Chapter 9. Patterns of Inheritance. Lectures by Gregory Ahearn. University of North Florida. Copyright 2009 Pearson Education, Inc.

Chapter 9 Patterns of Inheritance Lectures by Gregory Ahearn University of North Florida Copyright 2009 Pearson Education, Inc.

9.1 What Is The Physical Basis Of Inheritance? Inheritance occurs when genes are transmitted from parent to offspring. The units of inheritance are genes, which are segments of DNA of variable length.

9.1 What Is The Physical Basis Of Inheritance? Genes are segments of DNA at specific locations on chromosomes. A gene s physical location on a chromosome is called its locus. Each member of a pair of homologous chromosomes carries the same genes, located at the same loci. Different versions of a gene at a given locus are called alleles.

9.1 What Is The Physical Basis Of Inheritance? The relationship among genes, alleles, and chromosomes gene loci a pair of homologous chromosomes Both chromosomes carry the same allele of the gene at this locus; the organism is homozygous at this locus This locus contains another gene for which the organism is homozygous Each chromosome carries a different allele of this gene, so the organism is heterozygous at this locus the chromosome from the male parent the chromosome from the female parent Fig. 9-1

9.1 What Is The Physical Basis Of Inheritance? Mutations are the source of alleles. Differences in alleles at a given locus are due to mutations at that gene. If a mutation occurs in the cells that become sperm or eggs, it can be passed on from parent to offspring.

9.1 What Is The Physical Basis Of Inheritance? An organism s two alleles may be the same or different. A diploid organism has pairs of homologous chromosomes with two copies of each gene at a given locus. If both homologous chromosomes have the same allele at a locus, the organism is said to be homozygous.

9.1 What Is The Physical Basis Of Inheritance? An organism s two alleles may be the same or different (continued). If two homologous chromosomes have different alleles at a locus, the organism is heterozygous at that locus. The gametes of a homozygous individual are all the same at a particular locus, while gametes of a heterozygous individual would contain half one allele and half the other allele.

9.2 How Were The Principles Of Inheritance Discovered? The patterns of inheritance were discovered by an Austrian monk, Gregor Mendel. Fig. 9-2

9.2 How Were The Principles Of Inheritance Discovered? Doing it right: the secrets of Mendel s success Mendel was the first geneticist to employ three key steps in his experimentation: Choosing the right organism for the work Designing and performing experiments correctly Analyzing the data properly

9.2 How Were The Principles Of Inheritance Discovered? Mendel chose edible pea as the experimental subject for his experiments in inheritance. Pea egg cells in a pea flower fertilized by sperm from the same flower is called selffertilization. When sperm from one organism fertilizes eggs from a different organism, the process is called cross-fertilization.

9.2 How Were The Principles Of Inheritance Discovered? Mendel chose edible pea as the experimental subject for his experiments in inheritance (continued). Mendel studied individual characteristics of pea plants, such as flower color; these characteristics are called traits. He followed the inheritance of these traits for several generations, counting the numbers of offspring with each type of trait.

9.3 How Are Single Traits Inherited? True-breeding traits of organisms, such as purple flower color, are always inherited by all of their offspring that result from selffertilization. In one experiment, Mendel cross-fertilized white-flowered plants with purple-flowered plants. When he grew the resulting seeds, he found all the first-generation offspring, or the F 1 generation, produced purple flowers. What happened to the white color?

9.3 How Are Single Traits Inherited? Cross of pea plants that are true-breeding for white or purple flowers pollen pollen Parental generation (P) cross-fertilize true-breeding, purple-flowered plant true-breeding, white-flowered plant First-generation offspring (F 1 ) all purple-flowered plants Fig. 9-4

9.3 How Are Single Traits Inherited? The F 2 generation Next, Mendel allowed the F 1 flowers to selffertilize, collected the seeds, and grew the second generation, called the F 2 generation. Flowers in the F 2 generation were threefourths purple and one-fourth white, in a ratio of 3 purple to 1 white. This showed that the gene for white flowers was hidden in the F 1 generation, but appeared again in the F 2 generation.

