Chapter 15 The Chromosomal Basis of Inheritance

Mendelian inheritance has its physical basis in the behavior of chromosomes  In the early 1900s biologists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s “factors.”

Chromosome Theory of Inheritance  Mendelian genes have specific loci on chromosomes, and it is the chromosomes that undergo segregation and independent assortment.

Law of Segregation *Two alleles for each gene separate during meiosis. Law of Independent Assortment *Alleles of genes on non- homologous assort indepen- dently during meiosis

Linked Genes  Tend to be inherited together because they are located near each other on the same chromosome. The results are visible in crosses where the outcome deviates from those expected (from Mendel’s law of independent assortment) The results are visible in crosses where the outcome deviates from those expected (from Mendel’s law of independent assortment)

Evidence for linkage

Gene Maps  An ordered list of the genetic loci along a particular chromosome. The further apart two genes are, the higher the probability that a crossover will occur between them. The further apart two genes are, the higher the probability that a crossover will occur between them. Such map is called a linkage mapSuch map is called a linkage map

Linkage Mapping Using Recombination Data

A Partial Linkage Map for Fruit Flies

Gene Maps  These distances are relative to each other and are not actual physical distances.  A linkage map portrays the order of genes along a chromosome no the actual precise locations. Cytogenetic maps… Cytogenetic maps…

Chromosomal basis of sex

Inheritance of Sex-Linked Genes  Sex-linked versus X-linked Often spoke of interchangeably…though it is not Often spoke of interchangeably…though it is not X h – hemophilia X h – hemophilia X d – Duchenne muscular dystrophy X d – Duchenne muscular dystrophy

XX X Y

X inactivation in female mammals  Although female mammals, including humans, inherit two X chromosomes, one X chromosome in each cell becomes almost completely inactivated during embryonic development. Barr body Barr body

Calico Cats: Example of Barr Body ami.edu/dana/do x/calico.html

Abnormal Chromosome #  Nondisjunction Leads to aneuploidy: Leads to aneuploidy: Aneuploidy is the condition of having less than or more than the normal diploid number of chromosomes, and is the most frequently observed type of cytogenetic abnormality.Aneuploidy is the condition of having less than or more than the normal diploid number of chromosomes, and is the most frequently observed type of cytogenetic abnormality.

The most common type of aneuploidy  Monosomy- Turner's syndrome is the most common sex chromosome abnormality of human females. Its incidence in newborns is approximately 1 in 2,500 Turner's syndrome is the most common sex chromosome abnormality of human females. Its incidence in newborns is approximately 1 in 2,500  Trisomy – A common autosomal trisomy in humans in Down syndrome, or trisomy 21. A common autosomal trisomy in humans in Down syndrome, or trisomy 21. Down syndrome Down syndrome

Meiotic nondisjunction

Alterations of chromosome structure

The Key Points in Review  Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles.  Morgan traced a gene to a specific chromosome.

Key Points in Review  Linked genes tend to be inherited together because they are located on the same chromosome.  Independent assortment of chromosomes and crossing over produce genetic variation (recombinants)

Key Points in Review  Geneticists can use recombination data to map a chromosomes genetic loci.  Chromosomal basis for gender is dependent upon the organism.

Key Points in Review  Sex-linked genes have unique patterns of inheritance.  Alterations of chromosome numbers or structure cause some genetic disorders.

Morgan’s Fruit Flies  A tutorial A tutorial A tutorial