Mendel & Heredity

 Genes are segments on the chromosomes that are responsible for inherited traits such as eye color, hair color, skin color, height, etc.  Now we’re going to learn how these traits are in- herited by the study of genetics.

 I. Genetics: The branch of biology that studies heredity. Heredity: The passing of characteristics from parents to offspring. Mendel, an Austrian monk, carried out the 1st important studies of heredity. The “father” of genetics. He is credited with stating the – principle that traits among off- – spring result from combinations – of dominant and recessive genes. Traits :Characteristics that – are inherited.

 Monohybrid Cross: A cross involving 1 pair of contrasting traits. Mendel’s 1st experiments are called monohybrid crosses because the 2 parent plants differed by a single trait—height. Example: crossing a tall pea plant with a short pea plant.

 Mendel carried out his experiments in three steps: 1. Mendel allowed each variety of garden pea to self- pollinate for several generations. This method ensured that each variety was true-breeding (purebred) for a particular trait; that is all the offspring would show only one form of a particular trait. Self-pollination-occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or another flower of the same plant. For example: All the tall pea plants had to produce many generations of nothing but tall plants. All the short pea plants had to produce many generations of nothing but short plants. These true-breeding plants served as parental generation in Mendel’s experiments. Parental generation = P 1 generation (or P generation)

22.Mendel then cross-pollinated two P 1 generation plants that had contrasting forms of a trait, such as a tall and a short pea plant. Mendel called the offspring of the P 1 generation the 1 st filial generation, or F 1 generation. He then examined each F 1 plant and recorded the number of F 1 plants expressing each trait. OOffspring of the P 1 generation = 1 st Filial or F 1 generation CCross-pollination- involves flower of two separate plants.

 3. Finally, Mendel allowed the F 1 generation to self-pollinate. He called the offspring the F 1 generation plants and the 2 nd filial generation, of F 2 generation. Again, each F 2 plant was characterized and counted.

PP 1 = 1st generation  Tall x short FF 1 =2nd generation= Tall Tall Tall Tall FF 2 =3rd generation=Tall Tall Tall short **The “x” means crossed. Mendel transferred pollen from one plant to another plant. **All of the f generation is tall. It’s as if the shorter parent had never existed. ** ¾ (75%) of the F 2 generation is Tall & ¼ (25%) of the F 2 generation is short. A ratio of 3:1. *It was as if the short (recessive) trait had reappeared from nowhere! *A dominant trait appears in every generation of the offspring. A recessive trait does not.

IIII. Mendel’s Theories of Heredity—The foundation of genetics. 1. Mendel is known as the father/founder of genetics. 2. For each inherited trait, an individual has two copies of the gene—one from each parent. 3. Alleles are different gene forms (different versions). An individual receives one allele from each parent. Each allele can be passed on when the individual reproduces. 4. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance.

MMendel’s 3 rd Theory Cont. Mendel described the expressed form of the trait as dominant. And the trait not expressed when the dominant form of the trait was present was described as recessive Dominant:The expressed form of the trait. (The trait is visible). Observable trait of an organism that masks a recessive form of the train. Dominant genes (alleles) are represented by capital letters and the same letter in small case represents recessive genes. The dominant allele is always written first. Ex. If tallness is dominant over short pea plants. Tt would be used if the pea plant is tall.

Blonde Hair VS. Brown Hair

 Mendel’s 3 rd Theory Cont. Recessive: The trait that is not expressed when the dominant form of the trait is present. The hidden trait of an organism that is masked by a dominant trait. For every pair of contrasting forms of a trait that Mendel studied, the allele for one form of the trait was always dominant and the allele for the other form of the trait was always recessive. Example: Each of Mendel’s pea plants had 2 alleles that determined its height. A plant could have: 2 alleles for tallness, TT 2 alleles for shortness, tt 1 for tall and 1 for short, Tt

 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait.  When gametes unite during fertilization, each gamete contributes one allele. Each parent can contribute only one of the alleles because of the way gametes are produced during the process of meiosis (haploid). Sperm (1n) + Egg (1n) = Zygote (2n)

 Phenotype: The way an organism looks. It is the physical appearance of a trait.  Example: The phenotype of a tall plant is tall regardless of the genes it contains.  Genotype:The gene combination on organism contains (set of alleles). Genotype describes the genetic make up of a trait. The genotype of a tall plant that has 2 alleles for tallness is TT.  **You can’t always know an organism’s genotype simply by looking at its phenotype.

