12 inheritance, genes, and chromosomes. 12 inheritance, genes, and chromosomes 12.1 what are the...

12Inheritance, Genes, and

Chromosomes

12 Inheritance, Genes, and Chromosomes

12.1 What Are the Mendelian Laws of Inheritance?

12.2 How Do Alleles Interact?

12.3 How Do Genes Interact?

12.4 What Is the Relationship between Genes and Chromosomes?

12.5 What Are the Effects of Genes Outside the Nucleus?

12.6 How Do Prokaryotes Transmit Genes?

12 Inheritance, Genes, and Chromosomes

The population of Tasmanian devils was reduced by hunting and diseases, and the remaining individuals are closely related.

Now a type of cancer threatens the population, spreading due to the genetic relatedness.

Opening Question:

How can knowledge of genetics be used to save the Tasmanian devil?


Humans have been deliberately breeding plants and animals for thousands of years.

Two theories emerged to explain breeding experiments:

1. Blending inheritance—gametes contain hereditary determinants that blend in the zygote. Offspring phenotypes are intermediate.


2. Particulate inheritance—hereditary determinants are distinct and remain intact at fertilization.

Experiments performed by the monk, Gregor Mendel, supported the particulate theory.

Figure 12.1 Gregor Mendel and His Garden


Mendel’s theory of inheritance was published in 1866 but was largely ignored until 1900.

By that time chromosomes had been discovered and biologists realized that genes (hereditary determinants) might be carried on chromosomes.


Mendel worked with the garden pea, which has both male and female sex organs and normally self-fertilizes.


Mendel could control pollination and fertilization by removing the male organs and manually pollinating the flowers.


Pea plants have many varieties with easily recognized characteristics.

Character: observable physical feature (e.g., seed shape)

Trait: form of a character (e.g., round or wrinkled seeds)

Mendel worked with true-breeding varieties.


Mendel developed hypotheses to explain inheritance of different traits, then designed crossing experiments to test them.

• He transferred pollen from one plant to another: the parental generation, P

• The seeds and offspring were the first filial generation, F1


• In some experiments the F1 plants were allowed to self-pollinate and produce a second filial generation, F2


Mendel first performed monohybrid crosses: crossing parental varieties with contrasting traits for a single character.

• The F1 offspring were not a blend of the two parental traits. Only one of the traits was present (e.g., round seeds).

• Some F2 had wrinkled seeds. The trait had not disappeared because of blending.

These results supported the particulate theory.

Figure 12.2 Mendel’s Monohybrid Experiments (Part 1)


Mendel made monohybrid crosses for seven traits; all gave similar results.

The trait that occurred in the F1 and was more abundant in the F2 was called dominant, the other recessive.

In the F2 the ratio of dominant to recessive traits was about 3:1.


Mendel proposed that hereditary determinants (genes) occur in pairs and segregate from one another during formation of gametes.

He also proposed that each pea plant has two genes for each character, one inherited from each parent.


Diploid: the state of having two copies of each gene

Haploid: having just a single copy


Different traits arise from different forms of a gene (now called alleles).

• An organism that is homozygous for a gene has two alleles that are the same.

• An organism that is heterozygous for a gene has two different alleles. One may be dominant, (e.g., round [R]), and the other recessive, (e.g., wrinkled [r]).


Phenotype is the physical appearance of an organism.

Genotype is the genetic constitution of the organism.

Mendel proposed that the phenotype is the result of the genotype.


Mendel’s first law—

The law of segregation: the two copies of a gene separate during gamete formation; each gamete receives only one copy.

Working with Data 12.1: Mendel’s Monohybrid Experiments

Mendel’s monohybrid crosses were key to rejecting the blending theory of inheritance.

Mendel calculated ratios in the F2 generation, but did not do statistical analyses to determine whether the observed patterns might be due to chance alone.


Mendel’s data from the F2 generation after crossing green- and yellow-seeded plants:


Use the hypothesis that the ratio of yellow to green seeds in the F2 generation, 3:1, and perform a chi-square test to analyze the results for each plant in the table.

Question 1:

What can you conclude about this hypothesis from the individual plants?

How many crosses have P-values > 0.05?


Now total the data from all the plants and rerun the chi-square analysis.

Question 2:

What can you conclude?

What does your analysis indicate about the need for using a large number of organisms in studies of genetics?

