2 Gregor MendelAn Austrian monk, who in 1860 developed certain laws of heredity (rules for passing of traits from one generation to the next)Gregor Mendel investigated genetics at the organismal level. He experimented with pea plants and their offspring.Mendel's goal was to understand how traits are passed from one generation to the next.What is a trait?
3 By quantifying the patterns in which traits (your inherited characteristics) are inherited, Mendel developed two "laws".The law of segregation- states that each individual has two factors for each trait but can only pass on one to their kids.- today we call the traits genes and the factors alleles. (alternative forms of a gene)- alleles have the same position on a pair of homologous chromosomes (chromosomes for the same traits)
4 Mendel’s second law: The law of independent assortment The alleles segregate (separate)during the formation of the gametes (egg and sperm)and each gamete has only one allele from each pair the parent possesses . Each chromosome is passed on independent of the others (you have 23 pairs, 22 autosome and 1 sex chromosome)Fertilization gives each new individual two alleles for each trait again.
6 SummaryAlleles are alternative forms of a gene located at one site on a chromosome; alleles determine the traits of individuals (your genotype and your phenotype).Chromosomes and their alleles separate and assort independently when gametes form; this increases variety among offspring.
7 Gene loci - located on homologous chromosomes Each allelic pair, such as Gg or Zz, is located on homologous chromosomes at a particular gene locus, shown at the left. Following replication, each sister chromatid carries the same alleles in the same order, as shown at the right.
8 How do we inherit a single trait? We want to know the mode of inheritance.Dominant allele- it is expressed when present- it is designated with a capital (uppercase) letter- an example is W for widow’s peak.Recessive allele- it is only expressed in the absence of a dominant allele.- it is designated with a lowercase letter indicates- An example is w for continuous hairline.
9 Widow’s peakWW or WwIn humans, widow’s peak (top) is dominant over straight hairline (bottom).ww
11 Genotype versus 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 ww stands for homozygous recessive.Heterozygous means that the members of the allelic pair are differentfor example, Ww, a heterozygote
12 Phenotype refers to the physical or observable Phenotype refers to the physical or observable characteristics of the individual.Both WW and Ww result in widow’s peak,- the phenotype is a widow's peak- however, we have two genotypes resulting in the same phenotype.ww results in a straight hairline- in this case, the phenotype can only result from one genotype
13 Gamete FormationBecause homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles.If the allelic pair is Ww, a gamete would contain either a W or a w, but not both.Ww represents the genotype of an individual.Gametes are represented by W or w.
14 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.
15 P stands for the parental generation; F for the filial generation P stands for the parental generation; F for the filial generation. The F generation from this cross are all monohybrids.In a one-trait cross, if one parent is homozygous dominant and the other is homozygous recessive, all offspring of the filial generation with be heterozygous for that trait.
16 If you know the genotype of the parents, it is possible to determine the traits of the gametes. A Punnett square can then be used to determine the phenotypic ratio among the offspring.When a heterozygote (monohybrid) reproduces with another heterozygote (monohybrid), the phenotypic ratio of the offspring will be 3 expressing the dominant trait to 1 expressing the recessive trait (3:1 ratio).A 3 : 1 ratio means that there is a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype.The 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.)
17 Monohybrid crossA 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.
18 One-Trait Crosses and Probability Laws of probability alone can be used to determine results of a cross.The laws are:the probability that two or more independent events will occur together is the product of their chances occurring separately.2) the chance that an event that can occur in two or more independent ways is the sum of the individual chances.
19 In the cross of Ww x Ww, what is the chance of obtaining either a W or a w from a parent? Chance of W = ½, or chance of w = ½The probability of these genotypes is:The chance of WW = ½ x ½ = ¼The chance of Ww = ½ x ½ = ¼The chance of wW = ½ x ½ = ¼The chance of ww = ½ x ½ = ¼The chance of widow’s peak (WW, Ww, wW) is ¼ + ¼ + ¼ = ¾ or 75%.
20 The One-Trait Testcross PROBLEM: It is not always possible to discern a homozygous dominant from a heterozygous individual by inspection of phenotype.A testcross crosses the dominant phenotype with the recessive phenotype.If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.
21 One-trait testcross Possibility I 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.
22 Possibility IIBecause the offspring show a 1:1 phenotypic ratio, the individual is heterozygous as shown.
23 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.
24 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.
