Elizabeth Castellan
Chapter 15
The Chromosomal basis of Inheritance
An Example of Sex-Linkage
Mendelian inheritance has its physical basis in the behavior of chromosomes
Chromosomal theory of Inheritance
Mendelian genes have specific loci (positions) on chromosomes, and it is the
chromosomes that undergo segregation and independent assortment
Thomas Hunt Morgan – early 20th experimental embryologist
Used fruit flies to study genetics
Only have four chromosomes (3 pairs = autosomes and 1 pair =sex chromosomes)
He studied the eye color of the flies
(red = wild type = dominant, and white = mutant = recessive)
Morgan bred a white male with a wild type female
F1 generation = all wild type
F2 generation = 3:1 phenotypic ratio
However, all the females were wild-type
and ½ of the males were mutant
Conclusion = eye color also related to sex (gene on X, but not Y)
The eye-color gene was only on the X chromosome (XX = female, and XY = male)
For males a single copy of the mutant X chromosome would lead to a mutant phenotype
Females would have to have two copies of mutant X chromosome
This finding supported the Chromosomal Theory of Inheritance
Specific gene located on specific chromosome
Also concluded that genes on sex chromosomes had special patterns of Inheritance
Linked genes – tend to be inherited together because they are located near each other on the
same chromosome
When two alleles are close on the same chromosome, crossing over during meiosis may
not always affect the combination of these two alleles
The alleles will be together in the gametes more often
More parental phenotypes will be observed than expected
(50% recombination is normal)
Linkage Mapping Using Recombination Data
Genetic Map – an ordered list of the genetic loci along a
particular chromosome
Developed by Alfred H. Sturtevant
The farther apart two genes are, the higher the probability that a crossover will occur between
them and therefore the higher the recombination frequency
greater distance between the genes = more places for crossing over to occur
linkage map – a genetic map based on recombination frequencies
one map unit = 1% recombination frequency (dependent)
the highest recombination frequency is 50%
genetically unlinked even if physically linked (on same chromosome)
(independent)
Sex-linked Genes exhibit unique patterns of inheritance
(XX = female, and XY = male)
Sex chromosomes also have genes that are unrelated to sex
These genes are called sex-linked genes
Homozygous female will show the trait if a recessive trait (less females with recessive disorders)
Male with X chromosome with trait will show trait (only have one chromosome with trait)
Color-blindness
Duchene Muscular dystrophy
Hemophilia
X-inactivation in females
Females inherit two X chromosomes, however, one becomes inactive
Barr Body
Most genes in Barr body are not expressed
The inactivation occurs randomly in each cell present when they are inactivated
Some cells have inactive X from mother and others have father X inactive
Half the cells express alleles on one X and half express the other X
Alterations of Chromosome number or Structure cause some Genetic Disorders
Nondisjunction – the members of a pair of homologous chromosomes do not move apart
properly during meiosis I or sister chromatids fail to separate during meiosis II
One gamete receives two copies of the same chromosome and the other gamete no copy
Aneuploidy – when a normal and abnormal (0/2 copies) gametes fertilize together
Trisomy – the fertilized egg receives three copies of the chromosome
Monosomy – a chromosome is missing
Polyploidy – more than two complete chromosome sets
Alterations of Chromosomal Structure
Deletion – a chromosomal fragment is lost – missing certain genes
Duplication – a detached fragment attaches to another
homologous chromosome
Inversion – a detached part attaches backwards to the same chromosome
Translocation – a fragment attaches to a non-homologous chromosome
All could happen during crossing over
Disorders
Downs Syndrome – Trisomy of Chromosome 21 (body cells have 47 chromosomes)
Klinefelter Syndrome – males with extra X chromosome (XXY)
Turner Syndrome – Females with monosomy X (X)
Some Inheritance Patterns are Exceptions to the Standard Chromosome Theory
Genomic Imprinting – variation in phenotype depending from which parent the allele was
Inherited
(not sex-linkage)
The genes are imprinted differently in the sperm vs. the egg transferred to body cells
The genes are always imprinted the same way generation after generation
Inheritance of Organelle Genes
Extracellular genes – located in organelles in cytoplasm
(Mitochondria/Chloroplasts)
Do not display Medelian Inheritance patterns
Inherited from the Mother (passed on to a zygote from cytoplasm of the egg)