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 (F 1 )

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  F 1 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 I A I A, I A i I B I B, I B i I A I B 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??