Download presentation
Presentation is loading. Please wait.
Published byKarin Howard Modified over 9 years ago
1
Classical (Mendelian) Genetics Gregor Mendel
2
Vocabulary Genetics: The scientific study of heredityGenetics: The scientific study of heredity Allele: Alternate forms of a gene/factor.Allele: Alternate forms of a gene/factor. Genotype: combination of alleles an organism has.Genotype: combination of alleles an organism has. Phenotype: How an organism appears.Phenotype: How an organism appears. Dominant: An allele which is expressed (masks the other).Dominant: An allele which is expressed (masks the other). Recessive: An allele which is present but remains unexpressed (masked)Recessive: An allele which is present but remains unexpressed (masked) Homozygous: Both alleles for a trait are the same.Homozygous: Both alleles for a trait are the same. Heterozygous: The organism's alleles for a trait are different.Heterozygous: The organism's alleles for a trait are different.
3
History Principles of genetics were developed in the mid 19th century by Gregor Mendel an Austrian MonkPrinciples of genetics were developed in the mid 19th century by Gregor Mendel an Austrian Monk Developed these principles without ANY scientific equipment - only his mind.Developed these principles without ANY scientific equipment - only his mind. Experimented with pea plants, by crossing various strains and observing the characteristics of their offspring.Experimented with pea plants, by crossing various strains and observing the characteristics of their offspring. Studied the following characteristics:Studied the following characteristics: –Pea color (Green, yellow) –Pea shape (round, wrinkled) –Flower color (purple, white) –Plant height (tall, short) MONOHYBRID CROSS- cross fertilizing two organisms that differ in only one trait SELF-CROSS- allowing the organism to self fertilize (pure cross)
4
MENDEL’S CROSSES Made the following observations (example given is pea shape)Made the following observations (example given is pea shape) When he crossed a round pea and wrinkled pea, the offspring (F1 gen.) always had round peas.When he crossed a round pea and wrinkled pea, the offspring (F1 gen.) always had round peas. When he crossed these F1 plants, however, he would get offspring which produced round and wrinkled peas in a 3:1 ratio.When he crossed these F1 plants, however, he would get offspring which produced round and wrinkled peas in a 3:1 ratio. Started with pure plants ( P1) Then made a hybrid of two pure traits P1 X P1
5
Laws of Inheretance Law of Segregation: When gametes (sperm egg etc…) are formed each gamete will receive one allele or the other.Law of Segregation: When gametes (sperm egg etc…) are formed each gamete will receive one allele or the other. Law of independent assortment: Two or more alleles will separate independently of each other when gametes are formedLaw of independent assortment: Two or more alleles will separate independently of each other when gametes are formed
6
Punnett Squares Genetic problems can be easily solved using a tool called a punnett square.Genetic problems can be easily solved using a tool called a punnett square. –Tool for calculating genetic probabilities A punnett square
7
Monohybrid cross (cross with only 1 trait) Problem:Problem: Using this is a several step process, look at the following exampleUsing this is a several step process, look at the following example –Tallness (T) is dominant over shortness (t) in pea plants. A Homozygous tall plant (TT) is crossed with a short plant (tt). What is the genotypic makeup of the offspring? The phenotypic makeup ?
8
Punnet process 1.Determine alleles of each parent, these are given as TT, and tt respectively. 2.Take each possible allele of each parent, separate them, and place each allele either along the top, or along the side of the punnett square.
9
Punnett process continued Lastly, write the letter for each allele across each column or down each row. The resultant mix is the genotype for the offspring. In this case, each offspring has a Tt (heterozygous tall) genotype, and simply a "Tall" phenotype.Lastly, write the letter for each allele across each column or down each row. The resultant mix is the genotype for the offspring. In this case, each offspring has a Tt (heterozygous tall) genotype, and simply a "Tall" phenotype.
10
Punnett process continued Lets take this a step further and cross these F1 offspring (Tt) to see what genotypes and phenotypes we get.Lets take this a step further and cross these F1 offspring (Tt) to see what genotypes and phenotypes we get. Since each parent can contribute a T and a t to the offspring, the punnett square should look like this….Since each parent can contribute a T and a t to the offspring, the punnett square should look like this….
11
Punnett process continued Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1:2:1 genotypic ratio. We now have 2 different phenotypes (Tall & short) in a 3:1 Phenotypic ratio. This is the common outcome from such crosses.Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1:2:1 genotypic ratio. We now have 2 different phenotypes (Tall & short) in a 3:1 Phenotypic ratio. This is the common outcome from such crosses.
12
Dihybrid crosses Dihybrid crosses are made when phenotypes and genotypes composed of 2 independent alleles are analyzed.Dihybrid crosses are made when phenotypes and genotypes composed of 2 independent alleles are analyzed. Process is very similar to monohybrid crosses.Process is very similar to monohybrid crosses. Example:Example: –2 traits are being analyzed –Plant height (Tt) with tall being dominant to short, –Flower color (Ww) with Purple flowers being dominant to white.
