11 The Basic Principles of Heredity

Lecture Outline

I. Mendel= s principles of inheritance

A. Other plant breeders at the time of Mendel knew two main facts

1. Hybrid plants from true-breeding parents are similar

2. When hybrid offspring are self-fertilized, their offspring show a variety of traits

B. Mendel did his experiments primarily on the garden pea, Pisum sativum

1. Peas were easy to grow, and many varieties were readily available

2. Peas may be self- or cross-fertilized, as they have perfect flowers

a) Pollen from an anther may be dusted onto a stigma

b) Cross-fertilization may be prevented by bagging the flower

3. Mendel obtained and verified pea plants true-breeding lines

a) True-breeding plants, when self-pollinated, produce offspring all possessing the same visible trait as the parent plant (e.g., the same phenotype)

4. Mendel chose 7 clearly identifiable pairs of contrasting traits and the accompanying true-breeding plants

5. Mendel first crossed true-breeding plants with contrasting traits

a) These parental plants (the P generation) produced the first filial generation (the F1 generation)

b) The F1 generation plants all possessed the trait of only one of the two parents

c) When the F1 plants self-pollinated, the resulting F2 generation had characteristics of both of the P generation plants

d) These experiments showed that the hereditary factors had not been lost in the F1 generation, but were somehow masked

C. Mendel= s experiments disproved the idea of blending inheritance

1. Mendel proposed that inherited characteristics are controlled by two hereditary A factors@ (genes in modern terminology)

2. The term A dominant@ is now used for the traits seen in the F1 generation, the term A recessive@ for the trait hidden in the F1 generation

3. Alternate forms of a gene are now referred to as alleles

D. The principle of segregation states that alleles separate before gametes are formed

1. The alleles from the male and female gametes do not mix in any way (the principle of segregation)

E. Segregation is due to the separation of homologous chromosomes during meiosis.

Alleles occupy corresponding loci on homologous chromosomes

1. The term locus may refer to the location of a gene on a chromosome as well as to the type of gene controlling a characteristic

2. Alleles are the variations of a gene that govern the same feature

3. Alleles are denoted by a letter or letters; dominant genes are typically denoted by a capital letter, recessive genes with a lower case letter


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4. A monohybrid cross involves individuals with different alleles of a given locus

F. A monohybrid cross is the mating of two individuals that have different alleles of a gene at a single locus

1. Heterozygotes carry two different alleles of a particular locus; homozygotes carry identical alleles

2. Heterozygotes have a recessive and a dominant allele for a particular characteristic

G. A Punnett square predicts the ratios of genotypes and phenotypes of the offspring of a cross

1. Heterozygous organisms produce equal numbers of gametes possessing dominant or recessive alleles

2. Punnett squares allow you to determine the likely outcome of a genetic cross

H. The phenotype of an individual does not always reveal its genotype

1. Heterozygotes express the dominant trait; the recessive trait is masked

2. The appearance of the individual is the phenotype

3. The genetic makeup of the individual is the genotype

4. The phenotypes of the homozygous dominant individual and the heterozygous individual are typically identical

5. Traits that are dominant in one species may be recessive in another

I. A test cross can detect heterozygosity

1. A test cross is a cross between an individual expressing the dominant phenotype, and a homozygous recessive individual

a) Note that the homozygous dominant and heterozygote have indistinguishable phenotypes but different genotypes

b) If the dominant individual is heterozygous, you expect to get a 1:1 ratio of dominant to recessive offspring

c) If the dominant individual is homozygous, you expect to get all dominant offspring

J. A dihybrid cross involves individuals that have different alleles of two loci

1. If a dihybrid cross involves different loci on nonhomologous chromosomes, independent assortment occurs

2. A phenotypic ratio of 9:3:3:1 is expected

K. The principle of independent assortment states that the alleles of different loci on nonhomologous chromosomes are randomly distributed into gametes

1. Mendel= s dihybrid crosses illustrated this principle

2. The mechanics of meiosis are the basis for independent assortment

3. This process occurs during metaphase I of meiosis

L. Recognition of Mendel= s work

1. Mendel= s results were published in 1866 in the transactions of the Brunn Society for the Study of Natural Science

2. Mendel= s work was independently rediscovered by DeVries, Correns, and von Tschermak in 1900

3. In 1902, two independent researchers, Boveri and Sutton, associated Mendel= s work and meiosis which became the basis for the chromosome theory of inheritance


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II. Using probability to predict Mendelian inheritance

Genetic ratios are expressed as probabilities

A. The laws of probability predict the likelihood of genetic events

B. The product rule predicts the combined probabilities of independent events

1. Events are independent if the occurrence of one does not affect the probability of the occurrence of the other

2. To determine the probability of both events occurring, the independent probabilities are multiplied

C. The sum rule predicts the combined probabilities of mutually exclusive events

1. Outcomes that may be the result of either of two different events may be predicted by summing the probabilities

D. The laws of probability can be applied to a variety of calculations

1. Recall that the rules of chance have no A memory@

III. Inheritance and Chromosomes

A. Linked genes do not assort independently; this is supported by the work of T. H. Morgan and colleagues

1. Genes are located in a linear order on the chromosomes

2. The principle of independent assortment does not exactly apply to loci on the same homologous pair of chromosomes

3. Linkage is the phenomenon where groups of genes are inherited together because they are located on the same chromosome

4. Crossing-over may skew the proportions of expected phenotypic outcomes

B. The linear order of linked genes on a chromosome is determined by calculating the frequency of crossing-over

1. During crossing-over, during prophase I, segments of chromosomes are exchanged

2. The percentage of crossing-over is computed by adding the number of individuals in the recombinant classes, dividing by the total number of offspring, and multiplying by 100

3. Genes that are close together often are exchanged together

4. The distance between two genes of a chromosome is measured in map units

a) Map units measure the percentage of crossing-over

b) One map unit represents 1% recombination

5. Linked genes are composed of all of the genes on a particular chromosome, but because of crossing over, they may not all be inherited together

6. Linkage maps have been or are being determined for the fruit fly, the mouse, yeast, Neurospora (a mold), E. coli, and humans, to name a few

7. The Human Genome Project utilizes these techniques in mapping the human chromosomes

C. Sex is commonly determined by special sex chromosomes

1. The majority of animals have sex chromosomes that determine the sex of the individual

a) One sex will have a pair of identical sex chromosomes

(1) Human females have two X chromosomes

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b) The other sex has two different sex chromosomes

(1) Human males are of this type, having one X and one Y chromosome

c) The autosomes are not sex-determining chromosomes

2. The Y chromosome determines male sex in most species of mammals

a) The Y chromosome contains few, but important, genes for maleness, such as the srY gene (sex-reversal on Y)

(1) This gene causes other genes to A turn on,@ resulting in male characteristics in the embryo

b) A person with the XXY condition (Klinefelter syndrome) is male

c) A person with Turner syndrome (a single X chromosome) is phenotypically female

d) The X and Y chromosomes are not truly homologous, carrying different genetic components

e) Sperm containing an X chromosome and those containing a Y chromosome are produced in equal numbers; slightly more male human embryos are produced, slightly more male babies are born (106 boys for 100 girls)

f) Other species have 2 sex chromosomes, but different combinations of these chromosomes result in male- or femaleness

3. X-linked genes have unusual inheritance patterns

a) Genes located on the X chromosomes are called X-linked (older terminology: sex-linked)

b) Males are hemizygous for X-linked traits; they cannot be carriers for X-linked traits

c) X-linked traits may be denoted as XC for a dominant allele and Xc for a recessive allele; the Y chromosome has no superscripts

d) For most X-linked genes, the dominant form is the normal form

e) A female expressing an X-linked trait must have had a father with that trait and a heterozygous mother; therefore they are rare

f) A male expressing an X-linked trait typically had a father without that trait and a heterozygous (A carrier@ ) mother

4. Dosage compensation equalizes the expression of X-linked genes in males and females

a) Typical females have two X chromosomes; males have one

(1) Dosage compensation in mammals involves the inactivation of one X chromosome in female cells

(2) Fruit fly males accomplish this by making their single X chromosome more active

b) The Barr body is a dark area of highly condensed chromatin and is the single inactive X chromosome in female cells

c) The expression of a single X chromosome results in the mottled color of a calico cat (which are nearly always female)

d) Variegation is the result of X chromosome inactivation

(1) Inactivation happens randomly in cells, relatively early in embryological development

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(2) Resulting clones of cells have the same X chromosome inactivated

IV. Extensions of Mendelian genetics

g The relationship between genotype and phenotype is often complex

g A single pair of alleles may control several characteristics; in contrast, many alleles may control a single trait

A. Dominance is not always complete

1. Pink four o= clock flowers result from a cross between a plant with red flowers and a plant with white flowers

a) Incomplete dominance results when neither allele is dominant to the other and the phenotype is intermediate

2. Codominance results when the offspring has characteristics of both alleles

a) In cows, a reddish-coated cow mating with a white-coated cow will produce a calf with roan coloration, with both red and white hairs

b) The human ABO blood group alleles are an example

B. Multiple alleles for a locus may exist in a population

1. The human ABO blood group is also an example of multiple alleles

2. In rabbits, variations of the C gene result in rabbits with quite varied coat colors

C. A single gene may affect multiple aspects of the phenotype

1. The ability of a single gene to have multiple effects is referred to as pleiotropy

a) Many genetic diseases are pleiotropic (e.g., cystic fibrosis and sickle cell anemia)

D. Alleles of different loci may interact to produce a phenotype

1. These interactions result in variations from typical expected Mendelian ratios of crosses

2. Epistasis is the interaction between genes such that one gene can influence the effect of another gene

a) An example of this is seen in the coat color of Labrador retriever dogs

E. Polygenes act additively to produce a phenotype

1. Multiple, separate genes have similar and additive effects on the morphological feature

2. Height and skin color are simple examples in humans

3. The F1 generation has phenotypes intermediate between the homozygous parents, and the F2 generation shows wide variation in phenotypes

4. Bell-shaped curves (a normal distribution) are indicative of polygenes

F. Genes interact with the environment to shape phenotype

1. Human height is polygenic and also environmentally influenced

2. The range of phenotypic possibilities that can occur under different environmental conditions is the norm of reaction

3. This brings to mind the idea of nature versus nurture

a) Examples in the human species are the range of intelligence