Genetics is the study of inheritance Mendel’s laws of inheritance – Classical genetics Molecular Genetics – DNA structure and function

Mendel’s laws of inheritance Gregor Johann Mendel (1822-1884) is considered the father of genetics. Proposed the pattern of inheritance Mendel chose the garden pea to study the natural laws governing plant hybrids Mendel carried out two types of crosses 1. Self-fertilization Pollen and egg are derived from the same plant Naturally occurs in peas because a modified petal isolates the reproductive structures 2. Cross-fertilization Pollen and egg are derived from different plants Required removing and manipulating anthers Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Pea flowers

Contain the pollen grains, where the male gametes (sperm) are produced Contain the female gametes (eggs)

Provides storage material for the developing embryo

Mendel’s Experiments Mendel studied seven traits: Each trait showed two variants or forms found in the same species His first experiments crossed only two variants of one trait at a time. This is termed a monohybrid cross A pea plant contains two discrete hereditary factors, one from each parent - The two factors may be identical or different When the two factors of a single trait are different One is dominant and its effect can be seen The other is recessive and is not expressed During gamete formation, the paired factors segregate randomly so that half of the gametes received one factor and half of the gametes received the other This is Mendel’s Law of Segregation Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Mendelian factors are now called genes Alleles are different versions of the same gene An individual with two identical alleles is termed homozygous An individual with two different alleles, is termed heterozygous Genotype: RR rr Genotype refers to the specific allelic composition of an individual Phenotype refers to the outward appearance of an individual (Red vs white) Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Genes to Traits Molecular basis of Alleles Alleles are alternate forms of the same gene DNA ----------> Protein -----> Red Pigment -ATG CCT AGG- - Met-Pro-Arg- DNA -----------> Protein --X---> No Pigment -ATG CTT AGG- - Met-Leu-Arg

Test Cross : to determine if an individual expressing the dominant trait is a heterozygous or homozygous Make a cross between the individual expressing the dominant trait with a homozygous recessive individual Parent TT X tt (test parent) (Tall) (Dwarf) Gametes T T t t F1 Tt Tall Parent Tt X tt (test parent) Tall (Tall) (Dwarf) Gametes T t t t F1 Tt, t t (1:1) Tall : Dwarf (1:1)

Create an empty Punnett square Fill in the Punnett square with the possible genotypes of the offspring by combining the alleles of the gametes

Dihybrid cross (Two traits)

Independent assortment is also revealed by a dihybrid test-cross TtYy X ttyy Thus, if the genes assort independently, the expected phenotypic ratio among the offspring is 1:1:1:1 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

law of Independent assortment (Pattern of Inheritance) Based on the distribution of the different classes of the F2 progeny in a dihybrid cross, Mendel concluded that the genes assorted independently. This is the law of Independent assortment During gamete formation, the segregation of any pair of hereditary determinants is independent of the segregation of other pairs If one parent is RrYy and the other is rryy, the progeny will be of four types (Test cross) RrYy, Rryy, rrYy, rryy in the ratio of 1:1:1:1

Laws of Inheritance Law of segregation Law of Independent assortment During gamete formation, the paired factors segregate randomly so that half of the gametes receives one factor and half of the gametes receives the other If the parent is Rr, the gametes will be R, r in the ratio of 1:1 Law of Independent assortment During gamete formation, the segregation of any pair of hereditary determinants is independent of the segregation of other pairs If the parent is RrYy, the gametes could be of four types RY, Ry, rY, ry in the ratio of 1:1:1:1

Pedigree Analysis (Inheritance in Humans) Pedigree analysis is used to determine the pattern of inheritance of traits in humans When studying human traits, it is not ethical to control parental crosses Rather, we must rely on information from family trees or pedigrees Pedigree analysis is commonly used to determine the inheritance pattern of human genetic diseases Genes that play a role in disease may exist as A normal allele A mutant allele that causes disease symptoms Disease that follow a simple Mendelian pattern of inheritance can be Dominant Recessive Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

