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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??