David Sadava David M Hillis H Craig Heller

David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia. blu Genetica, biologia molecolare ed evoluzione S

Inheritance, Genes, and Chromosomes

What Are the Mendelian Laws of Inheritance? 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.

What Are the Mendelian Laws of Inheritance? Experiments performed by the monk, Gregor Mendel, supported the particulate theory. Mendel’s theory of inheritance was published in 1866 but was largely ignored until 1900. He 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.

What Are the Mendelian Laws of Inheritance? 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.

What Are the Mendelian Laws of Inheritance? 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, F 1 • In some experiments the F 1 plants were allowed to self-pollinate and produce a second filial generation, F 2

What Are the Mendelian Laws of Inheritance? Mendel first performed monohybrid crosses: crossing parental varieties with contrasting traits for a single character. • The F 1 offspring were not a blend of the two parental traits. Only one of the traits was present (e. g. , round seeds)— Mendel’s First Law. • Some F 2 had wrinkled seeds. The trait had not disappeared because of blending. These results supported the particulate theory.

What Are the Mendelian Laws of Inheritance? Mendel made monohybrid crosses for seven traits; all gave similar results. The trait that occurred in the F 1 and was more abundant in the F 2 was called dominant, the other recessive. In the F 2 the ratio of dominant to recessive traits was about 3: 1.

What Are the Mendelian Laws of Inheritance? 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. Mendel’s second law— Law of segregation: the two copies of a gene separate during gamete formation; each gamete receives only one copy.

What Are the Mendelian Laws of Inheritance? 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])

What Are the Mendelian Laws of Inheritance? 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. Allele combinations can be predicted using a Punnett square.

What Are the Mendelian Laws of Inheritance? 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.

What Are the Mendelian Laws of Inheritance? 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: F 1 individuals are crossed with homozygous recessive individuals (rr). His hypothesis accurately predicted the results of his test crosses.

What Are the Mendelian Laws of Inheritance? Mendel’s third 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) F 1 generation is Rr. Yy—all round yellow.

What Are the Mendelian Laws of Inheritance? Crossing the F 1 generation (double heterozygotes) is a dihybrid cross. Mendel asked whether, in the gametes produced by Rr. Yy, the traits would be linked, or segregate independently.

What Are the Mendelian Laws of Inheritance? • If linked, gametes would be RY or ry; F 2 would have three times more round yellow than wrinkled green (3: 1). • If independent, gametes could be RY, ry, Ry, or r. Y. F 2 would have nine different genotypes; phenotypes would be in 9: 3: 3: 1 ratio.

What Are the Mendelian Laws of Inheritance? 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.

What Are the Mendelian Laws of Inheritance? 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.

What Are the Mendelian Laws of Inheritance? 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.

What Are the Mendelian Laws of Inheritance? 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.

What Are the Mendelian Laws of Inheritance? 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

How Do Alleles Interact? 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.

How Do Alleles Interact? 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.

How Do Alleles Interact? Some alleles are neither dominant nor recessive—a heterozygote has an intermediate phenotype: incomplete dominance. In the F 2, the original phenotypes reappear, the alleles have not “blended. ”

How Do Alleles Interact? 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.

How Do Alleles Interact? Pleiotropic: one allele has multiple phenotypic effects. The heritable human disease phenylketonuria (PKU) 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.

How Do Genes Interact? 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.

How Do Genes Interact? Inbreeding: mating among close relatives; can result in offspring of low quality. Close relatives tend to have the same recessive alleles. A cross between two different true-breeding homozygotes can result in offspring with stronger, larger phenotypes. Called “hybrid vigor” or heterosis.

How Do Genes Interact? 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. Genes that determine these complex characters: quantitative trait loci.

What Is the Relationship between Genes and Chromosomes? In 1909, Thomas Hunt Morgan and students at Columbia University pioneered the study of the fruit fly Drosophila melanogaster. 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.

What Is the Relationship between Genes and Chromosomes? 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. Recombinant offspring phenotypes appear in recombinant frequencies.

What Is the Relationship between Genes and Chromosomes? Recombinant frequencies can be used to infer the location of genes on a chromosome, and make genetic maps.

What Is the Relationship between Genes and Chromosomes? Corn: each adult produces both male and female gametes—monoecious. Some plants and most animals are dioecious—male and female gametes produced by different individuals. 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.

What Is the Relationship between Genes and Chromosomes? The Y fragment in both cases contains SRY (sexdetermining 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, DAX 1, produces an anti-testis factor. In males, SRY inhibits the DAX 1 anti-testis factor. In females (who lack SRY), DAX 1 functions to inhibit maleness.

What Is the Relationship between Genes and Chromosomes? Secondary sex characteristics: the outward manifestations of sex. The gonads produce hormones (testosterone and estrogen) that control the development of these characteristics.

What Is the Relationship between Genes and Chromosomes? Sex chromosome abnormalities can result from nondisjunction in meiosis—aneuploidy (abnormal number of chromosomes). • XO—Turner syndrome • XXY—Klinefelter syndrome • 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.

What Is the Relationship between Genes and Chromosomes? A single copy of a gene is called hemizygous. 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.

How Do Prokaryotes Transmit Genes? 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. DNA passes from a donor cell to a recipient cell. The donor DNA lines up with the recipient’s DNA and crossing over can occur, changing the recipient’s genetic makeup.

How Do Prokaryotes Transmit Genes? Bacteria also have plasmids—small circular chromosomes. Plasmid genes fall into these categories: • Unusual metabolic functions • 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.

Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved