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Botany 1050

Introduction to Botany

Fall 2009

GENETICS

I.  Prior to Mendel blending concept of inheritance was prevalent

A. Offspring were thought to possess traits intermediate between those of parents

B. If red and white flowers produce pink, a return of red or white progeny was considered instability in genetic material

C. Blending theory was of no help to Charles Darwin's theory of evolution

1. Blending theory did not account for variation, and therefore could not explain species diversity

2. Particulate theory of inheritance, as proposed by Mendel, could account for presence of differences among members of a population generation after generation

3. Mendel's work was unrecognized until 1900; Darwin was never able to use it to support his theory of evolution

II. Gregor Mendel 

A. Austrian monk

B. Formulated two fundamental laws of heredity in the early 1860s

C. Previously, he had studied science and mathematics at the University of Vienna

D. Why Mendel was successful:

1. Mendel did a statistical study, probably because he had a mathematical background

2. He prepared his experiments carefully; he conducted preliminary studies:

a. Worked with garden pea, because peas were easy to cultivate, had a short generation time, and could be cross-pollinated

b. Peas also have the ability to be self-pollinated

c. Mendel chose true-breeding varieties for his experiments, i.e. all offspring like the parents and like each other

d. Mendel studied simple traits (e.g., seed shape and color, flower color, etc.)

3. Mendel followed inheritance of individual traits over more than one generation

E. Mendel's work has been republished on the web here

II.  Mendel's monohybrid crosses

A. Cross-pollination monohybrid crosses 

1. Hybrid = the product of parents that are true-breeding for distinctly different forms of a single trait

2. Monohybrid cross = cross between two parents true-breeding for distinct forms of a single trait

3. Mendel tracked each trait through two generations:

a. P generation is the parental generation in a breeding experiment

b. F1 generation is the first-generation offspring 

c. F2 generation is the second-generation offspring obtained by self-pollinating the F1 generation

B. Mendel's results:

1. Contrary to those predicted by a blending theory of inheritance

2. F1 plants resembled only one of the parents

3. Characteristic of other parent reappeared in about 1/4 of F2 plants; 3/4 of offspring resembled the F1 plants

4. Mendel saw that these 3:1 results were possible if:

a. F1 hybrids contained two factors (genes) for each trait, one dominant and one recessive

b. Factors separate when gametes formed; a gamete carried one copy of each factor

c. random fusion of all possible gametes occurred upon fertilization

5. From monohybrid results Mendel formulated his first law of inheritance:

a. Law of segregation = "hereditary characteristics are determined by discrete factors that appear in pairs, one pair being inherited from each parent. During the production of sex cells the pairs of factors are separated or segregated into different gametes"

b. These factors were later named genes

C. Modern genetics has an explanation

1. Each trait in a pea plant is controlled by two alleles = "alternative forms of a gene that occur at the same gene locus on homologous chromosomes":

a. Dominant allele masks or hides expression of a recessive allele; it is represented by an uppercase letter

b. Recessive allele is an allele masked by a dominant allele; it is represented by a lowercase letter

2. Gene locus = "the physical location of a gene on a chromosome"

3. In Mendel's cross, the parents were true-breeding; each parent had two identical alleles for a trait--they were homozygous, indicating they possess two identical alleles for a trait:

a. Homozygous dominant genotypes possess two dominant alleles for a trait, e.g.  TT

b. Homozygous recessive genotypes possess two recessive alleles for a trait, e.g. tt

4. After cross-pollination, all individuals of the F1 generation had one of each type of allele:

a. Heterozygous genotypes possess one of each allele for a particular trait, e.g. Tt

b. Recessive alleles are not expressed in a heterozygote 

5. Two organisms with different allele combinations can have same physical appearance, e.g. TT and Tt pea plants are both tall. Therefore, it is necessary to distinguish between alleles and appearance of organism:

a. Genotype refers to the genetic makeup of an individual, e.g. TT, Tt, or tt

b. Phenotype refers to the physical appearance of the individual, e.g. tall or short

