Basic Concepts in Genetics Mendel and the Gene Idea Basic Concepts in Genetics

What Is heredity The passing on of characteristics from parents to offspring Trait physical characteristic Genetics is the study of heredity

Chromosome Logical Structure Gene – segments of DNA that code for the basic units of heredity and are transmitted from one generation to the next Allele – genes that reside at the same locus on homologous chromosomes Alleles are the result of mutation!!!!!!

Genotypes Phenotypes At each locus (except for sex chromosomes) there are 2 genes. These constitute the individual’s genotype at the locus. The alleles that are present. The expression of a genotype is termed a phenotype. For example, hair color, weight, or the presence or absence of a disease.

genotype Genotype Gene(s) responsible for the trait The alleles that are present on each homologous chromosome that code for the trait

Phenotype Phenotype Expression of the characteristic The trait The way we “look” Red hair or Brown hair The expression of the gene

Patterns of Inheritance Gregor Mendel (1822-1884) is the called the “Father of Genetics” Researched with garden peas Developed the ideas that are the basis of genetics

Mendel used peas… Characters (inherited characteristic) are in two distinct forms (such as white and purple color) called traits. Not many traits Easy to keep track The male and female gametes are enclosed within the same flower – He could control the fertilization process Self-fertilization Cross-pollination The garden pea is small, grows easily, matures quickly and produces many offspring.

Mendelian genetics Height - short or TALL Seed color - green or YELLOW Mendel studied a number of characteristics in pea plants including: Height - short or TALL Seed color - green or YELLOW Seed shape - wrinkled or ROUND Seed coat color - white or GRAY Pod shape - constricted or SMOOTH Pod color - yellow or GREEN Flower position - terminal or AXIAL

Mendel was a careful researcher Carefully controlled experiments Studied one trait at a time Kept detailed data Cross - combining gametes from parents with different traits The offspring are called hybrids offspring of parents with different traits A monohybrid cross is one that looks at only one trait (let’s look at plant height – tall or short) Cross fertilization Pollen from one plant to fertilize another plant

Mendel’s monohybrid crosses Step One: Mendel allowed the peas to self-pollinate for several generations. What Did Mendel Find?

Mendel’s monohybrid crosses Step One: Each variety was true-breeding for a particular character. tall plants only produced tall plants These plants served as the parental generation. The P generation is the first two individuals that are crossed in a breeding experiment

Mendel’s monohybrid crosses Step Two: Mendel cross-pollinated two P generation plants with different traits The offspring were the first filial generation or F1 generation Mendel recorded the traits of the offspring What Did Mendel Find?

Mendel’s monohybrid crosses Tall plant crossed with short plants produced all tall offspring Purple flowers crossed with white flowers produced all purple offspring

Mendel’s monohybrid crosses Step Three: Finally, Mendel allowed the F1 generation to self-pollinate. He called the offspring of the F1 generation, the second filial generation, or F2 generation Again, Mendel recorded the traits of the offspring What Did Mendel Find?

Mendel’s monohybrid crosses The short plants reappeared!!!!!! Mendel found that 3 out of 4 (¾) of the offspring were tall & 1 out of 4 (¼) were short

Mendel’s monohybrid crosses Mendel found the same 1:3 ratio (1 out of 4) in the other traits as well!

Mendel’s results He discovered different laws and rules that explain factors affecting heredity

Theory of Heredity Before Mendel, people thought offspring were a blend of traits Tall x short = medium Mendel’s experiments did not support this theory Mendel’s work led him to the understanding that traits are carried in pairs (one from each parent)

questions What did Mendel cross? What are traits? What are gametes? What is fertilization? What is heredity? What is genetics?

Mendel’s 4 hypotheses Hypotheses 1 For each inherited character, an individual has two copies of each gene One on each chromosome Alternative versions of genes account for variations in inherited characters. Alleles are different versions of genes that impart the same characteristic.

Mendel’s 4 hypotheses Hypotheses 2 Gametes (sperm or egg) carry only one allele as a result of pair separation during meiosis Offspring inherit 2 alleles, 1 from each parent for each characteristic (i.e. height, color, etc.)

Mendel’s 4 hypotheses Hypotheses 3 The Rule of Dominance Some genes (alleles) are dominant and others are recessive The dominant allele, is fully expressed in the organism's appearance The recessive allele, has no noticeable effect on the organism's appearance.

