PATTERNS OF INHERITANCE GENETICS DEVELOPED FROM CURIOSITY ABOUT INHERITANCE GENETICS DEVELOPED FROM CURIOSITY ABOUT INHERITANCE • The Blending Hypothesis of Inheritance – Traits are variations of particular characteristics. • One plant might have red flowers, while another might have purple flowers. – The hypothesis in the early 1800s was the blending hypothesis, which explained how offspring inherited traits from both parents. • Crossing yellow and red flowered plants would produce orange flowered plants. GENETICS DEVELOPED FROM CURIOSITY ABOUT INHERITANCE • All offspring would then be orange. – This hypothesis was proven wrong after red flowered plants produced yellow flowers and some traits would disappear from one generation to the next. • Mendel’s Plant Breeding Experiments – Gregor Mendel, a priest, was one of the first to use the scientific method to the subject of inheritance, giving rise to genetics, the study of heredity. GREGOR MENDEL GENETICS DEVELOPED FROM CURIOSITY ABOUT INHERITANCE – Mendel developed the particulate hypothesis of inheritance after working with pea plants for seven years. – According to the hypothesis, parents pass on to their offspring genes responsible for the inherited traits. – These heritable factors retain their identity from generation to generation. – Mendel first identified true breeding plants that produced identical offspring in each generation after self fertilization. GENETICS DEVELOPED FROM CURIOSITY ABOUT INHERITANCE • Purple always produced purple and white always produced white. – A bag was tied around each flower to prevent cross pollination. – He then crossed the true breeding purple plant with the true breeding white plant. In a process called cross-fertilization. – Sperm of one plant fertilizes the eggs of a different plant. – He then wondered what the offspring traits would be (flower colors). MENDEL’S FERTILIZATION TECHNIQUE DIFFERENT PEA PLANT TRAITS REVIEW: CONCEPT CHECK 10.1, page 207 1. Explain how Mendel’s particulate hypothesis is different from the blending hypothesis of inheritance. 2. What is the difference between selffertilization and cross-fertilization? 3. Describe a pattern of inheritance that the blending hypothesis fails to explain. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE • Mendel’s Principle of Segregation – The offspring of two true breeding specimens are hybrids. – The parents are the P generation and the first offspring the F₁ generation for filial. – The F₂ generation is the product of the F₁ selffertilizing. – In one experiment, Mendel crossed purple flowered plants with white flowered plants, or a monohybrid cross where the parents differ in only one character. P, F₁, & F₂ GENERATIONS DOMINANT vs. RECESSIVE TRAITS P, F₁, & F₂ GENERATIONS P, F₁, & F₂ GENERATIONS MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE • The F₁ hybrids were all purple. • The F₂, after self-fertilization, were ¾ purple and ¼ white. – Mendel concluded that there were two factors for flower color, purple and white, each now called genes. – As seen in previous pictures, there were other characteristics in the pea plants that Mendel studied. – The same pattern appeared in each F₁ generation. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE – As a result, Mendel developed four hypothesis (use gene instead of factor): 1. There are alternative forms of genes (the gene for flower color was purple in one form and white in another) or alleles. 2. For each inherited character, the organism has two alleles for the gene that controls that character, one from each parent. a. b. Both alleles the same, the individual is homozygous. Both alleles different, the individual is heterozygous. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE 3. The trait is dominant if only one of the two different alleles affect or present that trait in the offspring, with the allele being recessive, or non-appearing. 4. The two alleles for a character segregate (separate) during the formation of gametes (sex cells) with each gamete carrying only one allele for each character. a. This is Mendel’s principle of segregation. b. As the gametes join during fertilization, allele pairs reform in the offspring. