2 Mendel’s LawsGregor Mendel was an Austrian monk who in 1860 developed certain laws of heredity after doing crosses between garden pea plants.Gregor Mendel investigated genetics at the organismal level.Examples of traits that can be observed at the organismal level include facial features (ex: big noses) that cause generations to resemble each other.
3 Gregor Mendel Mendel’s law of segregation: 1.) Each individual has two factors (called genes) for each trait (one from each parent).2.) The genes segregate (separate) during gamete formation (i.e., meiosis).3.) Each gamete contains only one gene for each trait (i.e., they are haploid).4.) Fertilization gives the new individual two genes for each trait (one from each parent, restores diploid state).
4 Diploid = Two copies of each type of chromosome Loci = Physical position of a gene on a chromosomeHomologous ChromosomesGenesFrom FatherFrom MotherAllele = Alternate forms of a gene:Alleles have the same position (locus) on a pair of homologous chromosomes
5 Alleles code for the same trait. Examples of alleles: curly or straight (alleles), hair type (gene)attached or unattached (alleles), ear lobe type (gene)Chromosomes segregate during the formation of the gametes and each gamete has only one chromosome from each pair.Fertilization gives each new individual two chromosomes again.
6 The Inheritance and Expression of a Single Trait A capital letter indicates a dominant allele, which is expressed when present.An example is W for widow’s peak.A lowercase letter indicates a recessive allele, which is only expressed only in the absence of a dominant allele.An example is w for a continuous or straight hairline.
7 STRAIGHT or CONTINUOUS HAIRLINE Widow’s peakWIDOW’S PEAKWWWwIn humans, widow’s peak (top) is dominant over straight hairline (bottom).STRAIGHT or CONTINUOUS HAIRLINEww
8 Genotype and Phenotype Genotype refers to the genes of an individual which can be represented by two letters or by a short descriptive phrase.Homozygous means that both alleles are the same; for example, WW stands for homozygous dominant and ww stands for homozygous recessive.
9 Heterozygous means that the members of the allelic pair are different—for example, Ww. Phenotype refers to the physical or observable characteristics of the individual – widow’s peak or straight hairline.Both WW and Ww result in widow’s peak, two genotypes with the same phenotype.
10 Gamete FormationBecause homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles (for a specific gene).If the allelic pair is Ww, the resulting gametes would contain either a W or a w, but not both – (gametes are haploid).Ww represents the genotype of an individual.Gametes that could be produced by this individual are W or w.
11 GenotypeAAAaAABbAaBbGametesA AA aAB AbAB Ab aB ab
12 One-Trait CrossesIn one-trait crosses, only one trait (such as type of hairline) is being considered.When performing crosses, the original parents are called the parental generation, or the P generation.All of their children are the filial generation, or F generation.Children are monohybrids when they are heterozygous for one pair of alleles.
13 MaleFemaleP stands for the parental generation; F for the filial generation. The F generation from this cross are all monohybrids.
14 If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring.WwWWWwwwThe 3:1 phenotypic ratio indicates three offspring with the dominant phenotype and 1 with the recessive phenotype. (The genotypic ratio would be 1:2:1.)
15 Genotypes of parents are known (both are heterozygous Ww) Monohybrid crossGenotypes of parents are known(both are heterozygous Ww)Genotypic RatioWW homozygous dominantWw heterzygousww homozygous recessivePhenotypic Ratiowidow’s peak1 straight hairlineA Punnett square diagrams the results of a cross. When the parents are heterozygous, each child has a 75% chance of having the dominant phenotype and a 25% chance of having the recessive phenotype.It is important to realize that chance has no memory; for example, if two heterozygous parents already have three children with a widow’s peak and are expecting a fourth child, this child still has a 75% chance of having a widow’s peak and a 25% chance of having a straight hairline.
16 The One-Trait Testcross It is not always possible to discern a homozygous dominant from a heterozygous individual by inspection of phenotype (they have the same phenotype – both will have widow’s peak).A testcross crosses the dominant phenotype with the recessive phenotype.If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.
17 One-trait testcross ? All offspring have dominant phenotype. Therefore thedominant parent (genotypewe are tying to figure out) mustbe homozygous dominant.A testcross determines if an individual with the dominant phenotype is homozygous or heterozygous. Because all offspring show the dominant characteristic, the individual is most likely homozygous as shown.
18 ? Offspring have dominant and recessive phenotypes. Therefore the dominant parent(genotype we are tying tofigure out) must beheterozygous dominant.Because the offspring show a 1:1 phenotypic ratio, the individual is heterozygous as shown.