Mendel’s 4 hypotheses Hypotheses 4 The two genes for each character segregate during gamete production. The genes are sorted into separate gametes, ensuring variation.  This sorting process depends on genetic recombination

mendel’s First law of heredity Law of Random Segregation A parent randomly passes only one allele for each trait to each offspring

Mendel’s second law of heredity Law of Independent Assortment Different gene pairs assort independently in gamete formation. This Law is only true for genes on separate chromosomes!

Law of independent assortment P GENERATION -True Breeding Parent Plants All purple (PP) All white (pp) Gametes will be either P or p FIRST FILIAL GENERATION F1 F1 are all purple because of dominance (Pp) SECOND FILIAL GENERATION F2 F2 results in a mathematically predictable 3:1 ratio

Mendelian Inheritance patterns Involve genes directly influencing traits Obey Mendel’s laws Law of segregation Law of independent assortment Include Dominant / recessive relationships Gene interactions Phenotype-influencing roles of sex and environment Most genes of eukaryotes follow a Mendelian inheritance pattern

Predicting Inheritance To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes. Each allele is represented by a letter UPPERCASE/CAPITAL letters are used to represent dominant genes. lowercase letters are used to represent recessive genes. Homozygous - the two alleles for a trait are the same (AA or aa) Heterozygous - the two alleles for a trait are different (Aa)

Predicting Inheritance For example: T = represents the gene for TALL in pea plants t = represents the gene for short in pea plants So: TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant.

PUNNETT SQUARES A diagram that predicts the outcome of a genetic cross It considers all the possible combinations of gametes 1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S WHY E COLI? What do you like to eat? WHEN YOU EAT YOU NEED ENZYMES TO BREAK DOWN THE FOOD YOU EAT EXPLAIN WHAT ENZYMES ARE THEY BREAK DOWN FOOD LIKE GLUCOSE AND LACTOSE WHAT ARE GLUCOSE AND LACTOSE?

PUNNETT SQUARES The alleles from one parent go here. The alleles from the other parent go here.

PUNNETT SQUARES T t

PUNNETT SQUARES T t

PUNNETT SQUARES T t

PUNNETT SQUARES T t Tt

PUNNETT SQUARES T t Tt

PUNNETT SQUARES F1 generation T t Tt

Interpreting the Results The genotype for all the offspring is Tt. The genotype ratio is: Tt – 4:4 = 100% heterozygous The phenotype for all the offspring is tall. The phenotype ratio is: tall – 4:4 = 100% tall T t Tt

PUNNETT SQUARES F2 generation T t TT Tt tt

Interpreting the Results This time the ratios are different! Tt. The genotype ratio is: TT – 1:4 Tt – 2:4 tt – 1:4 1:2:1 2 – homozygous 2 – heterozygous The phenotype ratio is: TT, Tt, Tt = 3 tall tt = 1 short 3:1 – tall : short T t TT Tt tt

Laws of probability help explain genetic events Genetic ratios are most properly expressed as probabilities: Probabilities range from 0 - an event is certain NOT to happen 1.0 - an event is certain to happen

Predicting Outcomes-probability The likelihood that a specific event will occur Probability = # of one kind of possible outcome total # of all possible outcomes a coin lands on “heads” 1 outcome Total possible outcomes = 2 heads or tails Possibility that the coin will land on heads = 1/2

Product law For simultaneous outcomes (this AND that) What is the chance that you will roll snake eyes with two dice? (1 and 1) Chance of rolling 1 with first die = 1/6 Chance of rolling 1 with second die = 1/6 Chance of rolling two 1’s = 1/6 X 1/6 = 1/36

Sum law For outcomes that can occur more than one way (this OR that) What is the chance that you will roll either a 1 or a 6 with one die? Chance of rolling 1 = 1/6 Chance of rolling 6 = 1/6 Chance of rolling 1 or 6 = 1/6 + 1/6 = 2/6 = 1/3

What fraction would we expect to be Probabilities Multiplication Rule The probability that two independent events, A and B, are realized simultaneously is given by the product of their separate probabilities What fraction would we expect to be Round AND Green 3/4 x 1/4 = 3/16

What fraction would we expect to be Probabilities Addition Rule The probability that one or the other of two mutually exclusive events, A or B, is the sum of their separate probabilities What fraction would we expect to be (Round and Green) OR (wrinkled and yellow) 3/16 + 3/16 = 6/16

Dihybrid cross F1 produces equal amounts of 4 possible genotypes F2 reveals even more genotypic possibilities (9:3:3:1) Dihybrid cross is equivalent to two monohybrid crosses (12:4 or 3:1) Illustrates the Law of Independent Assortment

