Chapter 2: Genetic Inheritance - Comprehensive Study Guide

2.1 THE CONCEPT OF GENETICS: CORE DEFINITIONS AND PRINCIPLES

Definition of Genetics: Genetics is defined as the science of inheritance. It is the study involving the inheritance of characteristics from one generation to the next. The biological basis of this inheritance involves the process of meiosis.
Gamete: A sex cell produced by meiosis. It contains a haploid number of chromosomes ( $n$ ). In plants and animals, these are specifically referred to as an egg or sperm.
Gene: A segment of DNA that serves as the basic unit of hereditary information. It functions to yield a protein or RNA product with a specific function.
Alleles: Alternative forms of a gene that govern variations of the same character.
- Dominant Allele: An allele that is always expressed when it is present, regardless of whether the genotype is homozygous or heterozygous.
- Recessive Allele: An allele that is not expressed when in the heterozygous state.
Locus: The specific physical location or place on a chromosome where the gene for a given trait occurs.
Parental Generation (P): Members of two different true-breeding lines that are crossed to produce offspring.
First Filial Generation ( $F1$ ): The first generation of hybrid offspring resulting from the cross between parents from two different true-breeding lines.
Second Filial Generation ( $F2$ ): The offspring resulting from the cross or self-fertilization of the $F1$ generation.
Pure Line / True-Breeding Line: A genetically pure strain of organisms in which all individuals are homozygous for the traits under consideration.
Genotype: The genetic makeup of an individual. There are two primary states:
- Homozygous: Possessing a pair of identical alleles for a particular locus.
- Heterozygous: Possessing a pair of unlike alleles for a particular locus.
Phenotype: The physical or chemical expression of an organism's genes. It is determined by the interaction between the organism's genes and the environment. Genes provide the potential, while the environment determines whether that potential is realized.

2.2 MENDELIAN GENETICS

Mendel’s Experimental Design:
- Gregor Mendel began his work in 1854 using the garden pea, Pisum sativum.
- Peas were chosen because they are easy to grow, produce large numbers of seeds quickly, and routinely self-fertilize. Experimental cross-fertilization is also easily performed.
- Mendel initially established true-breeding strains through self-fertilization before beginning his crosses.
Traits Selected for Study: Mendel selected traits with two distinct possibilities for phenotypes. Dominant phenotypes are marked here with an asterisk ( $*$ ):
- Flower color: $*$ Grey vs. White
- Seed coat color: $*$ Purple vs. White
- Seed color: $*$ Yellow vs. Green
- Seed shape: $*$ Smooth vs. Wrinkled
- Pod color: $*$ Green vs. Yellow
- Pod shape: $*$ Inflated vs. Pinched
- Stem height: $*$ Tall vs. Short
- Flower position: $*$ Axial vs. Terminal
Mendel’s First Law (Law of Segregation):
- Associated with Monohybrid Inheritance, which involves the inheritance of a single pair of contrasting characteristics (e.g., pure line tall vs. pure line dwarf).
- Conclusion: Internal factors (genes) occur in pairs. Only one factor from a pair can be represented in a single gamete. During the formation of $F1$ , one factor from each parent is carried by gametes. These factors retain their individuality; one is dominant and the other is recessive.
- Ratios: In a monohybrid cross of $F1 \times F1$ ( $Pp \times Pp$ ), the phenotype ratio of the $F2$ is $3:1$ . (e.g., $3\text{ Purple} : 1\text{ White}$ ).
Mendel’s Second Law (Law of Independent Assortment):
- Associated with Dihybrid Inheritance, which involves the inheritance of two pairs of different characteristics.
- Example Cross: Smooth yellow seeds ( $SSYY$ ) crossed with wrinkled green seeds ( $ssyy$ ).
- Phenotype Ratio: In the $F2$ generation of a dihybrid cross ( $SsYy \times SsYy$ ), the ratio is $9:3:3:1$ ( $9\text{ smooth yellow} : 3\text{ smooth green} : 3\text{ wrinkled yellow} : 1\text{ wrinkled green}$ ).
- Definition: Alleles of one gene segregate independently of the alleles of other genes. "Any one of a pair of characteristics may combine with either one of another pair."

TESTING AND CROSSING STRATEGIES

Testcross:
- Used to detect heterozygosity in an individual with a dominant phenotype but unknown genotype (e.g., $BB$ or $Bb$ ).
- The unknown individual is crossed with a homozygous recessive individual ( $bb$ ).
- Results: If all progeny show the dominant trait, the parent is homozygous dominant. If any progeny show the recessive trait, the parent is heterozygous.
- Phenotype Ratios: $1:1$ for monohybrid; $1:1:1:1$ for dihybrid.
Backcross:
- The crossing of a hybrid with one of its parents or a genetically similar individual.
- Purpose: To achieve offspring with a genetic identity closer to the parent (elite breeding line) or to transfer a specific trait from a donor parent. In animal breeding, this is known as a "topcross."
Reciprocal Cross:
- A pair of crosses where the phenotypes of the male and female are reversed compared to the first cross (e.g., White male $\times$ Black female followed by Black male $\times$ White female).
- Purpose: To determine if a gene is sex-linked. If results are the same, the gene is not sex-linked; if results differ, the gene is sex-linked.