Figure 12.3 Mendel’s Explanation of Inheritance (Part 1)


In the F2 generation, half of the gametes will have the R allele and the other half will have the r allele.

Allele combinations can be predicted using a Punnett square.

In-Text Art, Ch. 12, p. 236

Figure 12.3 Mendel’s Explanation of Inheritance (Part 2)


There are four possible combinations of alleles in the F2 generation: RR, Rr, rR, and rr.

If R is dominant, there are three ways to get round seeds, and only one way to get wrinkled seeds, resulting in the 3:1 phenotype ratio.


Genes are now known to be short sequences of DNA; a DNA molecule makes up a chromosome.

Alleles of a gene can separate during meiosis I.

Figure 12.4 Meiosis Accounts for the Segregation of Alleles (Part 1)

Figure 12.4 Meiosis Accounts for the Segregation of Alleles (Part 2)


Genes determine phenotypes through the proteins they encode.

Dominant genes are expressed; recessive genes may be mutated and no longer expressed, or encode non-functional proteins.

Wrinkled seed phenotype is due to absence of starch branching enzyme (SBE1).


One of Mendel’s hypotheses: there are two possible allele combinations (RR or Rr) for seeds with the round phenotype.

He tested this hypothesis by doing test crosses: F1 individuals are crossed with homozygous recessive individuals (rr).

His hypothesis accurately predicted the results of his test crosses.

Figure 12.5 Homozygous or Heterozygous?


Mendel’s second law—

Independent assortment: copies of different genes assort independently.

To test this he crossed true-breeding peas that differed in 2 characteristics: seed shape and color.

Round, yellow seeds (RRYY)

Wrinkled, green seeds (rryy)


F1 generation is RrYy—all round yellow.

Crossing the F1 generation (double heterozygotes) is a dihybrid cross.

Mendel asked whether, in the gametes produced by RrYy, the traits would be linked, or segregate independently.


• If linked, gametes would be RY or ry;

F2 would have three times more round yellow than wrinkled green.

• If independent, gametes could be RY, ry, Ry, or rY.

F2 would have nine different genotypes; phenotypes would be in 9:3:3:1 ratio.

Figure 12.6 Independent Assortment (Part 1)

Figure 12.6 Independent Assortment (Part 2)


The experiments supported the hypothesis of independent assortment.

It doesn’t always apply to genes located on the same chromosome.

But it is correct to say that chromosomes segregate independently during formation of gametes, and so do any two genes located on separate chromosome pairs.

Figure 12.7 Meiosis Accounts for Independent Assortment of Alleles (Part 1)

Figure 12.7 Meiosis Accounts for Independent Assortment of Alleles (Part 2)


One key to Mendel’s success was large sample sizes.

By counting many progeny, he was able to see clear patterns.

Later, geneticists began using probability calculations to predict ratios of genotypes and phenotypes, and statistical techniques to determine whether actual results matched predictions.


Probability:

• If an event is certain to happen, probability = 1.

• If an event cannot possibly happen, probability = 0.

• All other events have a probability between 0 and 1.


The multiplication rule—

Probability of two independent events happening together: multiply the probabilities of the individual events.

Tossing two coins: probability that both will come up heads =

½ × ½ = ¼

Figure 12.8 Using Probability Calculations in Genetics


The multiplication rule can be applied to a monohybrid cross:

F1 Rr plant self-pollinates; probability that gamete will have either gene is ½.

Probabilities of F2 genotypes:

RR = ½ × ½ = ¼

rr = ½ × ½ = ¼


The addition rule—

The probability of an event that can occur in two different ways is the sum of the individual probabilities.

In F2 there are two ways to get Rr,

thus ¼ + ¼ = ½

Result: 1:2:1 ratio of genotypes3:1 ratio of phenotypes


F2 in dihybrid crosses:

Probability of an F2 being round =

probability of heterozygote + probability of homozygote or ½ + ¼ = ¾

Joint probability that a seed will be round and yellow: ¾ × ¾ = 9/16


Human pedigrees can also show Mendel’s laws.

A pedigree is a family tree showing the occurrence of phenotypes and alleles.

Humans have small families, and so pedigrees don’t show the clear proportions that the pea plant phenotypes did.


But pedigrees can be used to determine whether a rare allele is dominant or recessive.

