Biology Chapter 12: Gene Inheritance
If the 2 genes in the previous question showed complete linkage, what would you predict for an F2 phenotypic ratio?
1 red tall: 2 pink tall: 1 white short
In a cross of Aa, Bb, cc X Aa, Bb, Cc, what is the probability of obtaining an individual with the genotype AA Bb Cc
1/16
What is the probability of obtaining an individual with the genotype bb from a cross between 2 individuals with the genotype Bb?
1/4
the F1 plants from the previous question are allowed to self-fertilize. The phenotypic ratio for the F2 should be:
3 purple; 1 white
Japanese 4 o'clocks that are red and tall are crossed to white short ones, producing an F1, that is pink and tall. If these genes assort independently and the F1 is self-crossed, what would you predict for the ratio of F2 phenotype?
3 red tall: 6 pink tall: 3 white tall: 1 red short: 2 pink short: 1 white short
Pedigree
A consistent graphic representation of matings and offspring over multiple generations for a particular genetic trait, such as albinism of hemophilia.
Punnett Square
A diagrammatic way of showing the possible genotypes and phenotypes of genetic crosses.
reciprocal cross
A genetic cross involving a single trait in which the sex of parents is reversed; for example, if pollen from a white-flowered plant is used to fertilize a purple flowered plant, the reciprocal cross would be pollen from a purple-flowered plant used to fertilize a white-flowered plant.
Testcross
A mating between a phenotypically dominant individual of unknown genotype and a homozygous "tester" done to determine whether the phenotypically dominant individual is homozygous or heterozygous for the relevant gene
ABO blood groups
A set of four phenotypes produced by different combinations of three alleles at a single locus; blood types are A, B, AB, and O, depending on which alleles are expressed as antigens on the red blood cell surface.
Mendel's model assumes that each trait is determined by a single factor with alternate forms. We now know that this is too simplistic and that
A single gene can affect more than one trait, and traits may be affected by more than one gene
dihybrid cross
A single genetic cross involving two different traits, such as flower color and plant height.
Quantitative trait
A trait that is determined by the effects of more than on gene; such as usually exhibits continuous variation rather than discrete either-or values.
Mendel's principle of segregation states that:
Alleles of a gene are segregated during gamete formation
Dominant
An allele that is expressed when present in either the heterozygous or the homozygous condition
Recessive
An allele that is only expressed when present in the homozygous condition, but being "hidden" by the expression of a dominant allele in the heterozygous condition.
Homozygous
Being a homozygote, having two identical alleles of the same gene; the term is usually applied to one or more specific loci, as in "homozygous with respect to the W locus"
incomplete dominance
Describes a case in which two or more alleles of a gene do not display clear dominance. The phenotype of a heterozygote is intermediate between the homozygous forms. For example, crossing red-flowered with white-flowered four o'clock yields pink heterozygous.
polygenic inheritance
Describes a mode of inheritance in which more than one gene affects a trait, such as height in human beings, polygenic inheritance may produce a continuous range of phenotypic values, rather than discrete either-or values.
Explain the genetic basis for observed alterations to Mendel's ratios
Genes may have more than two alleles, and these may not show simple dominance. In incomplete dominance, the heterozygote is intermediate between the two homozygotes, and in codominance the heterozygote shows aspects of both homozygotes, both of which alter the monohybrid ratio. The action of genes is not always independent, which can result in modified dihybrid ratios.
Heterozygous
Having two different alleles of the same gene; the term is usually applied to one or more specific loci, as in "heterozygous with respect to the W locus"
When you cross true-breeding tall and short tobacco plants you get an F1 that is intermediate in height. When this F1 is self-crossed it yields an F2 with a continuous distribution of heights: What is the best explanation for this data?
Height is determined by the additives effects of many genes.
Compare the segregation of alleles for different genes with the behavior of different homologues in meiosis
Independent assortment occurs because of the independent behavior of different homologous pairs of chromosomes during meiosis I.
Interpret data from testcrosses to infer unknown genotypes
Individuals showing the dominant phenotype can be either homozygous dominant or heterozygous. Unknown genotypes can be revealed using a testcross, which is a cross to a homozygous recessive individual. Heterozygotes produce both dominant and recessive phenotypes in equal numbers as a result of the testcross.
Epistasis
Interaction between two nonallelic genes in which one of them modifies the phenotypic expression of the other.
What properly distinguishes Mendel's investigation from previous studies?
