• Share
  • Email
  • Embed
  • Like
  • Save
  • Private Content
Transmission Genetics
 

Transmission Genetics

on

  • 1,870 views

This is a PowerPoint presentation for my students.

This is a PowerPoint presentation for my students.

Statistics

Views

Total Views
1,870
Views on SlideShare
1,870
Embed Views
0

Actions

Likes
1
Downloads
60
Comments
0

0 Embeds 0

No embeds

Accessibility

Upload Details

Uploaded via as Adobe PDF

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment
  • http://www.bio.davidson.edu/people/kahales/301Genetics/timeline.html
  • http://www.wwnorton.com/college/anthro/bioanth/ch1/chap1.htmJulian Okwu is a writer, photographer, father, husband, friend, brother, son, athlete, and dancer, among other things. http://www.jujustudiosblog.com/?p=217
  • http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%2013/chap13.html
  • http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookgenintro.html
  • http://anthro.palomar.edu/mendel/mendel_2.htm
  • http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/genetics-%20genes/mendelia.htm
  • http://anthro.palomar.edu/mendel/mendel_2.htm
  • http://anthro.palomar.edu/mendel/mendel_2.htm
  • http://anthro.palomar.edu/mendel/mendel_2.htm
  • http://anthro.palomar.edu/mendel/mendel_2.htm
  • http://www.mathsisfun.com/data/probability-tree-diagrams.htmlhttp://www.ict-teacher.com/Mathstutor/Probability.html#anchor619243
  • https://wikispaces.psu.edu/display/110Master/Predicting+Phenotypes+and+Genotypes
  • http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/genetics-%20genes/mendelia.htm
  • http://anthro.palomar.edu/mendel/mendel_2.htm
  • http://anthro.palomar.edu/mendel/mendel_3.htm
  • http://www.doctortee.com/dsu/tiftickjian/genetics/mendel-extensions.html
  • http://www.vce.bioninja.com.au/aos-3-heredity/inheritance/polygenic-inheritance.html

