+)
All red eyes – red is dominant
Bred F1 – classical 3:1 ratio
Finding: All the females had red eyes while half males had white and half had red eyes
Assumption: Eye color is linked to its sex
Females have 2 XX chromosomes, males have XY
Since white eyes is the mutant (w+) type and is recessive, and only females had the red eyes (w) because two versions can exist there was no second allele on an X chromosome to override the recessive allele

4. Chromosomes and Sex Inheritance
Males develop from zygotes containing one X and one Y
Y chromosome contains the SRY region – sex determining region
Required for development of the testes
Genes located on sex chromosomes are called sex linked genes
Y chromosomes are passed on virtually intact from father to son
Very few genes on the Y chromosomes-means very few disorders transferred on Y chromosome to son
Anatomical signs begin to develop at about 2 months in the embryo
Other types of sex determination
Grasshoppers, cockroaches and some other insects-only one sex chromosome X – females XX, males XO
Birds, fish, some insects – The sex chromosome in the egg determines the sex-ZW female, ZZ male
Bees and ants – haplo-diploid – no sex chromosomes, females develop fertilized eggs (diploid) and males non-fertilized (haploid)

5. X-Linked Inheritance X chromosome contains ~1,100 genes
Fathers pass all of their X alleles to their daughters
Mothers can pass X-linked alleles to sons or daughters
More males than females will exhibit X-linked disorders than females
Ex. Duchenne muscular dystrophy, Hemophilia, color blindness
If trait is sex linked recessive, female will only express if she is homozygous for the allele
Males can not be homozygous since they lack the other allele – they are therefore hemizygous – any male receiving a recessive allele will express the disorder

7. X Chromosome Inactivation
Early in development, one of the female chromosomes becomes inactivated
Insures the same dose of X-linked genes in males and females
Results in a Barr body
Selection of which X chromosome is completely random and independent during development
Barr body is a condensed object which lies along the inside of the nuclear envelope – most of the genes in the barr body are not expressed
After X chromosome inactivation all cells mitotically derived from that cell will have that same X chromosome inactivated – thus is she heterozygous for a trait, about half her cells will express one allele and half will express the other – mosaicism
Some human females will have patches of sweat glands and patches lacking

8. Gene Linkage and Recombination
Certain genes tend to be inherited together – linked genes
Some offspring inherit matches either of the parent phenotypes – parental types
Some will inherit traits that are new – recombinant types/recombinants
If two genes have a recombinance of 50% they are said to be on separate chromosomes
Crossing over leads to recombinants
The further the distance between two genes the more likely for recombinance

9. Mapping Genes
Alfred Sturtevant created an ordered list of the genes on a chromosome – genetic map
Used the recombinant frequency calculated from experiments using fruit flies to determine distance between them
The further the distance the greater the chance for crossing over between 2 genes-high recombination frequency
Created a linkage map that showed the position of the gene on the chromosome – expressed in map units
Different than a cytogenic map which has the actual position based on staining patterns of chromsomes

10. Abnormal Chromosome Numbers
Chromosomes fail separate during meiosis – nondisjunction event
Results in a gamete with either an extra or missing chromosome
If fertilization occurs the zygote will exhibit aneuploidy – abnormal chromosome number in a zygote
Monosomic – the zygote is missing a chromosome
Trisomic – the zygote has an extra chromosome
Some organisms will have extra complete chromosome sets – polyploidy
Ex. Plants, some fish and amphibians
Down syndrome
Trisomy 21
Characteristic facial feature, short stature, correctable heart defects, developmental delays
Shorter life span than most
Frequency increases with age under 30 increase .92 over 40 and continues to increase
Monosomy X
Turners syndrome
1 in 2500 births
Only known viable monosomy in humans
Sterile and do not mature
Normal intelligence
With estrogen, can develop seconday sex characterisitics
XXX
1 in 1000 births
Healthy, no unusual appearance
Slightly taller than average
At risk for learning disabilities
XYY
Below normal intelligence
Normal sexual development
Somewhat taller than average
XXY
Klinfelters syndrome
1: births
Testes are small, male is sterile
Even though X chromosome is inactivated, male will exhibit some female characteristics

11. Chromosome Mutations
Problems during crossing over or radiation can lead to chromosome damage
Deletion - Chromosome fragment lost
Duplication – detached fragment attaches to non-sister chromatid of a homologous chromosome
Inversion – chromosome reattaches but in the reverse order
Translocation – the fragment attaches to a nonhomologous chromosome