nucleotide Phosphate-yellow Sugar-blue Base-A,C,T,G

DNA is composed of four nucleotides DNA is made of chains of small subunits called nucleotides Each nucleotide has three components A phosphate group A deoxyribose sugar One of four nitrogen-containing bases Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

nucleotide Phosphate-yellow Sugar-blue Base-A,C,T,G

DNA is a double helix of two nucleotide strands (continued) James Watson and Francis Crick combined the X-ray data with bonding theory to deduce the structure of DNA They proposed that a single strand of DNA is a polymer consisting of many nucleotide subunits Within each DNA strand, the phosphate group of one nucleotide bonds to the sugar of the next nucleotide in the same strand The deoxyribose and phosphate portions make up the sugar-phosphate backbone

Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) Because of their structures and the way they face each other, adenine (A) bonds only with thymine (T) and guanine (G) bonds only with cytosine (C) Bases that bond with each other are called complementary base pairs Thus, if one strand has the base sequence CGTTTAGCCC, the other strand must have the sequence GCAAATCGGG

The parental DNA is unwound Parental DNA double helix New DNA strands are synthesized with bases complementary to the parental strands Each new double helix is composed of one parental strand (blue) and one new strand (red)

10.1 What Is the Physical Basis of Inheritance? Inheritance is the process by which the traits of organisms are passed to their offspring

10.1 What Is the Physical Basis of Inheritance? Genes are sequences of nucleotides at specific locations on chromosomes Inheritance is the process by which the characteristics of individuals are passed to their offspring A gene is a unit of heredity that encodes information needed to produce proteins, cells, and entire organisms Genes comprise segments of DNA ranging from a few hundred to many thousands of nucleotides in length The location of a gene on a chromosome is called its locus (plural, loci)

gene loci a pair of homologous chromosomes Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus gene loci This locus contains another gene for which the organism is homozygous Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus the chromosome from the male parent the chromosome from the female parent 10

10.1 What Is the Physical Basis of Inheritance? Genes are sequences of nucleotides at specific locations on chromosomes (continued) Homologous chromosomes carry the same kinds of genes for the same characteristics Genes for the same characteristic are found at the same loci on both homologous chromosomes Genes for a characteristic found on homologous chromosomes may not be identical Alternative versions of genes found at the same gene locus are called alleles

Figure 10-1 The relationships among genes, alleles, and chromosomes a pair of homologous chromosomes Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus gene loci This locus contains another gene for which the organism is homozygous Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus the chromosome from the male parent the chromosome from the female parent 12

10.1 What Is the Physical Basis of Inheritance? Mutations are the source of alleles Alleles arise as mutations—changes in the nucleotide sequence in genes If a mutation occurs in a cell that becomes a sperm or egg, it can be passed on from parent to offspring Most mutations occurring in the DNA of an organism initially appeared in the reproductive cells New mutations may have occurred in reproductive cells of the organism’s own parents, but this is quite a rarity

10.1 What Is the Physical Basis of Inheritance? An organism’s two alleles may be the same or different Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)

Figure 10-1 The relationships among genes, alleles, and chromosomes a pair of homologous chromosomes Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus gene loci This locus contains another gene for which the organism is homozygous Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus the chromosome from the male parent the chromosome from the female parent 15

Eukaryotic chromosomes usually occur in pairs The nonreproductive cells of many organisms have chromosomes in pairs, with both members of the pair being the same length. The chromosomes are the same length and have the same staining properties because they have the same genes arranged in the same order.

Eukaryotic chromosomes usually occur in pairs. sex chromosomes Eukaryotic chromosomes usually occur in pairs. An entire set of stained chromosomes from a single cell is called a karyotype. Fig. 8-6

Diploid cells contain 2n chromosomes. The number of different types of chromosomes in a species is called the haploid number and is designated n. In humans, n = 23. Diploid cells contain 2n chromosomes. Humans body cells contain 2n = 46 (2 x 23) chromosomes.

A typical human cell has 23 pairs of chromosomes. 22 of these pairs have a similar appearance and are called autosomes. Human cells also have a pair of sex chromosomes, which differ from each other in appearance and in genetic composition. Females have two X chromosomes. Males have one X and one Y chromosome.

