-Genetics - the study of heredity

I. Mendel’s Insight of Inheritance

A. Blended Theory - before Mendel, the thought was that the father’s information blended with the mother’s information during fertilization

B. Gregor Mendel - "Father of Genetics" ; Austrian monk; observed several pea plant traits over many generations; provided indirect evidence of how parents transmit genes to offspring

1. Mendel’s Scientific Approach

a. Why use the garden pea as a test organism?

II. Reproduces very quickly.

III. Self-fertilizing - the flowers produce sperm and eggs which fuse inside the same flower

IV. True-breeding - successive generations are just like the parents

in one or more traits

V. Cross-fertilizing - some pea flowers; the sperm and egg of different plants are brought together under controlled conditions

b. Mendel theorized that if he took a true-bred plant with a certain trait (ex. White flower) and eliminated its source of pollen (by cutting out the stamens from that plant) and then pollenated those plants with the pollen from a true-bred plant which displayed the opposite version of the trait (ex. Purple flower), then he could track the fate of the trait (ex. Flower color) over several generations. If there were patterns to the trait’s

inheritance, those patterns might tell him something about the heredity

material itself.

c. Some definitions used in genetics:

I. Gene - unit of information about a specific trait that are passed

from parent to offspring

II. Diploid - a pair of genes for each trait on a pair of homologous

chromosomes

III. Allele - the different molecular forms of a gene

IV. Homozygous - a condition when the alleles for a certain trait

are the same (AA or aa)

V. Heterozygous - a condition when the alleles for a certain trait

are different (Aa)

VI. Dominant - the allele that completely masks the expression of

the other allele; designated with a capital letter (A)

VII. Recessive - the allele that has it expression masked by the

dominant allele; designated with a lower case letter (a)

VIII. Homozygous Dominant - the individual has a pair of dominant alleles for a specific trait (AA)

IX. Homozygous Recessive - the individual has a pair of

recessive alleles for a specific trait (aa)

X. Genotype - the particular genes that are present in an individual

XI. Phenotype - an individual’s observed traits

XII. P - the parent generation of a genetic experiment

XIII. F1 - the first generation offspring

XIV. F2 - the second generation offspring

II. Mendel’s Principle of Segregation

A. Monohybrid Cross

a. Mendel took a true-breeding pea plant for a specific trait (white flower color)

and cross-fertilized (aka crossed) it with a true -breeding pea plant that had a

different expression of that trait (purple flower color). The resulting F1 generation all expressed the dominant phenotype for that trait (purple). When

he crossed two F1 offspring, for every four F2 offspring produced, he got three

that expressed the dominant phenotype and one that expressed the recessive

phenotype.

b. Conclusions:

I. Purple was the dominant allele and white was the recessive allele.

II. Since he started with true-bred parents, then the purple parent must

be homozygous dominant for that trait and the white parent must be

homozygous recessive for that trait.

III. Each parent would donate one allele each to the offspring. Therefore,

the only allele that the purple parent could donate would be dominant and

the only allele that the white parent could donate would be recessive. So

the resulting offspring would have a heterozygous genotype. Since, purple

flower color is dominant all of the offspring would express the purple

phenotype.

IV. When two heterozygous F1 offspring are crossed, each parent can donate one of two alleles (dominant and recessive). Therefore, for every

four F2 offspring produced, one individual would be homozygous dominant, two individuals heterozygous, and one individual homozygous recessive (1:2:1 genotypic ratio), while three would express the purple flower color and one would express the white (3:1 phenotypic ratio).

V. Since fertilization was suggested to be a chance event, Mendel could

use probability to help him predict the possible outcomes of crosses.

c. Geneticists use the Punnett square method to determine genotypic and

phenotypic ratios.

P: Purple Flower (PP) White Flower (pp)

F1: Purple Flower (Pp) x (Pp)

F2:

P p

PP (purple)	Pp (purple)
Pp (purple)	pp (white)

Genotypic Ratio: 1 PP: 2 Pp: 1 pp

Phenotypic Ratio: 3 purple: 1 white

-These ratios don’t work out exactly, but because Mendel did so many crosses most of the time the ratios fell close to the predicted ratios.

B. Testcross - a type of cross used to find out the genotype of an individual; by crossing it

with a homozygous recessive individual and then examining the resulting phenotypes, the

parent’s genotype can be ascertained.

