Chapter 12: Patterns of Inheritance Guided Notes

From his experiments, Mendel was able to understand four things about the nature of heredity:

- The plants he crossed did not produce progeny of intermediate appearance, as a hypothesis of blending inheritance would have predicted. Instead, different plants inherited each trait intact, as a discrete characteristic. - For each pair of alternative forms of a trait, one alternative was not expressed in the F1 hybrids, although it reappeared in some F2 individuals. The form of the trait that "disappeared" must be present, but not expressed in the F1 individuals. - The alternative forms of traits examined were segregated among the progeny of a particular cross, some individuals exhibiting one form of the trait and some the other. - These alternative forms of traits appear in the F2 generation in the ratio of 3/4 dominant to 1/4 recessive. This characteristic 3:1 segregation is referred to as the Mendelian ratio for a monohybrid cross.

Features of albinism

- sensitive to the sun - females and mails are affected equally - most affected individuals have unaffected parents - single affected parent usually does not have affected offspring - affected offspring are more frequent when parents are related

Figure 12.4 Mendel's seven traits.

1. Flower Color 2. Seed Color 3. Seed Texture 4. Pod Color 5. Pod Shape 6. Flower Position 7. Plant Height

Mendel usually conducted his experiments in three stages:

1. Mendel allowed plants of a given variety to self-cross for multiple generations to assure himself that the traits he was studying were indeed true-breeding--that is, transmitted unchanged from generation to generation. 2. Mendel then performed crosses between true-breeding varieties exhibiting alternative forms of traits. He also performed reciprocal crosses: using pollen from a white-flowered plant to fertilize a purple-flowered plant, then using pollen from a purple-flowered plant to fertilize a white-flowered plant. 3. Finally, Mendel permitted the hybrid offspring produced by these crosses to self-fertilize for several generations, allowing him to observe the inheritance of alternative forms of a trait. Most important, he counted the numbers of offspring exhibiting each trait in each succeeding generation.

Mendel's five-element model

1. Parents do not transmit physiological traits directly to their offspring. Rather, they transmit discrete information for the traits, what Mendel called "factors." We now call these factors genes. 2. Each individual receives one copy of each gene from each parent. We now know that genes are carried on chromosomes, and each adult individual is diploid, with one set of chromosomes from each parent. 3. Not all copies of a gene are identical. The different forms of the same gene are called alleles. When two haploid gametes containing the same allele fuse during fertilization, the resulting offspring is said to be homozygous. When the two haploid gametes contain different alleles, the resulting offspring is said to be heterozygous. 4. The two alleles remain discrete--they neither blend with nor alter each other. Therefore, when the individual matures and produces its own gametes, the alleles segregate randomly into these gametes. 5. The presence of a particular allele does not ensure that the trait it encodes will be expressed. In heterozygous individuals, only one allele is expressed (the dominant one), and the other allele is present but unexpressed (the recessive one).

Figure 12.3 How Mendel conducted his experiments.

1. The anthers are cut away on the purple flower. 2. Pollen is obtained from the white flower. 3. Pollen is transferred to the purple flower. 4. All progeny result in purple flowers.

The dominant-to-recessive ratio among the F2 plants was always close to (blank).

3:1

The F2 generation exhibits four types of progeny in a (blank) ratio

9:3:3:1

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 antigens are on the red blood cell surface.

ABO blood group

A recessive pedigree:

Albinism

Alleles: I^AI^A, I^Ai (I^A dominant to i) Sugars Exhibited: Galactosamine Donates and Receives: Receives A and O; Donates to A and AB

Blood Type: A

Alleles: I^AI^B (codominant) Sugars Exhibited: Both galactose and galactosamine Donates and Receives: Universal receiver; Donates to AB

Blood Type: AB

Alleles: I^BI^B, I^Bi (I^B dominant to i) Sugars Exhibited: Galactose Donates and Receives: Receives B and O; Donates to B and AB

Blood Type: B

Alleles: ii (i is recessive) Sugars Exhibited: None Donates and Receives: Receives O; Universal donor

Blood Type: O

What happened when the two varieties of the garden pea, Pisum sativum, were crossed?

Both varieties were true-breeding, meaning that the offspring produced from self-fertilization remained uniform from one generation to the next. All of the progeny (offspring) of the cross between the two varieties had yellow seeds. Among the offspring of these hybrids, however, some plants produced yellow seeds and others, less common, produced green seeds.

Figure 12.5 The F2 generation is a disguised 1:2:1 ratio.

By allowing the F2 generation to self-fertilize, Mendel found from the offspring (F3) that the ratio of F2 plants was 1 true-breeding dominant: 2 not-true-breeding dominant: and 1 true-breeding recessive.

Why did Mendel choose the garden pea?

First, many earlier investigations had produced hybrid peas by crossing different varieties, so Mendel knew that he could expect to observe segregation of traits among the offspring. Second, a large number of pure varieties of peas were available. Third, pea plants are small and easy to grow, and they have a relatively short generation time. A fourth advantage of studying peas is that both the male and female sexual organs are enclosed within each pea flower, and gametes produced by the male and female parts of the same flower can fuse to form viable offspring, a process termed self-fertilization.