Mendel quantified his results
Explain Mendel's principle of independent assortment
Mendel's analysis of dihybrid crosses revealed that the segregation of allele pairs for different traits is independent; this finding is known as Mendel's Principle of Independent Assortment.
Explain the advantages of Mendel's experimental system
Mendel's experiments with plants involved quantifying types of offspring and mathematically analyzing his observations. This quantification of results is what distinguished Mendel's research from that of earlier investigators, who only noted differences in a qualitative way.
Describe how assumptions in Mendel's model result in oversimplification
Mendel's model assumes that each trait is specified by one gene with only two alleles, no environmental effects alter a trait, and gene products act independently. All of these prove to be oversimplifications
Evaluate the outcome of monohybrid cross
Mendel's monohybrid crosses refute the idea of blending. One trait disappears in the first generation (F1), then reappears in a predictable ratio in the next (F2). The trait observable in the F1 is called dominant, and the other recessive. In the F2, the ratio of observed dominant offspring to recessive is 3:1, and this represents a ratio of 1 homozygous dominant to 2 heterozygous to 1 homozygous recessive.
Alleles
One of two or more alternative states of a gene
An organisms _____________ is/are determined by its________________.
Phenotype; Genotype
Describe explanations for inheritance prior to Mendel
Prior to Mendel, concepts of inheritance did not form a consistent model. The dominant view was of blending inheritance, in which traits of parents were carried by fluid and "blended" in offspring
true-breeding
Said of a breed of variety of organism in which offspring are uniform and consistent from one generation to the next. This is due to the genotypes that determine relevant traits being homozygous.
Explain Mendel's principle of segregation
The Principle of Segregation states that alleles segregate into different gametes, which randomly combine at fertilization. The physical basis for segregation is the separation of homologues during anaphase I of meiosis.
Phenotypes like height in humans, which show a continuous distribution, are usually the result of
The action of multiple genes on a single phenotype
Mendelian ratio
The characteristic dominant-to-recessive phenotypic ratios that Mendel observed in his genetics experiments. For example, the F2 generation in a monohybrid cross shows a ratio of 3:1; the F2 generation in a dihybrid cross shows a ratio of 9:3:3:1.
Genotype
The genetic constitution underlying a single trait or sets of traits
First filial (F1) generation
The offspring resulting from a cross between a parental generation (P); in experimental crosses these parents usually have different phenotypes.
Second filial (F2) generation
The offspring resulting from a cross between members of the first filial (F1) generation.
Compare the segregation of alleles with the behavior of homologues in meiosis
The physical basis for segregation is the separation of homologues during anaphase I of meiosis.
Phenotype
The realized expression of the genotype' the physical appearance or functional expression of a trait.
Explain the rule of addition and the rule of multiplication
The rule of addition states that the probability of either of two events occurring is the sum of their individual probabilities. The rule of multiplication states that the probability of two independent events both occurring is the product of their individual probabilities.
Rule of multiplication
The rule stating that for two independent events, the probability of both events occurring is the product of the individual probabilities
Rule of addition
The rule stating that for two independent events, the probability of either event occurring is the sum of the individual probabilities.
The principle of segregation
The two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent during fertilization.
Self-fertilization
The union of egg and sperm produced by a single hermaphroditic organism.
Apply the rules of probability to genetic crosses
These rules can be applied to genetic crosses to determine the probability of particular genotypes and phenotypes. Results can then be compared against these predictions.
Discuss a genetic explanation for continuous variation
Traits produced by the action of multiple genes (polygenic inheritance) have continuous variation. One gene can affect more than one trait (pleiotropy).
continuous variation
Variation in a trait that occurs along a continuum, such as the trait of height in human beings; often occurs when a trait is determined more than one gene.
Evaluate the outcome of a dihybrid cross
When individuals that differ in two traits are crossed, and their progeny are intercrossed, the result is four different types that occur in a ratio of 9:3:3:1, Mendel's dihybrid ratio.
The F1 generation of the monohybrid cross purple (PP) X white (pp) flower pea plants should
all have purple flowers
Independent Assortment
in a dihybrid cross, describes the random assortment of alleles for each of the genes. For genes on different chromosomes this results from the random orientation of different homologous pairs during metaphase 1 of meiosis. For genes on the same chromosome, this occurs when the two loci are far enough apart for roughly equal numbers of odd- and even numbered multiple cross over events.
Hybridization
the mating of unlike parents