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Transcript
Chromosomes and Genetics Early Discoveries There has always been a curiosity about the human body and how it works During the middle ages, some individuals would sneak into caves to dissect corpses Many would draw their observations Early Discoveries Early geneticists had to wait for the emergence of the light microscope before investigations could progress The discovery of the nucleus in 1831 was a big step towards the discoveries we have today Chromosomal Theory In 1902, Walter S. Sutton and Theodor Boveri each observed that chromosomes came in pairs that segregated (separated) during meiosis These separated chromosomes then formed new pairs when the egg and sperm united Chromosomal Theory The union of two different alleles in offspring and the formation of new alleles could now be explained and supported by scientific evidence Sutton and Boveri knew that expression of traits like eye colour was not tied to just the male or female sex cell They deduced that Mendel’s factors (alleles) must be found on the chromosomes Chromosomal Theory Humans have 44 autosomes (not involved in sex determination) and 2 sex chromosomes Sutton hypothesized that each chromosome carries genes Genes on the same chromosome are said to be linked genes Chromosomal Theory Chromosomal theory states: - chromosomes carry genes, the units of heredity - paired chromosomes segregate (separate) during meiosis. Each sex cell has half the number of chromosomes which is why gametes have only one of each of the paired alleles Each chromosome contains many different alleles and each gene has a specific locus on a particular chromosome Sex Linked Traits Thomas Morgan Hunt was among the first to use Drosophilia melanogaster to study principles of inheritance Fruit flies are ideal to study because: - Reproduce rapidly - Early mating age - Small size - Easy to tell male from female Sex Linked Traits When looking at flies, Morgan was interested in tracing the inheritance of the allele coding for white eyes He mated a white eye male with a red eye female and all offspring had red eyes He then mated to hybrids from the F1 generation This created ¾ red eyes and ¼ white eyes Sex Linked Traits Further examination showed that all the females had red eyes and only the males had white eyes Why?! Morgan turned to cytology (study of cell formation, structure, and function) Sex Linked Traits Previous researchers had found that females have 4 homologous pairs and males only have 3 → the fourth pair is only partially homologous Males had one X chromosome paired with a small hook shaped Y chromosome Females have two complete X chromosomes Sex Linked Traits Since X and Y are not completely homologous, it was found that they contain different genes Sex Linked Traits Morgan concluded that since the Y chromosome was missing genes, it did not carry the gene to determine eye colour Traits determined by genes located on sex chromosomes like eye colour are called sexlinked traits Sex Linked Traits Let’s look at the flies again: Red eye female → XRXR White eye male → XrY XR is allele for red eye dominant Xr is allele for white eye recessive No symbol on Y because it doesn’t have an allele for the trait Sex Linked Traits Sex Linked Traits Although Morgan did not find any white eyed females in his experiments, they do exist in nature This is rare because the female will need the recessive allele on both X chromosomes Human Sex Linked Traits In humans, it is estimated that the X chromosome carries between 100-200 genes while the Y chromosome has less than 100 genes Examples of sex linked traits in humans is red-green colour blindness, hemophilia, near-sightedness, night blindness Human Sex Linked Traits Recessive lethal X-linked disorders in humans, like infantile spinal muscular atrophy, occur more frequently in males Recessive lethal traits result in death or severe malformation of the offspring Barr Bodies A Barr body is a small, dark spot of chromatin located in the nucleus of a female mammalian cell These Barr bodies show that not all female cells are identical; some cells have one X chromosome inactive, while some have the other This means that some cells may express a certain trait while others express its alternate form Barr Bodies Example: If a female is heterozygous for anhidrotic ectodermal dysplasia, she will have patches of skin that contain sweat glands and patches that do not In normal skin, the X chromosome with the recessive allele is inactivated and sweat glands are produced In afflicted skin, the X chromosome with the recessive allele is activated so no sweat glands are produced Lab Exercise Page 632 Read through and analyze the pedigree chart Answer the Analysis questions This activity is homework and is due on Tuesday.