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Introduction to Mendelian Genetics Introduction to Mendelian Genetics
The Work of Gregor Mendel A. Genetics is the scientific study of heredity. Every living thing has a set of characteristics inherited from its parent or parents!
Who & Where: Austrian monk, “ Father of Genetics ”, born 1822 What: His work was important to the understanding of heredity. In charge of monastery ’ s garden; studied traits of pea plants B. Gregor Mendel
C. So Why Peas? Pea plant flowers are closed Self-fertilizing True-breeding Have 7 easily visible traits called phenotypes
Every time Mendel crossed 2 different traits, only ONE was seen in the offspring!
C. Mendel’s Principles 1.Principle of Dominance 2.Principle of Segregation 3.Principle of Independent Assortment 1.Principle of Dominance 2.Principle of Segregation 3.Principle of Independent Assortment
Back to Mendel’s Experiments…
If these hybrids self-pollinate.. The Next Generation The hidden trait returns! F1 generation: Tt x Tt Result?
Plants with different genotypes (TT and Tt) can have the same phenotype (“tall”). What happened to Recessive Allele?
Genotype: genetic makeup of organism Phenotype: physical characteristics of an organism GenotypePhenotype C c T P p
D. Principle of Dominance Definition: some alleles are dominant and others are recessive. The dominant gene shows up in the phenotype when present. Smooth peas S Example: Smooth peas S wrinkled peas s
During sex cell formation, alleles separates from each other Each gamete has one allele for each trait E. Principle of Segregation
Each trait is controlled by a gene that is in two contrasting formsEach trait is controlled by a gene that is in two contrasting forms The different forms of a gene are called alleles.The different forms of a gene are called alleles. Each trait is controlled by a gene that is in two contrasting formsEach trait is controlled by a gene that is in two contrasting forms The different forms of a gene are called alleles.The different forms of a gene are called alleles.
Homozygous: two identical alleles Example: TT or tt or SS or ss Heterozygous: two different alleles for the same trait Example: Tt or Ss Homozygous: two identical alleles Example: TT or tt or SS or ss Heterozygous: two different alleles for the same trait Example: Tt or Ss
II. Probability and Punnett Squares Why used? Punnett squares used to predict and compare genetic variations that will result from a cross The Rules: Dominant traits are the 1st letter and CAPITAL Recessive are the 2nd letter and lowercase
B. Practice 1) If a homozygous tall person was crossed with a homozygous short person, what are probably offspring? Tall= T short= t TT t t T T T T t t t t
Punnett Square 2) Cross two heterozygous tall parents. Tall= T short= t T TTTT T t t t t tt What is the genotype ratio? 1 TT: 2 Tt: 1 tt What is the phenotype ratio? 3 Tall: 1 short
Punnett Square 3) The long-eared allele (L) is dominant to the short-eared allele (l). Cross a homozygous long ear with a homozygous short-ear. Cross the F1 generation and give the F2 results.
III. Independent Assortment A. Mendel discovered that genes for different traits segregate independently during gamete formation Ex. Wrinkled/Smooth and Yellow/Green peas III. Independent Assortment A. Mendel discovered that genes for different traits segregate independently during gamete formation Ex. Wrinkled/Smooth and Yellow/Green peas
B. Dihybrid Cross: two traits being crossed at the same time Here is a heterozygous tall, heterozygous purple plant: T t P p
Tall and Purple are dominant to short and white.Example: Cross two heterozygous tall, heterozygous purple pea plants. T t P p x T t P p There are four possible gametes each parent can make.. Tall and Purple are dominant to short and white.Example: Cross two heterozygous tall, heterozygous purple pea plants. T t P p x T t P p There are four possible gametes each parent can make..
TTP t ptP T p P Tp tP pt TTPp TTPP TTPpTtPP TtPp TTppTtpp TtPPTtPpttPPttPp TtPpTtppttPp ttpp MOTHER FATHER
Results in fractions? Phenotypes: Tall, Purple? Tall, White? Short, Purple? Short, White? Results in fractions? Phenotypes: Tall, Purple? Tall, White? Short, Purple? Short, White? 9/16 3/16 1/16 heterozygous Ratio from heterozygous dihybrid cross is ALWAYS 9: 3: 3: 1 Alleles assort independently.