9.3 How Are Single Traits Inherited? Cross of F 1 plants with purple flowers Firstgeneration offspring (F 1 ) self-fertilize Secondgeneration offspring (F 2 ) 3/4 purple 1/4 white Fig. 9-5

9.3 How Are Single Traits Inherited? All the white-flowered plants in the F 2 generation only produced additional whiteflowered plants. Purple-flowered plants were of two types: About ⅔ were true-breeding for purple, while ⅔ produced both purple- and white-flowered offspring (ratio 3 purple/1 white). Therefore, the F 2 generation included ¼ truebreeding purple plants, ½ hybrid purple, and ¼ true-breeding white plants.

9.3 How Are Single Traits Inherited? The inheritance of dominant and recessive alleles on homologous chromosomes can explain the results of Mendel s crosses. Mendel s results allow us to develop a fivepart hypothesis to explain the inheritance of single traits. 1.Each trait is determined by pairs of distinct physical units called genes. There are two alleles for each gene, one on each homologous chromosome.

9.3 How Are Single Traits Inherited? 2. When two different alleles are present in an organism, the dominant allele may mask the expression of the recessive allele; however, the recessive allele is still present. 3. The two alleles of a gene segregate (separate) from one another during meiosis; this is known as Mendel s law of segregation. 4. Which allele ends up in any given gamete is determined by chance.

9.3 How Are Single Traits Inherited? 5. True-breeding (homozygous) organisms have two copies of the same allele for a given gene; hybrid (heterozygous) organisms have two different alleles for a given gene.

9.3 How Are Single Traits Inherited? The distribution of alleles in gametes homozygous parent gametes A A A A (a) Gametes produced by a homozygous parent heterozygous parent gametes A a A a (b) Gametes produced by a heterozygous parent Fig. 9-6

9.3 How Are Single Traits Inherited? In pea plants, purple is dominant to white. Letters can be used to describe the alleles (P is for the dominant allele; p is for the recessive allele). Homozygous purple plants are PP; homozygous white plants are pp.

9.3 How Are Single Traits Inherited? Homozygous purple plants are PP; homozygous white plants are pp. purple parent PP P + P white parent all P sperm and eggs pp p + p all p sperm and eggs (a) Gametes produced by homozygous parents Fig. 9-7a

9.3 How Are Single Traits Inherited? F 1 plants were produced by P and p gametes, making Pp F 1 hybrid offspring. sperm eggs F 1 offspring P + p Pp or p + P Pp (b) Fusion of gametes produces F 1 offspring Fig. 9-7b

9.3 How Are Single Traits Inherited? Next, Mendel crossed two F 1 hybrid plants (Pp x Pp). This cross made three types of F 2 offspring, with the following allele composition ¼ were PP; ½ were Pp; ¼ were pp.

9.3 How Are Single Traits Inherited? Fusion of gametes from the F 1 generation produces F 2 offspring. gametes from F 1 Pp plants sperm eggs F 2 offspring P + P PP P + p Pp p + P Pp p + p pp (c) Fusion of gametes from the F 1 generation produces F 2 offspring Fig. 9-7c

9.3 How Are Single Traits Inherited? Mendel s hypothesis was that two plants may look alike, called its phenotype, but have a different allele composition, called its genotype. In this case, purple plants had PP or Pp genotypes, but their phenotype (purple color) was the same. The F 2 generation could be described as having three genotypes (¼ PP, ½ Pp, and ¼ pp) and two phenotypes (¾ purple and ¼ white).

9.3 How Are Single Traits Inherited? Simple genetic bookkeeping can predict the genotypes and phenotypes of offspring. The Punnett square method is a convenient way to predict the genotypes and phenotypes of offspring.

