1. Mendel and the Gene Idea
Chapter 14
BCOR 012
January 20, 22, and 25, 2010

2. Mendel and the Gene Idea Chapter 14
I. Mendel and his contribution to biology
A. A brief biography of Mendel
B. Why his work was revolutionary
C. Mendel’s organism
D. Brief review of meiosis
E. So what did Mendel do and why is it
important?
F. Mendel's First Law of Inheritance, the law of segregation
G. Dihybrid Crosses and Independent Assortment

3. Gregor Mendel,

5. (Pisum sativum - Fabaceae)
Garden pea
(Pisum sativum - Fabaceae)

7. Mendel and the Gene Idea Chapter 14
I. Mendel and his contribution to biology
A. A brief biography of Mendel
B. Why his work was revolutionary
C. Mendel’s organism
D. Brief review of meiosis
E. So what did Mendel do and why is it
important?
F. Mendel's First Law of Inheritance, the law of segregation
G. Dihybrid Crosses and Independent Assortment

8. Each cell in the leaf of a pea plant has seven pairs of chromosomes
Each cell in the leaf of a pea plant has seven pairs of chromosomes. How many chromosomes in the egg cell? 14 28 7 21
The question cannot be answered with the information given

9. A short review of meiosis ...

10. Most organisms, including ourselves and garden peas, are
diploid. That means they have two sets of chromosomes per
cell.
One of these sets came from the mother, via the egg, and the
other came from the father, via the sperm.

11. Meiosis is also called
reductional division
because the number of
chromosomes in each
cell is reduced by half.
During meiosis, members
of the two sets separate
into separate cells.

16. Mendel’s Explanation of his Observations (in present-day language):
1) Alternative versions of genes (that is to say, different
alleles) account for variation in inherited characters.
2) For each character, an organism inherits two alleles,
one from each parent.
3) If an organism is heterozygous at a particular gene
locus, the organism’s appearance (phenotype) will be
determined by the dominant allele.
4) The two alleles for each character segregate during
gamete production.

17. This fourth point is also called Mendel’s Law of Segregation:
Alleles segregate into separate gametes during meiosis.

18. Some useful Genetic Vocabulary:
Homozygous: an individual having an identical pair of alleles at a locus is homozygous at that locus.
(example: PP )
Heterozygous: an individual having different alleles at a locus is heterozygous at that locus
(example: Pp)

19. P - purple color
(dominant allele)
p - white color
(recessive allele)
Note that the probability
of a white-flowered plant
in the F2 generation is
0.25, or 25%
(This is a
Punnett square.)

20. When solving a problem involving a cross in Mendelian genetics:
First ask yourself: what kind of gamete (genotype) can each parent produce?
And in what proportion will each be produced?
Then figure out all possible ways in which these gametes can combine at syngamy
Note that a Punnet square is one way to approach the problem, but not necessarily the most efficient way.

21. Here a Punnet square
has been used to
solve the problem
Another way to approach the problem:
Half (0.5) of the heterozygous mother’s eggs carry the P allele and half
carry the p allele.
Similarly, half of the heterozygous father’s sperm cells carry the P allele
and half carry the p allele.
Thus the progeny will include 0.5 P x 0.5 P = 0.25 PP, (0.5 P x 0.5 p)+ (0.5 P x 0.5 p) = 0.5 Pp, and 0.5 p x 0.5 p, = 0.25 pp

22. More useful Genetic Vocabulary:
Genotype: The specific genetic makeup of an organism, as contrasted with the actual characteristics of an organism (its phenotype).
Phenotype: The observable characteristics of an organism, as opposed to the set of genes it possesses (its genotype).

24. A testcross: a pea
plant showing the
dominant phenotype
- but whose genotype
is unknown - is bred
to a pea plant showing
the recessive pheno-
type.
The results permit the
unknown parent’s
genotype to be
determined.

