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Chapter 15: The Chromosomal Basis of Inheritance Mendel was the first to propose the idea that ________ are located on __________________. It is known that human somatic cells have _________ chromosomes and have approximately ____________ genes. What can you conclude about the number of genes on each chromosome? _____________________________________________________________________________ 1 Thomas Hunt Morgan In the early 1900’s Thomas Hunt Morgan worked with __________________ (fruit flies) and confirmed Mendel’s idea that genes are located at ____________ ____________ on chromosomes. Discoveries 1. Morgan discovered that the ____ and ____ chromosomes (sex chromosomes) are responsible for determining the sex of an organism XX = ____________ XY = ____________ 2. Morgan discovered that there are genes on the sex chromosomes which led to his most well known discovery; ______ linkage ____________ _______ – genes located on the sex chromosomes - ____________= located on the X chromosome - ____________= located on the Y chromosome _________ ______ – genes located on the same chromosome - linked genes tend to be inherited ____________ because the chromosome is passed on as a ______ - example: the genes for red hair and freckles are located on the same chromosome so people that have red hair usually have freckles Sometimes however, genes on the same chromosome can be inherited separately. This is due to _________ ______ _________ ______ – when segments of genes on homologous chromosomes trade places - crossing over does not create new genes of delete old ones 2 - crossing over only changes the locations of genes creating genetic variation Chromosome Mapping _________ ______ – a diagram showing the location of genes on a chromosome - if two genes are very close together it is _________ that the two genes (traits) will be separated during crossing over - if genes are far apart from one another it is _________ _________ that the two genes will separate during crossing over - the ___________ of crossing over between two linked genes is ____________ to the distance between them ______ ______ – used to determine the probability that crossing over between genes will occur - genes that split up due to crossing over 1% of the time are said to be ______ ______ ______ part 3 Genes A and B are linked at 18 map units between them. A heterozygous individual Ab/aB could possibly produce four gametes. List the gametes and the frequencies with which each gamete would be produced. Sex Chromosomes In humans sex is determined by the ______ chromosome in the male gamete (sperm) - if the father donates an X the zygote will be _________ - if the father donates a Y the zygote will be _________ However, other organisms have different chromosomal systems for determining sex 1. Grasshoppers = ______system 2. Birds = _________ system 3. Bees = ____ sex chromosomes - 32 chromosomes = ______ - 16 chromosomes = ______ 4 Sex-Linked Genes Sex chromosomes (especially the ______) have genes for many characteristics and proteins - sex-linked usually refers to ___________ characters ___ ______traits (sex-linked) – genes located on the X chromosome - __________________ – inability to see certain colors - _________________ – wasting away of muscles - _________________ – failure of blood to clot ______-_________ ______ – when the presence of sex hormones influence the expression of certain human traits - males and females can have the same _____________ but have completely different _____________ - example: ________________________ X-Inactivation Even though females have two X chromosomes, ______ of the X chromosomes in each cell becomes __________________ ______ ________ – an inactive X chromosome that is normally found in the nucleus of somatic cells of a female 5 Errors in Chromosome Inheritance __________________ – when chromosomes fail to separate during meiosis - causes gametes to lack (22) or have an extra chromosome (24) - _____________ – a zygote lacking a chromosome (45) - _____________ – a zygote with one extra chromosome (47) 1. _________________ – having an extra 21st chromosome - short stature, heart defects, mental retardation 2. _________________ – males with an extra X chromosome (XXY) - male sex organs, female characteristics, usually sterile 3. _________________– females that have only one X chromosome (XO) - sex organs don’t develop and secondary sex characteristics fail to develop 6