1. CHAPTER 11 Mendelian Genetics Text authored by Dr. Peter J. Russell
Slides authored by Dr. James R. Jabbur
CHAPTER 11
Mendelian Genetics

2. Drawing from the Deck of Genes
The question was asked: what genetic principles account for the passing of traits from parents to offspring? Two hypotheses emerged…
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 by breeding garden peas in carefully planned experiments (a very, very clever man indeed!)

3. Irony…
Figure 11.2 Gregor Johann Mendel, founder of the science of genetics.

4. Mendel’s experimental design
There are advantages of using pea plants for genetic study:
There are many varieties with distinct heritable features, or characters (such as flower color) & character variants, or traits (such as purple or white flowers)
The mating of plants can be controlled: cross-pollination can be achieved by dusting one plant with pollen from another
Mendel chose to track only those characters that varied in an either/or manner and used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)

5. The true-breeding parents are the P generation
In a typical experiment, Mendel mated two contrasting (white vs. purple flower color), true-breeding varieties in a process called hybridization
The true-breeding parents are the P generation
Seeds were produced from the mating and planted
The hybrid offspring are called the F1 generation
Further self-pollination of F1 individuals produces an F2 generation (and so on…)
Figure14.2 Crossing pea plants

6. Monohybrid Cross
When Mendel crossed contrasting, true-breeding parental smooth seed and wrinkled seed pea plants, all of the F1 hybrids were smooth (as shown in the previous slide)
When Mendel crossed the F1 hybrids, F2 plants with smooth seeds consistently outnumbered the F2 plants with wrinkled seeds at a ratio of 3:1

7. To support his hypothesis, Mendel then observed the same pattern of inheritance in other pea plant characters, each represented by two traits (i.e. purple vs. white flower color)
What Mendel called a “heritable factor” is what we now call a gene

8. The Model and the Principle of Segregation
The first concept is that alternative versions of genes account for variations in inherited characters. For example, the gene for flower color in pea plants exists in two versions, purple or white
These alternative versions of a gene are called alleles, residing at a specific locus on a specific chromosome
Allele for purple flowers
Homologous
pair of
chromosomes
Locus for flower-color gene
Allele for white flowers
Animation: Mendel’s Principle of Segregation

9. The second concept is that for each character, an organism inherits two alleles (one from each parent)
The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation. Alternatively, the two alleles at a locus may differ, as in the F1 hybrids
The third concept is that if the two alleles at a locus differ, the dominant allele determines the organism’s appearance, while the recessive allele has no noticeable effect on appearance. (In the previous flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant)

10. The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate during gamete formation. Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism
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. In this tool, a capital letter represents a dominant allele (S), and a lowercase letter represents a recessive allele (s)
(see next slides)

11. Figure 11. 8 The same cross as in Figures 11. 5 and 11
Figure 11.8 The same cross as in Figures 11.5 and 11.6, using genetic symbols to illustrate the principle of segregation of Mendelian factors. (a) Production of the F1 generation. (b) Production of the F2 generation.

12. Or… Self-fertilize 3 1 P Generation Appearance: Purple flowers
White flowers
Genetic makeup: PP pp
Gametes: P p
Or…
F1 Generation
Self-fertilize
Appearance:
Purple flowers
Genetic makeup: Pp
Gametes: P or p
Sperm
Figure 14.5 Mendel’s law of segregation
F2 Generation P p P PP Pp
Eggs p Pp pp 3 1

13. The branch diagram is an alternative approach to predicting the outcome of crosses
Figure 11.9 Using the branch diagram approach to calculate the ratios of phenotypes in the generation of the cross in Figure 11.8.

14. Phenotypes and Genotypes
An organism with two identical alleles is said to be homozygous for the gene controlling that character. An organism with two different alleles for a gene is said to be heterozygous. Heterozygotes are not true-breeding.
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 (physical appearance) and its genotype (genetic makeup)
In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes (see next slide)

15. Hereditary traits are under the control of genes (Mendel called them factors).
2. Genotype is the genetic makeup of an organism, a description of the genes it contains.
3. Phenotype is the characteristics that can be observed in an organism.
4. The phenotype is determined by the interaction of genes and the environment
Figure 11.1 Influences on the physical manifestation (phenotype) of the genetic blueprint (genotype): interactions with other genes and their products (such as hormones) and with the environment (such as nutrition).

16. Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous)2
Purple Pp
(heterozygous)
Figure 14.6 Phenotype versus genotype pp 1
White 1
(homozygous)
Ratio 3:1
Ratio 1:2:1

17. Testcrosses affirm Segregation
How can you tell the genotype of an individual with the dominant phenotype?
Such an individual must have one dominant allele, but the other allele could be dominant (homozygous) or recessive (heterozygous).
The answer is to carry out a testcross. Breed the mystery individual with a known homozygous recessive individual (talkin bout plants, yo)
If any offspring display the recessive phenotype, the mystery parent must be heterozygous

18. Test Cross Results Two possible ratios:
If the offspring are 100% dominant, the parent is homozygous
If 50% of the offspring have a dominant phenotype and 50% have a recessive phenotype, the parent is heterozygous
Figure Determining the genotypes of the F2 generation smooth seeds (Parent 1) of Figure 11.8 by testcrossing plants grown from the seed with a homozygous recessive wrinkled (ss) strain (Parent 2). (a) If Parent 1 is SS, then all progeny seeds are smooth. (b) If Parent 1 is Ss, then one-half of the progeny seeds are smooth and one-half are wrinkled.

