Download notes File

Chapter 13
Vocabulary:
Sex chromosomes
Female sex chromosomes
Male sex chromosomes
Autosomes
Dosage compensation
Genetic mosaics
Linked genes
Aneuploidy
Monosomics
Trisomics
Outline
13.1 Sex Linkage and the Chromosomal Theory of Inheritance
Chromosomal theory of inheritance – traits are carried on chromosomes
We take this for granted, but it had to be discovered at some point!
Sex chromosomes (X and Y)
Autosomes (everything else)
Sex linked genes appear on the X chromosome
13.2 Sex Chromosomes and Sex Determination
Autosomes are homologous
Sex chromosomes are not
Female carries XX
Male carries XY
Y chromosome only carries 330 genes
X chromosome carries 2,062 genes
Genes that appear on the X chromosomes can be passed from mother to sons and
daughters.
Can be passed from fathers only to daughters because fathers give a Y to their
sons
Recessive allele on X chromosome
Affects more males than females because males only have 1 X so trait is
expressed, females have another X that can mask the expression of a recessive trait
If mutated genes are on X chromosomes females have another X chromosome
that can carry the dominant allele and mask the expression of the mutant recessive gene.
Males only have 1 X chromosome and most of the traits are not on the Y chromosome so
they have no opportunity to mask it.
Ex. Color blindness and hemophilia A
In females one X chromosome is randomly selected for modification. This chromosome
will (with the exception of about 3 dozen genes) remain condensed and inactive. This
happens very early on in development (about 200 cells big). It is random within all these
early cells, but from then on all cells that are made from that cell will have the same
chromosome condensed and inactive. So in an adult some regions of the body are
expressing different x chromosomes. Mosaic. This means that if the alleles on the
chromosomes are not alike then different tissues can be expressing different forms of a
gene.
This happens so that males and females express the same levels of certain genes found on
the X chromosome. Dosage compensation
13.3 Exceptions to the Chromosomal Theory of Inheritance
Mitochondrial and chloroplast DNA is inherited only from the egg cell. Egg cells have
more cytoplasm and therefore organelles. This is maternal inheritance.
Differs from x-linked because sons and daughters get this DNA from mom; nobody gets
it from dad.
13.4 Genetic Mapping
Frequency of recombination is related to relative gene loci.
The closer 2 genes are the more likely they will stay together and be inherited together.
(linked)
If they are far enough apart multiple crossover events can lead to independent assortment.
Crossing over disrupts linkages
13.5 Selected Human Genetic Disorders
Nondisjunction leads to aneuploidy
Monosomics
Trisomics
Nondisjunction of sex chromosomes
Genomic imprinting – allele comes only from one parent
Pre-natal diagnosis:
Genetic counselors
Pedigrees
Amniocentesis
Chorionic villi sampling
Most important information to convey is how to work x-linked crosses. Mosaics, dosage
compensation, linkage, non-disjunction and euploidy are other highlights. Again don’t
ask them to memorize specific disorders or their genetics.