Chapter 23 Flashcards
What is an allele?
Definition: An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel’s law of segregation.
What is autosomal chromosomes?
Autosomal chromosome: Any chromosome except for the sex chromosomes. Humans have 44 autosomal chromosomes, or autosomes.
Barr bodies?
The small, dark-staining mass of inactive X chromosome within the nucleus of non-dividing cell.
Barr bodies are previously used as the basis for sex determination especially during the Olympics (see Barr test).
What is codominance?
Codominance refers to a relationship between two alleles of a gene. It occurs when the contributions of both alleles (genes) are clearly visible and do not overpower each other in the phenotype. This also means that the genotype is heterozygous.
what is a dihybrid?
The hybrid of parents that differ at only two gene loci, for which each parent is homozygous with different alleles.
what is a dihybrid cross?
n contrast to a monohybrid cross, a dihybrid cross is a cross between F1 offspring (first-generation offspring) of two individuals that differ in two traits of particular interest. For example, Bb × Bb (see the Punnett). Example: B = brown. b = blue. BB = Dark brown. Bb = Brown (not blue). bb = Blue.
A dihybrid cross is often used to test for dominant and recessive genes in two separate characteristics. Such a cross has a variety of uses in Mendelian genetics.
Dominant
An allele or a gene that is expressed in an organism’s phenotype, masking the effect of the recessive allele or gene when present or pertaining to) An organism that expresses the dominant gene (or allele).
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a polypeptide or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains
Hemizygous
A chromosome in a diploid organism is hemizygous when only one copy is present.[2] The cell or organism is called a hemizygote. Hemizygosity is observed when one copy of a gene is deleted, or in the heterogametic sex when a gene is located on a sex chromosome. For organisms in which the male is heterogametic, such as humans, almost all X-linked genes are hemizygous in males with normal chromosomes because they have only one X chromosome and few of the same genes are on the Y chromosome. In a more extreme example, male honeybees (known as drones) are completely hemizygous organisms. They develop from unfertilized eggs and their entire genome is haploid, unlike female honeybees, which are diploid. Transgenic mice generated through exogenous DNA microinjection of an embryo’s pronucleus are also considered to be hemizygous because the introduced allele is expected to be incorporated into only one copy of any locus. A transgenic can later be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotypes of each litter.
Heterozygous
Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), Wrinkled: (rr).
Homozygous
Having identical alleles for a given trait.
example:
AA or aa
Heterozygous
Having two different alleles for a trait.
Example:
Aa
Law of Segregation
States that homologs or alleles must separate during meiosis.
Law of Independent Assortment
States that genes do not influence each other during segregation.
Recessive
Hereditary factor that expresses itself in the phenotype only when the genotype is homozygous
Phenotype
Outward appearance of a organism caused by the genotype and environmental influences.
Genotype
Genes of an individual for a particualr trait or traits; often designated by letters, such as BB or Aa.
Monohybrid
An individual trait that is heterozygous.
Incomplete dominance
Occurs when the heterozygote is intermediate in phenotype between the two homozygotes.
Punnett Square
Grid created to calculate the expected results of simple genetic crosses.
