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CHAPTER 15
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
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
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
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)
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
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
Sex-linked gene inheritance pattern
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
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
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
Mosaic-ness
Back to Morgan & Flies
Linkage Inheritance Pattern
2 Characters: body color & wing size Body Color
b+ = Grey (wild type)
b = black (mutant)
Wing Sizevg+ = normal wings
vg = vestigial wings (Reduced wing size)
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
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)
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
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
Linkage Maps
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.
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
-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
-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
Chromosomal Structural Alteration
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