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Chapter 14: Mendel and the Gene Idea. Heredity. Heredity= passing of traits from parent to offspring Reproductive cells are only way traits are passed to offspring Changes in somatic cells will not be passed to next generation Genetics= scientific study of heredity.

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  • Heredity= passing of traits from parent to offspring
    • Reproductive cells are only way traits are passed to offspring
    • Changes in somatic cells will not be passed to next generation
  • Genetics= scientific study of heredity

What genetic principles account for the passing of traits from parents to offspring?

  • The “blending” hypothesis is the idea that genetic material from the two parents blends together
    • Kind of like blue and yellow paint blend to make green
  • The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
    • Heritable factors may be rearranged, but retain identity from one generation to the next
    • This hypothesis can explain the reappearance of traits after several generations
gregor mendel
Gregor Mendel
  • Gregor Mendel, Austrian monk
    • Documented a particulate mechanism in the 1800s byexperiments with garden pea plants
  • 1860s- Studied inheritance patterns in pea plants
    • Character= heritable feature with multiple variations
    • Trait= each type of variation for a character
    • At this time, chromosomes and genes had not been discovered yet
      • Used the term heritable factors to represent what we call a gene
gregor mendel1
Gregor Mendel
  • Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
  • Advantages of pea plants for genetic study
    • Short-life cycle
    • There are many varieties with distinct characters
gregor mendel2
Gregor Mendel
  • Parents pass characterson to offspring
    • Pea plants= one character is flower color
  • Parents can pass on different traits
    • Pea plants= purple or white flower color
gregor mendel3
Gregor Mendel
  • Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
  • Advantages of pea plants for genetic study
    • Short-life cycle
    • There are many varieties with distinct heritable characters
    • Mating can be controlled
      • Each flower has sperm-producing organs (stamens) and egg-producing organ (carpel)
      • Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another
gregor mendel4

In a typical experiment, Mendel mated two different true-breeding varieties=hybridization

Gregor Mendel
  • Mendel chose to track only those characters that occurred in two distinct alternative forms
  • He also used varieties that were true-breeding in P generation
    • Produce offspring of same variety when they self-pollinate
gregor mendel5
Gregor Mendel
  • The hybrid offspring of the P generation are called the F1 generation
  • When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced
gregor mendel6
Gregor Mendel
  • F1 Generation: 100% purple flowers
  • Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation
gregor mendel7
Gregor Mendel
  • Mendel reasoned that only the purple flower factor was affecting flower color in the F1hybrids
  • The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation
  • Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
pea plant experiments
Pea Plant Experiments
  • 7 Characters in Pea Plants
  • Crossed different varieties of each character
      • Hybrids
  • Genetic Test Cross
      • P generation
      • F1 generation
      • F2 generation
  • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2offspring
mendel s hypotheses based on 4 concepts
Mendel’s Hypotheses Based on 4 Concepts
  • There are alternate forms of genes (alleles) and these account for the variation of inherited characters.
  • Offspring inherit 2 alleles for each character, one from each parent.
  • When alleles are different, one allele determines appearance (dominant allele) and other has no effect (recessive allele).
  • Mendel’s Law of Segregation: Allele pairs separate during gamete production and sex cells carry one allele for each inherited character.
1 alternate forms of genes alleles account for variation
1. Alternate forms of genes (alleles) account for variation
  • Alleles= alternative forms of genes
  • Flower Color: 2 Forms
2 offspring inherit 2 alleles for each character one from each parent
2. Offspring inherit 2 alleles for each character, one from each parent.
  • 1 chromosome with alleles for genes are inherited from mom and one from dad
  • Alleles can be the same or different
    • Homozygous- alleles same
    • Heterozygous- alleles different
3 when alleles are different dominant allele masks recessive allele
3. When alleles are different, dominant allele masks recessive allele
  • Complete Dominance
  • Dominant Alleles (uppercase letters)
    • Determines appearance of offspring if present
      • Homozygous Dominant= PP
  • Recessive Alleles (lowercase letters)
    • Masked by dominant allele
    • Only determines appearance if both alleles are recessive
    • Homozygous recessive= pp
  • Heterozygous: 1 dominant, 1 recessive= Pp
frequency of dominant alleles
Frequency of Dominant Alleles
  • Dominant alleles are not necessarily more common in populations than recessive alleles
    • For example, one baby out of 400 in the United States is born with extra fingers or toes
  • The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage
  • In this example, the recessive allele is far more prevalent in the populationthan the dominant allele
exceptions to the rule
Exceptions to the Rule

Red = RR

  • Incomplete Dominance: Expression of recessive allele not completely masked in heterozygote
    • Snapdragon flower color
  • Co-dominance: Two alleles are dominant and expressed in heterozygote
    • Example: Blood Type: AB