 Homozygous:Two alleles for a trait are the same. (The presence of two identical alleles for a trait.)  Example—The true breeding tall plant that has two alleles for tallness (TT) would be homozygous for the trait height.  Because tallness is dominant, a TT individual is homozygous dominant for that trait.  A short plant would have 2 alleles for shortness (tt); it would always be homozygous recessive for the trait height.

 Heterozygous:(hybrid) Two alleles for a trait are different. (The presence of two different alleles for a trait.)  Example: The tall plant that has one allele for tallness and one allele for shortness (Tt) is heterozygous for trait height.

MMendel’s hypotheses brilliantly predicted the results of his crosses and also accounted for the ratios he observed. Because of their importance, Mendel’s ideas are often referred to as the laws of heredity LLaw of Dominance: The dominant gene hides the recessive gene; the recessive gene only shows when the dominant gene is NOT present. LLaw of Segregation: The first law of heredity describes the behavior of chromosomes during meiosis when homologous chromosomes, and then chromatids are separated. States that the two alleles for a trait segregate (separate) when gametes are formed.

MMendel’s Laws Cont. LLaw of Independent Assortment: States that the alleles of different genes separate independently of one another during gamete formation. Examples: The alleles for plant height separate independently of the alleles for flower color. We now know that this law applies only to genes that are located on different chromosomes or that are far apart on the same chromosomes. Today, however, scientists know that some of the parents’ characteristics are inherited together as a group because many genes are located together on the same chromosome. These are called linked genes, which were founded by Thomas Morgan & his graduate students. They studied the common fruit fly, Drosophila. EExample: There could be a short plant with green pods. There could also be a tall plant with green pods.

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State Test Quesiton Gregor Mendel pioneered studies of heredity in the 1800s. He believed that parent organisms could transmit any random combination of characteristics to their offspring. Today, however, scientists know that some of the parents’ characteristics are inhereited together as a group because—  certain genes attract one another and then stay together.  many genes are located together on the same chromosome.  pairs of chromosomes are joined together like the two sides of a zipper.  the chromosomes from one parent are dominant and those from the other are recessive.

PPunnett Squares: It is a diagram that predicts the possible offspring of crosses. We solve genetic problems and predict possible offspring by using the Punnett Squares. The simplest is four boxes inside a square. The possible gamete that one parent can produce is written along the top. The possible gamete that the other parent can produce is written along the left side. Each box inside the square is filled with two letters obtained by combining the allele along the top of the box with the allele along the side of the box. The letters in the boxes are the possible genotypes of the offspring.

A.Monohybrid Cross: A cross involving one pair of contrasting traits. Example #1: Cross a heterozygous tall plant with a homozygous short plant. T=dominant for tall and t=recessive for short. X Tttt T t tt Tt Tt tt Possible offspring: Genotype: 2 Tt; 2 tt Phenotype: 2 Tall; 2 short Genotype ratio _2:2______________ Phenotype ratio _2:2_____________ _50__ % tall _50__ % short

Example #2: Cross a heterozygous tall plant with a heterozygous tall plant. X Tt T t Tt TT Tt tt Possible offspring: Genotype: 1TT, 2Tt; 1tt Phenotype: 3 Tall; 1 short Genotype ratio _1:2:1______________ Phenotype ratio _3:1_____________ _75__ % tall _25__ % short

Example #3: A homozygous tall plant is crossed with a homozygous short plant. X TT tt T T tt Tt Tt Tt Possible offspring: Genotype: 4Tt Phenotype: 4 Tall Genotype ratio _4:0______________ Phenotype ratio _4:0_____________ _100__ % tall _0__ % short

Example #4: A homozygous tall plant bred to a heterozygous tall plant. X TTTt T T Tt TT Tt TTTt Possible offspring: Genotype: 2TT;2Tt Phenotype: 4 Tall Genotype ratio _2:2______________ Phenotype ratio _4:0_____________ _100__ % tall _0__ % short

Example #5: A dog that is heterozygous for curly hair is crossed with a dog that is homozygous recessive for straight hair. ss= straight hair SS=wavy hair Ss=curly hair X Ssss S s ss Ss ss ss Possible offspring: Genotype: 2Ss;2ss Phenotype: 2 curly; 2 straight Genotype ratio _1:1______________ Phenotype ratio _1:1_____________ _50__ % tall _50__ % short

Dihybrid Crosses: A cross involving 2 pairs of contrasting traits.  Ex: A female guinea pig is heterozygous for both fur color and coat texture is crossed with a male that has light fur color and is heterozygous for coat texture. What possible offspring can they produce?  Dark fur color is dominant (D) and light fur (d) is recessive. Rough coat texture (R) is  dominant, while smooth coat (r) is recessive.

