Chapter 12: Patterns of Inheritance

In polygenic inheritance, more than one gene can affect a single trait

Often, the relationship between genotype and phenotype is more complicated than a single allele producing a single trait.

A dominant pedigree: Juvenile glaucoma

One of the most extensive pedigrees yet produced traced the inheritance of a form of blindness caused by a dominant allele.

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.

Alleles

One of two or more alternative states of a gene.

Dihybrid Crosses: The Principle of Independent Assortment

The Principle of Segregation explains the behavior of alternative forms of a single trait in a monohybrid cross.

Mendelian Ratio

The characteristic 3:1 segregation is referred to as this for a monohybrid cross.

Early plant biologists produced hybrids and saw puzzling results

The first investigator to achieve and document successful experimental hybridizations was Josef Kolreuter, who in 1760 cross-fertilized (or crossed, for short) different strains of tobacco and obtained fertile offspring.

The human ABO blood group system

The gene that determines ABO blood types encodes an enzyme that adds sugar molecules to proteins on the surface of red blood cells.

Genotype

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

In epistasis, interactions of genes alter genetic ratios

The last simplistic assumption in Mendel's model is that the products of genes do not interact.

First Filial Generation (F1)

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

Second Filial Generation (F2)

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

Siamese cat.

The pattern of coat color is due to an allele that encodes a temperature-sensitive form of the enzyme tyrosinase.

Segregating

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

Analyzing a dihybrid cross.

The ratio of the four possible combinations of phenotypes is predicted to be 9:3:3:1, the ratio that Mendel found.

Phenotype

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

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 rule of multiplication

The second rule, and by far the most useful for genetics, deals with the outcome of independent events.

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. 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.

The model has five elements.

1. Parents do not transmit physiological traits directly to their offspring. 2. Each individual receives one copy of each gene from each parent. 3.Not all copies of a gene are identical. 4. The two alleles remain discrete-they neither blend with nor alter each other. 5.The presence of a particular allele does not ensure that the trait it encodes will be expressed.

The different combinations of the three alleles produce four different phenotypes:

1. Type A individuals add only galactosamine. 2. Type B individuals add only galactose. 3. Type AB individuals add both sugars and are IAIB heterozygotes (one genotype). 4. Type O individuals add neither sugar and are ii homozygotes (one genotype).

Pedigree

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

Punnett Square

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

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.

Monohybrid Crosses: The Principle of Segregation

A monohybrid cross is a cross that follows only two variations on a single trait, such as white- and purple-colored flowers.

Some human traits exhibit dominant/recessive inheritance

A number of human traits have been shown to display both dominant and recessive inheritance.

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 Traits

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.

The Mystery of Heredity

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.

The F2 generation exhibits both traits in a 3:1 ratio

After allowing individual F1 plants to mature and self-fertilize, Mendel collected and planted the seeds from each plant to see what the offspring in the second filial generation, or F2, would look like.

Recessive Genetic Traits

Albinism, Alkaptonuria, Red/Green color blindness, Cystic Fibrosis, Duchenne muscular dystrophy, Hemophilia, and Sickle cell anemia.

Extensions to Mendel

Although Mendel's results did not receive much notice during his lifetime, three different investigators independently rediscovered his pioneering paper in 1900, 16 years after his death.

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.

A recessive pedigree: Albinism

An example of inheritance of a recessive human trait is albinism, a condition in which the pigment melanin is not produced.

Phenotypes may be affected by the environment

Another assumption, implicit in Mendel's work, is that the environment does not affect the relationship between genotype and phenotype.

Height is a continuously varying trait.

Because many genes contribute to height and tend to segregate independently of one another, the cumulative contribution of different combinations of alleles to height forms a continuous distribution of possible heights, in which the extremes are much rarer than the intermediate values.

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

Mendel used mathematics to analyze his crosses

Born in 1822 to peasant parents in Austria, Gregor Mendel was educated in a monastery and went on to study science and mathematics at the University of Vienna, where he failed his examinations for a teaching certificate.

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: 1 not-true-breeding dominant: and 1 true-breeding recessive.

Pleiotropic

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

Traits in a dihybrid cross behave independently

Consider a cross involving different seed shape alleles (round, R, and wrinkled, r) and different seed color alleles (yellow, Y, and green, y).

The rule of addition

Consider a six-sided die instead of a coin: for any roll of the die, only one outcome is possible, and each of the possible outcomes are mutually exclusive.

Codominant

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 exhibit 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.

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'clocks yields pink heterozygotes.

Polygenic Inheritance

Describes a mode of inheritance 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.

The garden pea, Pisum sativum.

Easy to cultivate and able to produce many distinctive varieties, the garden pea was a popular experimental subject in investigations of heredity as long as a century before Gregor Mendel's experiments.