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE • Probability and Punnett Squares – Crossing true-breeding (homozygous) purple with white flowers will result in a purple (dominant) flower, P Purple(PP) X White(pp) F₁ Purple(Pp) where P is dominant and p is recessive, and is heterozygous. PENNY PROBABILITIES X axis: 2 sides of penny 2 (½H + ½T) Y axis: 2 sides of penny 1 (½H + ½T) ¼ HH ¼ HT ¼ HT ¼ TT PUNNETT SQUARES MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE – When the F₁ generation reproduces, each gamete will receive only one allele for flower color, P or p, with equal likelihood. – The gametes will combine randomly and form zygotes or pairs of alleles. – The likelihood or probability of each pair forming is key to the inheritance pattern in F₂. – Punnett squares aid in the calculation of probabilities by using grid patterns by showing all the possible outcomes of a genetic cross. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE • Genotype and Phenotype – Looking at the F₂ generation, the probabilities using the Punnett square shows that ¼ of the F2 plants are homozygous for purple allele (PP). – ½ (¼ +¼) of the F₂ are heterozygous (Pp), and be purple, the dominant color. – ¼ of the F₂ will be homozygous for the recessive trait (pp) and be white. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE – Therefore, according to Mendel’s hypothesis, the probabilities are ¾:¼ or 3:1, flower color (purple to white). – The phenotype is the observable trait and the genotype is the genetic makeup, or combination of alleles (PP, Pp, pp). – The phenotypic ratio is 3 purple to 1 white and the genotypic ratio is 1(PP):2(Pp):1(pp). MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE • The Testcross – A question arises when trying to determine the genotype of an organism. – For example, if a flower is purple, is it PP or Pp? – To determine this, a testcross is done, which is breeding an individual of an unknown genotype, but dominant phenotype, with a homozygous recessive individual. TESTCROSS TESTCROSS MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE – The appearance of the cross will reveal the genotype of the mystery plant. – This is because the homozygous recessive plant (pp) can contribute only a recessive allele to the offspring, therefore the phenotype will specify the allele contributed by the mystery plant. • Mystery homozygous (PP) X homozygous recessive (pp) yields heterozygous purple (Pp). • Mystery heterozygous (Pp) X homozygous recessive (pp) yields purple (Pp) and white (pp). MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE • Mendel’s Principle of Independent Assortment – Mendel studied seven characteristics of the peas, including shape and color. • Round shape dominant to wrinkled • Yellow color dominant to green – How about a dihybrid cross, that is, crossing organisms differing in two characters? DIHYBRID CROSS DIHYBRID CROSS DIHYBRID CROSS MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE – Mendel crossed true breeding round yellow seeds (RRYY) with true breeding wrinkled green seeds (rryy). – The first parent produced only RY gametes, the other ry. – The union produced RrYy, hybrid heterozygotes, with all having the dominant phenotype, round and yellow. – The hybrids grew into the F₁ plants, and were allowed to self-fertilize. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE – Four phenotypes were produced in the F₂, RY, rY, Ry, and ry, a ratio of 9 : 3 : 3 : 1. • In fact, looking at the dihybrid Punnett square, this shows the same outcome as two monohybrid crosses occurring at the same time. • Seed shape alone shows 12 plants with round seeds to every 4 with wrinkled seeds, which is the 3 : 1 seen in the monohybrid F₂ generation. – Mendel tested every pea characteristic in the dihybrid form and found that the 9:3:3:1 ratio persisted. MENDEL DISCOVERED THAT INHERITANCE FOLLOWS RULES OF CHANCE • As a result Mendel proposed the principle of independent assortment. • This states that gamete formation in F₂ crosses, a particular allele for one character can be paired with either allele of another character (R with a Y or y, r with a Y or y). REVIEW: CONCEPT CHECK 10.2, page 213 1. What are the two possible gametes produced by a plant that has the genotype Aa? Give the probability of each type of gamete. 2. Use a Punnett square to predict the genotypes produced if the plant in Question 1is self-fertilized. Calculate the probability of each outcome. 3. List all the possible genotypes of a pea plant with purple flowers and round seeds. 