Eukaryotic chromosomes usually occur in pairs. sex chromosomes Eukaryotic chromosomes usually occur in pairs. An entire set of stained chromosomes from a single cell is called a karyotype. Fig. 8-6

10.2 How Were the Principles of Inheritance Discovered? Gregor Mendel, an Austrian monk, discovered the common patterns of inheritance and many essential facts about genes, alleles, and the distribution of alleles in gametes and zygotes during sexual reproduction He chose the edible pea plant for his experiments, which took place in the monastery garden Mendel’s background allowed him to see patterns in the way plant characteristics were inherited

Figure 10-2 Gregor Mendel 22

10.2 How Were the Principles of Inheritance Discovered? Pea plants have qualities that make them a good organism for studying inheritance Pea flowers have stamens, the male structures that produce pollen, which in turn contain the sperm (male gametes); sperm are gametes and pollen is the vehicle Pea flowers have carpels, female structures housing the ovaries, which produce the eggs (female gametes) Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant

Figure 10-3 Flowers of the edible pea flower dissected to show its reproductive structures intact pea flower Carpel (female, produces eggs) Stamens (male, produce pollen grains that contain sperm) 24

10.2 How Were the Principles of Inheritance Discovered? Doing it right: The secrets of Mendel’s success (continued) Because of their structure, pea flowers naturally self-fertilize Pollen from the stamen of a plant transfers to the carpel of the same plant, where the sperm then fertilizes the plant’s eggs

10.2 How Were the Principles of Inheritance Discovered? Doing it right: The secrets of Mendel’s success (continued) Mendel was able to mate two different plants by hand (cross-fertilization) Female parts (carpels) were dusted with pollen from other selected plants

10.2 How Were the Principles of Inheritance Discovered? Doing it right: The secrets of Mendel’s success (continued) Unlike previous researchers, Mendel chose a simple experimental design He chose to study individual characteristics (called traits) that had unmistakably different forms, such as white versus purple flowers He started out by studying only one trait at a time

10.2 How Were the Principles of Inheritance Discovered? Doing it right: The secrets of Mendel’s success (continued) Mendel employed numerical analysis in studying the traits He followed the inheritance of these traits for several generations, counting the numbers of offspring with each type of trait By analyzing these numbers, he saw the basic patterns of inheritance emerge

10.3 How Are Single Traits Inherited? True-breeding organisms possess traits that remain inherited unchanged by all offspring produced by self-fertilization Mendel’s cross-fertilization of pea plants used true-breeding organisms Mendel cross-fertilized true-breeding, white-flowered plants with true-breeding, purple-flowered plants The parents used in a cross are part of the parental generation (known as P) The offspring of the P generation are members of the first filial generation (F1) Offspring of the F1 generation are members of the F2 generation

Parental generation (P) First-generation offspring (F1) pollen pollen Figure 10-4 Cross of pea plants true-breeding for white or purple flowers pollen Parental generation (P) pollen pollen cross-fertilize true-breeding, purple-flowered plant true-breeding, white-flowered plant First-generation offspring (F1) all purple-flowered plant 30

10.3 How Are Single Traits Inherited? Mendel’s flower color experiments Mendel allowed the F1 generation to self-fertilize The F2 was composed of 3/4 purple-flowered plants and 1/4 white-flowered plants, a ratio of 3:1 The results showed that the white trait had not disappeared in the F1 but merely was hidden Mendel then self-fertilized the F2 generation In the F3 generation, all the white-flowered F2 plants produced white-flowered offspring These proved to be true-breeding

10.3 How Are Single Traits Inherited? In the F3 generation, self-fertilized purple-flowered F2 plants produced two types of offspring About 1/3 were true-breeding for purple The other 2/3 were hybrids that produced both purple- and white-flowered offspring, again, in the ratio of 3 purple to 1 white Therefore, the F2 generation included 1/4 true-breeding purple-flowered plants, 1/2 hybrid purple, and 1/4 true-breeding white-flowered plants