9.3 How Are Single Traits Inherited? The Punnett square method Pp self-fertilize 1 2 P eggs 1 2 p 1 2 P sperm 1 4 PP 1 4 Pp 1 2 p 1 1 4 pp 4 pp Fig. 9-8

9.3 How Are Single Traits Inherited? Mendel s hypothesis can predict the outcome of new types of single-trait crosses. Mendel predicted the outcome of crossfertilizing Pp plants with homozygous recessive plants (pp) there should be equal numbers of Pp (purple) and pp (white) offspring.

9.3 How Are Single Traits Inherited? A Punnet square shows how this test cross results in the predicted offspring. if PP PP or Pp sperm unknown pollen if Pp pp all eggs p p eggs p eggs all sperm P 1 2 P all Pp sperm 1 2 Pp 1 2 p 1 2 pp Fig. 9-9

9.4 How Are Multiple Traits Inherited? Mendel next crossed pea plants that differed in two traits, such as seed color (yellow or green) and seed shape (smooth or wrinkled). He knew from previous crosses that smooth and yellow were both dominant traits in peas. His first cross was a true-breeding plant with smooth, yellow seeds (SSYY) to a truebreeding plant with wrinkled, green seeds (ssyy).

9.4 How Are Multiple Traits Inherited? Traits of pea plants studied by Gregor Mendel Trait Seed shape Seed color Pod shape Dominant form smooth yellow inflated Recessive form wrinkled green constricted Pod color green yellow Flower color purple white Flower location at leaf junctions at tips of branches Plant size tall (about 6 feet) dwarf (about 8 to 16 inches) Fig. 9-10

9.4 How Are Multiple Traits Inherited? All the offspring of this cross (F 1 generation) were SsYy and had smooth, yellow seeds (both dominant traits). F 1 plants were allowed to self-fertilize and produced F 2 offspring in the phenotypic ratio 9:3:3:1.

9.4 How Are Multiple Traits Inherited? Mendel concluded that multiple traits are inherited independently. Mendel realized that these results could be explained if the genes for seed color and seed shape were inherited independently. The independent inheritance of two or more distinct traits is called the law of independent assortment. Multiple traits are inherited independently because the alleles of one gene are distributed to gametes independently of the alleles of other genes.

9.4 How Are Multiple Traits Inherited? Predicting genotypes and phenotypes SsYy self-fertilize eggs 1 4 SY 1 4 Sy 1 4 sy 1 4 sy 1 4 SY 1 16 SSYY 1 16 SSYy 1 16 SsYY 1 16 SsYy sperm 1 4 1 4 Sy sy 1 16 1 16 SSyY ssyy 1 16 1 16 SSyy ssyy 1 16 1 16 SsyY ssyy 1 16 1 16 Ssyy ssyy 1 4 sy 1 16 ssyy 1 16 ssyy 1 16 ssyy 1 16 ssyy Fig. 9-11

9.4 How Are Multiple Traits Inherited? Independent assortment of alleles S Y chromosomes replicate s y pairs of alleles on homologous chromosomes in diploid cells S s Y y replicated homologues pair during metaphase of meiosis I, orienting like this or like this S s y Y meiosis I S Y s y S y s Y S Y s y S y s Y meiosis II S S s s S S s s Y Y y y y y Y Y SY sy Sy sy independent assortment produces four equally likely allele combinations during meiosis Fig. 9-12

9.4 How Are Multiple Traits Inherited? In an unprepared world, genius may go unrecognized. Gregor Mendel presented his theories of inheritance in 1865. His experiments made little impact on science during his lifetime. It was not until 1900 that three biologists Carl Correns, Hugo de Vries, and Erich Tschermak rediscovered Mendel s work and acknowledged its importance to science.

