Patterns of Inheritance - Past Paper

Patterns of Inheritance  Early Ideas of Heredity Before the 20th century, 2 concepts were the basis for ideas about heredity: -heredity occurs within species -traits are transmitted directly from parent to offspring This led to the belief that inheritance is a matter of blending traits fr...

Patterns of Inheritance  Early Ideas of Heredity Before the 20th century, 2 concepts were the basis for ideas about heredity: -heredity occurs within species -traits are transmitted directly from parent to offspring This led to the belief that inheritance is a matter of blending traits from the parents. 2  Early Ideas of Heredity Botanists in the 18th and 19th centuries produced hybrid plants. When the hybrids were crossed with each other, some of the offspring resembled the original strains, rather than the hybrid strains. This evidence contradicted the idea that traits are directly passed from parent to offspring. 3  Early Ideas of Heredity Gregor Mendel -chose to study pea plants because: 1. other research showed that pea hybrids could be produced 2. many pea varieties were available 3. peas are small plants and easy to grow 4. peas can self-fertilize or be cross- fertilized 4 5  Early Ideas of Heredity Mendel’s experimental method: 1. produce true-breeding strains for each trait he was studying 2. cross-fertilize true-breeding strains having alternate forms of a trait -perform reciprocal crosses as well 3. allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait 6  Gregor Mendel The Father of Genetics. 1850’s He worked with pea plants and noticed that if he crossed peas with different characteristics that some would be passed on to the next generation. *Used true breeding plants that would only produce a certain trait such as color He did not know how this happens only that it did. Did not know about alleles, genes or chromosomes 8 9  Monohybrid Crosses Monohybrid cross: a cross to study only 2 variations of a single trait Mendel produced true-breeding pea strains for 7 different traits -each trait had 2 alternate forms (variations) -Mendel cross-fertilized the 2 true-breeding strains for each trait 10  Monohybrid Crosses F1 generation (1st filial generation): offspring produced by crossing 2 true- breeding strains For every trait Mendel studied, all F1 plants resembled only 1 parent -no plants with characteristics intermediate between the 2 parents were produced 11  Monohybrid Crosses F1 generation: offspring resulting from a cross of true-breeding parents F2 generation: offspring resulting from the self-fertilization of F1 plants dominant: the form of each trait expressed in the F1 plants recessive: the form of the trait not seen in the F1 plants 12  Monohybrid Crosses F2 plants exhibited both forms of the trait in a very specific pattern: ¾ plants with the dominant form ¼ plant with the recessive form The dominant to recessive ratio was 3 : 1. Mendel discovered the ratio is actually: 1 true-breeding dominant plant 2 not-true-breeding dominant plants 1 true-breeding recessive plant 13 14  Monohybrid Crosses gene: information for a trait passed from parent to offspring alleles: alternate forms of a gene homozygous: having 2 of the same allele heterozygous: having 2 different alleles 15  Monohybrid Crosses genotype: total set of alleles of an individual PP = homozygous dominant Pp = heterozygous pp = homozygous recessive phenotype: outward appearance of an individual 16  Monohybrid Crosses Some human traits are controlled by a single gene. -some of these exhibit dominant inheritance -some of these exhibit recessive inheritance Pedigree analysis is used to track inheritance patterns in families. 17  TWO TYPES OF TRAITS: Some traits may be expressed and others may not be expressed at all. 19 20  Monohybrid Crosses Law of Segregation Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization. 21 22 23  Dihybrid Crosses Dihybrid cross: examination of 2 separate traits in a single cross -for example: RR YY x rryy The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait. 24  Dihybrid Crosses The F2 generation is produced by crossing members of the F1 generation with each other or allowing self-fertilization of the F1. -for example RrYy x RrYy The F2 generation shows all four possible phenotypes in a set ratio: 9:3:3:1 25  Dihybrid Crosses Law of Independent Assortment Two or more genes assort independently during gamete formation. In a dihybrid cross, the alleles of each pair of alleles segregate independently of any other pair of alleles. 26 27 28  Probability – Predicting Results Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities. When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4) ¼ + ¼ = ½ 29  Probability – Predicting Results Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities. When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is: probability of obtaiing rr = ¼ probability of obtaining yy = ¼ probability of rr yy = ¼ x ¼ = 1/16 30  Testcross Testcross: a cross used to determine the genotype of an individual with dominant phenotype -cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) -the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent 31 32  Extensions to Mendel Pleiotropy refers to an allele which has more than one effect on the phenotype. This can be seen in human diseases such as cystic fibrosis or sickle cell anemia. In these diseases, multiple symptoms can be traced back to one defective allele. 