Genetics Practice Exam Study Guide: Core Concepts, Linkage, Pedigrees, and Population Genetics

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BLOCK 1: BASICS

Genetic Crosses and Mendelian Ratios

Genetic crosses involving F1 plants heterozygous for a given allele can produce characteristic ratios in the F2 generation. Understanding these ratios is fundamental to classical genetics.

Monohybrid Cross: A cross between two individuals heterozygous for a single gene, typically yields a 3:1 phenotypic ratio in the F2 generation.
Dihybrid Cross: Involves two genes, producing a 9:3:3:1 ratio in the F2 generation if genes assort independently.
Reciprocal Cross: A pair of crosses involving switching the sexes of the parents to test for sex-linked inheritance.
Test Cross: Crossing an individual with a homozygous recessive to determine genotype.

Example: Crossing F1 heterozygotes for a gene (Aa x Aa) yields a 3:1 ratio of dominant to recessive phenotypes in the F2.

Meiosis and Gamete Formation

Meiosis is the process by which diploid cells produce haploid gametes, ensuring genetic diversity and proper chromosome number in offspring.

Diploid: Cells with two sets of chromosomes (2n).
Haploid: Gametes with one set of chromosomes (n).
During meiosis, each gamete receives one copy of each chromosome.

Example: A diploid cell (2n) undergoing meiosis produces gametes with n chromosomes.

Ploidy and Polyploidy

Ploidy refers to the number of sets of chromosomes in a cell. Polyploidy is common in plants and can affect genetic crosses.

Diploid: Two sets of chromosomes.
Tetraploid: Four sets of chromosomes.
Potatoes are tetraploid; thus, somatic cells have four copies of each chromosome.

Example: A tetraploid potato plant produces gametes with two copies of each chromosome.

Banana Genetics and Seedlessness

Commercial bananas are typically triploid and do not produce seeds due to irregular meiosis.

Triploid: Three sets of chromosomes.
Meiosis in triploids leads to unbalanced gametes, resulting in sterility.

Example: Bananas rarely produce seeds because their gametes are not viable.

Genotype and Gamete Diversity

The number of different gametes an organism can produce depends on its heterozygosity.

For n heterozygous gene pairs, the number of possible gamete types is .

Example: An individual heterozygous at 4 loci can produce different gametes.

Inheritance Patterns: Dominance and Sex-Linkage

Traits can be inherited in various patterns, including dominance and sex-linkage.

Dominant: Trait expressed when at least one dominant allele is present.
Recessive: Trait expressed only when two recessive alleles are present.
Sex-linked: Genes located on sex chromosomes (X or Y).

Example: Tongue rolling is a dominant trait; if both parents are heterozygous (Tt), their child may or may not express the trait.

Penetrance and Expressivity

Penetrance refers to the proportion of individuals with a genotype who express the phenotype. Expressivity is the degree to which a trait is expressed.

Incomplete Penetrance: Not all individuals with the genotype show the phenotype.
Variable Expressivity: Phenotype varies among individuals with the same genotype.

Example: Not all carriers of the BRCA1 mutation develop breast cancer due to incomplete penetrance.

BLOCK 2: LINKAGE AND GENE MAPPING

Genetic Linkage and Recombination

Genes located close together on the same chromosome tend to be inherited together, a phenomenon known as genetic linkage.

Linked Genes: Genes that do not assort independently.
Recombination Frequency: The proportion of recombinant offspring, used to estimate genetic distance.
Map Distance: Measured in centiMorgans (cM); 1 cM = 1% recombination.

Example: If two genes show 11.5% recombination, their map distance is 11.5 cM.

Test Crosses and Haplotype Analysis

Test crosses help determine gene linkage and haplotype combinations.

Haplotype: A combination of alleles at different loci on the same chromosome.
Test crosses between heterozygotes and homozygous recessive individuals reveal recombination events.

Example: A female fly heterozygous for two genes can produce four types of gametes if genes are unlinked.

Pedigree Analysis and Genetic Testing

Pedigrees are diagrams that show inheritance patterns across generations, useful for identifying autosomal and sex-linked traits.

Autosomal Dominant: Trait appears in every generation.
Autosomal Recessive: Trait may skip generations.
X-linked: Trait more common in one sex.

Example: Huntington's disease is autosomal dominant; each child of an affected parent has a 50% chance of inheriting the allele.

Mutation Types

Mutations can alter gene function and lead to genetic disorders.

Stop Codon Mutation: Premature termination of protein synthesis.
Splice Site Mutation: Alters mRNA processing.
Chromosome Translocation: Rearrangement of chromosome segments.
Trinucleotide Repeat Expansion: Repeats increase, causing disorders like Huntington's.
Aneuploidy: Abnormal number of chromosomes.

BLOCK 4: POPULATION GENETICS AND HUMAN EVOLUTIONARY GENETICS

Population Genetics Principles

Population genetics studies allele frequencies and their changes over time due to evolutionary forces.

Hardy-Weinberg Equilibrium: Describes allele and genotype frequencies in a non-evolving population.
Equation: where p and q are allele frequencies.
Genetic Drift: Random changes in allele frequency, especially in small populations.
Selection: Differential survival and reproduction of genotypes.

Example: In a population of 10,000 Hopi, if the frequency of the albinism allele is 0.01, the expected number of carriers is .

Fitness and Selection Coefficient

Fitness (w) measures reproductive success; selection coefficient (s) quantifies the reduction in fitness.

Equation:

Example: If s = 0.1, then w = 0.9.

Linkage Disequilibrium and Selective Sweeps

Linkage disequilibrium occurs when alleles at different loci are inherited together more often than expected by chance, often due to recent selection (selective sweep).

Selective Sweep: Rapid increase in frequency of a beneficial allele, reducing genetic variation nearby.

Example: The SLC24A5 allele in Europeans shows evidence of a selective sweep.

Genetic Disorders and Heterozygote Advantage

Some genetic disorders persist in populations due to heterozygote advantage, where carriers have increased fitness.

Sickle Cell Disease: Heterozygotes (carriers) are resistant to malaria.
Tay-Sachs Disease: Heterozygote advantage in certain populations.

Chi-Square Test for Hardy-Weinberg Equilibrium

The chi-square test assesses whether observed genotype frequencies conform to Hardy-Weinberg expectations.

Degrees of Freedom: Number of genotype classes minus number of alleles.
Equation:

Example: If is less than the critical value for the degrees of freedom, the population is in equilibrium.

Tables

Chi-Square Table (for Hardy-Weinberg Equilibrium)

df	0.9	0.5	0.1	0.05	0.01
1	0.02	0.45	2.7	3.8	6.6
2	0.1	1.4	4.6	5.9	9.2
3	0.58	2.4	6.3	7.8	13.2
4	1.1	3.6	7.8	9.5	14.9

Genotype Frequencies (Sickle Cell Example)

Genotype	Frequency
HbA/HbA	700/1000
HbA/HbS	270/1000
HbS/HbS	30/1000

Additional info:

Some context and definitions were inferred to clarify fragmented or brief points.
Examples and equations were added to ensure completeness and academic quality.