Which of the following choices represents the genotype of an individual diploid organism?
- Which of the following choices represents the genotype of an individual diploid organism?
- Which of the following might explain how two separate genes control a single trait?
- If two different alleles for the same trait have an equal effect on the phenotype, the alleles are
- An allele is
- Which of the following statements regarding genes is not true?
- Which of the following does not represent a phenotype?
Which of the following might explain how two separate genes control a single trait?
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Euchromatin refers to DNA that is transcriptionally active or relaxed. Heterchromatin refers to DNA that is transcriptionally inactive or condensed. Chromosomes are formed by pairs of chromatids that are joined at a centromere in a mature eukaryotic cell. Each chromatid has two arms, one long and one short. Telomeres are regions of high G-C content that are found at the end of each chromatid. These regions serve as buffers, preventing the DNA molecule from shortening during replication, and thus shortening with each round of cell replication. When a certain number of cell divisions, known as the Hayflick limit, is reached, the cell can no longer reproduce safely and dies.
Karyotypes depict the chromosomes in a cell of an organism. Karyotypes are normally done in a lab, where chromosomes are stained under a microscope, often with fluorescent dyes to distinguish between various chromosome regions. Photographs of a person’s condensed chromosomes are taken and digitally modified to reduce the size of individual chromosomes.
If two different alleles for the same trait have an equal effect on the phenotype, the alleles are
Mendel analyzed seven characteristics in his pea plants, each of which was represented as one of two variants, or traits. The physical expression of characteristics is done by the expression of genes borne on chromosomes. Peas have two identical or homologous copies of each chromosome, one from each parent, in their genetic makeup. Genes are ordered in the same linear order on each pair of homologous chromosomes. Peas, in other words, are diploid species, meaning that each chromosome is duplicated twice. The same can be said for a wide range of other plants and animals. Diploid species use meiosis to produce haploid gametes, which have one copy of each homologous chromosome and fuse to form a diploid zygote when fertilized.
For situations in which a single gene regulates a single trait, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. Alleles are gene variants that result from mutations and occur in the same relative positions on homologous chromosomes. Mendel studied the inheritance of genes with only two allele types, but in a natural population, it is normal to find more than two alleles for any given gene.
An allele is
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Which of the following statements regarding genes is not true?
The characteristics that Mendel analyzed in his pea plants were each represented as one of two versions, or traits, for example, green peas versus yellow peas). The physical manifestation of characteristics is achieved by the expression of genes—DNA sequences—carried on chromosomes, as we can see in more detail in later chapters. Peas have two identical (or homologous) copies of each chromosome, one from each parent (remember this concept from Chapter 11). Diploid species use meiosis to create haploid (1n) gametes that participate in fertilization by meiosis. A diploid organism has genetic copies that may or may not encode the same version of the characteristic while a single gene regulates a single characteristic, such as pea color. Alleles are the genetic differences between green and yellow peas, for example.
In a diploid organism, different alleles for a given gene interact to convey physical characteristics like pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism’s fundamental genetic makeup, that is, the combination of alleles, is called its genotype. When diploid species bear the same alleles for a given trait, they are said to be homozygous for the genotype; when they carry different alleles, they are said to be heterozygous. Homozygous dominant and heterozygous species with Mendelian expression (Section 12.1) will appear similar, that is, they will have different genotypes but the same phenotype. The recessive allele can only be found in homozygous recessive individuals.
Which of the following does not represent a phenotype?
Zygosity (/zasti/) is the degree to which both copies of a chromosome or gene have the same genetic sequence (the noun zygote comes from the Greek zygotos “yoked,” from zygon “yoke”). In other words, it is the degree to which alleles in an organism are identical.
Most eukaryotes are diploid, meaning they have two sets of chromosomes that match. The sequences at these loci may vary between the two chromosomes in a matching pair, and a few chromosomes may be mismatched as part of a chromosomal sex-determination scheme, but diploid species have the same loci on each of their two sets of homologous chromosomes. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they’re not the same, the organism is heterozygous for that gene. It is hemizygous if one allele is missing, and nullizygous if both alleles are missing.
The DNA sequence of a gene also differs from one individual to the next. Alleles are the names for these variants. Some genes have only one allele because there is little variation, and others have only one allele because deviation from it may be harmful or fatal. But most genes have two or more alleles. The prevalence of various alleles varies across the population. It’s possible that certain genes have alleles with similar distributions. Frequently, the various allele variations have little effect on the organism’s normal functioning. For certain genes, one allele may be normal while another is uncommon. In some cases, one allele causes disease while another allele is healthy.