19 1.) Both a man and woman are heterozygous for tongue rolling. Tongue rolling is dominant overnon-tongue rolling. What is the chance that theirchild will be a tongue roller?MaleFemaleTtGENOTYPETtT tGAMETEST tT tFEMALETTTtTtOffspring Phenotypes3 RollersNon-Roller3 of 4 chances for rollerchild (75% chance).MALETttt
20 2) Both you and your sibling are non-rollers and your parents are rollers. Tongue rolling is dominantover non-tongue rolling. What are the genotypes of yourparents?OFFSPRINGttPARENTSTtT tTtTTTtTttt
21 The Inheritance of Many Traits Independent AssortmentThe law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the gametes.This law is dependent on the random arrangement of homologous pairs at metaphase.
22 Segregation and independent assortment Segregation occurs because the homologous chromosomes separate during meiosis I. Also, independent assortment occurs. The homologous chromosomes line up randomly at the metaphase plate; therefore, the homologous chromosomes, and the alleles they carry, segregate independently during gamete formation. All possible combinations of chromosomes and alleles occur in the gametes.
23 Two-Trait CrossesIn two-trait crosses, genotypes of the parents require four letters because there two alleles for each trait.Gametes will contain one letter for each trait.When a dihybrid (heterozygous for both traits) reproduces with another dihybrid the phenotypic results are 9 : 3 : 3 : 1.
24 Widow’s Peak is dominant over Straight Hairline W w Short Fingers are dominant over Long FingersS sPhenotypeGenotypesWidow’s Peak / Short FingersWWSS WWSs WwSS WsSsWidow’s Peak / Long FingersWWss WwssStraight HL / Short FingerswwSS wwSsStraight HL / Long Fingerswwss
25 Dihybrid cross (two traits) Widow’s PeakShort FingersStraight HairlineLong Fingershomozygous dominanthomozygous recessiveF1 is the first filial generation, offspring of the parental cross. F2, or second filial generation, is the offspring of the cross of two F1 individuals.Since each F1 parent can form four possible types of gametes, four different phenotypes occur among the offspring in the proportions shown.The expected F2 phenotypic ratio is:9 widow’s peak and short fingers3 window’s peak and long fingers3 straight hairline and short fingers1 straight hairline and long fingersWidow’s PeakShort Fingers
27 The Two-Trait Testcross A testcross is done to determine genotype of individual that has dominant phenotypes (for both traits).(Homozygous dominant or heterozygous for the two traits under consideration).Cross heterozygote for both traits with homozygous recessive for both traits - results in 1 : 1 : 1 : 1 ratio.
28 Two-trait testcrossA testcross determines if the individual with a dominant phenotype is homozygous or heterozygous. If the individual is heterozygous as shown, there is a 25% chance for each possible genotype.
29 SELECTED TRAITS IN HUMAN HEREDITY Dominant Recessivenormal skin pigmentation albinismfreckles no frecklesbroad lips thin lipstongue roller non-tongue rollerPTC taster PTC non-tasterlarge eyes small eyesmigraine headaches no migraine headachesnormal foot arch flat feet
30 If a man that is homozygous recessive for eye size (i.e., has small eyes) and is homozygous dominant forfreckles (i.e., has freckles) has children with a womanthat is homozygous dominant for eye size (i.e., has largeeyes) and is homozygous recessive for freckles(i.e., does not have freckles), what are the potentialphenotypes and genotypes of their children?ManllFF IF only for gametesWomanLLff Lf only for gametesGENOTYPEPHENOTYPEIFIFAll are heterozygous for both traits and show large eyes with frecklesLfILFfILFfLfILFfILFf
31 If one of the children reproduces with another person that has the same genotype, what are the chances thatthey will have a child with large eyes and freckles?LlFfXLlFfLFLflFlflarge eyes/freckles9/16 or 56 %LFLLFFLLFfLlFFLlFfLarge eyes/no frecklesLf3/16LLFfLLffLlFfLlffSmall eyes/freckleslFLlFFLlFfllFFllFf3/16Small eyes/no freckleslfLlFfLlffllFfllff1/16
32 Patterns of Inheritance Genetic DisordersPatterns of InheritanceWhen studying human disorders, biologists often construct pedigree charts to show the pattern of inheritance of a characteristic within a family.Genetic counselors construct pedigree charts to determine the mode (dominant or recessive) of inheritance of a condition.
