Biology, 9th ed,Sylvia Mader Chapter 11 Chapter 11 Mendelian Inheritance Mendelian Inheritance

Human Genetic Disorders Outline Blending Inheritance Monohybrid Cross Law of Segregation Modern Genetics Genotype vs. Phenotype Punnett Square Dihybrid Cross Law of Independent Assortment Human Genetic Disorders

Formulated fundamental laws of heredity in early 1860s Gregor Mendel Austrian monk Studied science and mathematics at University of Vienna Conducted breeding experiments with the garden pea Pisum sativum Carefully gathered and documented mathematical data from his experiments Formulated fundamental laws of heredity in early 1860s Had no knowledge of cells or chromosomes Did not have a microscope

Gregor Mendel

Fruit and Flower of the Garden Pea

Garden Pea Traits Studied by Mendel

Theories of inheritance in Mendel’s time: Blending Inheritance Theories of inheritance in Mendel’s time: Based on blending Parents of contrasting appearance produce offspring of intermediate appearance Mendel’s findings were in contrast with this He formulated the particulate theory of inheritance Inheritance involves reshuffling of genes from generation to generation

Mendel’s Monohybrid Crosses: An Example

One-Trait Inheritance Mendel performed cross-breeding experiments Used “true-breeding” (homozygous) plants Chose varieties that differed in only one trait (monohybrid cross) Performed reciprocal crosses Parental generation = P First filial generation offspring = F1 Second filial generation offspring = F2 Formulated the Law of Segregation

Law of Segregation Each individual has a pair of factors (alleles) for each trait The factors (alleles) segregate (separate) during gamete (sperm & egg) formation Each gamete contains only one factor (allele) from each pair Fertilization gives the offspring two factors for each trait

Modern Genetics View Each trait in a pea plant is controlled by two alleles (alternate forms of a gene) Dominant allele (capital letter) masks the expression of the recessive allele (lower- case) Alleles occur on a homologous pair of chromosomes at a particular gene locus Homozygous = identical alleles Heterozygous = different alleles

Homologous Chromosomes

Genotype Versus Phenotype Refers to the two alleles an individual has for a specific trait If identical, genotype is homozygous If different, genotype is heterozygous Phenotype Refers to the physical appearance of the individual

Table listing all possible genotypes resulting from a cross Punnett Square Table listing all possible genotypes resulting from a cross All possible sperm genotypes are lined up on one side All possible egg genotypes are lined up on the other side Every possible zygote genotypes are placed within the squares

Punnett Square Showing Earlobe Inheritance Patterns

Try this one MONOHYBRID CROSS Cross a heterozygous tall plant with a heterozygous tall plant (use T = tall and t = short) Determine expected genotype and phenotype ratios.

Test cross determines genotype of individual having dominant phenotype Monohybrid Testcross Individuals with recessive phenotype always have the homozygous recessive genotype However, Individuals with dominant phenotype have indeterminate genotype May be homozygous dominant, or Heterozygous Test cross determines genotype of individual having dominant phenotype

IS MY DOG A PURE BREED? Do a test cross: G = golden retriever g = other

Not so much!

One-Trait Test Cross Unknown is Heterozygous

One-Trait Test Cross Unknown is Homozygous Dominant

Two-Trait Inheritance Dihybrid cross uses true-breeding plants differing in two traits Observed phenotypes among F2 plants Formulated Law of Independent Assortment The pair of factors for one trait segregate independently of the factors for other traits All possible combinations of factors can occur in the gametes

Try Mendel’s Classic Dihybrid Cross Cross two heterozygous tall, heterozygous green pod producing plants. Use a punnett square to show expected offspring and complete a phenotype ratio. Key: T = tall G = green pods t = short g = yellow pods

Two-Trait (Dihybrid) Cross

Hartford’s example

Rat Answers

Two-Trait Test Cross

Human Genetic Disorders Autosome - Any chromosome other than a sex chromosome Genetic disorders caused by genes on autosomes are called autosomal disorders Some genetic disorders are autosomal dominant An individual with AA has the disorder An individual with Aa has the disorder An individual with aa does NOT have disorder Other genetic disorders are autosomal recessive An individual with AA does NOT have disorder An individual with Aa does NOT have disorder, but is a carrier An individual with aa DOES have the disorder

Autosomal Recessive Pedigree Chart

Example: albinism

Autosomal Dominant Pedigree Chart

Autosomal Recessive Disorders Tay-Sachs Disease Progressive deterioration of psychomotor functions Cystic Fibrosis Mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous Phenylketonuria (PKU) Lack enzyme for normal metabolism of phenylalanine

Cystic Fibrosis Therapy

Autosomal Dominant Disorders Neurofibromatosis Tan or dark spots develop on skin and darken Small, benign tumors may arise from fibrous nerve coverings Huntington Disease Neurological disorder Progressive degeneration of brain cells Severe muscle spasms Personality disorders

A Victim of Huntington Disease

Huntington Disease: Normal and Diseased Brain

Sex-linked – disorders carried on the X chromosome Colorblindness Hemophilia Baldness (?)

