1. Mendel and the Gene Idea
Chapter 14
Mendel and the Gene Idea

2. Overview: Drawing from the Deck of Genes
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 (like blue and yellow paint blend to make green)
The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
Mendel documented a particulate mechanism through his experiments with garden peas
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3. Concept 14.1: Mendel Pea Experiments
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.
Advantages of pea plants for genetic study:
There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits
Mating of plants can be controlled
Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
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4. Figure A genetic cross
For his experiments, Mendel chose to CROSS POLLINATE (mate different plants to each other) plants that were TRUE BREEDING (meaning if the plants were allowed to self-pollinate, all their offspring would be of the same variety).
P generation – parentals; true-breeding parents that were cross-pollinated
F1 generation – (first filial) - hybrid offspring of parentals that were allowed to self-pollinate
F2 generation – (second filial) - offspring of F1’s

5. Key Terminology
Mendel chose to track only those characters that varied in an either-or manner
He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)
In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization
The true-breeding parents are the P generation
The hybrid offspring of the P generation are called the F1 generation
When F1 individuals self-pollinate, the F2 generation is produced
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6. The Law of Segregation
When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation
So where did the trait for the white flower go?
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7. What Mendel called a “heritable factor” is what we now call a gene
Mendel’s Reasoning
Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits
What Mendel called a “heritable factor” is what we now call a gene
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8. Table 14-1

9. Mendel’s Model – 4 Big Ideas
Alternative versions (different alleles) of genes account for variations in inherited characters.
For each character, an organism inherits two alleles, one from each parent.
If the two alleles differ, the dominant allele is expressed in the organism’s appearance, and the other, a recessive allele is masked.
(Law of Dominance)
Allele pairs separate during gamete formation. This separation corresponds to the distribution of homologous chromosomes to different games in meiosis.
(Law of Segregation)
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10. Fig. 14-4
Figure 14.4
The gene for a particular inherited character, such as color, resides at a specific locus (position) on a certain chromosome.
Alleles are variants of that gene. In the case of peas, the flower-color gene exists in two versions: the allele for purple flowers and the allele for white flowers.
This homologous pair of chromosomes represents an F1 hybrid, which inherited the allele for purple color from one parent and the allele for white flowers from the other parent.

11. MENDEL’S LAW OF SEGREGATION – TEXT PAGE 287:
Two alleles for each character separate during gamete formation. If the parent has two of the same alleles, then the offspring will all get that version of the gene.
But, if the parent has two different alleles for a gene, each offspringhas a 50% chance of getting one of the two alleles.

12. Figure 14.4 Mendel’s law of segregation (Layer 2)
The LAW OF SEGREGATION states that during the formation of gametes, the two traits carried by each parent separate.
Parent cell with full gene and Tt alleles.
Traits have separated during gamete formation from meiosis.

13. Mendel’s Law of Independent Assortment
States that each allele pairs of different genes segregates independently during gamete formation;
applies when genes for two characteristics are located on different pairs of homologous chromosomes
this means that, for example, a person having BROWN hair does NOT influence the likelihood of them getting BROWN eyes.
USEFUL ANIMATION:
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14. Mendel’s Law of Independent Assortment
TEXT PAGE 287:
Each pair of alleles from a homologous chromosome separate independently during gamete formation. The chance of being “G” does not influence the chance of being “Y” if these genes are located on separate chromosomes!
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15. Punnett Squares
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele
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16. Useful Genetics Vocabulary
An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character
Example: TT or tt for tall trait
An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character
Example: Tt for tall trait
Unlike homozygotes, heterozygotes are not true-breeding…they are HYBRIDS!
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17. Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous)
Fig. 14-6
Phenotype
Genotype PP
Purple 1
(homozygous) 3
Purple Pp
(heterozygous)
Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes 2
Purple Pp
(heterozygous) pp 1
White 1
(homozygous)
Ratio 3:1
Ratio 1:2:1

