1. Genetics Part I: Introduction
Nestor T. Hilvano, M.D., M.P.H.
(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain, Textbook, 2012.)

2. Learning Objectives Define key terminologies.
- alleles, dominant, recessive, homozygous, heterozygous, genotype, phenotype, P generation, F1 generation, and F2 generation
Give the translation of the following word stems.
– co-, di-, pleio-, poly-,gen-, and gamet-
3. Define monohybrid cross and illustrate an example.
4. Explain how Mendel’s law of segregation describes the inheritance of a single characteristic.
5. Explain how Mendel’s law of independent assortment applies to dihybrid cross. Illustrate an example of dihybrid cross.
6. Discuss how a testcross is performed to determine the genotype of an organism.

3. Wordstems co- together di- two pleio-more poly- many gen- produce
gamet- wife or husband

4. Science of Genetics
Blending hypothesis – characteristics from both parents blend in the offspring (like blue and yellow paint blend to make green)
Particulate hypothesis - is the idea that parents pass on discrete heritable units (genes)
- Gregor Mendel documented this through experiments
with garden peas; Father of modern genetics
Genetics:
- Trait (physical appearance specified by genes);
Ex: eye color, shape of thumb, height
- Types of Trait (set of genetic trait for each form);
Ex: Eye color= brown eyes(B) and blue eyes (b);
Thumb’s shape= straight thumb (H) and hitchhiker’s thumb (h)
- Heritable trait= passed from parent to offspring

5. Mendel’s Genetics Self-fertilization developed true-breeding varieties
Cross-fertilization between 2 different plants produced hybrids (genetic cross)
Hypothesized that there are alternative forms of genes for specific trait= Alleles
Each of us has 2 alleles- ex: (BB, bb, Bb);(HH, hh, Hh)
P generation= True-breeding (parents)
F1 generation= hybrid offspring
F2 generation= product of F1 cross
Monohybrid cross-1 trait (ex: flower color: purple and white)
Dihybrid cross- 2 traits (flower color: purple and white, and seed color: yellow and green)

6. Monohybrid Cross: Law of Segregation
Pair of genes segregate during formation of gametes; the genes pairs once again at fertilization
- Purple (PP)= (P) (P), white (pp)= (p) (p)
Homozygous genotype - identical alleles; Ex: PP (homozygous dominant); pp (homozygous recessive)
Heterozygous genotype - two different alleles (Ex: Pp)
Dominant allele (trait)- capital letter; determines appearance; ex: (P)
Recessive allele (trait) -lower case; no noticeable effect on appearance; ex: (p)
Phenotype – expression of trait or appearance; ex: purple, white
Genotype – actual genetic make-up; ex: PP, Pp, or pp
Punnett square – shows the possible combinations of alleles (genetic cross)
Figure 12.6 Inheritance of a Single Trait over Three Generations
Mendel crossed parent plants that were true-breeding (homozygous) for two discrete phenotypes (purple or white) of a particular trait (flower color). Such breeding trials are described as monohybrid crosses because the F1 plants are hybrid (heterozygous) for a single trait (flower color).

7. Monohybrid cross (1 trait) Ex: Flower color, P (purple); p (white)
♂ possible gametes
♀ possible gametes
Monohybrid cross (1 trait) Ex: Flower color, P (purple); p (white)
Possible combinations: PP (purple); Pp (purple); pp (white)
Phenotype ratio: 3:1 (3 purple:1 white)
Genotype ratio: 1:2:1
(1PP:2Pp:1pp)
Figure 12.7 The Punnett Square Method Is Used to Predict All Possible Outcomes of a Genetic Cross
Punnett squares chart the segregation (separation) of alleles into gametes and all the possible ways in which the alleles borne by these gametes can be combined to produce offspring.

8. Monohybrid Cross Ex. 1 Trait – eye color of fruitfly
dominant trait – Red (R)
recessive trait – white (r)
P generation: Red (RR) x white (rr)
F1 generation? All _______________
F2 generation? __________________
Fill in punnett square (diagram for predicting the results of genetic cross).
♂ possible gametes
Genotype ratio? ___________________
Phenotype ratio? __________________
♀ possible gametes

9. Law of Independent Assortment
Dihybrid cross: one pair segregate independently from the second pair during gamete formation (independent assortment)
F1 produce 4 gamete genotypes
F2 generation:
9/16 had round yellow seeds
3/16 had wrinkled yellow seeds
3/16 had round green seeds
1/16 had wrinkled green seeds
Genotype ratio: 1:2:1:2:4:2:1:2:1
Phenotype ratio: 9:3:3:1

10. Figure 12.8 Inheritance of Two Traits over Three Generations
Mendel used two-trait breeding experiments to test the hypothesis that the alleles of two different genes are inherited independently from each other. In one set of experiments, illustrated here, Mendel tracked the seed shape trait controlled by the R/r alleles and the seed color trait controlled by the Y/y alleles. The real test of the hypothesis came when Mendel examined the phenotypes of the offspring produced by crossing the heterozygous F1 plants (RrYy). As predicted by the hypothesis, two new phenotypic combinations were found among the F2 offspring: plants that made round, green seeds (R-yy) and plants that made wrinkled, yellow seeds (rrY-). The bottom panel summarizes the ratio of the two parental phenotypes and the two novel, nonparental phenotypes. A two-trait breeding experiment in which the F1 plants are double heterozygotes (heterozygous for both traits) is called a dihybrid cross.

11. Dihybrid Cross Ex. 2 traits – eye color and straight hair
dominant traits – Brown eye (B)
- Straight hair (S)
recessive traits – blue eye (b)
- curly hair (s)
P generation: BBSS x bbss
F1= BbSs x BbSs
F1 produce 4 genotypes: BS, Bs, bS, bs
F2=Phenotype ratio: 9:3:3:1
Give the phenotype of: BS= _____; Bs= _____;
bS= _____; bs= _____

12. Testcross
Use of the test is to identify the genotypes responsible for the phenotypes of the progenies after a cross
Crossing of unknown genotype (expressing dominant phenotype) and a homozygous recessive individual
Ex. Tall plant(T_) and dwarf plant(tt)
2 possibilities for Tall plant: TT or Tt
Do a cross of each possibility.
t t t
T T
T t

13. Homework
Define terms: genotype; phenotype; alleles; monohybrid cross; dihybrid cross; test cross; law of segregation
Differentiate homozygous alleles and heterozygous alleles, and dominant trait and recessive trait.
The allele for straight hair (S) is dominant to the allele for curly hair (s). A father with straight hair who is heterozygous (Ss) has 4 children with his wife who is also heterozygous (Ss). Construct a punnett square showing the location of gametes and offspring. Identify offspring as to genotype ratio and phenotype ratio. What is the phenotypic trait (expressed appearance) of child with ss alleles? with Ss alleles?