2.3 GENE INTERACTION (EXCEPTIONS TO MENDEL’S LAWS)

Codominance: Both alleles are dominant and expressed simultaneously in the heterozygote.
- Examples: ABO blood groups ( $A$ and $B$ antigens) and MN blood groups in humans. Roan cattle (red and white hairs together).
- Roan Cattle Cross: $RW \times RW$ results in a ratio of $1 \text{ Red} (RR) : 2 \text{ Roan} (RW) : 1 \text{ White} (WW)$ .
Incomplete Dominance: One allele is not completely dominant over the other, resulting in an intermediate phenotype in the heterozygote.
- Examples: Flower colors in Antirrhinum (Snapdragons), plumage color in chickens, and Palomino horses.
Multiple Alleles: When a single gene has more than two alternative forms. However, an individual can only carry two alleles because chromosomes occur in pairs.
- Example: Human ABO blood groups controlled by alleles $I^A$ , $I^B$ , and $i$ . $I^A$ and $I^B$ are codominant, while $i$ is recessive. Five multiple alleles are also found in Coleus.
Lethal Genes: Mutations in essential genes that result in death.
- Dominant Lethal: Death occurs in both homozygotes and heterozygotes (e.g., Huntington's disease).
- Recessive Lethal: Death occurs only in the homozygous state (e.g., fur color in mice where $YY$ is lethal, and cystic fibrosis, sickle-cell anemia, and achondroplasia in humans).
Polygenes: Characteristics produced by the combined effect of many different genes. This forms the genetic basis for continuous variation.
- Examples: Human height and weight.
Linked Genes: Genes located on the same chromosome that tend to be inherited together.
- They do not show independent assortment and do not yield the $9:3:3:1$ ratio.
- Recombinants: New combinations of traits resulting from crossing over during meiosis. Recombinant progeny appear in smaller numbers compared to parental types because crossing over is a random and less frequent process.

2.4 GENE MAPPING AND SEX-LINKAGE

Gene Mapping Calculations:
- The percentage cross-over value ( $\text{cov}$ ) is used to determine the distance between genes.
- Formula: $\text{\% cross over value} = \frac{\text{No. of recombinant individuals}}{\text{Total number of offspring}} \times 100$
- $1\text{\% cov}$ = $1\text{ map unit (m.u.)}$ .
Sex Determination Systems:
- Humans/Drosophila: Female is $XX$ (homogametic), Male is $XY$ (heterogametic).
- Birds: Female is $XY$ , Male is $XX$ .
- Insects (Grasshoppers): Female is $XX$ , Male is $XO$ .
Sex-Linked Genes: Usually carried on the $X$ chromosome and often recessive.
- Examples: Red-green color blindness and Hemophilia in humans; white eye color in Drosophila.
- X-Linked Characteristics: Traits occur more frequently in males. A male offspring will be affected $100\text{\%}$ if the mother has a sex-linked mutant trait (reciprocal inheritance).

2.5 PEDIGREE ANALYSIS AND 2.6 CHI-SQUARE TEST

Pedigree Analysis: A study of inheritance within a family tree.
- Symbols: Male = Square; Female = Circle; Shaded = Expresses trait.
- Generations are presented in rows (eldest on the left).
Chi-Square Test (\text{\chi^2}):
- A statistical test for "goodness of fit" to determine if the difference between observed and expected results is due to chance.
- Significance: Hypothesis is generally accepted if the probability ( $P$ ) is greater than $5\text{\%}$ ( $P > 0.05$ ).

QUESTIONS & DISCUSSION

Question (Eye Color): A blue-eyed man married a brown-eyed woman whose mother is blue-eyed. What percentage of children have blue eyes?
- Answer: P: $bb \times Bb$ . G: $b$ and ( $B$ , $b$ ). offspring: $Bb$ (brown) and $bb$ (blue). Ratio is $1:1$ , so $50\text{\%}$ will have blue eyes.
Question (Rabbit Dihybrid): Cross a dihybrid rabbit ( $SsBb$ ) with one heterozygous for length and homozygous for brown ( $Ssbb$ ).
- Genotype Ratio: $1\text{ SSBb} : 1\text{ ssbb} : 2\text{ SsBb} : 2\text{ Ssbb} : 1\text{ ssBb} : 1\text{ ssbb}$ .
- Phenotype Ratio: $3\text{ Short Black} : 3\text{ Short Brown} : 1\text{ Long Black} : 1\text{ Long Brown}$ .
Question (Blood Group Paternity): Which baby belongs to which parent?
- Parents $A$ and $B$ : Baby $AB$
- Parents $O$ and $O$ : Baby $O$
- Parents $AB$ and $O$ : Baby $A$
- Parents $B$ and $B$ : Baby $B$
Question (Tomato Cross): A purple stem, cut leaf plant (unknown) $\times$ green stem, uncut leaf ( $gguu$ ) yields $56$ purple/cut and $58$ green/cut.
- Inference: Since the ratio is roughly $1:1$ for stem color and all offspring have cut leaves, the unknown parent genotype must be $Gguu$ (if $U$ is cut). Note: Specific symbols depend on allele designation.
Linking Gene Explanation (Page 51): In a cross where $F2$ data does not follow the $9:3:3:1$ ratio (e.g., $4800$ Red Long, $390$ Red Round, $380$ White Long, $1400$ White Round), it indicates the genes are linked on the same chromosome.