4. List the four possible allele combinations in the gametes of a plant with genotype PpWw. THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS • Intermediate Inheritance – In the F₁ generations of Mendel, all the specimens looked like the dominant homozygous parent because there was one dominant allele to produce the dominant phenotype. – To see the recessive phenotype required two recessive alleles. – Some traits in organisms have neither allele as dominant. THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS – In these cases, the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes, or is the intermediate inheritance. • A prime example is the Andalusian chicken. • Black and white parents produce an F₁ hybrid called “blues” that have grayish-blue feathers. • Neither the black nor white allele is dominant, capital and lower case letters are not used. – C & B with a superscript B or W are used (CB or BW), with the heterozygous chicken being CB CW , and blue. INTERMEDIATE INHERITANCE THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS – This is not the blending hypothesis because the parent phenotypes can reappear in the F₂ generation. – In the picture previously, the predicted phenotypes in the F₂ are 1 black: 2 blue: 1 white, and this ratio 1:2:1 is also the genotype. • Multiple Alleles – We have looked at no more than two contrasting alleles for each inherited character, but in populations several alleles exist for genes. THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS – This expands the number of possibilities of genotypes and phenotypes. • Multiple alleles account for blood types. – These typically are A, B, AB, or O with the letter referring to the two carbohydrates (A and B) found on the surface of red blood cells (rbc). – So, a person’s rbc can be coated with one carbohydrate (type A), or the other (type B), both (type AB), or none (type O). – In the illustration to follow, there are various combinations for the three alleles. • IA (for carbohydrate A) • IB (for carbohydrate B) • i (for neither A or B) BLOOD TYPE ALLELES THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS • Each person inherits one of the alleles from each parent, with six possible ways to pair the alleles, or six possible genotypes. • The alleles IA and IB exhibit codominance, or a heterozygote expresses both traits. • This is different from intermediate inheritance because the phenotype is not intermediate but shows separate traits of both alleles. • Polygenic Inheritance – Mendel’s pea plants exhibited seven characters that occurred in two phenotypes. THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS – Sometimes, intermediate inheritance, codominance of alleles, or multiple types of alleles can lead to more than two phenotypes in a population. – Multiple genes affecting characters leads to much variation in phenotypes. – Polygenic inheritance is when two or more genes affect a single character. – Height and skin color are just two examples. • There could be 3 tall alleles (A, B, C) and 3 short alleles (X, Y, Z). POLYGENIC INHERITANCE THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS – AABBCC would be the tallest, AXBBCC maybe next, and XXYYZZ the shortest. – The number of genes that affect a character increases the potential combination of alleles. • The Importance of Environment – Environment has an affect on phenotype. – Trees and plants are examples where leaves, shape, and greenness are affected by wind and sunlight. THERE ARE MANY VARIATIONS OF INHERITANCE PATTERNS – Temperature can affect the fur color of a Siamese cat, where black is found on the ears, face, feet, and tail because these are cooler where black is predominant. – Nutrition can affect height, exercise body status, sunlight darkens skin. – The environment can even affect identical twins. – Blood type is not affected by environment but oxygen carrying capacity can be. PHENOTYPES AND THE ENVIRONMENT REVIEW: CONCEPT CHECK 10.3, page 217 1. For a trait with intermediate inheritance, what is the phenotypic ratio for F₂ offspring of a monohybrid cross? How is that different from a simple dominant-recessive cross? 2. Two parents have O blood. What blood type would you expect for their child? 3. What is the likely mechanism for inheritance for a character with large range of phenotypes? Explain. 