Heterozygous fur color & coat texture X Light fur color & heterozygous coat texture ________ X ________ DdRr ddRr DR DrdRdr dR dr dR dr DdRRDdRrddRRddRr DdRrDdrrddRrddrr DdRRDdRrddRRddRr DdRrDdrrddRrddrr Genotype: 2 DdRR 4 DdRr 2 Ddrr 2 ddRR 4 ddRr 2 ddrr Phenotype: 6 dark & rough 2 dark & smooth 6 light & rough 2 light & smooth

V. Determining Unknown Genotype: Test cross: A cross in which an individual whose phenotype is dominant, but whose genotypes is not known, is crossed with a homozygous recessive individual. In other words, a cross of an individual of an unknown genotype with an individual with a known genotype. Example: In guinea pigs, both BB and Bb result in a black coat. How could you determine whether a black guinea pig is homozygous (BB) or heterozygous (Bb)? Example: Y? (Could be Yy, or YY don’t know if heterozygous or homozygous dominant.) Y? X yy B b bb Bb bb bb bb B B Bb Bb Bb

Probabilities Can Also Predict The Expected Results of Crosses: Probability—is the likelyhood that a specific event will occur. Consider the possibility that a coin tossed into air will land on heads (1 possible outcome.) The total number of all possible outcome is two— heads or tails. Thus, the probability that a coin will land on heads is ½ or 50%. Probability = number of 1 kind of possible outcome total number of all possible outcomes

Example : Assume that in humans there is a 50/50 chance that a child will be a boy. If a certain mother and father have four sons, what are the chances that their fifth child will be a daughter. X XYXX X Y X X XX XY XY number of 1 kind of possible outcome OR 1 total number of all possible outcomes 2

Family Pedigrees can be used to study how traits are inherited  Pedigree:  A drawing that shows how a trait is inherited over several generations. It is a graphic representation of an individual’s family tree, where several generations may be illustrated.  A pedigree is helpful if the trait is a genetic disorder and the family members want to know if they are carriers of if their children might get the disorder. A genetic trait that appears in every generation of offspring is called dominant.  Scientists can determine several pieces of genetic information from a pedigree.  Autosomal trait—if a trait is autosomal, it will appear in both sexes equally. Autosomal does not involve the sex chromosomes.

 Each chromosome carries genes for certain traits. Recall that in humans, the diploid number of chromosomes is 46 or 23 pairs.  There are 22 pairs of matching homologous chromosomes called autosomes. Autosome is a chromosome other than X or Y sex chromosomes. The 2 chromosomes in a homologous pair of autosomes look exactly alike.  In a pair of homologous chromosomes, 1 comes from the mother, and one comes from the father. In other words, out of 46 chromosomes, 23 comes from mom and 23 from dad.  Homologous chromosomes have genes for the same traits arranged in the same order. However, because there are different possible alleles (versions) for the same gene, the two chromosomes in a homologous pair are not identical to each other.

TThe 23rd pair (sex chromosomes) of chromosomes differs in males and females. These two chromosomes determine the sex of an individual and are called sex chromosomes. SSex-linked trait— Traits controlled by genes located on sex chromosomes. IIn humans, the chromosomes that control the inheritance of sex characteristics are indicated by the letters X and Y. If you are a female, XX, your 23rd pair of chromosomes look alike. If you are a male, XY, your 23rd pair of chromosomes look different

State Test Question Humans have 23 pairs of chromosomes. How many pairs are sex chromosomes  1  2  3  4

 If a trait is sex linked, it is usually seen only in males. Most sex-linked traits are recessive.  Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked condition.  A female who carries a recessive allele on one X chromosome will not exhibit the condition if there is dominant allele on her other X chromosome.  She will express the recessive condition only if she inherits two recessive alleles. MOST TRAITS ARE NOT CONTROLLED BY SIMPLE DOMINANT/RECESSIVE ALLELES Patterns of heredity Can Be Complex:

 Polygenetic Trait: A trait controlled by two or more genes. Polygenic inheritance occurs when many genes interact to produce a single trait. The genes may be on the same chromosome or on different chromosomes, and each gene may have two or more alleles.  Example: Familiar examples of polygenic traits in humans include eye color, height, weight, hair, and skin color. All of these characteristics have degrees of intermediate conditions between one extreme and the other. When light-skinned people marry dark-skinned people, their offspring have intermediate skin colors. (F 1 Generation)  When these children marry and produce the F 2 generation, the resulting skin colors range from the light skin color to the dark skin color of the grandparent (the P 1 generation), with most children having an intermediate skin color.