What is albinism?

It is a condition in which the pigment melanin is not produced. It is known to result from mutations in multiple genes; the common feature is the loss of pigment from hair, skin, and eyes.

A dominant pedigree:

Juvenile glaucoma

Figure 12.7 Dominant pedigree for hereditary juvenile glaucoma.

Males are shown as squares and females are shown as circles. Affected individuals are shown shaded. The dominant nature of this trait can be seen in the trait's appearing in every generation, a feature of dominant traits.

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.

Mendelian ratio

Figure 12.8 Recessive pedigree for albinism.

One of the two individuals in the first generation must be heterozygous and individuals II-2 and II-4 must be heterozygous. Each affected individual, neither parent is affected, but both must be heterozygous (carriers). A double line indicates a consanguineous mating (between relatives) that, in this case, produced affected offspring.

The two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.

Principle of Segregation

A diagrammatic way of showing the possible genotypes and phenotypes of genetic crosses.

Punnett square

Who crossed two varieties of the garden pea, Pisum sativum?

T.A. Knight, an English landholder

What does juvenile glaucoma cause?

The disease causes degeneration of nerve fibers in the optic nerve, leading to blindness.

Before the 20th century, two concepts provided the basis for most thinking about heredity.

The first was that heredity occurs within species. The second was that traits are transmitted directly from parents to offspring.

Josef Kolreuter cross-fertilized different strains of tobacco and obtained fertile offspring. What happened?

The hybrids differed in appearance from both parent strains. When individuals within the hybrid generation were crossed, their offspring were highly variable. Some of these offspring resembled plants of the hybrid generation, but a few resembled the original strains. The variation observed in second-generation offspring contradicts the theory of direct transmission.

A number of assumptions are built into Mendel's model that are oversimplifications.

These assumptions include that each trait is specified by a single gene with two alternative alleles; that there are no environmental effects; and that gene products act independently.

One of two or more alternative states of a gene.

allele

Describes a case in which two or more alleles of a gene are each dominant to other alleles but not to each other. The phenotype of a heterozygote for codominant alleles exhibits characteristics of each of the homozygous forms. For example, in human blood types, a cross between an AA individual and a BB individual yields AB individuals.

codominance

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 by more than one gene.

continuous variation

A single genetic cross involving two different traits, such as flower color and plant height.

dihybrid cross

An allele that is expressed when present in either the heterozygous or the homozygous condition.

dominant

Interaction between two nonallelic genes in which one of them modifies the phenotypic expression of the other.

epistasis

The offspring resulting from a cross between a parental generation (P); in experimental crosses, these parents usually have different phenotypes.

first filial (F1) generation

Inheritance itself was viewed as traits being borne through (blank), usually identified as blood, that led to their blending in offspring. Why was this idea challenged?

fluid; This led to a paradox. If no variation enters a species from outside, and if the variation within each species blends in every generation, then all members of a species should soon have the same appearance.

The genetic constitution underlying a single trait or set of traits.

genotype

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" (that is, the genotype is W/w).

heterozygous

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" (i.e., the genotype is W/W or w/w).

homozygous

The mating of unlike parents.

hydridization

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'clocks yields pink heterozygotes.

incomplete dominance

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 orientations of different homologous pairs during metaphase I of meiosis. For genes on the same chromosomes, this occurs when the two loci are far enough apart for roughly equal numbers of odd- and even-numbered multiple crossover events.

independent assortment

A (blank) is a cross that follows only a single trait with two variations, such as white- and purple-colored flowers.

monohybrid cross

A consistent graphic representation of matings and offspring over multiple generations for a particular genetic trait, such as albinism or hemophilia.

pedigree

The realized expression of the genotype; the physical appearance or functional expression of a trait.

phenotype

Condition in which an individual allele has more than one effect on production of the phenotype.

pleiotropy

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.

polygenic inheritance

A trait that is determined by the effects of more than one gene; such a trait usually exhibits continuous variation rather than discrete either-or values.

quantitative trait

An allele that is expressed only when present in the homozygous condition, but is "hidden" by the expression of a dominant allele in the heterozygous condition.

recessive

A genetic cross involving a single trait in which the sex of the 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.

reciprocal cross

The rule stating that for two independent events, the probability of either event occurring is the sum of the individual probabilities.

rule of addition

The rule stating that for two independent events, the probability of both events occurring is the product of the individual probabilities.

rule of multiplication

The offspring resulting from a cross between members of the first filial (F1) generation.

second filial (F2) generation

The process by which alternative forms of traits are expressed in offspring rather than blending each trait of the parents in the offspring.

segregation

The union of egg and sperm produced by a single hermaphroditic organism.

self-fertilization

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.

testcross

Although hidden in the F1 generation,

the recessive trait had reappeared among some F2 individuals.

The example of ABO blood types in humans involves an allelic series with (blank) alleles.

three

Said of a breed or 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.

true-breeding

Although any diploid individual can carry only (blank) alleles for a gene, there may be more than two alleles in a population.

two