18. The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous
Why can’t we use homozygous dominant individuals for test crosses?
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19. TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp?
Fig. 14-7
TECHNIQUE 
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype: pp
Predictions
If PP
If Pp or
Sperm
Sperm
Figure 14.7 The testcross p p p p P P Pp Pp Pp Pp
Eggs
Eggs P p Pp Pp pp pp
RESULTS or
All offspring purple
1/2 offspring purple and
1/2 offspring white

20. Extending Mendelian Genetics for a Single Gene
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
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21. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
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22. P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW
Fig
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
Pink
F1 Generation
CRCW
Figure Incomplete dominance in snapdragon color
Gametes
1/2 CR
1/2 CW
Sperm
1/2 CR
1/2 CW
F2 Generation
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

23. Codominance

24. 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 than the population’s dominant allele
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25. Most genes exist in populations in more than two allelic forms
Multiple Alleles
Most genes exist in populations in more than two allelic forms
For example, the four phenotypes of the ABO blood group in humans are 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; the enzyme encoded by the i allele adds neither
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26. Red blood cell appearance Phenotype (blood group)
Fig
Allele
Carbohydrate IA A IB B i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Red blood cell
appearance
Phenotype
(blood group)
Genotype
The letters A, B, and O refer to 2 carbohydrates found on the surfaced of RED BLOOD CELLS.
Will often see the A,B designation as superscripts with a base of I;
O (since is recessive to A and B) is shown as i.
Matching compatible blood groups is critical – proteins called antibodies are produced against foreign blood factors.
Antibodies bind to foreign molecules and cause donated blood cells to clump together (agglutination).
IAIA or IA i A
IBIB or IB i B
IAIB AB ii O
(b) Blood group genotypes and phenotypes

29. Most genes have multiple phenotypic effects
Pleiotropy
Most genes have multiple phenotypic effects
The ability of a gene to affect an organism in many ways is called PLEIOTROPHY
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
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30. Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
Sickle cell is a disease caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells. When oxygen concentration of affected individual is low, the hemoglobin crystallizes into long rods.
Heterozygotes for sickle cell have increased resistance to malaria because the rod shape of blood interrupts the parasites life cycle. So, sickle cell is prevalent among African Americans.

31. Extending Mendelian Genetics for Two or More Genes
Some traits may be determined by two or more genes
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
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32. Fig 
BbCc
BbCc
Sperm
1/4
1/4
1/4
1/4 BC bC Bc bc
Eggs
1/4 BC
BBCC
BbCC
BBCc
BbCc
Figure An example of epistasis
For example, in mice and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
1/4 bC
BbCC
bbCC
BbCc
bbCc
1/4 Bc
BBCc
BbCc
BBcc
Bbcc
1/4 bc
BbCc
bbCc
Bbcc
bbcc 9
: 3
: 4

33. 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
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34. Fig 
AaBbCc
AaBbCc
Sperm
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
Figure A simplified model for polygenic inheritance of skin color
Additive effect of two or more genes on a single phenotypic character
1/8
Eggs
1/8
1/8
1/8
1/8
Phenotypes:
1/64
6/64
15/64
20/64
15/64
6/64
1/64
Number of
dark-skin alleles: 1 2 3 4 5 6

35. Concept 14.4: Patterns of Human Inheritance
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 endures as the foundation of human genetics
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36. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees
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37. (a) Is a widow’s peak a dominant or recessive trait?
Fig b
1st generation
(grandparents) Ww ww ww Ww
2nd generation
(parents, aunts,
and uncles) Ww ww ww Ww Ww ww
3rd generation
(two sisters)
WIDOW’S PEAK (dominant trait)
This pedigree traces the trait called widow’s peak through three generations of a family.
Notice in the 3rd generation that the second-born daughter lacks a widow’s peak, although both of her parents had the trait.
Such a pattern of inheritance supports the hypothesis that the trait is due to a DOMINANT allele.
If the trait were due to a recessive allele, and both parents had the recessive phenotype, then ALL of their offspring would also have the recessive phenotype. WW ww or Ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?