 Incomplete Dominance The phenotype of a heterozygote is intermediate between those of the two homozygotes (appearing halfway between the two parents). A condition in which a trait in an individual is intermediate between the phenotype of the two parents. Neither allele of the pair is completely dominant but combine and display a new trait. We use all capital letters on these problems to show that it is different from the traditional genetic cross.

 Example #1: In Caucasians, the child of a straight-haired parent and a curly-haired parent will have wavy hair. Straight and curly hair are homozygous dominant traits. Wavy hair is heterozygous and is intermediate between straight and curly hair. + =

Example #2: A cross between a homozygous red-flowered snapdragon is crossed with a homozygous white- flowered snapdragon.  RR=red  RW=pink  WW=white X RRWW R R WW RW Possible offspring: Genotype: 4RW Phenotype: 4 pink Genotype ratio _4:0______________ Phenotype ratio _4:0_____________

Codominance TThe equal expression of both alleles—both traits are displayed. IIn codominance, both alleles are expressed equally. CCodomant alleles cause the phenotype of both homozygotes to be produced in heterozygote individuals.

EExample #1: A cross between a homozygous red horse and a homozygous white horse results in heterozygous offspring with both red and white hairs in approximately equal numbers, producing the mixed color called roan. EExample #2: Crossing a black chicken with a white chicken results in a chicken that has black and white feathers, which appears checkered.

MMultiple Alleles A gene has three or more alleles for a trait. Blood type problems show this inheritance pattern. A capital “I” is used to show multiple alleles. Human blood types are determined by the presence or absence of certain molecules on the surfaces of red blood cells. Example: In the human population, the ABO blood groups (blood types) are determined by three alleles: I A, I B, and i on the surface of red blood cells. The I A allele produces surface molecule A. The I B allele produces surface molecule B. The i allele produces no surface molecules.

The I A and I B alleles are both dominant over I, which is recessive. But neither I A and I B is dominant over the other. When I A and I B are both present in the genotype, they are codominant (equal expression).

 When traits are controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene.  Different combinations of the 3 alleles I A, I B, and i result in four different blood phenotypes, A, AB, B, and O. Blood Types and Genotypes in the ABO Blood Group System AI A I A ; I A i BI B I B ; I B,i AB IAIBIAIB O ii Blood TypeGenotype Example: A child with type O has a mother with blood type A and a father with blood type B. The parental genotype for blood types must be __I A i; I B i__.

Traits Influenced by the Environment:  Environmental Influences: Making inheritance more difficult to understand are interactions between genes and the environment. The expression of some traits is affected by internal environments that are governed by age or sex. Expression of other traits is affected by external factors in the environment such as temperature, chemicals, or light. Example: Environmental influences fur color in the artic fox—in the winter it has white fur; it has dark fur in the summer.

Winter Summer

 Human Genetic Disorders Caused by Mutations: Changes in genetic material are called mutations. The harmful effects produced by inherited mutations are called genetic disorders. Many mutations are carried by recessive alleles in heterozygous individuals. This means two phenotypically normal people who are heterozygous carriers of a recessive mutation can produce children – who are homozygous – for the recessive allele.

Sickle Cell Anemia  An example of an autosomal recessive gene disorder is sickle cell anemia.  A condition caused by a mutated allele that produces a defective form of the protein hemoglobin.  Sickle cell anemia is caused by a recessive gene on chromosome 11.  In sickle cell anemia, the defective form of hemoglobin causes many red blood cells to bend into a sickle shape.  Normal RBCs last ~120 days in blood – stream, while sickle RBCs last ~ 10- – 20 days. Sickle cells tend to get stuck – in small blood vessels and block the – flow of blood.  Sickle cell anemia is a recessive disease..

Sickle Cell Anemia

EExample: A woman who is a carrier (Ss) for sickle cell has children with a man who is free of the disease (SS). What are the chances of the couple having a child with sickle cell anemia? X SS Ss S S S s SS Ss SS Ss ________ chance 0%

Tay-Sachs Disease  TSD is an extremely rare autosomal recessive genetic disorder. It causes deterioration of mental & physical abilities; destruction of the Central Nervous System, and fatty material builds up in the nerve cells of the brain.  The child appears normal at first. At 6 months, the child becomes blind, deaf, & unable to swallow over time. Muscle begin to atrophy & paralysis sets in; death by age 4.  No treatment or cure; found on chromosome 15.

