Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes.
When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. Figure 4. In Drosophila, the gene for eye color is located on the X chromosome. Clockwise from top left are brown, cinnabar, sepia, vermilion, white, and red. Red eye color is wild-type and is dominant to white eye color. Eye color in Drosophila was one of the first X-linked traits to be identified.
Thomas Hunt Morgan mapped this trait to the X chromosome in In flies, the wild-type eye color is red X W and it is dominant to white eye color X w Figure 4. Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous , because they have only one allele for any X-linked characteristic.
Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males. In an X-linked cross, the genotypes of F 1 and F 2 offspring depend on whether the recessive trait was expressed by the male or the female in the P 0 generation. With regard to Drosophila eye color, when the P 0 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 Figure 5. Figure 5. Punnett square analysis is used to determine the ratio of offspring from a cross between a red-eyed male fruit fly and a white-eyed female fruit fly.
What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color? Discoveries in fruit fly genetics can be applied to human genetics.
When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to percent of her offspring. In humans, the alleles for certain conditions some forms of color blindness, hemophilia, and muscular dystrophy are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects.
Parsons, P. The evolution of overdominance: Natural selection and heterozygote advantage. Nature , 7—12 link to article. Stratton, F. The human blood groups. Nature , link to article. Chromosome Theory and the Castle and Morgan Debate. Discovery and Types of Genetic Linkage. Genetics and Statistical Analysis. Thomas Hunt Morgan and Sex Linkage. Developing the Chromosome Theory.
Genetic Recombination. Gregor Mendel and the Principles of Inheritance. Mitosis, Meiosis, and Inheritance. Multifactorial Inheritance and Genetic Disease. Non-nuclear Genes and Their Inheritance. Polygenic Inheritance and Gene Mapping. Sex Chromosomes and Sex Determination. Sex Determination in Honeybees. Test Crosses. Biological Complexity and Integrative Levels of Organization.
Genetics of Dog Breeding. Human Evolutionary Tree. Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships.
Phenotype Variability: Penetrance and Expressivity. Citation: Miko, I. Nature Education 1 1 Why can you possess traits neither of your parents have? The relationship of genotype to phenotype is rarely as simple as the dominant and recessive patterns described by Mendel.
Aa Aa Aa. Complete versus Partial Dominance. Figure 1. Figure Detail. Multiple Alleles and Dominance Series. Summarizing the Role of Dominance and Recessivity. References and Recommended Reading Keeton, W. Heredity 35 , 85—98 Parsons, P. Nature , 7—12 link to article Stratton, F.
Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. One parent flower was red and the other was white. This is an example of which of the following forms of inheritance? In the example above, the flower has both red and white petals due to co-dominant inheritance pattern of the red and white petal alleles. Polygenic inheritance occurs when two or more genes control one characteristic, such as skin color, eye color, and adult height.
A sugar flower has white petals, and is homozygous for this trait. Another sugar flower is homozygous for red petals. The white and red sugar flowers are crossed, and produce offspring with pink petals.
Which of the following terms describes this type of inheritance? Incomplete dominance is described by a phenotype that is not completely dominant over another. Therefore, it will be a "blending" of colors in the case of this question, therefore the petals are pink. Codominance is when both dominant traits are expressed, therefore if white was considered dominant and red was also a dominant trait, the petals would have spots of white and red, with no pink.
Polygenic inheritance is described by one characteristic influenced by multiple genes, which is not the case in this problem. Finally, epistasis involves the suppression of genes, however in this problem color is not suppressed. If you've found an issue with this question, please let us know.
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Hanley Rd, Suite St. Louis, MO Subject optional. Email address: Your name:. Possible Answers: Dominance. Correct answer: Incomplete dominance. Explanation : A phenotypic "blending" of two traits is referred to as incomplete dominance, indicating that neither trait is truly dominant over the other.
Report an Error. Two pure breeding plants are crossed. One plant has red flowers and the other has white flowers. Possible Answers: Half of the offspring would have red flowers, and half would have pink.
Half of the offspring would have red flowers, and half would have white. All offspring would have both red and white flowers. Correct answer: All offspring would have pink flowers. Explanation : The genotypes of the offspring can be determined by crossing the red flowers, RR , with the white flowers, rr.
RR x rr Offspring: all offspring are Rr. Possible Answers: Black or white, depending on each individual offspring. Correct answer: Black and white spotted. Explanation : Codominance means that more than one type of dominant allele for the same gene is present. Possible Answers: The alleles for black and brown fur exhibit complete dominance. The alleles for black and brown fur exhibit incomplete dominance.
Correct answer: The alleles for black and brown fur exhibit incomplete dominance. Explanation : Incomplete dominance is when more than one type of dominant allele for the same gene is present. Possible Answers: Red and white. Correct answer: Red, white, and red-white spotted. Explanation : The possible genotypes for this trait are RR , Rr , and rr. We know that RR is red and rr is white, since these genotypes are homozygous. This gives three total phenotypes: red, white, and red-white spotted.
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