Pink = Rr

White = rr

4 mendel s law of segregation
4. Mendel’s Law of Segregation
  • Allele pairs separate during gamete production and sex cells carry one allele for each inherited character
  • Meiosis- haploid number of chromosomes
  • Fertilization- fusion of egg and sperm result in diploid offspring
    • Genotype= alleles present in offspring for character
      • Example: PP, pp, Pp
    • Phenotype= physical appearance as a result of alleles
      • Purple flowers, white flowers
4 mendel s law of segregation1
4. Mendel’s Law of Segregation
  • Mendel derived the law of segregation by following a single character
  • The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character
  • A cross between such heterozygotes is called a monohybrid cross
mendel s experimental crosses
Mendel’s Experimental Crosses

Cross homozygous dominant plant with homozygous recessive plant

P Generation



genotype and phenotype
Genotype and Phenotype
  • Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition
    • Phenotype=physical appearance
    • Genotype= genetic makeup
  • Flower color in pea plants:
    • PP and Pp plants have the same phenotype (purple) but different genotypes
  • Organisms with heterozygous genotype are considered “carriers” of the recessive allele
    • Not a true-breeding variety
    • Pea Plants Flower Color= Pp
  • Recessive allele is present, but not “visible” in phenotype
  • Can be passed on to next generation
punnett square
Punnett Square
  • Used to predict potential traits in offspring
    • 4 possible combinations of alleles
    • Each parent creates 2 potential gametes 2 x 2= 4
  • Need to know type of gametes produced by parent
    • Genotype= alleles for character
    • Each gamete has one allele
pea plants experiment
Pea Plants Experiment
  • Cross F1 generation to produce F2 generation
  • Phenotype: Purple Flowers
  • Genotype: Heterozygous
  • Cross Bb X Bb
  • F2 generation:
    • 1 BB (purple)
    • 2 Bb (purple)
    • 1 bb (white)
research in genetics
Research in Genetics
  • Testcross
    • Mating between unknown genotype and homozygous recessive genotype
    • Use Punnett Squares to predict offspring traits and compare to actual outcomes
mendel s law of independent assortment
Mendel’s Law of Independent Assortment
  • Mendel identified his second law of inheritance by following two characters at the same time
  • Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
  • A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
mendel s law of independent assortment1
Mendel’s Law of Independent Assortment
  • Mendel’s Law of Independent Assortment
    • Genes inherited independently of each other
    • Inheritance of one character independent from another character
  • This law applies only to genes on different chromosomes or those far apart on the same chromosome
    • Genes located near each other on the same chromosome tend to be inherited together
genetic inheritance
Genetic Inheritance
  • Mendel’s laws of segregation and independent assortment reflect the rules of probability
  • When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
    • In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
genetic inheritance1
Genetic Inheritance
  • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
  • Probability in an F1 monohybrid cross can be determined using the multiplication rule
  • Segregation in a heterozygous plant is like flipping a coin:
    • Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
genetic inheritance2
Genetic Inheritance

The addition rule= probability that any one of two (or more) exclusive events will occur is calculated by adding together their individual probabilities

Probability of heterozygote: ¼ + ¼ = ½

Probability of homozygote:

¼ + ¼ = ½

dihybrid crosses
Dihybrid Crosses
  • A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
  • Crosses with 2 characters of interest
    • RrYy x RrYy
      • In gametes, alleles for one character can be paired with either allele for other character
      • For each parents gametes, R can be paired with Y or y
      • 4 possible gamete types for each parent
        • RY, Ry, rY, ry
exceptions to the rule1
Exceptions to the Rule
  • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
  • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
exceptions to the rule2
Exceptions to the Rule
  • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
    • When alleles are not completely dominant or recessive
    • When a gene has more than two alleles
    • When a gene produces multiple phenotypes
exceptions to the rule3
Exceptions to the Rule

Red = RR

  • Incomplete Dominance: Expression of recessive allele not completely masked in heterozygote
    • Snapdragon flower color

Pink = Rr

White = rr

exceptions to the rule4
Exceptions to the Rule
  • Co-dominance: Two alleles are dominant and expressed in heterozygote
  • This occurs in blood types, which also has more than 2 alleles
    • Four phenotypes: A, B, O, AB
    • A and B codominant, both are dominant to O
  • Determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i
  • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate
  • Enzyme encoded by the i allele adds neither
  • Most genes have multiple phenotypic effects, a property called pleiotropy
  • For example, pleiotropic alleles are responsible for the multiple symptoms of some hereditary diseases
    • Cystic fibrosis
    • Sickle-cell disease
sickle cell disease
Sickle-cell disease
  • The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
    • Causes cell to collapse and form sickle shape
    • Clogs blood vessels
  • In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
  • Symptoms include physical weakness, pain, organ damage, and even paralysis
sickle cell disease1
Sickle-Cell Disease