25 Two-Trait CrossesIn two-trait crosses, genotypes of the parents require four letters because there is an allelic pair for each trait.Gametes will contain one letter of each kind in every possible combination.Crossing individuals who are heterozygous for two traits can produces four phenotypes.The ratio of these four phenotypes will be in a 9:3:3:1.
26 F1 is the first filial generation, offspring of the parental cross F1 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 fingers
27 F1 is the first filial generation, offspring of the parental cross F1 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 fingers
28 Two-Trait Crosses and Probability It is possible to use the two laws of probability to arrive at a phenotypic ratio for a two-trait cross without using a Punnett square.The results for two separate monohybrid crosses are as follows:Probability of widow’s peak = ¾Probability of short fingers = ¾Probability of straight hairline = ¼Probability of long fingers = ¼
29 The probabilities for the dihybrid cross: Probability of widow’s peak and short fingers= ¾ x ¾ = 9/16Probability of widow’s peak and long fingers= ¾ x ¼ = 3/16Probability of straight hairline and short fingers= ¼ x ¾ = 3/16Probability of straight hairline and long fingers= ¼ x ¼ = 1/16
30 Genetic Disorders Patterns of Inheritance When studying human disorders, biologists often construct pedigree charts to show the pattern of inheritance of a characteristic within a family.The particular pattern indicates the manner in which a characteristic is inherited.Genetic counselors construct pedigree charts to determine the mode of inheritance of a condition.
31 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?
32 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?
33 Autosomal Recessive Disorders Tay-Sachs DiseaseTay-Sachs disease is common among United States Jews of central and eastern European descent.An affected infant develops neurological impairments and dies by the age of three or four.Tay-Sachs results from a lack of hexosaminidase A and the storage of its substrate in lysosomes.Prenatal diagnosis of Tay-Sachs is possible following
34 Cystic FibrosisCystic fibrosis is the most common lethal genetic disorder among Caucasians.A chloride ion transport protein is defective in affected individuals.Normally when chloride ion passes through a membrane, water follows.In cystic fibrosis patients, a reduction in water results in a thick mucus which accumulates in bronchial passageways and pancreatic ducts.Genetic testing for the CF gene is possible.
35 Phenylketonuria (PKU) Individuals with phenylketonuria lack an enzyme needed for the normal metabolism of phenylalanine, coded by an allele on chromosome 12.Newborns are regularly tested for elevated phenylalanine in the urine.If the infant is not put on a phenylalanine-restrictive diet in infancy until age seven when the brain is fully developed, brain damage and severe mental retardation result.
36 Autosomal Dominant Disorders NeurofibromatosisSmall benign tumors, made up largely of nerve cells, occur under skin or on various organs.The effects can range from mild to severe, and some neurological impairment is possible; this disorder is variably expressive.The gene for this trait is on chromosome 17.Neurofibromatosis is variably expressive; most individuals have mild symptoms, but some have severe symptoms.
37 Huntington DiseaseIndividuals with Huntington disease experience progressive degeneration of the nervous system and no treatment is presently known.Most patients appear normal until middle age.The gene coding for the protein huntingtin contains many more repeats of glutamines than normal.The gene for Huntington disease is located on chromosome 4.
38 Polygenic and Multifactorial Inheritance Polygenic traits are governed by more than one gene pair.Several pairs of genes may be involved in determining the phenotype.Multifactorial traits are governed by more than one gene and environmental factors.
39 Height is a multifactorial trait. 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 multifactorial inheritance.A range of phenotypes exist and several possible phenotypes fall between the two extremes of very dark and very light.The distribution of these phenotypes follows a bell-shaped curve.
41 Multifactorial traits and disorders Many 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.Many behaviors, such as phobias, are also likely due to the combination of genes and the effects of the environment.
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.Each individual inherits only two alleles for these genes.
43 ABO Blood Types A person has two alleles. They can be: - two A antigen (blood type A)- two B antigen (blood type B)- one each of A and B antigens (blood type AB)- or the O antigen (blood type O)Human blood types can betype A (IAIA or IA i)type B (IBIB or IBi)type AB (IAIB)or type 0 (ii).The 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 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.Incomplete dominance is exhibited when the heterozygote shows not the dominant trait but an intermediate phenotype, representing a blending of traits.Such a cross would produce a phenotypic ratio of 1:2:1.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 hemoglobinHbS 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 HbA/HbA individuals are normal; HbS/HbS individuals have sickle-cell diseaseHbA/HbS 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.