19. unknown known TECHNIQUE RESULTS Dominant phenotype, unknown genotype:
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype: pp
Predictions
If pp
If pp or
Sperm
Sperm
If:
Homozygous
Dominant
Genotype
(PP)
If:
Heterozygous
Genotype
(Pp) p p p p P P Pp Pp Pp Pp
Eggs
Eggs
Figure 14.7 The testcross P p Pp Pp pp pp
RESULTS or
All offspring purple
1/2 offspring purple and
1/2 offspring white

20. Mendel’s Law of Independent Assortment
Mendel derived the law of segregation by following a single character. The F1 offspring produced in this cross were monohybrids, (individuals heterozygous for one character)
Mendel identified the law of independent assortment by following two characters at the same time. He crossed 2 true-breeding parents, differing in two characters, producing dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross can determine whether two characters are linked on the same chromosome or not. (This concept is called Linkage)
Animation: Mendel’s Principle of Independent Assortment

21. Linked Unlinked Genes Genes (YR or yr) Dihybrid cross shown
P Generation
YYRR
yyrr
Mate homozygous dominant with
homozygous recessive…
Gametes YR  yr
Produce heterozygous offspring
that you mate with each other…
F1 Generation
YyRr
Hypothesis of
dependent
Assortment
(linkage)
Hypothesis of
independent
Assortment
(non-linkage)
Predictions
Predicted
offspring of
F2 generation
Sperm or
1/4 YR
1/4 Yr
1/4 yR
1/4 yr
Sperm
1/2 YR
1/2 yr
1/4 YR
Linked
Genes
(YR or yr)
YYRR
YYRr
YyRR
YyRr
Unlinked
Genes
1/2 YR
YYRR
YyRr
1/4 Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Eggs
Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?
1/2 yr
YyRr
yyrr
1/4 yR
YyRR
YyRr
yyRR
yyRr
3/4
1/4
1/4 yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9:3:3:1
(see next slide)
315
108
101 32
Phenotypic ratio is approximately 9:3:3:1

22. another look at genotypes
Figure 11.12a The principle of independent assortment in a dihybrid cross. This cross, actually done by Mendel, involves the smooth, wrinkled and yellow, green character pairs of the garden pea. (a) Production of the F1 generation. (b) The F2 genotypes and 9:3:3:1 phenotypic ratio of smooth, yellow : smooth, green : wrinkled, yellow : wrinkled, green, derived by using the Punnett square. (Note that, compared with previous figures of this kind, only one box is shown in the F1 instead of four. This is because only one class of gametes exists for Parent 2 and only one class for Parent 1. Previously, we showed two gametes from each parent, even though those gametes were identical.)
another look at genotypes

23. Branch diagram of a dihybrid cross
Figure Using the branch diagram approach to calculate the F2 phenotypic ratio of the cross in Figure

24. Thus, the law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes
When genes are linked together on the same chromosome they can not independently assort from each other. (in other words, they are inherited together)

25. Trihybrid cross
Crosses involving three independently assorting character pairs are a trihybrid
There are 64 possible combinations of the eight
different gamete types contributed by each parent, creating 27 different genotypes in the F2
There will be eight different phenotypes, in a predicted ratio of 27:9:9:9:3:3:3:1

26. Branch diagram of a trihybrid cross
Figure Branch diagram derivation of the relative frequencies of the eight phenotypic classes in the F2 of a trihybrid cross.

27. Statistical Analysis of Genetic Data
Data is presented as a null hypothesis (where the observed and predicted results are considered the same)
The Chi Square Test (χ2 ) checks for goodness of fit between the predicted and observed results to determine whether differences are owed due to chance
If the discrepancy is not due to chance, the “null hypothesis” must be rejected and the experiment validated
Equation to calculate chi-square: χ2 = ∑ d2/e
χ2 = chi-square value
∑ = sum of
d = deviation; difference between the observed & expected (o-e)
e = expected
Degrees of freedom (df) in a test involving n classes = n-1
Chi Square analysis does not validate a hypothesis, it validates the significance of the data! (next slide)

29. 3.43
A P value of less than 5% (0.05) indicates a poor fit, results are rejected

30. Mendelian Genetics in Humans
Pedigree analysis is the study of the phenotypic records of a family over several generations
Commonly used pedigree symbols are shown on the right
Figure Photographs of (a) normal hands and (b) hands with brachydactyly.

31. Recessive Human Genetic Traits
Recessive human genetic traits are common in the human population and are usually the result of a mutation causing the loss or modification of a gene product
Most affected individuals have 2 normal looking parents who are both heterozygotes (carriers)
The trait frequently skips a generation
The harmful recessive gene is masked by the normal dominant allele
Mating between the 2 heterozygotes (“carriers”) produces 1/4th progeny with recessive traits.
When both parents are affected all progeny exhibit the trait

33. Dominant Human Genetic Traits
Dominant traits are also well documented
A mutation may produce a dominant phenotype by causing a function to be gained because of an altered gene product capable of a new activity
Characteristics of the dominant inheritance of a relatively rare trait include:
Affected individuals have at least one affected parent (Aa or in very vare cases, AA)
The trait is present in every generation (has to be!)
Offspring of an affected heterozygote will be:
½ affected and ½ wild-type

35. Autosomal recessive Autosomal dominant Albinism Cystic fibrosis
Tay-Sachs disease
Sickle cell anemia
alkaptonuria
Autosomal dominant
Autosomal dominant polycystic kidney disease
Achondroplasia
Brachydactyly
Marfan syndrome
Huntington’s disease