33  Extensions to Mendel Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes. Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes. 34 35 Purebred red snapdragons were crossed with purebred white snapdragons all offspring were pink Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes. Red crossed with white makes pink. Incomplete Dominance In humans, straight hair and curly hair are incompletely dominant traits that result in hybrids that have wavy hair. Cross a straight hair with a wavy hair. What are the chances of having a curly haired child? What are the chances of having a straight hair child? 1. Go over Incomplete/codominance wkst 2. Sex linked traits  Co- dominacnce Co-dominance: the heterozygote shows some aspect of the phenotypes of both homozygotes. Black crossed with white makes gray.  Codominance The human ABO blood group system demonstrates: -multiple alleles: there are 3 alleles of the I gene (IA, IB, and i) -Co-dominance: IA and IB are dominant to i but codominant to each other 42  Three alleles are possible at one single gene locus IA, IB, and i. The enzyme produced by these alleles either adds or does not add a sugar molecule to a protein found in the membrane of the red blood cells. IA alleles add galactosamine, IB adds galactose, and i does not add any sugar. These Protein /Sugar complex act as an antigen.  Each has two alleles, so Blood type A could be ( IA IA or IA i), Type B ( IB IB or IB i), Type AB ( IA IB) Type O (i i ) The immune system is tolerant to its own antigens but makes antibodies to those that differ. This causes agglutination or clumping of and lysis of foreign red blood cells.  If Blood type A receives type B blood than it is recognizes as foreign and attacked causing them to clump. Same is true if B or AB types are transfused. If either of these Blood types is given a transfusion with type O blood than there are no antigens so the O blood will be tolerated. That is why Type O blood is known as the Universal Donor. Because Blood type AB has both antigens ,neither will be foreign, so these patients may receive blood from any of the blood groups. AB then is known as the universal recipient.  Other Antibodies are; IgM, Rh, and IgG antibodies. IgM work on foreign blood antigens such as carbohydrates on bacteria, even if these carbohydrates are found in our cellular makeup. But they do not act on the carbohydrates that make up the cell. Rh Factor or Rh antigen : The protein is either present, Rh positive or absent, Rh negative on the surface of RBC. A Rh neg. person who receives a Rh pos. transfusion produces antibodies to the foreign antibodies.  Blood Types A,B,O blood types A and B are dominant over O – Co-dominant to each other O blood type is recessive RESULTING GENOTYPE PHENOTYPES I I A A Type A I i A Type A IBIB Type B IBi Type B IAIB Type AB ii Type O 48  Extensions to Mendel The expression of some genes can be influenced by the environment. for example: coat color in Himalayan rabbits and Siamese cats -an allele produces an enzyme that allows pigment production only at temperatures below 30oC 49 Extensions to Mendel 50  Extensions to Mendel Mendel’s model of inheritance assumes that: -each trait is controlled by a single gene -each gene has only 2 alleles -there is a clear dominant-recessive relationship between the alleles Most genes do not meet these criteria. 51  Extensions to Mendel One Gene affects the phenotype of another gene, because their products interact. Epistasis: one gene can interfere with the expression of another gene Labrodor Retrievers: B or b is for fur color and E or e is for if it will be deposited. BBEE; BBEe; BbEe will be a black lab. bbEE or bbEe will be a chocholate lab BBee: Bbee; bbee; will all be a yellow lab. (Key ee) 52  Epistasis Epistasis – one allele hides/suppresses another allele  Extensions to Mendel Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait. The phenotype is an accumulation of contributions by multiple genes. For example – human height 700 genetic variations with over 180 genes Skin Pigment: https://www.sciencedaily.com/releases/2017/10/171012143324.htm These traits show continuous variation and are referred to as quantitative traits. 54  Polygenic Phenotype depends on alleles in multiple genes – Skin color, height, eye color – Continuous progression in expression of traits 56  Another example of a polygenic trait: Hair Color – Hair color is controlled by alleles on chromosomes 3, 6, 10, and 18. – The more dominant alleles that appear in the genotype, the darker the hair!  Sex Determination Thomas Hunt Morgan – studied fruit flies in the early 1900’s; Columbia University. Proved meiosis works in animals.  Sex Determination Observed that one pair of chromosomes was different between males and females – Large one named “X” chromosome – Smaller one named “Y” chromosome – XX = female; XY = male  Sex Chromosomes In many organisms, the Y chromosome is greatly reduced or inactive. genes on the X chromosome are present in only 1 copy in males sex-linked traits: controlled by genes present on the X chromosome Sex-linked traits show inheritance patterns different than those of genes on autosomes. 