13
Dihybrid cross example The cross with a pure-breeding (homozygous) Tall,Purple plant with a pure-breeding Short, white plant should look like this.The cross with a pure-breeding (homozygous) Tall,Purple plant with a pure-breeding Short, white plant should look like this. F1 generation
14
Dihybrid cross example continued Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f1 generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. The cross should look like this. (The mathematical “foil” method can often be used here)Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f1 generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. The cross should look like this. (The mathematical “foil” method can often be used here) F2 Generation
15
Dihybrid cross example continued Note that there is a 9:3:3:1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits.Note that there is a 9:3:3:1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits. Also note that this also indicates that these alleles are separating independently of each other. This is evidence of Mendel's Law of independent assortmentAlso note that this also indicates that these alleles are separating independently of each other. This is evidence of Mendel's Law of independent assortment
16
PROBABILITY Definition- Likelihood that a specific event will occur Probability = number of times an event happens number of opportunities for event to happen
17
What if you don’t know the GENOTYPE? Perform a TEST CROSS- cross with a homozygous recessive individual If no recessive traits appear than unknown individual was HOMOZYGOUS DOMINANT
18
TEST CROSS If the unknown individual was heterozygous than 50% of the offspring should have the recessive phenotype.
19
INCOMPLETE DOMINANCE When neither allele is completely recessive Example RR ---- red roses rr---------- white roses Rr-------pink roses In the HETEROZYGOUS individual both alleles are still visible – but not fully visible
20
Other Factors: Incomplete Dominance Some alleles for a gene are not completely dominant over the others. This results in partially masked phenotypes which are intermediate to the two extremes.Some alleles for a gene are not completely dominant over the others. This results in partially masked phenotypes which are intermediate to the two extremes.
21
Other Factors: Continuous Variation Many traits may have a wide range of continuous values. Eg. Human height can vary considerably. There are not just "tall" or "short" humans
22
CODOMINANCE When the HETEROZYGOUS INDIVIDUAL fully shows both alleles. Example is blood type Blood Type A is dominant Blood Type B is dominant Blood Type O is recessive to both A and B Blood Type AB- is heterozygous for A and B
23
Multiple Alleles Phenotypes are controlled by more than 2 variances for a trait ABO Blood typing –Humans have multiple types of surface antigens on RBC's –The nature of these surface proteins determines a person's Blood Type. –There are 3 alleles which determine blood type I A, I B, or I O. This is referred to as having multiple alleles –Human blood types are designated as A, B or O. Type A denotes having the A surface antigen, and is denoted by I A Type B denotes having the B surface antigen, and is denoted by I B Type O denotes having neither A or B surface antigen, and is denoted by I O –There are several blood type combinations possible A B AB (Universal recipient) O (Universal donor)
24
Punnett Square for blood typing A A O B O AB AO BO OO
25
Blood & Immunity A person can receive blood only when the donor's blood type does not contain any surface antigen the recipient does not. This is because the recipient has antibodies which will attack any foreign surface protein.A person can receive blood only when the donor's blood type does not contain any surface antigen the recipient does not. This is because the recipient has antibodies which will attack any foreign surface protein. Thus, Type AB can accept any blood types because it will not attack A or B surface antigens. However, a type AB person could only donate blood to another AB person. They are known as Universal Recipients.Thus, Type AB can accept any blood types because it will not attack A or B surface antigens. However, a type AB person could only donate blood to another AB person. They are known as Universal Recipients. Also, Type O persons are Universal donors because they have NO surface antigens that recipients' immune systems can attack. Type O persons can ONLY receive blood from other type O persons.Also, Type O persons are Universal donors because they have NO surface antigens that recipients' immune systems can attack. Type O persons can ONLY receive blood from other type O persons. There is another blood type factor known as Rh.There is another blood type factor known as Rh. People are either Rh+ or Rh- based on a basic dominant/recessive mechanism.People are either Rh+ or Rh- based on a basic dominant/recessive mechanism. Not usually a problem except with pregnancy.Not usually a problem except with pregnancy. It is possible that an Rh- mother can carry an Rh+ fetus and develop antibodies which will attack & destroy the fetal bloodIt is possible that an Rh- mother can carry an Rh+ fetus and develop antibodies which will attack & destroy the fetal blood This usually occurs with 2nd or 3rd pregnancies, and is detectable and treatable.This usually occurs with 2nd or 3rd pregnancies, and is detectable and treatable.
26
Other Factors Gene interaction:Gene interaction: –Many biological pathways are governed by multiple enzymes, involving multiple steps.(Examples the presence of a HORMONE) If any one of these steps are altered. The end product of the pathway may be disrupted. Environmental effects:Environmental effects: –Sometimes genes will not be fully expressed owing to external factors. Example: Human height may not be fully expressed if individuals experience poor nutrition.