4. Give three examples of human physical characters affected by environment. MEIOSIS EXPLAINS MENDEL’S PRINCIPLES • Chromosome Theory of Inheritance – From the 1800s to the beginning of the 20th century, scientists discovered mitosis and meiosis and saw the similarities between the behavior of chromosomes and Mendel’s heritable factors. – From this came the chromosome theory of inheritance, which states that genes are located on chromosomes, and the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns. MEIOSIS EXPLAINS MENDEL’S PRINCIPLES – Remember, during meiosis the chromosomes undergo segregation and independent assortment, Mendel’s two primary principles. – Every diploid individual, pea plants or humans, have two sets of homologous chromosomes. • One from the male • One from the female – The alleles of a gene are in the same location on the homologous chromosomes, or gene locus. GENE LOCUS CHROMOSOME THEORY OF INHERITANCE MEIOSIS EXPLAINS MENDEL’S PRINCIPLES – The homologous chromosomes can either have the same allele or a different one at the locus, making the organism either homologous or heterozygous for each gene. • Genetic Linkage and Crossing Over – Genes located on separate chromosomes sort independently of each other during meiosis. – What happens to genes located on the same chromosome? MEIOSIS EXPLAINS MENDEL’S PRINCIPLES – These alleles would normally not be sorted into gametes independently but crossing over can separate them. – Genetic linkage is the tendency of the alleles on one chromosome to be inherited together. – Closeness to each other (genes) increases linkage. – The further apart, crossover is more likely. – Scientists can map distance between genes by looking at the proportion of gametes with recombined alleles. MEIOSIS, LINKAGE, RECOMBINATION REVIEW: CONCEPT CHECK 10.4, page 219 1. Draw a picture of the possible arrangements of chromosomes in the gametes produced by a pea plant with genotype RRYy. Label the gene loci. (Remember that these genes are located on separate chromosomes). 2. Explain how the distance between two gene loci on the same chromosome affects genetic linkage. SEX-LINKED TRAITS HAVE UNIQUE INHERITANCE PATTERNS • Sex-Linked Genes – The sex chromosomes are the X and Y chromosomes in many species. – Eggs contain the X chromosome while the sperm contains both the X and Y. – Any gene located on the sex chromosome is a sexlinked gene. • In humans, most of these are found on the X chromosome. SEX-LINKED GENES SEX-LINKED TRAITS HAVE UNIQUE INHERITANCE PATTERNS – Thomas Hunt Morgan, while studying fruit flies, discovered sex-linked genes. • Normally they have red eyes, while white is rare. • He crossed a white eyed male with a red eyed female, and the F₁ offspring had red eyes (red eye dominant allele). • The F₂ crossed with each other showed the 3 : 1 phenotypic ratio of red to white. • What surprised him was that none of the F₂ offspring that had white eyes was female, indicating that eye color must be related to sex. SEX-LINKED TRAITS HAVE UNIQUE INHERITANCE PATTERNS – Morgan then thought that the gene involved must be located on the X chromosome, as there was no corresponding eye color locus on the Y chromosome. – The female(XX) carry two copies of the gene for this character, and the male only one(XY). – White is recessive, so females have to have the while allele on both X chromosomes, while the male needs only one on its X chromosome. SEX-LINKED GENES SEX-LINKED GENES SEX- LINKED GENES XRXR x XrY ↓ XRXr + XRY + XRXr + XRY [F₁ Generation] ↓ XRXR + XRY + XRXr + XrY [F₂ Generation] SEX-LINKED TRAITS HAVE UNIQUE INHERITANCE PATTERNS • Sex-Linked Disorders – Red-green color blindness and hemophilia are sexlinked, and, like the fruit fly, are seen more often in males. – Females must inherit two of the alleles, one from each parent, to exhibit the trait. SEX-LINKED DISEASES REVIEW: CONCEPT CHECK 10.5, page 221 1. Describe why one would expect to find more white eyed fruit male fruit flies than whit eyed females in natural populations. 2. A man with hemophilia and a homozygous non hemophiliac woman have a son. Is it possible that the son will inherit hemophilia? Why not? THE NUCLEUS CONTAINS AN INFORMATION RICH GENOME • DNA Packing in a Single Cell – Each chromosome consists of one DNA molecule, which is MUCH longer than the nucleus that houses it. • All the DNA in the 46 chromosomes placed end to end would be 2 M long. – A genome is the complete set of genetic material in an organism, defined by its order of bases. – Because of the unique property that DNA has for folding, all of the genome can fit into the nucleus HISTONES NUCLEOSOMES THE NUCLEUS CONTAINS AN INFORMATION RICH GENOME of a single cell. – The DNA is wrapped around a protein called a histone. • Then it is wrapped in a tight helical fiber