Tay-Sachs Disease

Cystic Fibrosis An autosomal recessive gene disorder found to affect chromosome 7. This disorder causes production of a protein that makes a person’s body produce unusually thick, sticky mucus. This mucus can lead to severe infections, & it prevents enzymes from the pancreas from helping the body to break down food & absorb nutrients. This person usually has a persistent cough, frequent lung infections, salty tasting skin, infertility, and weight loss even when he/she or she eats well, & shortness of breath. There is no known cure; therefore, cystic fibrosis is often fatal. However, medical treatment may help patients live longer.

Huntington’s Disease  Huntington’s disease is a genetic disorder caused by a dominant allele located on an autosome.  In time, HD causes loss of muscle control, un- controllable physical spasms, severe mental illness, and eventually death.

 Example: If one parent is heterozygous for the trait and the other has normal alleles, what are their chances of having an offspring with the disease? X Hh hh H h hh Hh hh ________ chance 50%

Phenylketonuria (PKU)  A recessive disorder that results from the absence of an enzyme that converts one amino acid, phenylalanine cannot be broken down, it and its by-products accumulate in the body and result in severe damage to the central nervous system and mental retardation.

PKU

Sex-Linked Genetic Disorders  Human traits are determined by genes that are carried on the sex chromosomes.  Two traits that are sex-linked are color blindness and hemophilia.  When a characteristic is sex-linked, it occurs most commonly in males. A condition that primarily affects males is X-linked genetic disorders.

Sex-Linked Genetic Disorders  Hemophilia is a recessive genetic disorder caused by a mutation on the X chromosome. A recessive genetic disorder is hemophilia—a condition that impairs the blood’s ability to clot. If a mutation appears on the X chromosome, which a male receives from his mother, he does not have a normal gene on the Y chromosome to compensate. Therefore, he will develop hemophilia.

X XX h XY X XXhXh XX XhXXhX XYXhYXhY Possible offspring: Genotype: X h X; X h Y;XX; XY Phenotype: 1 female carrier; 1 infected male; 1 normal female; 1 normal male Example 1: A female is a carrier for hemophilia XX h – What will the offspring exhibit? Y

X XhXhXhXh XY XhXh X X h X XhYXhY XhXXhX X h Y Possible offspring: Genotype: 2X h X; 2X H Y; Phenotype: 2 female carriers; 2 infected males Example 2: A female has hemophilia disease X h X h – What will the offspring exhibit? XhXh Y

X XX c XY X Y XXc XX c XXc XYXcYXcY Possible offspring: Genotype: XX; XY; X c X; X c Y Phenotype: 1 normal female; 1 normal male; 1 female carrier; 1 colorblind male Color Blindness- affects about 8% of the male population The female who is the carrier XX c can expect-

Change in Chromosome Number  Karyotype Array of the chomosomes found in an individual’s cells arranged in order of size and shape. Helpful in diagnosing Down’s Syndrome. Homologous chromsomes are pairs of chromsomes containing genes that code for the same trait. The number of chromosomes can be studied using a karyotype.

 Each of an individual’s 46 chromosomes has thousands of genes. Since genes play an important role in determining how a person’s body develops and functions, the presence of all 46 chromosomes is essential for normal development and function. Humans who are missing even one of the 46 chromo- somes do not survive.

 A karyotype is helpful in diagnosing Down’s Syndrome (trisomy 21). People with Down’s syndrome have 47 chromosomes.  A karyotype can tell it there is an extra copy of chromosome 21, a missing 23 rd chromosome, more than one X chromosome. A karyotype can’t show individual genes on chromosomes.  The nuclei of cells (somatic cells) are examined at the stage of mitosis when they are most visible that is, at metaphase. The chromosomes are stained and photographed under the light microscope, as shown in part A. Pairs of homologous chromsomes are then arranged according to a standard numbering system from 1 to 22 and X, Y. Karyotype analysis reveals any gross chromosomal abnormalities

 Nondisjunction The failure of chromosome pairs to separate properly during cell division. Examples of disorders:  Down’s Syndrome (trisomy)  Cat Eye Syndrome (trisomy)  Cri du chat (monosomy)  Turner’s Syndrome

Down’s Syndrome

 Klinefelter’s Syndrome About 1 in every 500 males (1:1000) are born with an extra X chromosome, producing an XXY genotype. Because a Y chromosome is present, individuals are male. People with klinefelter syndrome are usually taller than average, have longer-than-average limbs, and are sterile. Often, men with this syndrome exhibit some degree of mental retardation.

Klinefelter’s Syndrome