Transmission Genetics Transmission Genetics Presentation Transcript

  • The Study of Heredity • Transmission (classical) genetics studies how an individual organism inherits its genetic makeup and how it passes its genes on to the next generation. o Population genetics studies the gene pool. o Molecular genetics studies the structure, organization, and function of the gene.
  • Darwin and Heredity • Charles Darwin wanted to develop an explanation of evolution based on hereditary principles. • But at the time (1859), as Darwin noted in The Origin of Species, “the laws governing inheritance are for the most part unknown.”
  • • During the 19th century, most naturalists accepted the blending model of inheritance: an individual’s traits are an intermediate mixture of their parents’ traits. o Although this may be visibly true in many cases, it does not explain why favorable variations are not rapidly lost within a few generations of breeding within the population. The Blending Model of Inheritance It is obvious to anyone that children resemble a mix of their parent’s features.
  • The Particulate Model of Inheritance • But Gregor Mendel discovered that traits are determined by "factors" (now called alleles) and that alleles do not blend when inherited; they remain distinct and separate in the offspring. This is called the particulate model of inheritance. o Traits may appear to blend, but not alleles. Thus, genetic variation is not lost and persists over generations, maintaining the raw material for natural selection to occur. A mixed race couple who gave birth to daughters of very different appearance.
  • Gregor Mendel • Mendel inferred the existence of genes and alleles by cross- breeding pea plants and observing the ratios of offspring with different traits. o He was an Austrian monk. o He studied science and mathematics (including statistics) at the University of Vienna from 1851 to 1853. Mendel’s knowledge of statistics later proved valuable in his research on heredity. o He began breeding peas in 1857. He kept very accurate records of his research and used very large sample sizes. o His research was largely ignored until 1900, after chromosomes and meiosis had been discovered.
  • Basic Terminology in Genetics: • Character. A detectable feature of an organism, such as flower color, that is controlled by a gene(s). • Trait. A variant of a character, such as purple flowers. • Gene. A unit of heredity composed of DNA occupying a specific position on a chromosome; a gene controls one or more characters. • Alleles. Copies of the same gene; alternative forms are responsible for different traits. • Homozygous. Having two identical alleles of a given gene. • Heterozygous. Having two different alleles of a given gene. • Phenotype. The detectable feature(s) of an individual. • Genotype. The genetic makeup of an individual; the allele(s) possessed for a given gene(s).
  • Mendel’s Three Principles of Inheritance • Inheritance can be explained by several concepts first proposed by Mendel that form the foundation of modern genetics: o An organism carries two copies (alleles) of each gene. o The principle of dominance. When the alleles in a pair are different, one (dominant) allele hides the effects of the other (recessive) allele. o The principle of segregation. During gamete formation, each member of an allele pair separates (segregates) into one-half of the gametes. The paired condition is restored by the random fusion of gametes at fertilization. o The principle of independent assortment. During gamete formation, different allele pairs are often observed to separate (segregate) independently of one another.
  • MENDEL’S SEVEN CHARACTERS AND TRAITS
  • Genetic Notation • Generally, the capitalized first letter of the dominant trait is used to represent the dominant allele; the lowercase letter indicates the recessive allele. This table shows some examples of how genetic notation is used in pea plants (). o Round seeds (R) is dominant to wrinkled seeds (r); and yellow seeds (Y) is dominant to green seeds (y). Genotype Phenotype (peas) RR Seed is round Rr Seed is round rr Seed is wrinkled RRYY Seed is round and yellow RrYy Seed is round and yellow rryy Seed is wrinkled and green Rryy Seed is round and green rrYy Seed is wrinkled and yellow
  • • Although how gametes are produced was unknown when he lived, Mendel’s principles of segregation and independent assortment are actually a direct consequence of meiosis: o Both alleles for a gene and homologs occur in pairs. o The segregation of alleles during gamete formation corresponds to the segregation of homologs during anaphase I. o The independent assortment of alleles corresponds to the independent assortment of homologs during anaphase I, provided that the allele pairs are on different chromosomes (or far apart on the same chromosome). Mendel and Meiosis
  • • Now, imagine that the allele pair Bb is on a short chromosome and the allele pair Ss is on a long chromosome:
  • o The principle of segregation says that a gamete can get either B or b, but not both. Likewise, it can get either S or s, but not both. o The principle of independent assortment says that a gamete that gets B can get either S or s. Likewise, a gamete that gets b can get either S or s.
  • The Punnett Square • One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented in 1905 by an English mathematician named Reginald Punnett, using what we now call a Punnett square. Think of a Punnett square as a simple graphical way of discovering all of the possibilities that can occur during fertilization.
  • • For example, consider a cross between two plants, each with the genotype Rr. o The character is seed shape: allele R for round is dominant to allele r for wrinkled. • According to Mendel’s principle of segregation, each gamete produced by an individual with the genotype Rr randomly receives either allele R or allele r. o Thus, the probability of a given gamete getting either allele from a pair in the parent is ½ (or 0.5 or 50 %). Single-Character Genetic Crosses
  • • There are four possible ways fertilization can occur in this example:
  • • Setting up and using a Punnett square: 1) Draw a grid of perpendicular lines to form a table; the number of rows and columns equals the number of genetically different gametes from each parent. This is a cross of Rr × Rr.
  • R r R r This is a cross of Rr × Rr. • Setting up and using a Punnett square: 1) Draw a grid of perpendicular lines to form a table; the number of rows and columns equals the number of genetically different gametes from each parent. 2) Put the genotypes of the possible gametes from one parent across the top, and those from the other parent down the left side.
  • R r R RR Rr r Rr rr • Setting up and using a Punnett square: 1) Draw a grid of perpendicular lines to form a table; the number of rows and columns equals the number of genetically different gametes from each parent. 2) Put the genotypes of the possible gametes from one parent across the top, and those from the other parent down the left side. 3) Fill in the boxes by copying the row and column-head letters across and down, respectively, into the empty squares. This is a cross of Rr × Rr.
  • • If two or more events can occur such that none of them affects the occurrence of the others, then the probability of all of them occurring is the product of the probabilities of each of them occurring individually. This is called the Law of Multiplication. The Law of Multiplication p(A and B) = p(A) × p(B)