For rare dominant alleles:

• Every affected person has an affected parent.

• About half of the offspring of an affected parent are also affected.

Figure 12.9 Pedigree Analysis and Inheritance (Part 1)


For rare recessive alleles:

• Affected people can have two parents who are not affected.

• Only a small proportion of people are affected: about ¼ of children whose parents are both heterozygotes.

• There has usually been a marriage of relatives

Figure 12.9 Pedigree Analysis and Inheritance (Part 2)


Mendel’s laws are still valid today, and he laid the groundwork for future genetic studies.

But we have learned that things are often more complex:

• Over time genes accumulate differences and new alleles arise.

• There may be more than two alleles for one character.


• Alleles don’t always show simple dominant-recessive relationships.

• A single allele may have several phenotypic effects.


New alleles arise through mutations: stable, inherited changes in the genetic material.

The allele present in most of the population is called the wild type. Other alleles are mutant alleles.

Wild-type and mutant alleles reside at the same locus (specific position on a chromosome).


A genetic locus is polymorphic if the wild-type allele is present less than 99% of the time.

Any one individual has 2 alleles at a locus, but there may be many alleles in a population.

Multiple alleles often show a hierarchy of dominance.


Coat color in rabbits is determined by multiple alleles of the C gene:

• C determines dark gray

• cchd determines chinchilla

• ch determines Himalayan (point restricted)

• c determines albino

Figure 12.10 Multiple Alleles for Coat Color in Rabbits


Some alleles are neither dominant nor recessive—a heterozygote has an intermediate phenotype: incomplete dominance.

In the F2, the original phenotypes reappear, the alleles have not “blended.”

Figure 12.11 Incomplete Dominance Follows Mendel’s Laws


Codominance: two alleles produce phenotypes that are both present in the heterozygote.

The ABO blood group system results from three different alleles that encode an enzyme that adds specific groups to oligosaccharides on red blood cell surfaces.


The three alleles, IA, IB, and IO produce different versions of the enzyme.


The oligosaccharides act as antigens, molecules that are recognized by specific antibodies.

People make antibodies in the blood serum which react with foreign proteins—this protects the body from invasion by “non-self” molecules or organisms.


People in the A group make A antigen, and anti-B antibodies.

People in the B group make B antigen and anti-A antibodies.

People in the AB group make both A and B antigens, and neither antibody. The IA and IB alleles are codominant.


The enzyme in the O group is inactive, so neither antigen is made; they have both anti-A and anti-B antibodies.

Knowledge of blood groups is extremely important in determining compatibility of blood types for blood transfusions.

Figure 12.12 ABO Blood Reactions Are Important in Transfusions


Pleiotropic: one allele has multiple phenotypic effects.

The heritable human disease phenylketonuria results from a mutation in the gene for a liver enzyme that converts the amino acid phenylalanine to tyrosine.


Phenylalanine builds up to toxic levels, and affects development.

The mutated allele is pleiotropic: it results in mental retardation, and reduced hair and skin pigmentation.


Epistasis: phenotypic expression of one gene is influenced by another gene.

Coat color in Labrador retrievers:

For alleles B (black) and b (brown) to be expressed, allele E (pigment deposition) must be expressed.

An ee dog is yellow regardless of which B alleles are present. E is said to be epistatic to B.

Figure 12.13 Genes May Interact Epistatically (Part 1)

Figure 12.13 Genes May Interact Epistatically (Part 2)


Inbreeding: mating among close relatives; can result in offspring of low quality.

Close relatives tend to have the same recessive alleles.

Inbreeding is a concern for very small populations of endangered species.


A cross between two different true-breeding homozygotes can result in offspring with stronger, larger phenotypes.

Called “hybrid vigor” or heterosis.

Hybridized corn and other crops and animals have led to increased food production.

Figure 12.14 Hybrid Vigor in Corn


The mechanism of heterosis is debated.

Dominance hypothesis: extra growth can be explained by lack of inbreeding depression; hybrids are unlikely to be homozygous for deleterious recessive alleles.

Overdominance: new allele combinations result in superior traits.


Environment also affects phenotype.

Light, temperature, nutrition, etc. can affect expression of the genotype.

“Point restriction” coat patterns in Siamese cats and rabbits: the enzyme that produces dark fur is inactive at higher temperatures. Nose, ears, etc. are cooler, and thus darker in color.