27
Chapter 12--Sex Linkage All chromosomes are homologous except on sex chromosomes.All chromosomes are homologous except on sex chromosomes. Sex chromosomes are either X or Y.Sex chromosomes are either X or Y. If an organism is XX, it is a female, if XY it is male.If an organism is XX, it is a female, if XY it is male. If a recessive allele exists on the X chromosome. It will not have a corresponding allele on the Y chromosome, and will therefore always be expressedIf a recessive allele exists on the X chromosome. It will not have a corresponding allele on the Y chromosome, and will therefore always be expressed
28
is an important tool for studying inherited diseases uses family trees and information about affected individuals to: figure out the genetic basis of a disease or trait from its inheritance pattern predict the risk of disease in future offspring in a family (genetic counseling) PEDIGREE ANALYSIS
29
How to read pedigrees Basic patterns of inheritance 1. autosomal, recessive 2. autosomal, dominant 3. X-linked, recessive 4. X-linked, dominant (very rare)
30
How to read a pedigree
31
Sample pedigree - cystic fibrosis female male affected individuals
32
Autosomal dominant pedigrees 1. The child of an affected parent has a 50% chance of inheriting the parent's mutated allele and thus being affected with the disorder. 2. A mutation can be transmitted by either the mother or the father. 3. All children, regardless of gender, have an equal chance of inheriting the mutation. 4. Trait does not skip generations
33
Autosomal dominant traits There are few autosomal dominant human diseases (why?), but some rare traits have this inheritance pattern ex. achondroplasia (a sketelal disorder causing dwarfism)
34
AUTOSOMAL RECESSIVE 1. An individual will be a "carrier" if they posses one mutated allele and one normal gene copy. 2. All children of an affected individual will be carriers of the disorder. 3. A mutation can be transmitted by either the mother or the father. 4. All children, regardless of gender, have an equal chance of inheriting mutations. 5. Tends to skip generations
35
Autosomal recessive diseases in humans Most common ones Cystic fibrosis Sickle cell anemia Phenylketonuria (PKU) Tay-Sachs disease
36
Autosomal Recessive
37
X-Linked Dominant 1. A male or female child of an affected mother has a 50% chance of inheriting the mutation and thus being affected with the disorder. 2. All female children of an affected father will be affected (daughters possess their fathers' X-chromosome). 3. No male children of an affected father will be affected (sons do not inherit their fathers' X-chromosome).
39
X-LINKED Recessive 1. Females possessing one X-linked recessive mutation are carriers 2. All males possessing an X-linked recessive mutation will be affected (why?) 3. All offspring of a carrier female have a 50% chance of inheriting the mutation. 4. All female children of an affected father will be carriers (why?) 5. No male children of an affected father will be affected
41
Sex linkage example Recessive gene for white eye color located on the X w chromosome of Drosophila.Recessive gene for white eye color located on the X w chromosome of Drosophila. All Males which receive this gene during fertilization (50%) will express this.All Males which receive this gene during fertilization (50%) will express this. If a female receives the X w chromosome. It will usually not be expressed since she carries an X chromosome with the normal geneIf a female receives the X w chromosome. It will usually not be expressed since she carries an X chromosome with the normal gene
42
Human Sex Linkage Hemophilia:Hemophilia: –Disorder of the blood where clotting does not occur properly due to a faulty protein. –Occurs on the X chromosome, and is recessive. Thus a vast majority of those affected are males.Thus a vast majority of those affected are males. –First known person known to carry the disorder was Queen Victoria of England. Thus all those affected are related to European royalty.
43
LINKAGE GROUPS (pg 222) Definition-genes that are located on the same chromosome. Discovered by Thomas Hunt Morgan. Made a dihybrid cross with heterozygous fruit flies ( Gray body and Long wings) GgLl x GgLl = predicted a 9:3:3:1 ratio What ratio did he get?
44
Answer He only got two combinations. Gray body with long wings - DOMINANT white body with short wings- RECESSIVE And they were in a 3:1 ratio just like a standard MONOHYBRID cross. Conclusion– these GENES must be on the same chromosome.
45
Further studies of Morgan Wanted to find out which traits were linked together on the same chromosome. Linked many traits together (remember that fruit flies have only 4 chromosomes)
46
During his many linkage studies found some mutations While working with the gray body and long wing linkage. Occasionally he had some flies come out Gray body with short wings and White body with long wings How could this be?
47
CROSSING OVER- forms new genetic combinations Gray body White body Short wings Long wings Gray body White body Short wings Long wings
48
CHROMOSOME MAPPING New question- where are the genes located on a chromosome? How far apart are the genes on a chromosome?
49
Using the rate of CROSSING OVER to determine location.
50
CHROMOSOME MAPPING The PERCENTAGE of crossing over is equal to ONE MAP UNIT on a chromosome.
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.