Due to the stochastic inactivation, the patterning of orange and black fur is a distinctive quality of calicos. While the genetic information for the orange or black coat color exists in all cells, they are not equally expressed.
Around , Thomas Hunt Morgan began to explore the genetics of what was to become a model organism, Drosophila melanogaster Fruit fly. This small organism had a relatively short life cycle, great fecundity and was easily managed. From these flies that normally have red eye coloring, he and his students found white-eyed mutants. The lab noted that white-eyed flies were almost exclusively male.
This gender imbalance led Morgan to believe that the trait was sex-linked. In , Morgan published a paper that described the inheritance patterns of 5 eye-colors in Drosophila Morgan, Drosophila follows a sex determination based on the ratio of X:A chromosomes and not by the presence of a Y as in mammals.
A ratio results in a female and a ratio results in a male where the Y is ignored. By cataloging many mutations in the lab, he was able to construct a map of gene locations. His paper specifically stated that some traits were sex-linked and therefore residing on the sex chromosome. When performing crosses of white-eyed males to wild-type females, he continued to find white-eyed trait only in males. However, in the subsequent cross of females from that generation with white-eyed males, the presence of white-eyed males and females were revealed.
This indicated that the white-eyed trait was recessive and resided on the X chromosome. Morgan received the Nobel Prize in Physiology or Medicine in for his inference of chromosomes being a physical mechanism for packaging genetic information in the cells.
Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. Metabolism 2. Cell Respiration 3. Photosynthesis 9: Plant Biology 1. Xylem Transport 2. Phloem Transport 3. Plant Growth 4. Plant Reproduction Genetics 1. Meiosis 2. The "a" r ecessive allele will be expressed in his phenotype The "a" recessive allele will not be expressed in her phenotype male female There are about 1, human X-linked genes.
Most of them code for something other than female anatomical traits. Many of the non-sex determining X-linked genes are responsible for abnormal conditions such as hemophilia , Duchenne muscular dystrophy , fragile-X syndrome , some high blood pressure, congenital night blindness, G6PD deficiency, and the most common human genetic disorder, red-green color blindness.
X-linked genes are also responsible for a common form of baldness referred to as "male pattern baldness". None of their boys will inherit the harmful allele. Only girls receive X chromosomes from their fathers. Queen Victoria of England was a carrier of the gene for hemophilia. She passed the harmful allele for this X-linked trait on to one of her four sons and at least two of her five daughters. Her son Leopold had the disease and died at age 30, while her daughters were only carriers.
As a result of marrying into other European royal families, the princesses Alice and Beatrice spread hemophilia to Russia, Germany, and Spain. By the early 20th century, ten of Victoria's descendents had hemophilia. All of them were men, as expected. Queen Victoria with her husband and nine children in By comparison to the X chromosome, the much smaller Y chromosome has only about 26 genes and gene families.
With regard to Drosophila eye color, when the P 1 male expresses the white-eye phenotype and the female is homozygous red-eyed, all members of the F 1 generation exhibit red eyes. Now, consider a cross between a homozygous white-eyed female and a male with red eyes. Sex-linkage studies provided the fundamentals for understanding X-linked recessive disorders in humans, which include red-green color blindness and Types A and B hemophilia.
Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected.
Carrier females can manifest mild forms of the trait due to the inactivation of the dominant allele located on one of the X chromosomes. However, female carriers can contribute the trait to their sons, resulting in the son exhibiting the trait, or they can contribute the recessive allele to their daughters, resulting in the daughters being carriers of the trait.
Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are, therefore, not transmitted to subsequent generations. Learning Objectives Distinguish between sex-linked traits and other forms of inheritance.
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