1. CHAPTER 15

2. You Must Know
How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance
The unique pattern of inheritance in sex-linked genes
How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders

3. Linking Mendel and Chromosomes
Today we know that Mendel’s “hereditary factors” are located on chromosomes
So we can link Mendelian genetics to modern genetics through the genes that lie on the chromosome
Chromosome Theory of Inheritance
Mendelian genes have specific loci (positions) on chromosomes
Chromosomes undergo Segregation & Independent Assortment

5. T.H. Morgan studied flies
He studied Drosophila Melanogaster – Fruit fly
They _____ like flies
Take 2 weeks to breed
Hundreds of offspring per brood
Only 4 pairs of chromosomes
3 autosomes and 1pair of sex chromosomes

6. Converting to Morgan speak
Phenotype
Dominant = wild type
Recessive = mutant
Wild-type – normal or typical W+
Mutant type – not normal W
For example, vg+ = wild type for body size (full body)
vg = mutant (vestigial body, smaller size)

7. Morgan gets unMendelian results
He crossed wild type (red eyes) with a mutant (white eyes), but did not get Mendelian results, or did he?
Basis for his questioning Mendelian Genetics
P = Red-eyed female + White-Eyed male
F1 = ALL Red eyes
F2 = 3:1 ratio of red to white eyes, BUT only MALES = white eyes

10. Gene Linkage
Linked genes – genes located on the same chromosome that tend to be inherited together
Linked genes do not adhere to the laws of Mendelian genetics

11. Sex-linked gene inheritance pattern

12. Notes on Sex-linked Traits
Female = XX Male = XY
Fathers pass sex-linked traits to daughters NOT sons
Fathers pass the y-chromosome to sons
Female expression is like any other trait
Since Males have only 1 chromosome, they express the allele on their 1 X-chromosome
Sex-linked traits have NOTHING to do with sex
CANNOT use heterozygous and homozygous

13. Known Sex-linked Disorders
Duchenne muscular dystrophy
Progressively weakening of muscles and loss of coordination
Hemophilia
Blood that is unable to clot normally
Due to absence of proteins required for proper clotting

14. X Inactivation in Females
Although females receive 2 copies of alleles, one chromosome becomes inactivated during embryonic development
Due to XX
Chromosome inactivation is Random
Inactivation is due to methylation
So females and males are operating on only 1 copy
Barr Body – inactivated chromosome
Lies on the inside of the nuclear envelope

15. Mosaic-ness

16. Back to Morgan & Flies

17. Linkage Inheritance Pattern
2 Characters: body color & wing size
Body Color
b+ = Grey (wild type)
b = black (mutant)
Wing Size
vg+ = normal wings
vg = vestigial wings (Reduced wing size)

18. Some more vocab… Linked Genes – located on same chromosome
Tend to be inherited together
Genetic Recombination – Offspring with new combination of genes inherited from parents
Parental Phenotype – offspring phenotype = at least one of the parental phenotypes
Recombinants – offspring phenotype NOT = either parental phenotype

20. Notes on the Results 2,300 offspring
Far higher proportion of parental phenotypes than expected from independent assortment
Genes are inherited together
There were also recombinants or non-parental phenotypes as well
Conclusion = Partial linkage & Genetic recombination (recombinants or recombinant types)

21. Recombinant Frequency
If the genes are located on different chromosomes, then the recombination frequency should be 50%
In the flies, the recombinant frequency was less than 50%; it was about 17%
Evidence of that the 2 genes lied on the same chromosome
So some linkage but incomplete

22. Crossing Over
Crossing Over explains why some linked genes get separated during meiosis
Crossing Over occurs in Meiosis I
Farther apart 2 genes = Higher P(Crossing Over)
Linkage Map – genetic map based on the percentage of cross-over events
Map unit – 1% recombination frequency
Used ONLY for relative distances on the chromosome

23. Linkage Maps

24. Explain
We know that Mendel’s seed color and flower color were on the SAME chromosome, but they did not behave as linked genes. Explain.

25. Nondisjunction
Nondisjunction – mishap where pairs of homologs do not move apart properly during meiosis
Could happen in Meiosis I or when Sister chromatids fail to separate correctly in Meiosis II
One gamete receives 2 of the same type of chromosome, while another receives no copy

27. -somy Aneuploidy – abnormal number of chromosomes
Nondisjunction could result in a cell with 2n+1
Here this cell would be considered aneuploid, and considered trisomic for that individual chromosome
-somy = different number of an INDIVIDUAL chromosomes
Trisomy – 3 copies of a chromosome
Monosomy – only 1 copy of a chromosome
2n – 1

28. -ploidy Alteration of an ENTIRE CHROMOSOMAL SET Called polyploidy
Triploidy = 3n
Tetraploidy = 4n
Polyploidy plants are fairly common, animals are less common
Polyploids are more normal than aneuploids
Hence, 1 chromosome extra or fewer is more disruptive, than an entire set of chromosomes extra or fewer

29. Chromosomal Structural Alteration

30. Human Disorders caused by Chromosomal Alteration
Down Syndrome
1 of 700
Trisomy 21 (each cell has 47 chromosomes, not 46)
Risk increases with maternal age
Klinefelter Syndrome
Male have extra X chromosome
Possess male sex organs, but are sterile
IF Female, 3 chromosomes (XXX) = healthy & normal
Turner Syndrome
Female with only 1 X chromosome
Sterility, but only known monosomy in humans