Mendel and Heredity

Inheritance The passing of traits from parents to offspring. Humans have known about inheritance for thousands of years.

Genetics The scientific study of the inheritance. Genetics is a relatively “new” science (about 150 years).

Gregor Mendel Father of Modern Genetics.

Reasons for Mendel's Success Used an experimental approach. Applied mathematics to the study of natural phenomena. Kept good records.

Mendel was a pea picker. He used peas as his study organism.

Why Use Peas? Short life span. Bisexual. You can control mating. Cross- and self-pollinating. (You can eat the failures).

Cross-pollination Two parents. Results in hybrid offspring where the offspring may be different than the parents.

Self-pollination One flower as both parents. Natural event in peas. Results in pure-bred offspring where the offspring are identical to the parents.

Mendel's Work Used seven characters, each with two expressions or traits. Example: Character - height Traits - tall or short.

Monohybrid or Mendelian Crosses Crosses that work with a single character at a time. Example - Tall X short Purple X white

P Generation The Parental generation or the first two individuals used in a cross. Example - Tall X short Mendel used reciprocal crosses, where the parents alternated for the trait.

Offspring F1 - first filial generation. F2 - second filial generation, bred by crossing two F1 plants together or allowing a F1 to self-pollinate.

Another Sample Cross P1 Tall X short (TT x tt) F1 all Tall (Tt) F2 3 tall to 1 short (1 TT: 2 Tt: 1 tt)

Results - Summary In all crosses, the F1 generation showed only one of the traits regardless of which was male or female. The other trait reappeared in the F2 at ~25% (3:1 ratio).

Mendel's Hypothesis 1. Genes can have alternate versions called alleles. 2. Each offspring inherits two alleles, one from each parent.

Mendel's Hypothesis 3. If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent. Comment - do not use the terms “strongest” to describe the dominant allele.

Mendel's Hypothesis 4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation

Law of Segregation

Vocabulary Phenotype - the physical appearance of the organism. Genotype - the genetic makeup of the organism, usually shown in a code. T = tall t = short

Helpful Vocabulary Homozygous - When the two alleles are the same (TT/tt). Heterozygous- When the two alleles are different (Tt).

6 Mendelian Crosses are Possible Cross Genotype Phenotype TT X tt all Tt all Dom Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res TT X TT all TT all Dom tt X tt all tt all Res TT X Tt 1TT:1Tt all Dom Tt X tt 1Tt:1tt 1 Dom: 1 Res

Test Cross Cross of a suspected heterozygote with a homozygous recessive. Ex: T_ X tt If TT - all dominant If Tt - 1 Dominant: 1 Recessive

Dihybrid Cross Cross with two genetic traits. Need 4 letters to code for the cross. Ex: TtRr Each Gamete - Must get 1 letter for each trait. Ex. TR, Tr, etc.

Number of Kinds of Gametes Critical to calculating the results of higher level crosses. Look for the number of heterozygous traits.

Dihybrid Cross TtRr X TtRr Each parent can produce 4 types of gametes. TR, Tr, tR, tr Cross is a 4 X 4 with 16 possible offspring.

Law of Independent Assortment The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2nd trait. Inheritance of height is independent of the inheritance of flower color.

Probability Genetics is a specific application of the rules of probability. Probability - the chance that an event will occur out of the total number of possible events.

Variations on Mendel 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Epistasis 5. Polygenic Inheritance

Incomplete Dominance When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents. Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white

Result No hidden Recessive. 3 phenotypes and 3 genotypes (Hint! – often a “dose” effect) Red = CR CR Pink = CRCW White = CWCW

Another example

Codominance Both alleles are expressed equally in the phenotype. Ex. MN blood group MM MN NN

Result No hidden Recessive. 3 phenotypes and 3 genotypes (but not a “dose” effect)

Multiple Alleles When there are more than 2 alleles for a trait. Ex. ABO blood group IA - A type antigen IB - B type antigen i - no antigen

Result Multiple genotypes and phenotypes. Very common event in many traits.

Alleles and Blood Types Type Genotypes A IA IA or IAi B IB IB or IBi AB IAIB O ii

Comment Rh blood factor is a separate factor from the ABO blood group. Rh+ = dominant Rh- = recessive A+ blood = dihybrid trait

Epistasis When 1 gene locus alters the expression of a second locus. 1st gene: C = color, c = albino 2nd gene: B = Brown, b = black

Gerbils

In Gerbils CcBb X CcBb Brown X Brown F1 = 9 brown (C_B_) 3 black (C_bb) 4 albino (cc__)

Result Ratios often altered from the expected. One trait may act as a recessive because it is “hidden” by the second trait.

Epistasis in Mice

Polygenic Inheritance Factors that are expressed as continuous variation. Lack clear boundaries between the phenotype classes. Ex: skin color, height

Genetic Basis Several genes govern the inheritance of the trait. Ex: Skin color is likely controlled by at least 4 genes. Each dominant gives a darker skin.

Result Mendelian ratios fail. Traits tend to "run" in families. Offspring often intermediate between the parental types. Trait shows a “bell-curve” or continuous variation.

Genetic Studies in Humans Often done by Pedigree charts. Why? Can’t do controlled breeding studies in humans. Small number of offspring. Long life span.

Human Recessive Disorders Several thousand known: Albinism Sickle Cell Anemia Tay-Sachs Disease Cystic Fibrosis PKU Galactosemia

Sickle-cell Disease Most common inherited disease among African-Americans. Single amino acid substitution results in malformed hemoglobin. Reduced O2 carrying capacity. Codominant inheritance.

Tay-Sachs Eastern European Jews. Brain cells unable to metabolize type of lipid, accumulation of causes brain damage. Death in infancy or early childhood.

Cystic Fibrosis Most common lethal genetic disease in the U.S. Most frequent in Caucasian populations (1/20 a carrier). Produces defective chloride channels in membranes.

Recessive Pattern Usually rare. Skips generations. Occurrence increases with consaguineous matings. Often an enzyme defect.

Human Dominant Disorders Less common then recessives. Ex: Huntington’s disease Achondroplasia Familial Hypercholsterolemia

Inheritance Pattern Each affected individual had one affected parent. Doesn’t skip generations. Homozygous cases show worse phenotype symptoms. May have post-maturity onset of symptoms.

Sex-Linked Traits Trait that is on a sex chromosome…more X linked traits than Y because X is much larger Sex chromosomes: genes that determine the gender of an individual Autosomes: remaining genes

Common sex linked diseases Hemophilia Duchenne muscular dystrophy Red-green color blindness

Genetic Screening Risk assessment for an individual inheriting a trait. Uses probability to calculate the risk.

Carrier Recognition Fetal Testing Newborn Screening Amniocentesis Chorionic villi sampling Newborn Screening

Fetal Testing Biochemical Tests Chromosome Analysis

Amniocentesis Administered between 11 - 14 weeks. Extract amnionic fluid = cells and fluid. Biochemical tests and karyotype. Requires culture time for cells.

Chorionic Villi Sampling Administered between 8 - 10 weeks. Extract tissue from chorion (placenta). Slightly greater risk but no culture time required.

Newborn Screening Blood tests for recessive conditions that can have the phenotypes treated to avoid damage. Genotypes are NOT changed. Ex. PKU

Newborn Screening Required by law in all states. Tests 1- 6 conditions. Required of “home” births too.

Multifactorial Diseases Where Genetic and Environment Factors interact to cause the Disease. Becoming more widely recognized in medicine.

Ex. Heart Disease Genetic Diet Exercise Bacterial Infection