  • • For example, two numbered bags of beans are each half red beans and half white beans. If randomly drawing a red bean from bag #1 is denoted R1 and randomly drawing a red bean from bag #2 is denoted R2, then: p(R1 and R2) = p(R1) × p(R2) = ½ × ½ = ¼ (or 0.25).
  • • Since fertilization amounts to selecting a sperm and selecting an egg, and which sperm is selected does not affect which egg is selected, we can compute the probability of a given outcome by multiplying the probability of selecting that sperm and the probability of selecting that egg. R (0.5) r (0.5) R (0.5) RR (0.25) Rr (0.25) r (0.5) Rr (0.25) rr (0.25) This is a cross of Rr × Rr.
  • • Question: What is the probability of one offspring being homozygous recessive? • Answer:
  • • Question: What is the probability of one offspring being homozygous recessive? • Answer: In other words, what is the probability of selecting a sperm with a recessive allele and an egg with a recessive allele? Using the Law of Multiplication, the overall probability is ¼ (or 0.25).
  • • If two or more events cannot occur at the same time, the probability of either of them occurring is the sum of the probabilities of each of them occurring individually. This is called the Law of Addition. The Law of Addition p(A or B) = p(A) + p(B)
  • • For example, a single die has 6 different faces, so a toss of one die cannot result in two faces showing at the same time. Since the probability of any particular outcome occurring on one toss of a die is ⅙, therefore: p(getting “4” or getting “6”) = p(getting “4”) + p(getting “6”) = ⅙ + ⅙ = ⅓ (or 0.33).
  • • Question: What is the probability of one offspring being a heterozygote? • Answer:
  • • Question: What is the probability of one offspring being a heterozygote? • Answer: When a sperm fertilizes an egg, there are two ways in which a heterozygote may be produced: either allele R may be in the egg and allele r in the sperm, or allele R may be in the sperm and allele r in the egg. Since only one of these alternatives can occur in a single fertilization event, we use the Law of Addition. The overall probability is ½ (or 0.5).
  • • Question: What is the probability of one offspring having round seeds? • Answer:
  • • Question: What is the probability of one offspring having round seeds? • Answer: When a sperm fertilizes an egg, there are three ways in which an offspring with round seeds may be produced: either allele R may be in the egg and allele r in the sperm, or allele R may be in the sperm and allele r in the egg , or allele R may be in both the egg and the sperm. Since only one of these alternatives can occur in a single fertilization event, we use the Law of Addition. The overall probability is ¾ (or 0.75).
  • • A Punnett square can also be used to predict the results of a genetic cross involving more than one character. For example, suppose we cross two plants, each with the genotype RrYy. o The second character is seed color: allele Y for yellow is dominant to allele y for green. • Since alleles of different characters segregate independently during gamete formation, there is the same probability that the R (or r) allele will segregate with the Y allele as with the y allele. o Thus, all four combinations of gametes (RY, Ry, rY, ry) are equally probable. Two-Character Genetic Crosses
  • • Thus, RY (0.25) Ry (0.25) rY (0.25) ry (0.25) RY (0.25) RRYY (0.0625) RRYy (0.0625) RrYY (0.0625) RrYy (0.0625) Ry (0.25) RRYy (0.0625) RRyy (0.0625) RrYy (0.0625) Rryy (0.0625) rY (0.25) RrYY (0.0625) RrYy (0.0625) rrYY (0.0625) rrYy (0.0625) ry (0.25) RrYy (0.0625) Rryy (0.0625) rrYy (0.0625) rryy (0.0625) This is a cross of RrYy × RrYy.
  • "Exceptions" to Mendel’s Inheritance Patterns • Today, Mendel's findings are still fundamental to the science of genetics, although many important exceptions to them have been discovered. • Does this mean that Mendel was "wrong"? NO, it means that we know more today about diseases, genes, and heredity than we did 150 years ago!
  • X-linked Inheritance • X-linked inheritance means that the gene causing a trait is located solely on the X sex chromosome. An allele on the X chromosome is represented by a superscript following "X", such as XR. o In mammals, females have two X chromosomes while males have one X and one Y, but there are very few genes located on the Y-chromosome. o Traits due to recessive X-linked alleles, such as red-green color blindness in humans, are more common in males since there is no chance of males being heterozygous. o Males transmit an X-linked allele to their daughters, but not to their sons.
  • Incomplete Dominance • Mendel discovered complete dominance, in which the heterozygous phenotype is identical to one of the homozygous phenotypes. With incomplete dominance, the heterozygous phenotype is quantitatively intermediate between both homozygous phenotypes.
  • o For example, when a red-flowered snapdragon (RR) and a white- flowered snapdragon (rr) are crossed, the offspring (Rr) is neither white nor red—it is pink. Although the homozygous phenotypes seem to blend in heterozygotes, the alleles do not; both parental phenotypes reappear in later generations.
  • Epistasis • Epistasis refers to two separate genes that control a single character, but one gene masks the phenotypic expression of the other gene. o It is not the same thing as dominance, in which one allele of a gene masks the expression of another allele of the same gene.
  • o For example, coat color in Labrador retrievers is controlled by two genes. Alleles B and b of a pigment gene determine black and brown, respectively. Allele E of a separate gene allows pigment to be deposited in the fur, but allele e blocks pigment deposition and results in yellow fur. Genotype Phenotype E_B_ Black coat color E_bb Brown coat color ee_ _ Yellow coat color
  • Polygenic Inheritance • Polygenic inheritance (not epistasis) refers to a single character that is controlled by more than one gene, but each allele contributes equally. So, the phenotypes vary quantitatively between two extremes, producing continuous variation.
  • • An example of a polygenic character is human stature. o The combined size of all of the body parts from head to foot determines the height of an individual. The sizes of all of these body parts are, in turn, determined by numerous genes. There is an additive effect.
  • Pleiotropy • Pleiotropy is the control by a single gene of several distinct and seemingly unrelated phenotypic traits. o Pleiotropy is a consequence of the fact that most proteins have multiple roles in different cell types; so, any change in the gene that that controls such a protein can potentially have wide-ranging effects in a variety of tissues. o An example in humans is phenylketonuria (PKU). The multiple phenotypes associated with PKU include mental retardation, eczema, and pigment defects that make affected individuals lighter skinned. PKU is due to a mutation in the gene for the enzyme that is necessary to convert the essential amino acid phenylalanine to tyrosine.
  • Environment and Phenotype • The phenotype for a character may depend on environment as well as on genotype. For example: o Hydrangea flowers with the same genotype range from blue-violet to pink, depending on soil acidity.
  • o Siamese cats have a gene that codes for dark pigments – this gene is more active at low body temperatures. Parts of the body that are colder will develop the darker pigmentation – ears, feet, nose, and tail of the cats. o In humans, the environment plays an important role in characters such as intelligence, height, and body weight, and also in many diseases. Usually, about 10% of an individual's height is due to the environment.