9.5 How Are Genes Located on the Same Chromosome Inherited? Genetic linkage is the inheritance of genes as a group because they are on the same chromosome. Genes that are located on the same chromosome are inherited together, rather than sorted independently. In peas, the gene for flower color and the gene for pollen shape occur on the same chromosome and are inherited together. Because the two genes are located on the same chromosomes, they tend to end up in gametes together, and are then expressed in the plants.

9.5 How Are Genes Located on the Same Chromosome Inherited? Homologous chromosomes of the sweet pea, showing the genes for flower color and pollen shape flower color gene pollen shape gene purple allele, P long allele, L red allele, p round allele, l Fig. 9-13

9.5 How Are Genes Located on the Same Chromosome Inherited? Crossing over can create new combinations of linked alleles. Genes on the same chromosome do not always stay together. During prophase I of meiosis, homologous chromosomes sometimes exchange parts in the process, called crossing over. Crossing over produces a new allele combination on both homologous chromosomes. Therefore, the chromosomes of each haploid daughter cell receives different combinations of alleles from those of the parent cell.

9.6 How Is Sex Determined? Offspring sex is determined by a special pair of chromosomes called the sex chromosomes. In mammals, females have two X chromosomes and males have an X chromosome and a Y chromosome. While the X chromosomes look alike, the Y chromosomes are much smaller than the X chromosomes.

9.6 How Is Sex Determined? Photomicrograph of human sex chromosomes Y chromosome X chromosome Fig. 9-14

9.6 How Is Sex Determined? During gamete formation, the sex chromosomes segregate so that each female gamete gets one X, but the male gametes get either an X or a Y. All animals have one pair of sex chromosomes and a variable number of other chromosomes, called autosomes. A Punnett square cross shows how offspring sex is determined from the segregation of the sex chromosomes.

9.6 How Is Sex Determined? Sex determination in mammals female parent X 1 X 2 eggs X 1 X 2 X 1 X m X 2 X m male parent X m X m Y sperm female offspring X 1 Y X 2 Y Y male offspring Fig. 9-15

9.7 How Are Sex-Linked Genes Inherited? Genes that are found on one sex chromosome but not on the other are called sex-linked. Because females have two X chromosomes, they can be either homozygous or heterozygous for genes on the X chromosome. Males only have one X chromosome, and therefore express all the alleles they have on their X chromosome.

9.7 How Are Sex-Linked Genes Inherited? If a man inherits one defective, recessive allele on his X chromosome, he will show the defective phenotype. A female, however, may be phenotypically normal because one of her two X chromosomes may display a functional, dominant allele.

9.7 How Are Sex-Linked Genes Inherited? In humans, the most familiar genetic defects caused by recessive alleles on X- chromosome genes are hemophilia and redgreen color blindness. Fig. 9-16

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? When a heterozygous phenotype is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance. Human hair texture is influenced by a gene with two incompletely dominant alleles, C 1 and C 2. A person with two copies of the C 1 allele has curly hair; two copies of the C 2 allele produces straight hair; heterozygotes with C 1 C 2 genotype have wavy hair.

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? mother Two wavy-haired people could have the following children: ¼ curly (C 1 C 1 ), ½ wavy C 1 C 2 eggs (C 1 C 2 ), and ¼ straight (C 2 C 2 ). father C 1 sperm C 1 C 2 C 1 C 1 C 1 C 2 C 1 C 2 C 2 C 1 C 2 C 2 C 2 Fig. 9-17

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? A single gene may have multiple alleles. A single individual can have only two alleles for any gene, one on each homologous chromosomes. However, within all the members of a species there could be dozens of alleles for every gene.

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? A single gene may have multiple alleles (continued). Human blood types A, B, and O arise as a result of three different alleles of a single gene on chromosome 9; this gene codes for an enzyme that adds sugar molecules to recognition proteins on the surfaces of red blood cells. A person may have one of six genotypes: AA, BB, AB, Ao, Bo, and oo.

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits?

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? Alleles A and B are dominant to o. People with type AB blood have both A and B proteins and have type AB blood. When heterozygotes express phenotypes with both of the homozygotes, the pattern of inheritance is called codominance.

