1. 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)

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

4. 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

5. 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

6. 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)

7. 10.1 What Is the Physical Basis of Inheritance?
Inheritance is the process by which the traits of organisms are passed to their offspring

8. 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)

9. 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

10. 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

11. 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)

12. 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

13. 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

14. Figure 10-2 Gregor Mendel 14

15. 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

16. 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) 16

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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 22

23. 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

24. 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

25. 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 25

26. 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 26

27. 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

28. 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)

29. 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

30. 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

31. 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)

32. 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 32

33. Figure 10-7b Fusion of gametes produces F1 offspring
sperm
eggs P p Pp or p P pP
Fusion of gametes produces F1 offspring 33

34. 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 34

35. 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

36. 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)

37. 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

38. 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

39. Figure 10-8 Determining the outcome of a single-trait crossPp
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 39