Figure 10-5 Self-fertilization of F1 pea plants with purple flowers First- generation offspring (F1) self-fertilize Second- generation offspring (F2) 3/4 purple 1/4 white 33

Figure 10-6 The distribution of alleles in gametes homozygous parent gametes A A A A Gametes produced by a homozygous parent heterozygous parent gametes A a A a Gametes produced by a heterozygous parent 34

10.3 How Are Single Traits Inherited? The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses (continued) There are two alleles for a given gene characteristic (such as flower color) Let P stand for the dominant purple-flowered allele: A homozygous purple-colored plant has two alleles for purple flower color (PP) and produces only P gametes Let p stand for the recessive white-flowered allele: A homozygous white-colored plant has two alleles for white flower color (pp) and produces only p gametes

10.3 How Are Single Traits Inherited? The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses (continued) A cross between a purple-flowered plant (PP) and a white-flowered plant (pp) produces all purple-flowered F1 offspring, with a Pp genotype Dominant P gametes from purple-flowered plants combined with recessive p gametes from white-flowered plants to produce hybrid purple-flowered plants (Pp)

10.3 How Are Single Traits Inherited? The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses (continued) The F1 offspring were all heterozygous (Pp) for flower color When the F1 offspring were allowed to self-fertilize, four types of gametes were produced from the Pp parents Sperm: Pp Eggs: Pp

10.3 How Are Single Traits Inherited? The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses (continued) A heterozygous plant produces equal numbers of P and p sperm and equal numbers of P and p eggs When a Pp plant self-fertilizes, each type of sperm has an equal chance of fertilizing each type of egg Combining these four gametes into genotypes in every possible way produces offspring PP, Pp, Pp, and pp The probabilities of each combination (and therefore the genotypic fraction each genotype is of the total offspring) are 1/4 PP, 1/2 Pp, and 1/4 pp

10.3 How Are Single Traits Inherited? The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel’s crosses (continued) The particular combination of the two alleles carried by an individual is called the genotype For example, PP or Pp The physical expression of the genotype is known as the phenotype (for example, purple or white flowers)

Figure 10-7a Gametes produced by homozygous parents purple parent PP P P all P sperm and eggs white parent pp p p all p sperm and eggs Gametes produced by homozygous parents 40

Figure 10-7b Fusion of gametes produces F1 offspring sperm eggs P p Pp or p P pP Fusion of gametes produces F1 offspring 41

Fusion of gametes from the F1 generation produces F2 offspring Figure 10-7c Fusion of gametes from the F1 generation produces F2 offspring gametes from F1 Pp plants F2 offspring sperm eggs PP P P p Pp P p P pP p p pp Fusion of gametes from the F1 generation produces F2 offspring 42

10.3 How Are Single Traits Inherited? Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring The Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametes First, assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive) Determine the gametes and their fractional proportions (out of all the gametes) from both parents

10.3 How Are Single Traits Inherited? Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring (continued) Write the gametes from each parent, together with their fractional proportions, along each side of a 2 x 2 grid (Punnett square) Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete (e.g., 1/4 PP, 1/4 Pp and 1/4 pP, and 1/4 pp)

10.3 How Are Single Traits Inherited? Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring (continued) Add together the fractions of any genotypes of the same kind (1/4 Pp + 1/4 pP = 1/2 Pp total) From the sums of all the different kinds of offspring genotypes, create a genotypic fraction 1/4 PP, 1/2 Pp, 1/4 pp is in the ratio 1 PP : 2 Pp : 1 pp

10.3 How Are Single Traits Inherited? Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring (continued) Based on dominant and recessive rules, determine the phenotypic fraction A genotypic ratio of 1 PP : 2 Pp : 1 pp yields 3 purple-flowered plants : 1 white-flowered plant

Figure 10-8 Determining the outcome of a single-trait cross Pp self-fertilize P eggs p offspring genotypes genotypic ratio (1:2:1) phenotypic ratio (3:1) sperm eggs P P P PP PP PP Pp sperm P p Pp purple Pp p P pP p p p pp pp white pP pp Punnett square of a single-trait cross Using probabilities to determine the offspring of a single-trait cross 47