NON-MENDELIAN INHERITANCE Many genes do not follow a Mendelian inheritance pattern Incomplete Dominance Co-dominance Multiple alleles Pleiotropy Polygenic Inheritance Lethal Dominance Environmental Influence on Gene Expression

Incomplete dominance The phenotype of the heterozygous genotype is intermediate between the phenotypes of the homozygous genotypes E.g. snapdragons Heterozygotes differ from homozygotes Predictable 1:2:1 ratio Different than “blending” hypothesis No testcross necessary

codominance Both alleles are dominant and affect the phenotype in two different but equal ways Andalusian chickens show this pattern of inheritance. If you cross a black (BB) chicken With a white (WW) chicken You get black+white speckled (BW) chicken

Multiple alleles More than two possible alleles controlling one trait Example3 alleles in blood type – OAB 4 possible phenotypes = O, A, B, AB 6 possible genotypes Note: this is also an example of co-dominance

Multiple alleles Genes that have more than two alleles More than two alleles exist in a given population However, any one individual only has two of these alleles Example - Coat color in rabbits (4 alleles) Chinchilla Rabbit Himalayan Rabbit Full Color Rabbit Albino Rabbit

Pleiotropy Gene influences multiple characteristics A singe gene influences more than one phenotypic trait. Genes that exert effects on multiple aspects of physiology or anatomy are pleiotropic

Polygenic Inheritance Multiple genes have an additive effect on a single character in the phenotype Example: Skin Color or height Usually is described by a bell-shaped curve with majority clustered in the middle

Examples of Polygenic Inheritance

Lethal dominance T/t x T/t = T/T T/t t/t 1 : 2 : 1 ratio at conception 0 : 2 : 1 ratio at birth

Characters influenced by the environment pH of the soil will change the color of hydrangea flowers from blue to pink

Characters influenced by the environment Temperature will affect color change in fur

Characters influenced by the environment Body temperature affects color in Siamese cats Height is affected by nutrition

How do we determine inheritance of human traits Pedigree Analysis In humans, pedigree analysis is used to determine individual genotypes and to predict the mode of transmission of single gene traits

Some examples of dominant and recessive traits in humans (at one gene locus)

Autosomal dominant traits Huntington disease is a progressive nerve degeneration, usually beginning about middle age, that results in severe physical and mental disability and ultimately in death Every affected person has an affected parent Two unaffected parents will not produce affected children. (aa x aa) Both males and females are affected with equal frequency. Pedigrees show no Carriers.

Examples of autosomal dominant disorders Dwarfism Polydactyly and Syndactyly Hypertension Hereditary Edema Chronic Simple Glaucoma – Drainage system for fluid in the eye does not work and pressure builds up, leading to damage of the optic nerve which can result in blindness. Huntington’s Disease – Nervous system degeneration resulting in certain and early death. Onset in middle age. Neurofibromatosis – Benign tumors in skin or deeper Familial Hypercholesterolemia – High blood cholesterol and propensity for heart disease Progeria – Drastic premature aging, rare, die by age 13. Symptoms include limited growth, alopecia, small face and jaw, wrinkled skin, atherosclerosis, and cardiovascular problems but mental development not affected.

Autosomal recessive traits Albinism = absence of pigment in the skin, hair, and iris of the eyes Most affected persons have parents who are “normal” (Aa x Aa) The parents are heterozygous for the recessive allele and are called carriers (Aa) Approximately 1/4 of the children of carriers are affected (aa) Close relatives who reproduce are more likely to have affected children. Both males and females are affected with equal frequency. Pedigrees show both male and female carriers.

Examples of autosomal recessive traits Congenital Deafness Diabetes Mellitus Sickle Cell anemia Albinism Phenylketoneuria (PKU) – Inability to break down the amino acid phenylalanine. Requires elimination of this amino acid from the diet or results in serious mental retardation. Galactosemia – enlarged liver, kidney failure, brain and eye damage because can’t digest milk sugar Cystic Fibrosis – affects mucus and sweat glands, thick mucus in lungs and digestive tract that interferes with gas exchange, lethal. Tay Sachs Disease – Nervous system destruction due to lack of enzyme needed to break down lipids necessary for normal brain function. Early onset and common in Ashkenazi Jews; results in blindness, seizures, paralysis, and early death.

Why we look the way we look... Mendel and Heredity