Figure 12.15 The Environment Influences Gene Expression


Effects of genes and environment on phenotype:

• Penetrance: proportion of individuals with a certain genotype that show the phenotype

• Expressivity: degree to which genotype is expressed in an individual


The pea characters Mendel studies were discrete and qualitative.

For more complex characters, phenotypes vary continuously over a range—quantitative, or continuous, variation.

Quantitative variation is usually due to both genes and environment.

Figure 12.16 Quantitative Variation


Genes that determine these complex characters: quantitative trait loci.

Identifying these loci can help improve crop yields, understand disease susceptibility and behavior.


In 1909, Thomas Hunt Morgan and students at Columbia University pioneered the study of the fruit fly Drosophila melanogaster.

Much genetic research has been done with Drosophila because of its size, ease of breeding, and short generation time.


Some crosses performed with Drosophila did not yield expected ratios according to the law of independent assortment.

Some genes were inherited together; the two loci were on the same chromosome, or linked.

All of the loci on a chromosome form a linkage group.

Figure 12.17 Some Alleles Do Not Assort Independently (Part 1)

Figure 12.17 Some Alleles Do Not Assort Independently (Part 2)


Absolute linkage is rare—genes on the same chromosome do sometimes separate.

Genes may recombine during prophase I of meiosis by crossing over.

Chromosomes exchange corresponding segments. The exchange involves two chromatids in the tetrad; both chromatids become recombinant.

Working with Data 12.2: Some Alleles Do Not Sort Independently

Thomas Hunt Morgan studied linked genes by doing F1 × homozygous recessive test crosses.

They hypothesized that genes are linked together on chromosomes and that crossing over during meiosis gives rise to less frequent phenotypes.


Morgan first performed a dihybrid cross between black, normal-winged flies (bbVgVg) and gray, vestigial-winged flies (BBvgvg).

The F1 flies were interbred, yielding the F2 phenotypes shown in the table (Experiment 1).

Working with Data 12.2, Table 1


Question 1:

Compare these data (Experiment 1) with the expected data in a 9:3:3:1 ratio by using the chi-square test.

Are there differences, and are they significant?


To quantify linkage, Morgan crossed homozygous black, normal-winged females with homozygous gray, vestigial-winged males.

The results of this test cross are shown in the table (Experiment 2).


Question 2:

Are these genes linked (Experiment 2)?

If they are linked, what is the map distance between the genes?

Explain why these data are so different from the data shown in Figure 12.17.

Figure 12.17 Some Alleles Do Not Assort Independently


In a third experiment, Morgan crossed two strains of flies that were homozygous for the body color and wing genes.

The F1 flies were all gray and normal-winged, and these were interbred.

The results are shown in the table (Experiment 3).


Question 3:

What were the genotypes and phenotypes of the original parents that produced the F1? (Experiment 3)

Figure 12.18 Crossing Over Results in Genetic Recombination (Part 1)

Figure 12.18 Crossing Over Results in Genetic Recombination (Part 2)


Recombinant offspring phenotypes appear in recombinant frequencies:

Divide number of recombinant offspring by total number of offspring.

Recombinant frequencies are greater for loci that are farther apart.

Figure 12.19 Recombinant Frequencies


Recombinant frequencies can be used to infer the location of genes on a chromosome, and make genetic maps.


Gene sequencing has made mapping less important, but it is still a way to verify that a particular DNA sequence corresponds with a particular phenotype.

Linkage has allowed biologists to identify genetic markers linked to important genes: important in crop breeding and identifying people carrying medically significant mutations.


In some cases, parental origin of an allele is important, and thus an understanding of sex determination.

Corn: each adult produces both male and female gametes—monoecious.

Some plants and most animals are dioecious—male and female gametes are produced by different individuals.


In most dioecious organisms, sex is determined by differences in the chromosomes.

In many animals, sex is determined by a single pair of sex chromosomes which differ from one another.

Both sexes have two copies of all other chromosomes, called autosomes.

Table 12.2


Mammals:

• Female has two X chromosomes (XX)

• Male has one X and one Y (XY)

Male mammals produce two kinds of gametes—half carry a Y and half carry an X.

The sex of the offspring depends on which gamete fertilizes the egg.