25. Mendel and the Gene Idea Chapter 14
I. Mendel and his contribution to biology
A. A brief biography of Mendel
B. Why his work was revolutionary
C. Mendel’s organism
D. Brief review of meiosis
E. So what did Mendel do and why is it
important?
F. Mendel's First Law of Inheritance, the law of segregation
G. Dihybrid Crosses and Independent Assortment

27. Mendel’s Law of Independent Assortment:
Pairs of alleles segregate independently during meiosis.

28. Mendel and the Gene Idea Chapter 14
II. The Relationship Between Genotype and Phenotype
A. Incomplete Dominance
B. Codominance
C. Pleiotropy: Epistasis
D. Polygenic traits
E. Environmentally Induced Variation
III. Pedigree Analysis in Humans
A. Recessive alleles
B. Dominant alleles
IV. Tools for detection of genetic disorders
A. Pedigree analysis and risk assessment
B. Amniocentesis
C. Chorionic villus sampling

29. The Relationship Between Genotype and Phenotype

30. The inheritance of flower color in snapdragons provides
a useful example of
incomplete dominance.
In incomplete dominance,
a heterozygous genotype
creates an intermediate
phenotype.

31. Human Blood Types
In the human population, there are three alleles determining blood groups: IA IB i
Of course, any given individual will possess only one
(if homozygous at the blood group locus) or two
(if heterozygous at that locus) of these. Your genotype
at the blood group locus determines your blood type.

32. Human Blood Types
Phenotype Genotype
Type A IA IA or IA i
Type B IB IB or IB i Type AB IA IB
Type O ii
Human blood group alleles IA and IB demonstrate codominance.
In codominance, neither phenotype is dominant. Instead, the individual expresses both phenotypes.

33. Epistasis is the condition in which the genotype
at one locus affects the expression of the genotype
at another locus.

34. Coat color in mice: an example of epistatic interaction between
two loci.
Locus C determines
whether pigment is
deposited or not
Locus B determines
coat color, but its
expression depends on the
genotype at the C locus.

35. < water level
The submersed and emersed leaves of water marigold (Megalodonta beckii, Asteraceae) demonstrate an environmental contribution to the phenotype.

36. Mendel and the Gene Idea Chapter 14
II. The Relationship Between Genotype and Phenotype
A. Incomplete Dominance
B. Codominance
C. Pleiotropy: Epistasis
D. Polygenic traits
E. Environmentally Induced Variation
III. Pedigree Analysis in Humans
A. Recessive alleles
B. Dominant alleles
IV. Tools for detection of genetic disorders
A. Pedigree analysis and risk assessment
B. Amniocentesis
C. Chorionic villus sampling

37. Human Pedigree Analysis: the Conventions

38. Pedigree Analysis of a Benign Condition in Humans: the
Attached Earlobe I II
III
Q. Is the trait inherited as a dominant or a recessive condition?
A. Recessive. You can tell because the affected daughter in generation III was born to unaffected parents.

39. For those traits exhibiting recessive gene action:
affected individuals may be born to unaffected parents
the trait is not manifested in every generation
The pedigree of a family with cystic fibrosis, a recessively inherited disorder.

40. The pedigree of a family with Huntington’s disease, a dominantly inherited disorder
For those traits exhibiting dominant gene action:
affected individuals have at least one affected parent
the phenotype generally appears every generation
two unaffected parents only have unaffected offspring

41. Woody Guthrie, American folk hero and composer of “This Land is
His son, the 60s legend Arlo
Guthrie, did not inherit the disease.
Woody Guthrie, American folk hero and composer of “This Land is
Your Land,” died of Huntington’s disease in 1967.

42. Mendel and the Gene Idea Chapter 14
II. The Relationship Between Genotype and Phenotype
A. Incomplete Dominance
B. Codominance
C. Pleiotropy: Epistasis
D. Polygenic traits
E. Environmentally Induced Variation
III. Pedigree Analysis in Humans
A. Recessive alleles
B. Dominant alleles
IV. Tools for detection of genetic disorders
A. Pedigree analysis and risk assessment
B. Amniocentesis
C. Chorionic villus sampling

44. Know the Difference: meiosis vs. mitosis gene vs. allele
genotype vs. phenotype
homozygous vs. heterozygous
dominant vs. recessive
monohybrid vs. dihybrid

45. Solving Problems in Mendelian Genetics
A corn plant of genotype XxYyZz is crossed with a plant of
genotype XXYyzz. Assume the genes assort independently.
Of their offspring, what proportion will:
a. be heterozygous at all three loci? 1/2 X 1/2 X 1/2 = 1/8
b. be homozygous at all three loci? 1/2 X 1/2 X 1/2 = 1/8
c. be homozygous for the dominant
allele at all three loci? 1/2 X 1/4 X 0 = 0
d. have the genotype XXYyZz? 1/2 X 1/2X 1/2 = 1/8