and coiled. • The Human Genome Project – Remember that the DNA double helix is a long chain of nucleotides containing base pairs (A, T, C, G) – The order is different for each species and can even be different within the species. THE NUCLEUS CONTAINS AN INFORMATION RICH GENOME – By 1990, new DNA technology allowed scientists to sequence the human genome that was completed in a rough draft in 2000. – Knowing just the sequence is only part of the picture as knowing the functions of the polypeptide chains in important. – As we will see with Darwin, this project and sequencing has allowed scientists to compare the human genome to other species and embryonic development. HUMAN GENOME PROJECT THE NUCLEUS CONTAINS AN INFORMATION RICH GENOME – Researchers all over the world can enter the database for any research that they are doing. – New medical treatments and medicines come from knowing the genome. – Reworking genetic sequences in genetic diseases is now possible. REVIEW: CONCEPT CHECK 12.1, page 249 1. Draw a simple diagram showing the different levels of DNA-packing within a nucleus. 2. How might a complete map of the human genome be useful? ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS • Down Syndrome – Normal human karyotypes have 46 chromosomes, or 23 pair. – What if there are three chromosomes at the 21st pair, instead of two? – This is called a trisomy 21, which results from an error during either stage of meiosis but more commonly meiosis I. – Most times , embryos with abnormal numbers of TRISOMY 21 OR DOWN SYNDROME ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS – – – – chromosomes results in spontaneous abortions commonly known as a miscarriage. Trisomy 21 embryos , for the most part, do survive. Its incidence is 1:700. The syndrome associated with this defect is Down syndrome, named after John Langdon Down who described it in 1866. The symptoms include characteristic facial features, short height, heart defects, an impaired FEATURES OF DOWN’S SYNDROME ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS immune system, and differing degrees of mental disability. – Their life spans might be shorter than normal, but they live happy and sometimes very productive lives. • Nonseparation of Chromosomes – Trisomy 21 and other errors of chromosomes number are usually caused by homologous chromosomes or sister chromatids failing to NONDISJUNCTION ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS – separate during meiosis, called nondisjunction. – If it occurs in meiosis I, it usually occurs during anaphase I, as well as anaphase II during meiosis II. – The result is gametes with abnormal numbers of chromosomes. – For example, the sperm could fertilize an egg having an extra chromosome 21, resulting in a zygote with 3 copies of this chromosome rather than 2. ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS – The total number of chromosomes after fertilization would be 47, not 46. – Then, as you will remember, all the cells replicate with 47 chromosome, resulting in an embryo with 47 chromosomes. – The cause of nondisjunction is not fully known, but occurs more frequently the later in life pregnancy occurs. – This would indicate that the older the mother is, there is a difference in egg cell production. ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS – Remember meiosis begins at puberty but has its origin in the pre-egg cells in a girl’s ovaries before birth. – There is one egg produced and released each month until menopause, around 50 y.o. – The possibility then exists for a cell to be stopped in the middle of meiosis for many years. – The longer this period is, the greater the chance that errors such as nondisjunction will occur, at the completion of meiosis. MATERNAL AGE AND DOWN SYNDROME ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS – One hypothesis for the cause, then, is the damage to this cell during this lag time. • Damaged Chromosomes – Changes in chromosome structure can also lead to disorders. • Chromosomes can break, leading to new arrangements that affect the genes. – Duplication occurs when a part of the chromosome is repeated. ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS • When not fatal, they result in developmental abnormalities. – Deletion occurs when fragments of chromosomes are lost. • If a portion of the chromosomes is lost, then the genes that code for those proteins are also lost. • As proteins are essential to the body, losing some might have deleterious effects on the body. – Inversion is when a fragment is reversed from the original. DELETION, INVERSION, TRANSLOCATION ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS • Because most of the genes are still present, these are less likely to cause harmful effects. • Translocation happens when a fragment of one chromosome attaches to a nonhomologous chromosome. • This can even occur in two different chromosomes exchanging parts. • Jumping Genes – These are single genes that can move around that Barbara McClintock, an American geneticist, discovered this in the 1940s. BARBARA McCLINTOCK AND “JUMPING GENES” ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS – She studied the genetic variation in corn and found that some genetic material had the ability to move from on location to another in a chromosome, or to an entirely different chromosome. – This is not the same as translocation where a whole piece of the chromosome moves but where a gene moves. – If they landed in the middle of other genes, and they did, they could disrupt them. TRANSPOSONS ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS • Examples include pigment genes in corn, or spotted kernels. – These ‘jumping genes” are now called transposons. – Data now shows that all organisms have transposons. – Dr. McClintock won the Nobel prize for this in 1983. – The transposon includes a gene that codes for an enzyme which catalyzes movement of the gene by ACCIDENTS AFFECTING CHROMOSOMES CAN CAUSE DISORDERS attaching to the ends of the transposon and another site on the DNA. – The enzyme then cuts the DNA and catalyzes insertion of the transposon at the new site, and, perhaps, disrupting the sequence of the new gene. REVIEW: CONCEPT CHECK 12.2, page 253 1. What is the relationship between trisomy 21 and Down syndrome? Describe how nondisjunction can result in trisomy 21. 2. List and define four types of damage to chromosome structure that can cause disorders. 3. What is a “jumping gene,” or transposon? 4. How is a mother’s age related to the probability of nonseparation of chromosomes in her gametes? MENDEL’S PRINCIPLES APPLY TO HUMANS • Working With Human Pedigrees – Human subjects cannot control matting capabilities such as we can with plants and animals. – What is done is to analyze the genetic patterns in previous, current, and past family members. – A trait such as attached or unattached earlobes can be genetically transmitted to future generations. HUMAN PEDIGREES MENDEL’S PRINCIPLES APPLY TO HUMANS – As you can see from the previous slide, Mendel’s principles can be applied to this trait, knowing what is dominant and recessive. – A pedigree is like a family tree but it records and traces the occurrence of phenotypic characters in a family. – Genotypes can be determined by examining the patterns of the phenotype. HUMAN PEDIGREES MENDEL’S PRINCIPLES APPLY TO HUMANS • Disorders Inherited as Recessive Traits – Pedigree analysis becomes more important when the inherited alleles can cause serious problems. – There are over one thousand genetic disorders that have Mendelian inheritance patterns. – A single gene, either dominant or recessive, cause these disorders. • Most human genetic disorders are recessive. – The severity ranges from mild to lethal. MENDEL’S PRINCIPLES APPLY TO HUMANS – Albinism (lack of skin and hair pigmentation) is non-lethal while Tay-Sachs disease (nervous system disorders is. – Carriers are heterozygous for the trait, that is they have one copy of the allele for the recessive disorder. – Using Mendel’s genetics and Punnett squares, one can predict the probabilities of children of two carriers that will have the disorder. ALBINISM TAY-SACHS DISEASE INHERITED DEAFNESS CYSTIC FIBROSIS MENDEL’S PRINCIPLES APPLY TO HUMANS • Disorders Inherited as Dominant Traits – This group is smaller in number than recessive inheritance. – Extra fingers and toes and achondroplasia (dwarfism) are but two. • In achondroplasia, the torso may be normal but the arms and legs are short. • The incidence is 1:25,000 • All people with this disorder are heterozygous (a single copy of the allele) dominant. ACHONDROPLASIA MENDEL’S PRINCIPLES APPLY TO HUMANS • Homozygous for this condition is fatal. – Dominant alleles in the population, that are fatal, are rare because the affected individual dies before producing any offspring. – Recessive alleles that are fatal are more common because they can pass from generation to generation undetected as