Normal Blood Cell

  • Heterozygotes= produce both types of blood cells
    • Heterozygotes have sickle-cell trait
      • Usually healthy but may suffer some symptoms of disease
  • Sickle-cell disease affects one out of 400 African-Americans
    • Unusual to have high frequency of an allele with detrimental effects in homozygotes


sickle cell disease2
Sickle-Cell Disease

Normal Blood Cell

  • Reason: “Heterozygote Advantage”
    • Less susceptible to the malaria parasite, so there is an advantage to being heterozygous in areas where malaria is common
    • Keeps allele in population despite detrimental effects of homozygous condition


  • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
  • In Labrador retrievers and many other mammals, coat color depends on two genes
    • One gene determines the pigment color (with alleles for black or brown)
    • The other gene (with alleles for color or no color) determines whether the pigment will be deposited in the hair
polygenic inheritance
Polygenic Inheritance
  • Quantitative characters are those that vary in the population along a continuum
  • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
  • Skin color in humans is an example of polygenic inheritance
environmental variation in phenotypes
Environmental Variation in Phenotypes
  • Another exception arises when phenotype of a character depends on environment as well as genotype
    • Multifactorial characters: genetic and environmental factors collectively influence phenotype
  • Norm of reaction: phenotypic range of a genotype influenced by the environment
    • Usually broadest for polygenic characters
  • Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
human genetics
Human Genetics
  • Humans are not good subjects for genetic research
    • Generation time is too long
    • Parents produce relatively few offspring
    • Breeding experiments are unacceptable
  • However, basic Mendelian genetics still serves as the foundation of human genetics
genetic traits in humans
Genetic Traits in Humans

Pedigree= Family tree of genetic traits for multiple generations

  • Pedigrees can also be used to make predictions about future offspring
  • We can use the multiplication and addition rules to predict the probability of specific phenotypes
  • Many genetic disorders are inherited in a recessive manner
    • Recessively inherited disorders show up only in individuals homozygous for the allele
      • Carriersare heterozygous individuals with 1 copy of recessive allele but are phenotypically normal
      • Most individuals with recessive disorders are born to carrier parents
  • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
human genetics1
Human Genetics
  • For a growing number of diseases, tests are available that identify carriers and help define the odds of offspring having disease more accurately
    • Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components, making predictions for multifactorial diseases difficult
  • Now that the human genome has been described, research is focusing on identifying mutations in sequence and how they affect health
    • Advances in understanding the human genome will potentially lead to new medicines, treatments, and preventive care
current genetic research
Current Genetic Research
  • Breast Cancer
    • BRCA1 and BRCA2- human genes part of a group of genes called tumor suppressors
      • Stabilize DNA and prevent uncontrolled growth
    • Mutations in these genes increase risk of hereditary breast and ovarian cancers
    • Mutations can be found in men and women and may increase risk of other types of cancers
    • Blood test can detect genetic mutation
      • Not all mutations are harmful, many types are neutral
current genetic research1
Current Genetic Research
  • Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders
    • Individuals can evaluate chances of development of genetic disorders present in close family members
    • Potential parents with genetic disorders can determine the chances of passing genetic disorder on to offspring
current genetic research2
Current Genetic Research
  • Punnett Square: Predicting the chance of passing genetic disorder on to offspring
    • If a woman has a blood test and finds she has a recessive mutation for a genetic disorder (heterozygous), what are the chances of her offspring having the mutation if her husband is homozygous dominant?



Genotype Probability:

50% AA (no mutation)

50% Aa (mutation carrier)



  • In a large population, chance of mating with a relative is low because there are many choices
    • Most societies and cultures have restrictions against marriages between close relatives
  • Sexual reproduction between non-related individuals increases or maintains high genetic diversity
  • In a small population, chance of mating with a relative is high because there are few choices
    • Increases frequency of deleterious recessive genotypes
    • Exposes rare deleterious mutations
example white tigers
Example: White Tigers
  • White coloration in Bengal tigers
    • 1 in 15,000 wild births
  • Homozygous recessive condition (aa)
  • In 1951, one white male tiger was captured in the wild
  • Most white tigers in zoos are descendants of this original tiger
  • Use a Punnett Square to find probabilities of offspring from the following matings:
  • White male (aa) x Orange female (AA)
  • White male (aa) x Orange daughter (Aa)
  • White male (aa) x White daughter (aa)





example white tigers1
Example: White Tigers
  • Over time, repeated matings between close relatives to produce white tiger decreased genetic diversity and evidence of inbreeding depression was observed
    • Birth defects and low reproductive success
    • Weak immune system- more susceptible to disease
    • Poor eyesight and hearing
    • Weak bones
  • In 2011, American Zoological Associationbanned breeding of white tigers