Chapter 15: the chromosomal basis of inheritance

• Mendel proposed the existence of hereditary factor
• chromosomes theory of inheritance states Mendelian genes have specific positions on chromosomes & chromosomes undergo segregation and independent assortment
• First, solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan who was an embryologist
• The advantages of using fruit flies: prolific producers, new generation can be bred every two weeks and lastly, they only have 4 pairs of chromosomes
• wild type are normal phenotypes for a character
• traits alternative to the wild type are called mutant phenotypes
• Morgan determined that the white-eyed mutant allele must be located on the X chromosomes
• There are two varieties of sex chromosomes: a larger X chromosomes and a smaller Y chromosomes
• the SRY gene on the Y chromosomes codes for a protein for the development of male anatomical features (testes)
• females are XX and males are XY
• a gene that is located on either sex chromosomes is called a sex-linked gene
• genes on the Y chromosomes are called Y-linked genes
• genes on the X chromosomes are called X-linked genes
• X chromosomes have genes for many character-unrelated to sex
• Y chromosomes mainly encodes genes related to sex determination
• X-linked gene follow specific patterns of inheritance
• For a recessive X-linked trait to be expressed a female needs two copies of the allele and a male need only one copy of the allele
• Duchenne muscular dystrophy is a disease characterized by weakening muscles and loss of coordination
• Hemophilia is an X-linked recessive disorder defined by the absence of one or more of the proteins required for blood clotting
• genes located on the same chromosomes that tend to be inherited together are called linked genes
• Morgan states genes do not assort independently and reasons they are on the same chromosomes
• genetic recombination is the production of offspring with combination of traits differing from either parent
• recombinant chromosomes bring alleles together in new combinations in gametes
• a genetic map is an ordered list of the genetic positions along a particular chromosomes
• Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them & therefore the higher the recombination frequency
• a linkage map is a genetic map of chromosomes based on recombination frequencies
• distances between genes can be expressed as map units
• the cytogenetic map indicates the positions of genes for chromosomal features
• aneuploidy results from the fertilization of gametes in which nondisjunction occurs
• monosomic zygote has only one copy of a particular chromosome
• a trismic zygote has three copies of a particular chromosomes
• polyploidy is a condition in which an organisms has more than two complete set of chromosomes
• triploidy (3N) has three sets of chromosomes
• tetraploidy (4n) has four sets of chromosomes
• deletion removes a chromosomal segment
• duplication repeats a segment
• inversion reverses orientation of a segment within a chromosomes
• translocation moves a segment from one chromosome to another
• Klinefelter syndrome is the result of an extra chromosomes in a male, producing XXY individuals
• Monosomy X called Turner syndrome produces X0 females who are sterile, known as viable monosomy in humans