This lesson looks at population attributes, regulation, and growth. It also covers population genetics, particularly genetic variations, natural selection, genetic drift, genetic migration, and speciation Gregor Mendel's studies into Monohybrid and Dihybrid crossing and Charles Darwin's study of evolution and natural selection have led to studies that actively manipulate the phenotype of offspring by selective breeding in animals and plants.
The evolution of the species of the genus "Homo" led to the emergence of modern humans. Find out more about human evolution in this tutorial that elaborates on the different Homo species in the early geologic time.
This tutorial looks at sex determination via the sex chromosomes, X and Y. Read it to get more info on X and Y chromosomes and the genetic traits inherited via these two This tutorial investigates the genetic diversity in more detail. 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. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, recessive X-linked traits appear more frequently in males than females. In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male.
This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous. 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 Figure Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted to subsequent generations.
Figure The son of a woman who is a carrier of a recessive X-linked disorder will have a 50 percent chance of being affected. A daughter will not be affected, but she will have a 50 percent chance of being a carrier like her mother.
Occasionally, a nonfunctional allele for an essential gene can arise by mutation and be transmitted in a population as long as individuals with this allele also have a wild-type, functional copy. The wild-type allele functions at a capacity sufficient to sustain life and is therefore considered to be dominant over the nonfunctional allele.
In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the nonfunctional allele. Because the gene is essential, these individuals might fail to develop past fertilization, die in utero , or die later in life, depending on what life stage requires this gene.
An inheritance pattern in which an allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered non-lethal phenotype is referred to as recessive lethal. For crosses between heterozygous individuals with a recessive lethal allele that causes death before birth when homozygous, only wild-type homozygotes and heterozygotes would be observed.
The genotypic ratio would therefore be In other instances, the recessive lethal allele might also exhibit a dominant but not lethal phenotype in the heterozygote.
For instance, the recessive lethal Curly allele in Drosophila affects wing shape in the heterozygote form but is lethal in the homozygote. A single copy of the wild-type allele is not always sufficient for normal functioning or even survival. The dominant lethal inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age.
Individuals with mutations that result in dominant lethal alleles fail to survive even in the heterozygote form. Dominant lethal alleles are very rare because, as you might expect, the allele only lasts one generation and is not transmitted. However, just as the recessive lethal allele might not immediately manifest the phenotype of death, dominant lethal alleles also might not be expressed until adulthood. Once the individual reaches reproductive age, the allele may be unknowingly passed on, resulting in a delayed death in both generations.
People who are heterozygous for the dominant Huntington allele Hh will inevitably develop the fatal disease. When true-breeding or homozygous individuals that differ for a certain trait are crossed, all of the offspring will be heterozygotes for that trait. If the traits are inherited as dominant and recessive, the F 1 offspring will all exhibit the same phenotype as the parent homozygous for the dominant trait. If these heterozygous offspring are self-crossed, the resulting F 2 offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive.
Because homozygous dominant and heterozygous individuals are phenotypically identical, the observed traits in the F 2 offspring will exhibit a ratio of three dominant to one recessive.
Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles of a gene to exist in a population.
In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two.
Finally, some alleles can be lethal. Recessive lethal alleles are only lethal in homozygotes, but dominant lethal alleles are fatal in heterozygotes as well. Skip to main content. Genetics and Inheritance. Search for:. Characteristics and Traits Learning Objectives By the end of this section, you will be able to: Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems Develop a Punnett square to calculate the expected proportions of genotypes and phenotypes in a monohybrid cross Explain the purpose and methods of a test cross Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, recessive lethals, multiple alleles, and sex linkage.
Art Connection Figure 2. Art Connection Figure 3. Evolution Connection Multiple Alleles Confer Drug Resistance in the Malaria Parasite Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae Figure 7a , and is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia.
Art Connection Figure 9. Link to Learning Watch this video to learn more about sex-linked traits. What are the genotypes of the individuals labeled 1, 2 and 3?
The gene for flower position in pea plants exists as axial or terminal alleles. Given that axial is dominant to terminal, list all of the possible F 1 and F 2 genotypes and phenotypes from a cross involving parents that are homozygous for each trait.
Express genotypes with conventional genetic abbreviations. Use a Punnett square to predict the offspring in a cross between a dwarf pea plant homozygous recessive and a tall pea plant heterozygous. What is the phenotypic ratio of the offspring? Can a human male be a carrier of red-green color blindness? Answers You cannot be sure if the plant is homozygous or heterozygous as the data set is too small: by random chance, all three plants might have acquired only the dominant gene even if the recessive one is present.
If the round pea parent is heterozygous, there is a one-eighth probability that a random sample of three progeny peas will all be round.
Individual 1 has the genotype aa. Individual 2 has the genotype Aa. Individual 3 has the genotype Aa. Half of the female offspring would be heterozygous X W X w with red eyes, and half would be homozygous recessive X w X w with white eyes. Half of the male offspring would be hemizygous dominant X W Y withe red yes, and half would be hemizygous recessive X w Y with white eyes.
Because axial is dominant, the gene would be designated as A. F 1 would be all heterozygous Aa with axial phenotype. F 2 would have possible genotypes of AA , Aa , and aa ; these would correspond to axial, axial, and terminal phenotypes, respectively.
Clockwise from the top left, the genotypes listed within the boxes will be Tt , Tt , tt , and tt. The phenotypic ratio will be 1 tall:1 dwarf. No, males can only express color blindness.
They cannot carry it because an individual needs two X chromosomes to be a carrier. Sumiti Vinayak, et al. Licenses and Attributions. CC licensed content, Shared previously. Britannica English: Translation of characteristic for Arabic Speakers.
Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free! Log in Sign Up. Save Word. Essential Meaning of characteristic. Full Definition of characteristic Entry 1 of 2. Definition of characteristic Entry 2 of 2. Choose the Right Synonym for characteristic Adjective characteristic , individual , peculiar , distinctive mean indicating a special quality or identity.
Examples of characteristic in a Sentence Noun the ability to fashion tools and other characteristics that distinguish human beings from other animals Adjective He responded to their comments with characteristic good humor. Recent Examples on the Web: Noun Seriously Stewardship is an important characteristic of a quality board advisor.
First Known Use of characteristic Noun , in the meaning defined at sense 1 Adjective , in the meaning defined above. History and Etymology for characteristic Noun see character entry 1 Adjective see character entry 1. Learn More About characteristic.
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