1. The chromosomal basis of inheritance
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.
How genomic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian inheritance.

3. Chromosome Theory of Inheritance
Genes have a specific location (loci) on chromosomes
Chromosomes segregate and assort independently

4. I. Sex – linked genes
Genes located on the sex chromosomes (X or Y in humans)
Discovered by Thomas Hunt Morgan while working with fruit flies

5. Inheritance of X-linked genes
Males receive one copy of the X-linked gene from their mother and a Y chromosome (without that gene) from their father (can’t use homozygous/heterozygous)
Females receive two copies of the gene – one from their mother and one from their father
Traits represented as
superscripts on the X

6. Most sex-linked traits/disorders are recessive
Heterozygous females are called “carriers” – they don’t display the trait, but their sons may
Females need two copies of the allele to express the trait, males only need 1 copy

8. Hemophilia – lack clotting factors in blood
X-linked Disorders
Duchenne muscular dystrophy – progressive weakening of the muscles, death by early 20s
Hemophilia – lack clotting factors in blood
Color blindness
Interesting documentary on hemophilia and historical royal families if you’re interested (on your own time)

9. X – inactivation
Only one X chromosome worth of X-linked genes is necessary (as demonstrated by males only needing one)
In females, one X (randomly selected) in each cell is inactivated by methylation
Inactive X condenses into
a Barr body

10. Linked genes
Genes located in the same chromosome and are therefore more likely to be inherited together
Only way they are not inherited together is if crossing over occurs between the two loci
The farther apart two genes are on a chromosome, the more likely it is that crossing over will occur between them

11. Genetic recombination – production of offspring with a different combination of genes than either parent

12. Linkage Map – show where genes are located on a chromosome based on the percentage of crossover events (1 map unit = 1% recombination frequency)

13. Sample Problem
In fruit flies, gray body is dominant to black, and normal wings are dominant to vestigial. Flies heterozygous for both traits were crossed with flies that were black with vestigial wings. The results are below.
How do you know these genes are linked?
How many map units apart are these two genes?
Phenotype
Number of flies
Gray/normal
482
Black/vestigial
472
Gray/vestigial
103
Black/normal 92
Total = Recombinants = /1149 = 17% 17 map units

14. Alteration of chromosome number
Nondisjunction – failure of homologous chromosomes or sister chromatids to separate properly during meiosis
Trisomy = 3 copies of one chromosome
Monosomy = 1 copy of one chromosome
Results in aneuploidy – incorrect number of chromosomes

15. Human disorders from chromosomal alterations
Most miscarried early in pregnancy (bigger the chromosome, more likely that is)
Down Syndrome – Trisomy 21-
characteristic facial features,
short, heart defects,
developmental delays

16. Klinefelter Syndrome – XXY – generally sterile, some developmental delays
Turner Syndrome – XO – reproductive organs do not mature, sterile, developmental delays

17. Polyploidy – complete extra set(s) of chromosomes (3n, 4n, +)
Rare in animals, but commons in plants
Results in extra large fruits

18. Structural alterations of chromosomes
Occur when crossing over is not completed correctly
1. Deletion – loss of a fragment of a chromosome
2. Duplication – extra copies of fragments of a chromosome

19. Inversion – part of a chromosome breaks off and reattached backwards
4. Translocation – a piece of one chromosomes breaks off and attaches to a non-homologous chromosome

20. Exceptions to standard Mendelian Inheritance
Epigenetics – environmental factors influence gene expression

21. Exceptions to standard Mendelian Inheritance
Genomic imprinting – traits vary depending on whether they are inherited from mom or dad
During gamete formation, one allele is turned off by methylation

22. Exceptions to standard Mendelian Inheritance
Mitochondrial DNA – only passed from mother to child