Sex chromosome abnormalities can result from nondisjunction in meiosis:

• Pair of homologous chromosomes fail to separate in meiosis I

• Pair of sister chromatids fail to separate in meiosis II

Result is aneuploidy—abnormal number of chromosomes.


In humans:

• XO—the individual has only one sex chromosome; female, sterile, with mental abnormalities (Turner syndrome).

• XXY—Klinefelter syndrome, affects males and results in sterility and overlong limbs.

Suggests gene for maleness is on the Y chromosome.


Other abnormalities helped pinpoint the gene location:

• Some women are XY but lack a small piece of the Y chromosome.

• Some men are XX but a small piece of the Y chromosome is attached to another chromosome.


The Y fragment in both cases contains SRY (sex-determining region on the Y chromosome).

Primary sex determination:

If SRY protein is present, the embryo develops testes.

If there is no SRY, the embryo develops ovaries.


A gene on the X chromosome, DAX1, produces an anti-testis factor.

In males, SRY inhibits the DAX1 anti-testis factor.

In females (who lack SRY), DAX1 functions to inhibit maleness.


Secondary sex characteristics: the outward manifestations of sex.

The gonads produce hormones (testosterone and estrogen) that control the development of these characteristics.


Fruit flies normally have three pairs of

autosomes and 1 pair of sex chromosomes.

X and Y chromosomes are not true homologs; many genes on X are not present on Y.

In-Text Art, Ch. 12, p. 251


Sex-linked genes were discovered in fruit flies:

A gene for eye color is on the X chromosome, the Y doesn’t have it.

A single copy of a gene is called hemizygous.

Figure 12.20 Eye Color Is a Sex-Linked Trait in Drosophila (Part 1)

Figure 12.20 Eye Color Is a Sex-Linked Trait in Drosophila (Part 2)


Sex-linked inheritance: inheritance of a gene carried on a sex chromosome.

In mammals the X chromosome is larger and carries more genes than the Y, so sex-linked genes are usually on the X chromosome.


Pedigrees of X-linked recessive phenotypes show:

• Phenotype appears much more often in males

• Daughters who are heterozygous are carriers

• Mutant phenotype can skip a generation if it passes from a male to his daughter

Figure 12.21 Red-Green Color Blindness Is a Sex-Linked Trait in Humans


Mitochondria and plastids contain small numbers of genes, remnants of the genomes of endosymbiotic prokaryotes.

Mitochondria and plastids are inherited only from the mother.

Eggs contain cytoplasm and organelles, but the only part of the sperm to enter the egg is the nucleus.


There may be hundreds of mitochondria or plastids in a cell, so it’s not diploid for organelle genes.

Organelle genes tend to mutate faster than nuclear genes and have multiple alleles.


Organelle genes are important for their assembly and function, mutations can result in new phenotypes.

Some plastid gene mutations affect chlorophyll synthesis, resulting in a white phenotype.

Inheritance follows a non-Mendelian, maternal pattern.

Figure 12.22 Cytoplasmic Inheritance


Bacteria exchange genes by conjugation; requires physical contact between cells.

A sex pilus extends from one cell to another, and brings them together.

Genetic material can pass through a thin cytoplasmic bridge called the conjugation tube.

Figure 12.23 Bacterial Conjugation and Recombination (A)


DNA passes from a donor cell to a recipient cell; there is no reciprocal DNA transfer.

The donor DNA lines up with the recipient’s DNA and crossing over can occur, changing the recipient’s genetic makeup.

Figure 12.23 Bacterial Conjugation and Recombination (B)


Bacteria also have plasmids—small circular chromosomes.

Plasmid genes fall into these categories:

• Unusual metabolic functions—e.g., breaking down hydrocarbons

• Antibiotic resistance genes (R factors)

• Genes for making a sex pilus


Plasmids can move between the cells during conjugation.

Plasmids can replicate independently of the main chromosome, or be integrated into the main chromosome.

Figure 12.24 Gene Transfer by Plasmids

12 Answer to Opening Question

The Tasmanian Devil Genome Project is dedicated to preserving the species.

Matings are planned between the most genetically diverse individuals to maximize heterozygosity.

A captive population from an area where devils are cancer-free is being developed, and are being genotyped.

12 inheritance, genes, and chromosomes. 12 inheritance, genes, and chromosomes 12.1 what are the...

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