Use FOIL to set up these examples: FfPp: SSTt: DdRR: FPFpfPfp ST St DR dR
IV. Beyond Pure Dominance…. Some alleles are not simply dominant or recessive.. A. Incomplete dominance: Alleles are expressed as a blend. Each allele has a capital letter. Red= R Yellow= Y
Red=R White=W 1. Cross a red flower with a white flower, showing incomplete dominance. RR W W R R R R W W W W Genotype: 100% RW Phenotype: PINK!
D. Co-dominance seen separatelyBoth traits dominate, seen separately! Red Horse White Horse
Give you ROAN!
1. Example of Codominant Problem Red feathers are codominant to white feathers in chickens. C R = red C W = white Cross a homozygous Red with a homozygous white feathered chicken. CRCR CRCR CWCW CWCW CRCR CWCW CRCR CWCW CRCR CWCW CRCR CWCW PHENOTYPE: 100% Red and white mixed feathers CRCR CWCW GENOTYPE: 100%
One trait, many allele options! But remember: an individual cannot inherit more than two actual alleles, even if more than two possible alleles exist. Example: Blood type A, B, AB, O!
Blood Type Problem I Cross a homozygous Type A with a heterozygous Type B. What are the possible phenotypes of offspring? IBIB IAIA IBIB i IAIA IAIA IAIA i Phenotypes: 50% Type AB 50% Type A
Blood Type Problem II Cross a heterozygous Type A man with a heterozygous Type B woman. Is it possible for them to have an O child? IBIB i IBIB i IAIA IAIA IAIA i Phenotypes: 25% Type AB 25% Type A 25% Type B 25% Type O IBIB i ii
Blood Type Problem III Cross a heterozygous Rh+ man with a Rh- woman. What are the possible phenotypes of offspring? Rh - Rh + Rh - Phenotypes: 50% Type + 50% Type - Rh -
Rabbits have 4 basic colors (alleles!) brown chinchilla or grey It is recessive to brown. himalayan or white with black tips. It is recessive to both brown and chinchilla. albino It is recessive to all.
AIbino Himalayan Chinchilla Full color
D. Polygenic Traits Traits produced by many genes with many alleles Most human traits are polygenic Most variety of expression There are 3 genes that contribute to skin color.. And many alleles for each gene!
More examples: Height Weight Intelligence Eye color
V. Sex Determination In humans, the X and Y chromosomes control the sex of offspring. Outcome is always 50% chance of a male, and 50% chance of a female
Sex-linked traits Traits controlled by genes on the sex chromosomes are called sex-linked. Alleles for sex-linked traits are written as superscripts on the X chromosomes only. Traits controlled by genes on the sex chromosomes are called sex-linked. Alleles for sex-linked traits are written as superscripts on the X chromosomes only. XRXR XRXR XrXr y Example: Red eyes in fruit flies found in females Males tend to have white eyes, which is recessive.
X and Y sex chromosomes are non- homologous Any allele on the X chromosome will NOT be masked by a matching allele on the Y chromosome.
Why are sex-linked disorders more common in males than in females? Males have just one X chromosome containing an allele. So all X-linked alleles are automatically expressed in males, even if they are recessive.
Color blindness Color blindness Duchenne Muscular Dystrophy Duchenne Muscular Dystrophy Hemophilia Hemophilia C. Examples of Sex-Linked
Frank and Awilda at Breakfast Frank: Are you sure you want to wear that new shirt to work today? A green and red shirt like that would be better for Christmas, not for St. Patrick's Day. Awilda: Oh no! Not again! I hate being color blind! I really thought this shirt was just different shades of green. Where's the red? At Dinner That Night Awilda: We should try to find a way to make sure we only have sons, no daughters. I don't want to have any daughters who might be color blind and have so many problems like I do. Color blindness wouldn't matter so much for a boy.
Frank: Remember, the doctor said that, since I'm not color blind, none of our daughters would be color blind, only our sons. Awilda: That doesn't make any sense. Our daughters should be color blind like me and our sons should be normal like you. Frank: No, the doctor said the gene for color blindness is on the X chromosome, so only our sons will inherit your colorblindness. Awilda: I don't agree. Girls have more X chromosomes than boys, so girls should be more likely to be color blind.