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? Alleles A and B are dominant to o (continued). People with type AB blood have both A and B proteins and have type AB blood. When heterozygotes express phenotypes with both of the homozygotes, the pattern of inheritance is called codominance.

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? A single trait may be influenced by several genes. Many physical traits are governed not by single genes, but by interactions among two or more genes, a phenomenon called polygenic inheritance. The more genes that contribute to a single trait, the greater the number of phenotypes and the finer the distinctions among them.

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? Human eye color is controlled by at least three genes. At least three, and possibly dozens, of genes affect human skin pigmentation, and exposure to sun further alters skin color. Fig. 9-18

9.8 Do Mendelian Rules Of Inheritance Apply To All Traits? The environment influences the expression of genes. The Himalayan rabbit has the genotype for black fur everywhere on this body. An enzyme that produces the black pigment is inactive at temperatures above 93 F (34 C). Most of its body is above 93 F and its fur is pale; however, its ears, nose, tail, and feet are cooler, and these areas have black fur. Fig. 9-19

9.9 How Are Human Genetic Disorders Investigated? Medical geneticists are especially interested in genes that influence our susceptibility to disease. Human geneticists study the inheritance of diseasecausing alleles by searching medical, historical, and family records to study past crosses. Pedigrees are diagrams showing the genetic relationships among a set of related individuals. Careful analysis of pedigrees reveals whether a particular trait is inherited in a dominant, recessive, or sex-linked pattern.

I II III (a) A pedigree for a dominant trait I II???? III IV (b)??? A pedigree for a recessive trait How to read pedigrees I, II, III = generations = male = female = parents = offspring or = shows trait or = does not show trait or = known carrier (heterozygote) for recessive trait? or? = cannot determine the genotype from this pedigree Fig. 9-20

9.10 How Are Single-Gene Disorders Inherited? Some human genetic disorders are caused by recessive alleles. Many genes encode information to synthesize enzymes or structural proteins in cells, and a defective allele in such a gene may cause damaged or inactive protein. In some cases, a defective gene may be masked when one normal allele is also present and makes enough functional protein.

9.10 How Are Single-Gene Disorders Inherited? Some human genetic disorders are caused by recessive alleles (continued). A heterozygote will remain healthy, but people who inherit two copies of a recessive, defective allele will have the disorder. Muscular dystrophy is a fatal muscle degeneration of young boys that occurs with two defective recessive alleles of such a protein-coding gene.

9.10 How Are Single-Gene Disorders Inherited? A defective allele for hemoglobin synthesis causes sickle-cell anemia. In sickle-cell anemia, there is a substitution of one nucleotide that results in a single, incorrect amino acid in the hemoglobin molecule. Under these conditions, the hemoglobin cannot transport oxygen correctly and tissues do not receive enough oxygen. The sickled cells are more fragile than normal red blood cells and die before their time, causing anemia.

9.10 How Are Single-Gene Disorders Inherited? People homozygous for sickle-cell allele synthesize only defective hemoglobin; many of their red cells become sickled. Fig. 9-21

9.10 How Are Single-Gene Disorders Inherited? A defective allele for hemoglobin synthesis causes sickle-cell anemia (continued). Heterozygotes have half normal and half abnormal hemoglobin, but have few sickled cells and are not seriously affected. Because only homozygous recessives usually show symptoms, sickle-cell anemia is a recessive disorder. Individuals who are heterozygous for the sickle-cell allele are resistant to malaria.

9.10 How Are Single-Gene Disorders Inherited? Some human genetic disorders are caused by dominant alleles. Many genetic diseases are caused by dominant alleles, in which a single defective allele is enough to cause the disorder. For dominant diseases to be inherited, at least one parent must suffer from the disease but live long enough to have children. Some diseases, like Huntington disease, do not appear until after the affected person has reproduced.