33 Pedigree Analysis: determine how a genetic disorder is inherited, chances of offspring having a genetic disorder.Unaffected maleUnaffected femaleAffected MaleAffected female“UNION”OFFSPRING
34 Genetic Disorders: medical conditions caused by alleles inherited from parents, hereditary disorder.Autosomal Genetic Disorders: genetic disorders caused byAlleles on autosomal chromosomes (non-sexChromosomes – similar to somatic).Autosomal Disorders can be:1) Autosomal Dominant2) Autosomal RecessiveAutosomal Dominant AA or Aa have disorder (phenotype)aaAaAaaa
35 Autosomal Recessive: aa have disorder (phenotype) AA or AaaaaaaaaaAa
36 Autosomal recessive pedigree chart (Tay-sachs disease, Cystic fibrosis, PKU) CARRIER –Has allele butis unaffected** HOW DO YOU KNOW INDIVIDUALIS HETEROZYGOUS?Autosomal recessive disorders have these characteristics:Affected children can have unaffected parents.Heterozygotes (Aa) have a normal phenotype.Two affected parents will always have affected children.Affected individuals with homozygous dominant mates will have unaffected children.Close unaffected relatives who reproduce are more likely to have affected children if they have joint affected relatives.Both males ad females are affected with equal frequency.How would you know the individual at the asterisk is heterozygous?
37 Autosomal dominant pedigree chart (Neurofibromatosis, Huntington disease) ** HOW DO YOU KNOW INDIVIDUALIS HETEROZYGOUS?Autosomal dominant disorders have these characteristics:Affected children will have at least one affected parent.Heterozygotes (Aa) are affected.Two affected parents can produce an unaffected child.Two unaffected parents will not have affected children.Both males and females are affected with equal frequency.How would you know the individual at the asterisk is heterozygous?
38 Polygenic Inheritance Polygenic traits are governed by more than one gene pair (e.g., several pairs of genes may be involved in determining the phenotype).
39 Polygenic inheritance Such traits produce a continuous variation representing a bell-shaped curve (Ex: height in humans).When you record the heights of a large group of young men, the values follow a bell-shaped curve. Such a continuous distribution is due to control of a trait by several sets of alleles. Environmental effects (i.e., differences in nutrition) are also involved.
40 Skin ColorThe inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of polygenic inheritance.A range of phenotypes exist from very dark to very light.The distribution of these phenotypes also follows a bell-shaped curve.
41 Polygenic DisordersMany human traits, like allergies, schizophrenia, hypertension, diabetes, cancers, and cleft lip, appear to be due to the combined action of many genes plus environmental influences.
42 Multiple Allelic Traits Inheritance by multiple alleles occurs when more than two alternative alleles exist for a particular gene locus.A person’s blood type is an example of a trait determined by multiple alleles (A, B, and O).***Each individual inherits only two alleles for these genes.
43 ABO Blood TypesA person can have an allele for an A antigen (blood type A) or a B antigen (blood type B), both A and B antigens (blood type AB), or no antigen (blood type O) on the red blood cells.Human blood types can be type A (IAIA or IA i), type B (IBIB or IBi), type AB (IAIB), or type 0 (ii).Alleles: A, B, OThe Rh factor is inherited separately from ABO blood types. When you are Rh positive, your red blood cells have a particular antigen, and when you are Rh negative, that antigen is absent. The Rh-positive allele is dominant over the Rh-negative allele.
44 Inheritance of blood type…. (Who’s your daddy?) A mating between blood type A and blood type B can result in any one of four blood types.
45 Incompletely Dominant Traits Codominance means that both alleles are equally expressed in a heterozygote. (Ex: sickle cell anemia)Incomplete dominance is exhibited when the heterozygote doesn’t show the dominant trait but shows an intermediate phenotype, representing a blending of traits. (Ex: curly, wavy, or straight hair)The alleles A and B in ABO blood groupings are codominant. An example of incomplete dominance is seen in degree of hair curliness.
46 Incomplete dominanceAmong Caucasians, neither straight nor curly hair is dominant. When two wavy-haired individuals reproduce, each offspring has a 25% chance of having either straight or curly hair and a 50% chance of having wavy hair, the intermediate phenotype.
47 Sickle-Cell DiseaseSickle-cell disease is an example of a human disorder controlled by incompletely dominant alleles.Sickle cell disease involves irregular, sickle shaped red blood cells caused by abnormal hemoglobin.HbA represents normal hemoglobin; and HbS represents the sickled condition.Individuals with sickle cell trait survive malaria because the sickling of some red blood cells causes a leakage of potassium, which is toxic to the malarial parasite.
48 HbAHbA individuals are normal; HbSHbS individuals have sickle-cell disease and HbAHbS individuals have the intermediate condition called sickle-cell trait.Heterozygotes have an advantage in malaria-infested Africa because the pathogen for malaria cannot exist in their blood cells.This evolutionary selection accounts for the prevalence of the allele among African Americans.