Help Frank to explain to Awilda why the doctor was right by answering the following questions. 1. What are the genotypes of Awilda and Frank? (Since the allele for color blindness is recessive and located on the X chromosome, use the symbol X c for an X chromosome with the allele for color blindness and X C for an X chromosome with the normal allele.) Awilda:Frank: X c X C y
2. Draw the Punnett square for this couple and their children. In this Punnett Square, circle each daughter and use arrows to indicate any colorblind offspring. XX y X c C C = normal vision c = colorblind c XC XcXC Xc XC XcXC Xc Xc yXc yXc yXc y
3. Write an explanation to help Awilda understand why their daughters will not be colorblind like their mother. 4. Explain why their sons will be colorblind even though their father has normal vision. 5. Explain why having two X chromosomes decreases a person’s risk of color blindness, instead of increasing their risk, as Awilda fears.
Practice Problems Hemophilia is an X-linked recessive disease. Cross a heterozygous female with a normal male. Hemophilia is an X-linked recessive disease. Cross a heterozygous female with a normal male. Duchenne Muscular Dystrophy is an X- linked recessive disease. Cross a heterozygous female with a normal male. Duchenne Muscular Dystrophy is an X- linked recessive disease. Cross a heterozygous female with a normal male.
Examples of Sex-linked Diseases Colorblindness
D. Sex-Limited Traits A few traits are not caused by genes on the X or the Y chromosome but still occur in only one sex of animals –Examples Antlers in deer- only bucks have antlers Milk yield in bovines is a trait expressed by only cows (females) Eggs in chickens
E. Sex-Influenced Some traits are sex-influenced because of genes that interact with a substance (like hormones) that is not produced equally in males and females –Example: early pattern baldness
Baldness Sample Problem Baldness is a dominant trait. Heterozygous men are bald, BUT heterozygous women have all hair. Cross a Heterozygous woman with a normal hair male. Bb x bb b b bBGenotype - Phenotype If all girls? If all boys? B b b B bb
Human Genetic Disorders
Symptoms:learning difficulties, mental retardation, a characteristic facial appearance, and poor muscle tone Detection/ Frequency? 1 in 1000 live born infants Mode of Inheritance/ Chromosome Chromosome 21, nondisjunction TreatmentPhysical therapy for muscle weakness, heart is checked regularly for problems, educational therapy PrognosisMay have shortened life span Down Syndrome
Marfan Syndrome Symptoms:Myopia, retinal detachment, bone overgrowth and loose joints, may have long thin arms and legs, bent chest inwards or outwards Detection/ Frequency? occurring 1 in 10,000 to 20,000 individuals Mode of Inheritance/ Chromosome Autosomal dominant, Chromosome 15 TreatmentSurgery to correct skeletal problems, sight issues fixed with glasses, must avoid contact sports
Phenylketonuria (PKU) Symptoms:causes increase of phenylalanine in blood - results in mental retardation, heart problems, small head size (microcephaly) and developmental delay Detection/ Frequency? 1 in 10,000 to 1 in 15,000 newborn babies Mode of Inheritance/C hromosome TreatmentLimiting dietary intake of phenylalanine Prognosis
Symptoms: Detection/ Frequency? Mode of Inheritance/C hromosome Treatment Prognosis
Symptoms: Detection/ Frequency? Mode of Inheritance/C hromosome Treatment Prognosis
11.5 Linkage & Gene Maps Thomas Hunt Morgan, 1910 Research fruit flies Found 50+ Drosophilia genes Many of them “ linked ” together All the genes from one group were inherited together Chromosomes assort independently, not the genes
How did Mendel miss this linkage? By pure luck, the 6 genes he looked at were on different chromosomes Gene Maps Crossing-over sometimes separates genes on the same chromosomes onto homologous chromosomes. –Occasionally separate and exchange linked genes and produce new combinations
The farther apart two genes are, the more likely they are to be separated by a crossover in meiosis. Alfred Sturtevant created a gene map showing the locations of each known gene on one of the Drosophila chromosomes