Phenotype reveals genotype without test cross Incomplete Dominance Heterozygote has phenotype intermediate between that of either homozygote Homozygous red has red phenotype Homozygous white has white phenotype Heterozygote has pink (intermediate) phenotype Phenotype reveals genotype without test cross

Incomplete Dominance

Multiple Allelic Traits Some traits controlled by multiple alleles The gene exists in several allelic forms (but each individual only has two) ABO blood types The alleles: IA = A antigen on red cells, anti-B antibody in plasma IB = B antigen on red cells, anti-AB antibody in plasma I = Neither A nor B antigens, both antibodies Phenotype (Blood Type) Genotype A (actually AA or AO) IAIA or IAi B (actually BB or BO) IBIB or IBi AB IAIB O (actually OO) ii

Inheritance of Blood Type

Answers to blood type problems Blood Type Mystery Answers to blood type problems

QUICK REVIEW Incomplete vs. Codominance Mendel proposed independent assortment (not always the case) * linked genes (w/crossing over at times) Mendel proposed complete dominance (not always the case) * incomplete and codominance occurs

AB blood type is also example of codominance

Blood Type: good for complete dom., codominance and multiple alleles

Complete the chart

Polygenic Inheritance Occurs when a trait is governed by two or more genes having different alleles Each dominant allele has a quantitative effect on the phenotype These effects are additive Result in continuous variation of phenotypes

Height in Human Beings

Frequency Distributions in Polygenic Inheritance

Terminology Pleiotropy Codominance Epistasis A gene that affects more than one characteristic of an individual Sickle-cell (incomplete dominance) Codominance More than one allele is fully expressed ABO blood type (multiple allelic traits) Epistasis A gene at one locus interferes with the expression of a gene at a different locus Human skin color (polygenic inheritance)

Pleiotropy

Epistasis – disrupts the expected

Environment and Phenotype: Himalayan Rabbits

SOLVING GENETICS PROBLEMS Rules of Probability *segregation and independent assortment of alleles during meiosis and fertilization are random events *if we know genotype of parents we can predict the EXPECTED genotypes of offspring using rules of probability

The probability of all possible outcomes for an event must add up to 1 Probability Scale Scale range: 0-1 The probability of all possible outcomes for an event must add up to 1

EVENT PROBABILITY *tossing heads w/normal coin………………1/2 *tossing coins Common Examples EVENT PROBABILITY *tossing heads w/normal coin………………1/2 *tossing coins 1

Probability of rolling 3 on a six-sided die? 1/6 Try a common one Probability of rolling 3 on a six-sided die? 1/6 Probability of rolling a number other than 3? 5/6

Random events are independent of one another! If a couple has 6 sons and tries for the little girl: there is still a ½ chance of getting their wish. Regardless of the past events!

Rule of Multiplication The probability that independent events will occur simultaneously is the product of their individual probs.

A: egg w/ t = ½ sperm w/ t = ½ Ex 1 - monohybrid Q: In a Mendelian cross between two heterozygous pea plants (Tt), what is the probability that the offspring will be homozygous recessive? A: egg w/ t = ½ sperm w/ t = ½ overall probability that two will unite is ½ x ½ = ¼

A: prob. of egg w/ YR = ½ x ½ = ¼ prob. of sperm w/ YR = ½ x ½ = ¼ Ex 2. Dihybrid cross Q: For YyRr x YyRr, what is the probability of an F2 plant having genotype YYRR? A: prob. of egg w/ YR = ½ x ½ = ¼ prob. of sperm w/ YR = ½ x ½ = ¼ Overall prob. for offspring: ¼ x ¼ = 1/16

Ex. 3 Trihybrid Q: What is the probability that a trihybrid cross between two organisms with genotypes AaBbCc and AaBbCc produce an AABBCc offspring? A: Look at each allele pair separately Aa x Aa: prob. of AA = ¼ Bb x Bb: prob. of BB = ¼ Cc x Cc: prob. of Cc = ½ Overall that this will occur simultaneously: ¼ x ¼ x ½ = 1/32

Rule of Addition The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways. Think heterozygotes!

Ex. 1: monohybrids Q: In a Mendelian cross between pea plants that are heterozygous (Pp), what is the probability of the offspring being heterozygous? A: there are two different ways for this to occur- 1) sperm gives P and egg gives p OR 2) egg gives P and sperm gives p

So… sperm gives P and egg gives p ½ X ½ = ¼ 2) egg gives P and sperm gives p OVERALL PROB. of way 1 or way 2 ¼ + ¼ = ½

A: Look at each allele pair separately Aa x Aa: prob. of aa = ¼ Ex 2 Trihybrid Q: What is the probability that a trihybrid cross between two organisms with genotypes AaBbCc and AaBbCc produce an aabbcc offspring? A: Look at each allele pair separately Aa x Aa: prob. of aa = ¼ Bb x Bb: prob. of bb = ¼ Cc x Cc: prob. of cc = ¼ Overall that this will occur simultaneously: ¼ x ¼ x ¼ = 1/64

ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr Ex. 3 Trihybrid Q: PpYyRr x Ppyyrr What is the chance that the offspring will show at least two recessive phenotypes out of the three traits? A: break down the possibilities of all the genotypes that are homozygous recessive for at least two traits: ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr

PpYyRr x Ppyyrr ppyyRr ¼ x ½ x ½ = 1/16 ppYyrr ¼ x ½ x ½ = 1/16 Ppyyrr ½ x ½ x ½ = 2/16 PPyyrr ¼ x ½ x ½ = 1/16 ppyyrr ¼ x ½ x ½ = 1/16 6/16 or 3/8 chance of an offspring showing at least 2 of the recessive traits

Human Genetic Disorders Review Blending Inheritance Monohybrid Cross Law of Segregation Modern Genetics Genotype vs. Phenotype Punnett Square Dihybrid Cross Law of Independent Assortment Human Genetic Disorders

Biology, 9th ed,Sylvia Mader Chapter 11 Ending Slide Chapter 11 Mendelian Inheritance Mendelian Inheritance