Mendel and the Gene Idea

Gregor Mendel: The Man

Austrian monk Began breeding

peas in 1857 to study inheritance

Kept very accurate records of his laboratory work and used very large sample sizes

Why Peas?

Available in many varieties Flower color, seed color, flower

position, pod color, seed shape, pod shape, stem length

Mendel could control which plants mated with which

Peas grow quickly!

Mendel’s Procedure

Crossed male pea parts (stamens) and female pea parts (carpels) with opposite traits

Carpel matures to first-generation offspring (F1)

Mendel’s Predictions

Crossing purple and white flowers would result in an intermediate phenotype Mendel thought light purple flowers

would be the result!

Mendel’s Findings

Alternative versions of genes (different alleles) account for variations in inherited characters The gene for flower color (example)

exists in two versions – purple and white

Each version is called an allele

Mendel’s Findings

For each characteristic, an organism inherits two alleles, one from each parent An allele is a part of

a chromosome Each parent

contributes one chromosome of each homologous pair

Mendel’s Findings

If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.

Mendel’s Findings The two alleles for each

trait segregate during gamete production. An egg cell or sperm cell

receives only one allele Each parent passes on

only one of his/her 2 alleles

This is Mendel’s Law of Segregation

Some Terminology

Punnett Square: A diagram used to predict the results of a genetic

cross Homozygous vs. Heterozygous:

Homozygous/pure: identical alleles (HH or hh) Heterozygous/hybrid: different alleles (Hh)

Genotype vs. Phenotype: Genotype: genetic makeup (Tt) Phenotype: physical appearance (tall)

TestCross

A testcross is used to determine the genotype of a parental organism

Cross the organism with the unknown genotype with an organism with the recessive phenotype

Law of Independent Assortment

Each pair of alleles segregates into gametes independently Just because an organism gets one allele

doesn’t mean it will get a certain other one

Example: Seed color (yellow or green) vs. Seed shape

(round or wrinkled) Yellow is NOT always with Round, etc… Blonde hair does not HAVE to go with blue

eyes

Probability

Probability scale ranges from 0 to 1 If an event is certain to happen, it has a

probability of 1 If an event is certain NOT to happen, it

has a probability of 0 Getting heads on a coin toss is ½ (one

out of two)

The Rule of Multiplication

To determine the chance that two or more independent events will occur together in a specific combination, compute the probability for each independent event and then multiply the individual probabilities to get the overall probability

Example: Rolling two dice and rolling a 3 on each 1/6 X 1/6 1/36

The Rule of Addition

The probability of an event that can occur in two or more different ways is the sum of the separate possibilities of those ways

Rolling an odd number using a dice: 1/6 + 1/6 + 1/6 = 3/6 (or ½)

Incomplete Dominance

F1 hybrid is intermediate between the two parents

1:2:1 ratio red: pink: white

Codominance

Both alleles are separately manifested in the phenotype Example: Horses

Brown hairs Black hairs Brown and Black hairs

Multiple Alleles

Genes that exist in more than two allelic forms

Example: ABO Blood Typing IAIA, IAi IBIB, IBi IAIB

ii

Pleiotropy

Def’n: The ability of a gene to affect an

organism in many ways Example:

Alleles that cause sickle-cell anemia also cause other symptoms

Epistasis

Def’n: A gene at one locus

(location) alters the phenotypic expression of a gene at another locus (location)

BB/Bb/bb determines coat color…BUT…

CC/Cc/cc determines pigment or not

Polygenic Inheritance

Many characteristics, including human skin color and height, vary along a continuum among the population

Polygenic inheritance is the effect of two or more genes put together on a single phenotypic characteristics

Example: Height determined by 3 genes AABBCC: very tall person (6’2”) aabbcc: very short person (4’11”) AaBbCc: intermediate height person (5’5”)

Pedigrees A pedigree is a family tree that shows the

interrelationships of parents and children across the generations

Used to predict patterns in the future (risk assessment)

Recessive Genetic Disorders

Cystic Fibrosis (cc) Recessive disorder; most common in

Caucasians Cc (carrier)

Tay-Sachs Disease (tt) Recessive disorder; most common in

Ashkenazi Jews Sickle-Cell Anemia (aa)

Recessive disorder; most common among African-Americans

Societal Factors…

The prevalence of recessive genetic disorders greatly increases when closely-related relatives interbreed

This is why many countries and cultures have laws against intermarriage among close relatives (cousins, etc.)

Dominant Genetic Disorders

Dwarfism: DD or Dd = dwarf phenotype

Huntington’s Disease: Aa or AA Current research can now tell us

whether or not a person has Huntington’s before symptoms set in

Ethical dilemma??