Introduction

Every living creature is a product of the long evolutionary history of life on Earth.

Heredity and family resemblance.

Family resemblances are often strong-a visual manifestation of the mechanism of heredity.

Practical considerations for use of the garden pea

For his experiments, Mendel chose the garden pea, the same plant Knight and others had studied.

Mendel's Principle of Independent Assortment explains dihybrid results

From a total of 556 seeds from self-fertilized dihybrid plants, he observe the following results: 1. 315 round yellow (R_Y_) 2. 108 round green (R_yy) 3. 101 wrinkled yellow (rr Y_) and 4. 32 wrinkled green (rr yy).

Mendel's Principle of Segregation explains monohybrid observations

From his experiments, Mendel was able to understand four things about the nature of heredity: 1. The plants he crossed did not produce progeny of intermediate appearance, as a hypothesis of blending inheritance would have predicted. 2. For each pair of alternative forms of a trait, one alternative was not expressed in the F1 hybrids, although it appeared in some F2 individuals. 3. The pairs of alternative traits examined were segregated among the progeny of a particular cross, some individuals exhibiting one trait and some the other. 4. These alternative traits were expressed in the F2 generation in the ratio of 3/4 dominant to 1/4 recessive.

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

Incomplete dominance.

In a cross between a red-flowered (genotype CRCR) Japanese four o'clock and a white-flowered one (CWCW), neither allele is dominant.

Principle of Independent Assortment

In a dihybrid cross, the alleles of each gene assort independently.

How Mendel conducted his experiments.

In a pea plant flower, petals enclose both the male anther (containing pollen grains, which gave rise to haploid sperm) and the female carpel (containing ovules, which give rise to haploid eggs).

The F2 generation exhibits four types of progeny in a 9:3:3:1 ratio

In analyzing these results, we first consider the number of possible phenotypes.

Incomplete Dominance

In incomplete dominance, the heterozygote is intermediate in appearance between the two homozygotes.

Using a testcross to determine unknown genotypes.

Individuals with a dominant phenotype, such as purple flowers, can be either homozygous for the dominant allele, or heterozygous.

Epistasis

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

Dominant pedigree for hereditary juvenile glaucoma.

Males are shown as squares and females are shown as circles.

Genes may have more than two alleles

Mendel always looked at genes with two alternative alleles.

Mendel's seven traits.

Mendel studied how differences among varieties of peas were inherited when the varieties were crossed.

Mendel's experimental design

Mendel was careful to focus on only a few specific differences between the plants he was using and to ignore the countless other differences he must have seen.

The 3:1 ratio is actually 1:2:1

Mendel went on to examine how the F2 plants passed traits to subsequent generations.

Dominance is not always complete

Mendel's idea of dominant and recessive traits can seem hard to explain in terms of modern biochemistry.

The principle of segregation

Mendel's model account for the ratios he observed in a neat and satisfying way.

Dominant Genetic Traits

Middigital hair, Brachydactyly, Huntington disease, Phenylthiocarbamide (PTC) sensitivity, Camptodactyly, Hypercholesterolmia (the most common human Mendelian disorder), and Polydactyly.

Codominance

Most genes in a population possess several different alleles and often no single allele is dominant; instead, each allele has its own effect, and the heterozygote shows some aspect of the phenotype of both homozygotes.

In pleiotropy, a single gene can affect more than one trait

Not only can more than one gene affect a single trait, but a single gene can affect more than one trait.

Probability: Predicting the Results of Crosses

Probability allows us to predict the likelihood of the outcome of random events.

Dihybrid cross probabilities are based on monohybrid cross probabilities

Probability analysis can be extended to the dihybrid case.

True-Breeding

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.

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.

ABO blood groups illustrate both codominance and multiple alleles.

There are three alleles of the I gene: IA, IB, and i.

Mendel's five-element model

To explain these results, Mendel proposed a simple model that has become one of the most famous in the history of science, containing simple assumptions and making clear predictions.

The Testcross: Revealing Unknown Genotypes

To test his model further, Mendel devised a simple and powerful procedure called the testcross.

The Punnett square allows symbolic analysis

To test his model, Mendel first expressed it in terms of a simple set of symbols.

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.

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 by more than one gene.

A dihybrid Punnett square

We can then construct a Punnett square with these gametes to generate all possible progeny.

Two probability rules help predict cross results

We can use probability to make predictions about the outcome of genetic crosses using only two simple rules.

The F1 generation exhibits only one of two traits, without blending

When Mendel crossed white-flowered and purple-flowered plants, the hybrid offspring he obtained did not have flowers of intermediate color, as the hypothesis of blending inheritance would predict.