Chromosomes and Chromatids

Chromosome and Chromatid Counting

Understanding chromosome and chromatid numbers is fundamental in genetics, especially when analyzing cell division processes such as mitosis and meiosis.

• Haploid (1n): A cell with one set of chromosomes. Example: If 1n = 12, then the diploid number (2n) is 24.

• Diploid (2n): A cell with two sets of chromosomes.

• Chromosome Number: Determined by counting centromeres.

• Chromatid Number: Determined by counting the arms of chromosomes (each chromosome in X form has two chromatids).

Chromatids in Mitosis

During metaphase of mitosis in a diploid cell (1n = 6):

• Diploid means 2n = 12 chromosomes.

• In metaphase, chromosomes are in X form, so there are 24 chromatids.

Chromatids in Meiosis

Meiosis involves two divisions, resulting in haploid cells.

• Start: Diploid cell (2n = 12 chromosomes, 24 chromatids).

• After Meiosis I: Chromosome number halves (1n = 6 chromosomes, 12 chromatids).

• After Meiosis II: Each cell has 6 chromosomes and 6 chromatids.

Pedigree Analysis

Identifying Inheritance Patterns

Pedigrees are diagrams that show the inheritance of traits across generations. They are used to determine whether traits are autosomal or sex-linked, and dominant or recessive.

• Autosomal Recessive: Affected children can have unaffected parents.

• X-linked Recessive: More males are affected; trait is often passed through females.

• Incomplete Penetrance: Not all individuals with the genotype express the phenotype.

Example: Pedigree Analysis

Look for affected children with unaffected parents to identify recessive inheritance. X-linked recessive traits often show more affected males and are transmitted through carrier females.

Punnett Squares and Probability

X-linked Punnett Squares

Punnett squares are used to predict the outcome of genetic crosses. X-linked crosses require careful attention to sex chromosomes.

• RR x rr: All offspring are Rr.

• XRXR x XrY: All offspring show dominant phenotype.

• Test Cross: Female test cross yields half males with recessive phenotype; male test cross yields all males recessive, all females dominant.

• XrXr x XRY: All males are recessive, all females dominant.

Dihybrid Punnett Squares

Dihybrid crosses involve two genes and can be analyzed using Punnett squares.

• Dihybrid TtPp x TtPp: Ratio is 9:3:3:1.

• Dihybrid test cross TtPp x ttpp: Ratio is 1:1:1:1.

• Monohybrid Tt x Tt: Ratio is 3:1.

• Monohybrid Tt x tt: Ratio is 1:1.

Probability Calculations

Probability in genetics can be calculated using sum, product, and binomial rules.

• SUM Rule: For mutually exclusive events.

• PRODUCT Rule: For sequential events (e.g., first tall, then short).

• BINOMIAL Rule: For unordered events (e.g., 4 out of 7).

Binomial Example: Cat Coat Patterns

Cross: Tt x Tt (mackerel tabby dominant, classic tabby recessive)

• Expected: 75% mackerel, 25% classic.

• Probability of 5 out of 8 classic tabby kittens:

Use the binomial formula:

• n = 8, x = 5, p = 0.25 (classic), q = 0.75 (mackerel)

Chi Square Analysis

Chi Square Test

The chi square test is used to compare observed and expected results in genetic crosses.

• Expected: Theoretical ratio from Punnett square.

• Observed: Actual data from experiment.

Chi square formula:

• O = observed value

• E = expected value

Chromosome Mapping

Test Cross and Recombination Frequency

Chromosome mapping in eukaryotes uses test crosses to determine recombination frequency.

• Test Cross: Ratio is 1:1:1:1.

• Parentals: Offspring with parental genotype.

• Nonparentals: Offspring with recombinant genotype.

• Recombination Frequency:

• Map Units (cM): 1% recombination = 1 cM.

Alternative equation:

Extensions of Mendelian Genetics

Lethal Alleles and Epistasis

Extensions include lethal alleles and gene interactions such as epistasis.

• Lethal Alleles: Can result in a 2:1 ratio in offspring.

• Epistasis: Interaction between two genes affects phenotype.

Complementation Groups

Gene Complementation

Complementation tests determine whether mutations are in the same gene or different genes.

• Same gene: Mutations do not complement.

• Different gene: Mutations complement.

Bacterial Genetics

Bacterial Conjugation

Bacterial conjugation is a process of genetic exchange involving F factors.

• F+: Donor cell with fertility plasmid.

• F-: Recipient cell without plasmid.

• Hfr: High frequency recombination cell.

• F': Cell with F plasmid carrying additional genes.

• Merozygote: Partial diploid cell formed during conjugation.

Hfr Conjugation

Hfr cells transfer chromosomal genes to F- cells during conjugation, allowing mapping of bacterial genes.

Viral Life Cycles

Bacteriophage Genetics

Bacteriophages are viruses that infect bacteria and are used in genetic mapping and analysis.

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome.

Additional info: Viral life cycles are important for understanding genetic exchange and mapping in bacteria.

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