Chapter 10 Genetics

Heritable Traits Heredity is the tendency for traits (features) to be passed from parent to offspring

Gregor Mendel Gregor Mendel performed experiments with garden peas Peas are ideally suited to the study of heredity White Self-fertilize Purple

Mendel’s Experimental Design First he established true-breeding varieties and named these pure lines the P generation

Mendel’s Experimental Design Next he crossed two different varieties and named the resulting offspring the F1 generation The trait expressed in the F1 generation known as the dominant trait

Mendel’s Experimental Design Finally he allowed the plants from the F1 generation to self-fertilize and named the resulting offspring the F2 generation The trait not expressed in the F1 generation, but reemerges in the F2 generation is known as the recessive trait

Seven Characteristics Studied

Patterns Emerged Mendel found a consistent proportions of expressed traits in the F2 generation 3/4 expressed the dominant trait while 1/4 expressed the recessive trait the dominant : recessive ratio 3:1

F3 Generation 1/4 of plants from the F2 that were dominant were true-breeding dominant 1/2 of plants showed both traits 1/4 of the plants from the F2 that were recessive were true-breeding recessive

True Ratio He determined that the 3:1 ratio that he observed in the F2 generation was, in fact, a disguised 1:2:1 ratio

Mendel Proposes Theories Hypothesis 1 parents do not transmit traits directly to their offspring parents transmit information about the trait in the form of genes Hypothesis 2 each parent contains two copies of a gene the two copies of the gene may or may not be the same

Mendel Proposes Theories Hypothesis 3 alternative forms of a gene lead to alternative traits the alternative forms of a gene are known as alleles An individual with 2 identical alleles is said to be homozygous (homo = same) An individual with 2 different alleles is said to be heterozygous (hetero = different)

Genotype vs Phenotype Genotype indicates the alleles an individual possesses a dominant trait is expressed as a capital letter eg. P = purple flowers a recessive trait is expressed as a lower case letter eg. p = white flowers Phenotype is the physical appearance of an individual depending on their genotype a homozygous dominant individual has genotype PP, and phenotype purple flowers a homozygous recessive individual has genotype pp, and phenotype white flowers a heterozygous individual has genotype Pp, and phenotype purple flowers

Genotype vs Phenotype Genotype Phenotype PP Pp pp Homozygous Dominant Heterozygous Homozygous Recessive PP Pp pp Genotype Phenotype

Punnett Square The results from a cross between a true-breeding, white-flowered plant (pp) and a true breeding, purple-flowered plant (PP) can be visualized with a Punnett square Lists the possible gametes from one individual with the possible gametes from the other individual on the opposite side The genotypes of potential offspring are represented within the square

A Punnett square analysis

Punnett Square of Flower Color

Testcross Mendel devised the testcross with a homozygous recessive individual in order to determine the genotype of unknown individuals in the F2 generation Heterozygous, ½ offspring dominant phenotype, ½ recessive phenotype Homozygous dominant, all offspring dominant phenotype

Mendel’s Laws Mendel’s First Law: Segregation the two alleles of a trait separate from each other during the formation of gametes, so that half of the gametes will carry one copy and half will carry the other copy

Dihybrid Cross Investigates the inheritance pattern for 2 different characteristics when crossing individuals who are true-breeding for two different characters, the F1 individual that results is a dihybrid after the dihybrid individuals self-fertilize, there are 16 possible genotypes of offspring

Analysis of a dihybrid cross

Independent Assortment Mendel’s Second Law: Independent Assortment genes located on different chromosomes are inherited independently of one another

Why Some Traits Don’t Show Mendelian Inheritance Continuous variation Characteristics can show a gradation in phenotypes This type of inheritance is called polygenic The gradation in phenotypes is called continuous variation

Why Some Traits Don’t Show Mendelian Inheritance Pleiotropic effects an allele that has more than one effect on a phenotype is considered pleiotropic these effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle-cell disease

Pleiotropic effects of the cystic fibrosis gene, cf

Why Some Traits Don’t Show Mendelian Inheritance Environmental effects the degree to which many alleles are expressed depends on the environment some alleles are heat-sensitive Arctic foxes only produce fur pigment when temperatures are warm

Why Some Traits Don’t Show Mendelian Inheritance Incomplete dominance not all alternative alleles are either fully dominant or fully recessive incomplete dominance produces a heterozygous phenotype that is intermediate between those of the parents each allele is represented, but essentially only at 50%

Incomplete dominance

Why Some Traits Don’t Show Mendelian Inheritance Codominance a gene may have more than two alleles in a population both alleles are expressed equally these alleles are said to be codominant

Codominance and Blood Type The gene that determines ABO blood type in humans exhibits more than one dominant allele the gene that encodes blood type, designated I, has three alleles: IA,IB, and i different combinations of the three alleles produce four different phenotypes, or blood types (A, B, AB, and O) both IA and IB are dominant over i and also codominant