III. Mendel’s Principle of Independent Assortment

A. Dihybrid Cross - genetic experiment where two different genes are examined at the same time

B. Punnett square method

P: Purple, Tall Pea White, Short Pea

(PPTT) (pptt)

F1: Purple, Tall Pea

(PpTt) X (PpTt)

F2:

PT Pt pT pt

Genotypic Ratio: 1 PPTT: 2 PPTt :1 PPtt :4 PpTt: 2 PpTT: 2 Pptt: 1 ppTT: 2 ppTt: 1 pptt

Phenotypic Ratio: 9 purple, tall: 3 purple, short: 3 white, tall: 1 white, short

IV. Some Exceptions to the Rule

A. Incomplete Dominance - one allele is not fully dominant over its partner

P: Red Snapdragon (RR) X White Snapdragon (rr)

F1: Pink Snapdragon (Rr)

F2: 1 Red (RR): 2 Pink (Rr): 1 White (rr)

B. Codominance - a pair of nonidentical alleles specify two phenotypes and both are expressed in the heterozygous condition (EX. ABO blood types)

A type - I^AI^A and I^Ai

B type - I^BI^B and I^Bi

AB type - I^AI^B

O type - ii

F. Continuous Variation in a Population - generally the individuals of a population show a range of small differences in most traits. The greater number of genes and environmental factors that influence a trait, the more continuous will be the expected distribution of all the versions of that trait. (EX. Eye color and Height)

V. Sex Determination in Humans

-male-determining gene - found on the Y chromosome; if this gene is present, then testes

form from the precursor reproductive tissue; if this gene is absent, then ovaries

automatically form from the precursor reproductive tissue

VI. Early Questions About Gene Locations

A. X-linked Genes

1. Thomas Hunt Morgan - Kentuckian (UK biol.bldg.) - experiments with fruit

flies to determine that each gene has a specific location (locus) on a chromosome

2. Genes found only on the X chromosome

3. EX. Fruit Fly Eye Color

X^RX^R X^ry

(Homozygous, (White-eyed

Red-eyed Female) Male)

F1:

X^RX^r X^Ry

(Heterozygous, (Red-eyed Male)

Red-eyed Female)

F2:

B. Y-linked Genes

1. Found only on the Y chromosome

1. EX. Male-determining Gene

VII. Human Genetic Disorders

1. Genetic Abnormality - rare, uncommon version of a trait

2. Genetic Disorder - an inherited condition that results in mild to severe medical

problems; don’t disappear from population because:

a. Mutations

b. Heterozygotes (carriers) who can transmit the recessive allele for a trait

to offspring

1. Autosomal Recessive Disorders

a. Albinism - absence of pigmentation

b. Sickle-cell Anemia - severe tissue and organ damage

c. Galactosemia - galactose not broken down; brain, liver, and eye

1. Autosomal Dominant Disorders

a. Progeria - rare aging disorder caused by mutation of a gene

b. Huntington Disorder - progressive deterioration of nervous system

c. Achondroplasia - leads to stillbirth in homozygous dominants; hetero-

zygotes cannot form cartilage properly resulting in abnormally short arms

and legs (type of dwarfism)

1. X-linked Recessive Disorders

a. Hemophilia A - blood-clotting disorder; when injury occurs blood does

not clot causing severe blood loss; caused by a mutation

b. Duchene Muscular Dystrophy - muscles enlarge with fat and connective

tissue while muscle tissue atrophies: muscles continue to weaken until

early death

1. X-linked Dominant Disorder

Faulty Enamel Trait - failure of teeth enamel to develop properly

1. Changes in Chromosome Number

a. Aneuploidy - having one extra or one less chromosome

1. Lethal in humans. 99% die before birth, the rest die within a month of birth.

9. Change in Number of Sex Chromosomes

1. Turner Syndrome - inheritance of an X chromosome without another X or Y partner; nondisjunction affecting sperm formation (75%of the cases); almost all abort spontaneously early in pregnancy; only females (XO)

Symptoms:

VIII. Prenatal Diagnosis - can detect some disorders

A. Amniocentesis - a sample of the amnionic fluid is withdrawn and the cells are

cultured and analyzed

B. Chorionic Villi Sampling (CVS) - cells removed from the chorion (the sac around the

amnion)