Chapter 12 Learning Outcomes/Vocab

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 crosses

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.

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.

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"

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.

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.

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 for 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

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. Plant hybridizers before Mendel, however, had already cast doubt on this model by observing characteristics in hybrids that seemed to change in second-generation 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.

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

Hybridization

The mating of unlike parents

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.