A recessive pattern of inheritance makes two important predictions 1. Two normal heterozygous individuals will have, on average, 25% of their offspring affected 2. Two affected individuals will produce 100% affected offspring A dominant pattern of inheritance predicts that An affected individual will have inherited the gene from at least one affected parent Alternatively, the disease may have been the result of a new mutation that occurred during gamete formation Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

A family pedigree for albinism

An albino family

A family pedigree of dwarfism

The father (dwarf) must be heterozygous and did not pass the dwarfism gene to his children

Dominant versus Recessive Inheritance Pattern? B

PROBABILITY AND STATISTICS The laws of inheritance can be used to predict the outcomes of genetic crosses For example Animal and plant breeders are concerned with the types of offspring produced from their crosses Parents are interested in predicting the traits that their children may have This is particularly important in the case of families with genetic diseases Of course, it is not possible to definitely predict what will happen in the future Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Probability The probability of an event is the chance that the event will occur in the future Probability = Number of times an event occurs Total number of events For example, in a coin flip Pheads = 1 heads (1 heads + 1 tails) = 1/2 = 50% Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Sum rule The probability that one of two or more mutually exclusive events will occur is the sum of their respective probabilities Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

If two heterozygous (Dede Ctct) mice are crossed Then the predicted ratio of offspring is 9 with normal ears and normal tails 3 with normal ears and crinkly tails 3 with droopy ears and normal tails 1 with droopy ears and crinkly tail These four phenotypes are mutually exclusive A mouse with droopy ears and a normal tail cannot have normal ears and a crinkly tail Question What is the probability that an offspring of the above cross will have normal ears and a normal tail or have droopy ears and a crinkly tail? Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Applying the sum rule Step 1: Calculate the individual probabilities P(normal ears and a normal tail) = 9 (9 + 3 + 3 + 1) = 9/16 P(droopy ears and crinkly tail) = 1 (9 + 3 + 3 + 1) = 1/16 Step 2: Add the individual probabilities 9/16 + 1/16 = 10/16 10/16 can be converted to 0.625 Therefore 62.5% of the offspring are predicted to have normal ears and a normal tail or droopy ears and a crinkly tail Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Product rule The probability that two or more independent events will occur is equal to the product of their respective probabilities Note Independent events are those in which the occurrence of one does not affect the probability of another Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Consider the disease congenital analgesia Recessive trait in humans Affected individuals can distinguish between sensations However, extreme sensations are not perceived as painful Two alleles P = Normal allele p = Congenital analgesia Question Two heterozygous individuals plan to start a family What is the probability that the couple’s first three children will all have congenital analgesia? Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Applying the product rule Step 1: Calculate the individual probabilities This can be obtained via a Punnett square P(congenital analgesia) = 1/4 Step 2: Multiply the individual probabilities 1/4 X 1/4 X 1/4 = 1/64 1/64 can be converted to 0.016 Therefore 1.6% of the time, the first three offspring of a heterozygous couple, will all have congenital analgesia Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

What are the chances of having progeny with the genotype AA bb Cc Aa Bb CC X Aa bb Cc What are the chances of having progeny with the genotype AA bb Cc Chance of AA is ½ x ½ + ¼ Chance of bb is ½ x 1 = ½ Chance of Cc is 1 x ½ = ½ Probability that an offspring will be AA bb Cc is P = ¼ x ½ x ½ = 1/16

What are the chances that T will be an albino? A couple, denoted R and S, are concerned about the possibility that they will have a child (T) with albinism, an autosomal recessive condition. S, the prospective father, is an albino, and R, the prospective mother, has 2 albino siblings. What are the chances that T will be an albino? The risk depends on two factors: probability that R is heterozygous and the probability that she will transmit this allele to T if she is a carrier R’s parents must be Aa Aa x Aa = 1AA : 2 Aa : 1aa = chance that R is Aa is 2/3 Chances that R will pass on a to T is 2/3 x1/2 = 1/3 R is aa Chances of T being aa is 1/3 x 1 = 1/3