Multiple alleles controlling the ABO blood groups

Chromosomes Are the Vehicles The chromosomal theory of inheritance –chromosomes carry the information for traits (genes) Supported by evidence: similar chromosomes pair with one another during meiosis reproduction involves the initial union of only eggs and sperm each gamete contains only copy of the genetic information and combines to form a diploid cell

Linkage Linkage is defined as the tendency of close-together genes to segregate together the farther apart two genes are from each other on the same chromosome, the more likely crossing over is to occur this would lead to independent segregation the closer that two genes are to each other on the same chromosome, the less likely that crossing over will occur between them

Linkage and Crossing Over These two chromosomes could segregate independently into 2 separate cells as is, or…. During meiosis I, crossing over could occur, separating far genes from on another, but having no effect on genes close to each other These two hybrid chromosomes could then segregate independently into 2 separate cells (gametes)

Human Chromosomes Each human somatic cell normally has 46 chromosomes, or 23 pairs 22 of the 23 pairs are perfectly matched in both males and females and are called autosomes 1 pair are the sex chromosomes females are designated XX while males are designated XY the genes on the Y chromosome determine “maleness” a female passes on an X to offspring, males may pass X (50%) or Y (50%)

Nondisjunction Sometimes errors occur during meiosis Nondisjunction is the failure of chromosomes to separate correctly during either meiosis I or meiosis II this leads to aneuploidy, an abnormal number of chromosomes most of these abnormalities cause a failure to develop or an early death before adulthood in contrast individuals with an extra copy of chromosome 21

Nondisjunction

Nondisjunction OR meiosis Monosomy Trisomy Sperm Ovum (egg) Sperm M P M P Monosomy OR M P Ovum Sperm Trisomy

Trisomy 21 or Down Syndrome 3 4 5 6 7 8 9 11 12 X Y 1 13 14 15 16 17 18 19 20 21 22 (b) (a) 10

Nondisjunction of Sex Chromosomes Nondisjunction of the X chromosome creates three possible viable conditions XXX female (triple X) usually taller than average but other symptoms vary XXY male (Klinefelter syndrome) sterile male with many female characteristics and diminished mental capacity XO female (Turner syndrome) sterile female with webbed neck and diminished stature

Nondisjunction of the X chromosome

Nondisjunction OR X meiosis Turner X XX XX Y XX XXY Klinefelter Sperm Ovum X Ova (eggs) meiosis M P Turner X XX OR M P Ovum Sperm XX Y XX Klinefelter XXY

Nondisjunction of Sperm Nondisjunction of the Y chromosome also occurs in such cases, YY gametes are formed, leading to XYY males these males are fertile and may be aggressive

Studying Pedigrees To study human heredity, scientists examine crosses that have already been made they identify which relatives exhibit a trait by looking at family trees or pedigrees often one can determine whether a trait is sex-linked or autosomal and whether the trait’s phenotype is dominant or recessive

A general pedigree

A pedigree of albinism Is the trait sex-linked or autosomal? V Generation IV II III Unaffected Carrier Affected Female Male KEY: Is the trait sex-linked or autosomal? Is the trait dominant or recessive? Is the trait by a single gene or several?

Pedigree of color blindness Generation II III IV Is the trait sex-linked or autosomal? Is the trait dominant or recessive? Is the trait by a single gene or several?

Mutations Accidental changes in genes are called mutations mutations occur only rarely and almost always result in recessive alleles in some cases, produce harmful effects called genetic disorders

Some Important Genetic Disorders

Hemophilia: Sex-linked Trait Hemophilia is a recessive, blood-clotting disorder (do not make the protein clotting factor VIII) This type of hemophilia is sex-linked

Sex-linked Genetic Disorder

Sickle-cell Trait: Recessive Trait Affected individuals are homozygous recessive and carry two copies of mutated gene that produces a defective version of hemoglobin the hemoglobin sticks together and forms rod-like structures that produce a stiff red blood cell with a sickle shape the cells cannot move through the blood vessels easily and tend to clot incomplete dominance occurs, affected individual makes 50% of defective gene, not enough to cause disease, but resistant to malaria, prevalent in African-decent

Sickle-cell disease

The sickle-cell allele confers resistance to malaria

Huntington’s Disease: Dominant Trait it causes progressive deterioration of brain cells every individual who carries the allele expresses the disorder but most persons do not know they are affected until they are more than 30 years old

Huntington’s disease is a dominant genetic disorder

Genetic Screening Genetic screening can allow prenatal diagnosis of high-risk pregnancies amniocentesis is when amniotic fluid is sampled and isolated fetal cells are then grown in culture and analyzed 1 in 600 tests may result in miscarriage chorionic villus sampling is when fetal cells from the chorion in the placenta are removed for analysis 1 in 100 tests may result in miscarriage

Amniocentesis

Inquiry & Analysis Is woolly hair sex-linked or autosomal? Is woolly hair dominant or recessive? Why Woolly Hair Runs in Families