61 62  Sex Linkage Sex Linkage: the presence of a gene on a sex chromosome (X or Y) X-linked genes: genes found on the X chromosome – X chromosome carries more genes Y-linked genes: genes found on the Y chromosome  Fruit Fly Eye Color Fruit flies normally have red eyes – Red is dominant; white is recessive A few males have white eyes  Morgan’s Fruit Fly Experiments Red-eyed female (XRXR) x White-eyed male (XrY) XR XR XRXr XRXr RESULTS: Xr F1 generation – red-eyed all XRY XRY Y  Morgan’s Conclusions Gene for eye color is carried on the X chromosome = eye color is an X-linked trait Y chromosome does not carry a gene for eye color Red-eyed = XRXR, XRXr , XRY White-eyed = XrXr, XrY In fruit flies red eye color (R) is dominant to white eyes (r) and is a sex linked trait. A heterozygous red eye female mates with a red eye male. 1.How many will have red eyes? 2.What percent will have white eyes? 3.How many will be female and red eyed? In humans colorblindness (b) is an example of a sex-linked recessive trait. A male with colorblindness marries a female who isn’t colorblind and does not carry the allele. What is the chance they will have a child that is colorblind?  Sex Chromosomes and Barr Bodies Barr Body: Two X Chromosomes are found in every cell of a mammalian females. One will become inactive in each cell of the body. Either from The male or female parent. The inactivation of X chromosome is random in each embryonic cell. (Father or Mother lineage.) Giving a mosaic of traits. Each cell will divide through mitosis. Ex. Patchy coloration in female tortoise shelled collored cats, and females with patchy sweat glands. 69 70  Human Genetic Disorders Muscular Dystrophy: Sex linked. 71  Huntington Disease 3-7 per 100,000 people of European Ancestry. Less common in Japanese Chinese or African ancestry. Autosomal Dominant disorder only need one copy of the gene. Mutation in the HIT gene. Degeneration of the nerves in the Brain. Causes jerking, Twitching, pycheatric problems, etc. No cure.  Sickle-Cell Anemia About 1 in 12 African Americans and 1-100 Hispanic Americans are carriers. Mutation of the Hemoglobin Beta gene on Chromosome 11. Mutant Red Blood Cells. The damaged gene causes the cells to stick together and to become stiff. Cells clump together and damage organs of the body. These cell die fast and the bone marrow cannot produce enough RBC. Only cure is bone marrow transplants.  Hemophilia 1 in 5000 male births. 1/3 of the births happen to families with no history. Sex-linked = X linked This is a bleeding disorder, where the affected people cannot clot the blood. Treatment is that patients are given injections of the clotting factors  Muscular Dystrophy Disorder where the body fails to produce Dystrophin, which allows the muscle to grow and function. Sex Linked. Develop symptoms by are 2-3 and are in a wheel chair by age 12. 9 different forms of MD. All have different times of onset. No Treatment for any form.  Tay Sachs Autosomal Recessive: Mutation in the HEXA Gene. Destroys the neurons in the brain and spinal chord. Child appears normal until the ages of 3-6 months. Loss of muscle control and child loses ability to roll over, sitting and crawling. Sight and Hearing problems. Prevalent in people of Eastern European Jews, Amish, Cajun, and French Canadian communities. No Cure.  Cystic Fibrosis Autosomal Recessive Disorder Inherited disease of the secretory gland that make mucus and sweat. Individuals produce a very thick mucus. May effect the Lungs, skin, pancreas, liver, and intestines.  Recessive Affects the lungs, pancreas, liver, and intestine Characterized by – accumulation of thick, sticky mucus – coughing or shortness of breath – poor growth and weight gain – frequent chest infections – Salty skin https://www.youtube.com/user/CysticFibros isUSA  PKU Phenylketonuria Autosomal Recessive Disorder of Metabolism. Caused by Phenalalenine build up. Can’t breakit down. Women who have high levels of phenylalanine during pregnancy are at high risk for having babies born with mental handicapps, heart problems, small head size (microcephaly) and developmental delay. This is because the babies are exposed to their mother's very high levels of phenylalanine before they are born.  Albinism Recessive defect of melanin production results in little or no color in the skin, hair, and eyes Achondroplasia common cause of dwarfism Sporadic mutation in approximately 75% of cases (associated with advanced paternal age) Or dominant genetic disorder Unlikely homozygous child will live past a few months of its life  Human Genetic Disorders Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring. Some genetic disorders can be diagnosed during pregnancy. amniocentesis collects fetal cells from the amniotic fluid for examination chorionic villi sampling collects cells from the placenta for examination 82  Amniocentesis Amniotic fluid withdrawn 84  How is genetic testing done? blood, hair, skin, amniotic fluid, or other tissue Look for changes in chromosomes, DNA, proteins 86