38. (b) Is an attached earlobe a dominant or recessive trait?
Fig c
1st generation
(grandparents) Ff Ff ff Ff
2nd generation
(parents, aunts,
and uncles) FF or Ff ff ff Ff Ff ff
3rd generation
(two sisters)
ATTACHED EARLOBES (recessive trait)
This is the same family, but in this case we are tracing the inheritance of a recessive trait, attached earlobes.
Notice that the first-born daughter in the 3rd generation has attached earlobes, although both of her parents lack that trait (they have free earlobes).
Such a pattern is easily explained if the attached-lob phenotype is due to a RECESSIVE allele.
If it were due to a dominant allele, then at least one parent would also have had the trait. ff FF or Ff
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?

39. Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)
Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
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40. Parents Normal Normal Aa Sperm A a Eggs Aa AA A Normal (carrier)
Fig
Parents
Normal
Normal Aa  Aa
Sperm A a
Figure Albinism: a recessive trait
Eggs Aa AA A
Normal
(carrier)
Normal Aa aa a
Normal
(carrier)
Albino

41. Cystic Fibrosis (Recessive Disorder)
Cystic fibrosis is the most common lethal genetic disease in the United States striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
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42. Sickle-Cell Disease (Recessive Disorder)
Sickle-cell disease affects one out of 400 African-Americans
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include physical weakness, pain, organ damage, and even paralysis
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43. Dominantly Inherited Disorders
Some human disorders are caused by dominant alleles
Dominant alleles that cause a lethal disease are rare and arise by mutation
Achondroplasia is a form of dwarfism caused by a rare dominant allele
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44. Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd
Fig
Parents
Dwarf
Normal Dd  dd
Sperm
Figure Achondroplasia: a dominant trait D d
Eggs Dd dd d
Dwarf
Normal Dd dd d
Dwarf
Normal

45. Huntington’s Disease (Dominant Disorder)
Huntington’s disease is a degenerative disease of the nervous system
The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
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46. Fetal Testing for Genetic Disorders
In amniocentesis, the liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
Technology is Providing New Tools for Genetic Testing and Counseling
Carrier recognition with genetic screening and Fetal testing:
-ultrasound and sonograms
-amniocentesis
-chorionic villi sampling
-fetoscopy
-blood/urine tests of newborns
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47. (b) Chorionic villus sampling (CVS)
Fig
Amniotic fluid
withdrawn
Centrifugation
Fetus
Fetus
Suction tube
inserted
through
cervix
Placenta
Placenta
Chorionic
villi
Uterus
Cervix
Fluid
Bio-
chemical
tests
Figure Testing a fetus for genetic disorders
Fetal
cells
Several
hours
Fetal
cells
Several
hours
Several
weeks
Several
weeks
Several
hours
Karyotyping
(a) Amniocentesis
(b) Chorionic villus sampling (CVS)

48. Probabilities Practice
What is the probability that the genotype Aa will be produced by the parents Aa x Aa?
What is the probability that the genotype ccdd will be produced by the parents CcDd x CcDd?
1/16
What is the probability that the genotype Rr will be produced by the parents Rr x rr?
What is the probability that the genotypes TTSs will be produced by the parents TTSs x TtSS?
1/4

49. Genetics Practice Problems
How many unique gametes could be produced through independent assortment by an individual with the genotype AaBbCCDdEE? 8
What is the expected genotype ratio for a dihybrid heterozygous cross?
9:3:3:1
2x2x1x2x1 = 8

50. You should now be able to:
Define the following terms: true breeding, hybridization, monohybrid cross, P generation, F1 generation, F2 generation
Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype
Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F2 generation
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51. Define and give examples of pleiotropy and epistasis
Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance
Define and give examples of pleiotropy and epistasis
Explain why lethal dominant genes are much rarer than lethal recessive genes
Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling
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