Mendelian Genetics and Population Genetics: Study Guide and Practice

Mendelian and Human Genetics

Key Concepts in Mendelian Genetics

Mendelian genetics forms the foundation of classical genetics, describing how traits are inherited from one generation to the next. Understanding these concepts is essential for analyzing genetic crosses and predicting inheritance patterns.

• Blending Theory of Inheritance: An early, incorrect theory proposing that offspring are a 'blend' of parental traits.

• Particulate Theory of Inheritance (Mendel’s Theory): Mendel proposed that inheritance is controlled by discrete units (genes) that retain their identity across generations.

• Parental, F1, and F2 Generations:

  • Parental (P): The original true-breeding individuals in a cross.

  • F1 (First Filial): The first generation of offspring from the parental cross.

  • F2 (Second Filial): The offspring from crossing two F1 individuals.

• Allele: Alternative forms of a gene found at the same locus on homologous chromosomes.

  • Dominant: An allele that masks the expression of another (recessive) allele.

  • Recessive: An allele whose expression is masked by a dominant allele.

• Gene: Mendel defined a gene as a unit of inheritance; today, it is a DNA segment coding for a protein or RNA.

• Diploid: Cells with two sets of chromosomes (2n), typical of somatic cells.

• Haploid: Cells with one set of chromosomes (n), typical of gametes.

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).

  • Homozygous dominant: Two dominant alleles (e.g., AA).

  • Homozygous recessive: Two recessive alleles (e.g., aa).

  • Heterozygous: One dominant and one recessive allele (e.g., Aa).

• Phenotype: The observable traits of an organism, resulting from genotype and environment.

• Segregation of Alleles: During gamete formation, alleles for a gene separate so each gamete carries only one allele for each gene (Mendel’s First Law).

• Incomplete Dominance: Neither allele is fully dominant; heterozygotes show an intermediate phenotype (e.g., red x white flowers produce pink).

• Codominance: Both alleles are fully expressed in the heterozygote (e.g., AB blood type).

• Multiple Alleles: More than two possible alleles exist for a gene (e.g., ABO blood group).

• Laws of Probability in Genetic Crosses: Used to predict the likelihood of genotypes and phenotypes in offspring.

• Single Factor Cross: A cross involving one gene (monohybrid cross).

• Double Factor Cross: A cross involving two genes (dihybrid cross); demonstrates the Law of Independent Assortment—genes on different chromosomes assort independently during meiosis.

Human Genetics

• Karyotype: A visual display of all chromosomes in a cell, used to detect chromosomal abnormalities.

• Autosomes: The 22 pairs of chromosomes not involved in sex determination.

• Sex Chromosomes: X and Y chromosomes that determine biological sex.

• Non-disjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy (e.g., Down syndrome).

• Autosomal Dominant Inheritance: Trait appears in every generation; affected individuals have at least one affected parent.

• Autosomal Recessive Inheritance: Trait can skip generations; affected individuals may have unaffected parents.

• X-linked Recessive Inheritance: More common in males; females are typically carriers.

• X-linked Dominant Inheritance: Affected males pass trait to all daughters, not sons; affected females can pass to both.

• Mitochondrial Inheritance: Traits inherited through mitochondrial DNA, passed from mother to all offspring.

• Pedigrees: Diagrams showing inheritance patterns across generations; used to infer genotypes and modes of inheritance.

Major Concepts and Applications

• Chromosome Structure: Chromosomes consist of DNA and proteins; each chromatid contains an identical DNA molecule with thousands of genes.

• Genetic Crosses: Ability to perform and interpret single and double factor crosses, including incomplete dominance, codominance, multiple alleles, and X-linked traits.

• Pedigree Analysis: Interpreting inheritance patterns and predicting genotypes using family trees.

Population Genetics and Microevolution

Evolution by Natural Selection

Population genetics studies the genetic composition of populations and how it changes over time, forming the basis for understanding evolution.