the heterozygote before two carriers happen to produce a homozygous offspring. MENDEL’S PRINCIPLES APPLY TO HUMANS – There are lethal dominant disorders that don’t cause death until adulthood. – In these cases, the lethal gene could have already been transmitted before the malady is known. – Huntington’s disease is one of these disorders. • In this case, a degeneration of the nervous system occurs and does not begin until middle age, and is eventually fatal. • This disease has been tracked to chromosome 4 with a test now available to detect this. • Treatments by genetic engineering are already in the works. HUNTINGTON’S DISEASE MENDEL’S PRINCIPLES APPLY TO HUMANS • Sex-Linked Disorders – Previously, you learned most sex-linked alleles are on the X chromosome. • The male receives sex-linked alleles from his mother. • The homologous Y chromosome is always inherited from the father. – The male, therefore, needs only one allele while the female needs two to exhibit a trait. – As previously noted, red-green colorblindness is MENDEL’S PRINCIPLES APPLY TO HUMANS sex-linked and involves a break down of light sensitive cells in the eyes. – While rare, it is not impossible for females to exhibit the trait. – This would happen when the carrier female married a male with the trait. – 50% of the children, male or female, would have the trait. WHAT COLORBLIND PEOPLE SEE MENDEL’S PRINCIPLES APPLY TO HUMANS • Predicting and Treating Genetic Disorders – Genetic counselors collect and analyze data about inheritance patterns and predict, explain, the results and significance. – Standard tests are performed on every baby born in the US. – One of these is Phenylketonuria, a recessive trait in which the individual does not possess the amino acid phenylalanine. PHENYLKETONURIA MENDEL’S PRINCIPLES APPLY TO HUMANS – Babies that test positive for this can be placed on low phenylalanine diets that prevents the mental disability that usually results from this disorder. REVIEW: CONCEPT CHECK 12.3, page 259 1. What information is collected to create a pedigree for a particular trait? 2. Give examples for a recessive disorder, a dominant disorder, and a sex-linked recessive disorder, an describe how each is inherited. 3. What does a genetic counselor do? GENETIC CHANGES CONTIBUTE TO CANCER • Cancer Genes – For the cell cycle to function properly, there are two classes of genes that direct the production of proteins for growth and division. – One class of genes produce proteins called growth factors which initiate cell division. – The other class, tumor-suppressive genes, produces proteins that stop cell division in certain situations. GENETIC CHANGES CONTIBUTE TO CANCER – This happens when cells contain damaged DNA or when cells exceed a specific amount of space. – Cancer can develop when several mutations to these genes occurs. – First, a mutation results in producing too much growth factor or increasing growth factor activity. – This gene may become a oncogene, or cancer causing gene. – These cells divide more frequently. GENETIC CHANGES CONTIBUTE TO CANCER – Second, mutations to tumor-suppressor genes can cause cancer. – Remember, these stop cell division, and if there is no control to stop them, the cells will continue to divide out of control. • “Inherited” Cancer – Cancer is always a genetic disease in that it always results from changes to DNA. – The mutations that cause cancer start in the organ where the cancer originated. MUTATIONS AND CANCER MUTATIONS AND CANCER MUTATIONS AND CANCER GENETIC CHANGES CONTIBUTE TO CANCER – These mutations do not affect the cells that give rise to eggs or sperm, so they are not passed from parent to child. – There, however, can be a mutation to one or more of these same genes in a cell that leads to a gamete. – These, then can be passed on to the offspring and the risk of getting cancer. • One of these is the mutated version of the BRAC1 gene, a tumor-suppressing gene. GENETIC CHANGES CONTIBUTE TO CANCER • Women who have this are at a greater risk for breast cancer. • It doesn’t mean that if one has this gene they will develop cancer, just that they have a head start on accumulating more mutations that can lead to cancer. REVIEW: CONCEPT CHECK 12.4, page 261 1. Compare and contrast the two classes of genes involved in regulating the cell cycle. 2. Describe how a woman inheriting a mutated BRCA1 gene is at a higher risk for breast cancer. 3. What is an oncogene? What effect does an oncogene have on a cell?