D. Punnett square 

1. Provides a simple method to calculate probable results of a genetic cross

2. All possible types of sperm alleles are lined up vertically, all possible egg alleles are lined up horizontally; every possible combination is placed in squares

III.  Mendel's dihybrid crosses

A. Dihybrid cross = between two parents true-breeding for distinct forms of a two traits

B. E.g. Mendel crossed tall peas that had green pods with short peas that had yellow pods and then self-pollinated the F1's:

C. In the F2 generation he got the following ratios:

1. 9/16 of the offspring dominant for both traits, tall and green pods

2. 3/16 of the offspring dominant for one trait and recessive for the other trait, tall and yellow pods

3. 3/16 of the offspring dominant and recessive opposite of the previous proportions, short and green pods

4. 1/16 of the offspring recessive for both traits, short and yellow pods.

D. Dihybrid crosses predict phenotypic ratios of 9:3:3:1 in F2 offspring

E. Mendel's dihybrid crosses allowed him to formulate his second law:

Principle of independent assortment = "the inheritance of a pair of factors for one trait is independent of the simultaneous inheritance of factors for other traits, such factors assort independently, as though no other factors were present"

IV. Note: genetic ratios are only predictions and they are subjected to the laws of probability, which only apply to large sample sizes. Also note that it is necessary to convert predicted ratios to percentages in order to compare with actual results

SOME EXCEPTIONS TO MENDEL'S LAWS:

I. Linkage - if two different genes are located relatively close to each other on the same chromosome they can not segregate independently.

II. Dominance - Mendel actually investigated complete dominance, i.e. dominant gene totally masks the recessive.

A. Incomplete dominance - a blending of the effects of two alleles which creates an intermediate form. E.g. RR - red, rr - white, Rr - pink. At first this appears to be blending inheritance - but use a Punnett square to predict the colors obtained from crossing two pinks (Rr).

B. Codominance - both genes are expressed, e.g. human AB blood types.

III. Multiple alleles - three or more alleles for same gene, e.g. three alleles for human blood groups A, B, O with O the recessive and A and B codominants. 

A. Human blood types exhibit both codominance and multiple alleles.

B. There are about 100 alleles for hair color in mammals.

IV. Polygenic Inheritance - traits are determined by more than one pair of genes.  

A. E.g. human eye color and height. Therefore generally get continuous variation in characteristics and a bell shaped curve. 

V. Pleiotrophy - one allele can affect two or more traits. 

A. E.g. sickle cell anemia is caused by a single mutation in the hemoglobin gene. It produces a number of effects.

VI. Environment may affect the expression of genes.

A. Siamese cats have dark ears, nose and paws because they are colder than rest of the body. The enzyme responsible for the dark color only functions at lower temperatures (recall effect of temperature on enzymes). 

B. Above water versus submerged leaves on some plants are markedly different.

C. Identical twins have identical genes but exercise, diet and a number of other factors may cause them to have different phenotypes.

D. Moth larvae hatching, spring vs. summer

VII. Cytoplasmic inheritance - recall that plastids and mitochondria have their own DNA. Therefore some characteristics may be under the control of genes in the cytoplasm. 

A. In plants cytoplasmic inheritance from plastids produces the variegated leaves of some leaves, e.g. Coleus. 

B. Male sterility in some cultivated plants is due to a mitochondrial gene. This is useful for producing artificial hybrids because the stamens do not have to be removed to prevent self-pollination.

VIII. Sex chromosomes - in many organisms, particularly animals, the males and females may have one pair of chromosomes that are different and they determine the sex of the individual. These are called sex chromosomes.

A. Humans have 22 pairs of autosomes and one pair of sex chromosomes. In humans the male, Y chromosome, carries a master gene called SRY (Sex determining Region of the Y chromosome) which causes the development of male hormones and reproductive organs.

 

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