Genetics Study Guide: Mendelian Inheritance, Chromosome Theory, and Genetic Mapping

Mendelian Genetics

Types of Genetic Crosses

Genetic crosses are fundamental experiments used to study inheritance patterns. The type of cross depends on the number of genes and traits being analyzed.

• Monohybrid cross: Involves one gene with two alleles (e.g., Aa x Aa).

• Dihybrid cross: Involves two genes, each with two alleles (e.g., AaBb x AaBb).

• Testcross: Crossing an individual with a dominant phenotype with a homozygous recessive individual to determine genotype.

• Backcross: Crossing an F1 hybrid with one of its parental genotypes.

Example: The cross AaBb x aabb is a testcross for two genes.

Phenotypic Ratios in Mendelian Crosses

Assuming complete dominance and independent assortment, the expected phenotypic ratios for F2 generation are:

• Monohybrid cross (Aa x Aa): 3:1 (dominant:recessive)

• Dihybrid cross (AaBb x AaBb): 9:3:3:1 (two traits, four phenotypes)

• Testcross (AaBb x aabb): 1:1:1:1 (equal proportions of all possible phenotypes)

Formula: For a dihybrid cross, the phenotypic ratio is $9:3:3:1$.

Independent Assortment and Segregation

Mendel's laws describe how alleles are inherited:

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Application: These laws explain the ratios observed in genetic crosses.

Chromosome Theory and Cell Division

Chromosome Number and Ploidy

Chromosome number and ploidy are key concepts in genetics:

• Diploid (2n): Two sets of chromosomes (e.g., human somatic cells, 2n=46).

• Haploid (n): One set of chromosomes (e.g., human gametes, n=23).

Example: Metaphase of mitosis in humans shows 2n=46 chromosomes.

Stages of Cell Division

Cell division occurs via mitosis or meiosis, each with distinct stages:

• Mitosis: Produces two identical diploid cells.

• Meiosis: Produces four haploid gametes; includes two divisions (meiosis I and II).

• Key stages: Prophase, metaphase, anaphase, telophase, cytokinesis.

Example: Metaphase I of meiosis: homologous chromosomes align at the equator.

Blood Groups and Human Genetics

ABO Blood Group Inheritance

The ABO blood group is determined by three alleles (IA, IB, i) and exhibits codominance.

• Type A: IAIA or IAi

• Type B: IBIB or IBi

• Type AB: IAIB (codominant)

• Type O: ii

Application: Blood type inheritance can be used in paternity testing.

Genetic Linkage and Mapping

Linkage and Recombination

Genes located close together on the same chromosome tend to be inherited together (linked). Recombination frequency is used to estimate genetic distance.

• Recombination frequency: Percentage of recombinant offspring; 1% = 1 map unit (centimorgan).

• Linked genes: Recombination frequency < 50%.

• Unlinked genes: Recombination frequency ≈ 50%.

Formula: $\text{Recombination frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total offspring}} \times 100\%$

Genetic Mapping Problems

Geneticists use testcrosses and progeny analysis to map gene locations:

• Calculate recombination frequencies between gene pairs.

• Use three-point testcrosses to determine gene order.

Phenotype	Number Observed
Hairless	259
Jointless	49
Anthocyaninless, hairless	268
Jointless, hairless	39
Normal	260
Anthocyaninless, jointless	52

Main purpose: This table is used to determine linkage arrangement and recombination frequencies among three genes.

Probability in Genetics

Calculating Probabilities

Probability is used to predict outcomes of genetic crosses.

• Product rule: Probability of independent events occurring together is the product of their individual probabilities.

• Sum rule: Probability of mutually exclusive events is the sum of their probabilities.

Example: Probability that the first three offspring of a dihybrid cross are all females: $ (1/2)^3 = 0.125 $

Sex Determination and Sex-Linked Inheritance

Sex Chromosomes and Disorders

Sex determination in humans is based on the presence of X and Y chromosomes.

• XX: Female

• XY: Male

• XYY, XXY: Abnormal karyotypes, can result in syndromes (e.g., Klinefelter's, Jacob's).

Example: An individual with XXY is typically a male with Klinefelter syndrome.

Modes of Inheritance

Traits can be inherited in different ways:

• Autosomal dominant

• Autosomal recessive

• X-linked dominant

• X-linked recessive

• Holandric (Y-linked)

Pedigree analysis is used to determine the mode of inheritance.

Bacterial Genetics

Gene Transfer in Bacteria

Bacteria can exchange genetic material via several mechanisms:

• Transformation: Uptake of free DNA from the environment.

• Conjugation: Direct transfer of DNA between bacteria via pilus.

• Transduction: Transfer of DNA by bacteriophages.

• Sexduction: Transfer of F factor during conjugation.

Example: An F+ cell can donate the F factor to an F- cell during conjugation.

Bacterial Phenotypes and Nutritional Requirements

Bacterial strains may have specific nutritional requirements:

• Auxotroph: Requires additional nutrients for growth.

• Prototroph: Can grow on minimal medium.

• Pseudotroph: False or non-standard nutritional requirement.

Example: A methionine auxotroph cannot grow without methionine.

Molecular Genetics

DNA Structure and Replication

DNA is a double helix composed of nucleotides:

• Purines: Adenine (A), Guanine (G)

• Pyrimidines: Cytosine (C), Thymine (T)

• Base pairing: A-T, G-C

• Antiparallel strands: DNA strands run in opposite directions (5' to 3' and 3' to 5').

Additional info: In RNA, uracil (U) replaces thymine.

Gene Mapping by Interrupted Mating

Interrupted mating experiments are used to map bacterial genes:

• Genes are transferred in a linear fashion from donor to recipient.

• Time of entry is used to map gene order.

Formula: $ a \rightarrow b \rightarrow c \rightarrow d $ (order of gene transfer)

Summary Table: Key Genetic Terms

Term	Definition
Dihybrid cross	Cross involving two genes, each with two alleles
Testcross	Cross with homozygous recessive to determine genotype
Auxotroph	Bacterial strain requiring additional nutrients
Prototroph	Bacterial strain that grows on minimal medium
Recombination frequency	Percentage of recombinant offspring
F factor	Plasmid that enables bacterial conjugation