9.10 How Are Single-Gene Disorders Inherited? How can a defective allele be dominant to a normal allele? Some dominant alleles may encode proteins that carry out new, toxic reactions. Other dominant alleles may encode a protein that is overactive, performing its function at inappropriate times and places. Some dominant alleles may encode defective proteins that interfere with the action of the normal protein.

9.10 How Are Single-Gene Disorders Inherited? Huntington disease in a dominant inherited disorder that causes a slow, progressive deterioration of parts of the brain, leading eventually to death. Symptoms do not typically appear until 30 to 50 years of age. Geneticists isolated the Huntington gene in 1993 and identified its protein product, which they called huntington. Mutant huntington seems to interfere with normal huntington, and form large aggregates in nerve cells that eventually kill the cells.

9.11 How Do Errors In Chromosome Number Affect Humans? Abnormal numbers of sex chromosomes cause some disorders. In normal males, X and Y chromosomes pair up during meiosis, and sperm normally carry either an X or a Y chromosome. Nondisjunction of sex chromosomes in a male produces sperm that have two sex chromosomes (XX, YY, or XY) or that have no sex chromosomes at all (designated O).

9.11 How Do Errors In Chromosome Number Affect Humans? Abnormal numbers of sex chromosomes cause some disorders (continued). Nondisjunction in a female can produce XX or O eggs instead of normal eggs with a single X. When these defective sperm or eggs fuse, the resulting offspring have abnormal numbers of sex chromosomes. The most common abnormalities are XO, XXX, XXY, and XYY.

9.11 How Do Errors In Chromosome Number Affect Humans? Turner syndrome (XO) A female baby born with only one X chromosome has a condition called Turner syndrome. Such women fail to menstruate and have slow development of secondary sexual characteristics. They also have more X-linked recessive disorders, such as hemophilia and color blindness, than do XX women.

9.11 How Do Errors In Chromosome Number Affect Humans? Trisomy X (XXX) Most of these women have no detectable defects. There is a higher incidence of below-normal intelligence in such women. Such women are fertile and can have normal XX female children.

9.11 How Do Errors In Chromosome Number Affect Humans? Klinefelter syndrome (XXY) This disorder is in males born with two X chromosomes and one Y chromosome. At puberty they show mixed secondary sexual characteristics. The men are usually infertile because of a low sperm count.

9.11 How Do Errors In Chromosome Number Affect Humans? Jacob syndrome (XYY) Males with this disorder have a high level of testosterone, severe acne, and are tall compared to normal male height.

9.11 How Do Errors In Chromosome Number Affect Humans? Abnormal numbers of autosomes cause some disorders. Nondisjunction of autosomes produces eggs or sperm that are missing an autosome, or that have two copies of an autosome. Embryos with only one copy of any autosome abort early in development. A baby with three copies of chromosome 21 (trisomy) can live to adulthood.

9.11 How Do Errors In Chromosome Number Affect Humans? Trisomy 21 (Down syndrome) Inheritance of an extra copy of chromosome 21 is called trisomy 21, or Down syndrome. Individuals with Down syndrome display low resistance to infectious diseases, heart malformations, and varying degrees of mental retardation.

9.11 How Do Errors In Chromosome Number Affect Humans? Individuals with Down syndrome have several distinctive physical characteristics, which include weak muscle tone, a small mouth, and distinctively shaped eyelids. Fig. 9-22

9.11 How Do Errors In Chromosome Number Affect Humans? The frequency of nondisjunction in gametes increases with age. There is some increase in defective sperm with increasing age of the father. The mother s age is more significant in the probability of Down syndrome. However, because younger women tend to have more children than older women, the younger women still have the majority of Down syndrome babies.

9.11 How Do Errors In Chromosome Number Affect Humans? Down syndrome frequency increases with maternal age. 400 300 number per 10,000 births 200 100 0 10 20 30 40 50 age of mother (years) Fig. 9-23