• Darwin and Wallace’s Theory: Evolution occurs by natural selection, where environmental pressures and natural variation lead to differential survival and reproduction.

• Microevolution: Short-term changes in allele frequencies within a population.

• Macroevolution: Long-term, large-scale evolutionary changes, such as speciation.

Sources of Genetic Variation

• Mutations: Random changes in DNA that create new alleles; can be harmful, neutral, or beneficial.

• Sexual Reproduction: Shuffles existing alleles through independent assortment, fertilization, and crossing over.

• Nondisjunction and Chromosomal Abnormalities: Errors during meiosis can alter chromosome number or structure.

Gene Pool and Allele Frequencies

• Gene Pool: The total collection of alleles in a population.

• Allele Frequency: The proportion of a specific allele among all alleles for a gene in a population.

• Measuring Allele Frequencies: Can be estimated from phenotypic data, protein, or DNA sequences.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg Law provides a mathematical model for genetic equilibrium in populations, assuming no evolutionary forces are acting.

• Allele Frequency Equation:

  $p + q = 1$

  where p = frequency of dominant allele, q = frequency of recessive allele.

• Genotype Frequency Equation:

  $p^2 + 2pq + q^2 = 1$

  where p2 = homozygous dominant, 2pq = heterozygous, q2 = homozygous recessive.

• Steps for Calculating Frequencies:

  1. Determine the frequency of the homozygous recessive genotype (q2) from phenotypic data.

  2. Calculate q by taking the square root of q2.

  3. Find p using p = 1 - q.

  4. Calculate genotype frequencies: p2, 2pq, q2.

  5. Check that p2 + 2pq + q2 ≈ 1.

• Hardy-Weinberg Equilibrium Conditions:

  • Large population size

  • Random mating

  • No mutation

  • No natural selection

  • No migration (gene flow)

Factors Disrupting Genetic Equilibrium

• Mutation: Introduces new alleles.

• Migration (Gene Flow): Movement of alleles between populations.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

  • Bottleneck Effect: Sudden reduction in population size alters allele frequencies.

  • Founder Effect: New population established by a small group, leading to different allele frequencies.

• Natural Selection: Differential survival and reproduction changes allele frequencies.

Types of Natural Selection

• Stabilizing Selection: Favors average phenotypes.

• Directional Selection: Favors one extreme phenotype (e.g., dark fur in rock pocket mice).

• Disruptive Selection: Favors both extremes over the average.

• Sexual Selection: Favors traits that increase mating success.

Application: Hardy-Weinberg and Natural Selection

• The Hardy-Weinberg equation can be used to analyze genetic composition at a given time and can be modified to include selection coefficients to model the effects of natural selection.

• Example: The rock pocket mice case study demonstrates how directional selection alters allele frequencies in response to environmental changes.

Practice Data: Class Traits and Hardy-Weinberg Calculations

The following table summarizes class data for several traits, showing genotype counts, allele frequencies, and genotype frequencies. Use these data to practice Hardy-Weinberg calculations.

Trait	Genotype(s)	Class Data (n)	p	q	p2	2pq	q2
Free earlobe	EE, Ee	26	0.54	0.46	0.29	0.50	0.21
Attached earlobe	ee	7	0.54	0.46	0.29	0.50	0.21
Non hitchhiker’s thumb	HH, Hh	13	0.22	0.78	0.05	0.34	0.61
Hitchhiker’s thumb	hh	20	0.22	0.78	0.05	0.34	0.61
Mid-digital hair	MM, Mm	10	0.17	0.83	0.03	0.28	0.70
No mid-digital hair	mm	23	0.17	0.83	0.03	0.28	0.70
Tongue rolling	RR, Rr	31	0.75	0.25	0.56	0.38	0.06
No tongue rolling	rr	2	0.75	0.25	0.56	0.38	0.06
Right handed	AA, Aa	30	0.70	0.30	0.49	0.42	0.09
Left handed	aa	3	0.70	0.30	0.49	0.42	0.09
Widow’s peak	WW, Ww	16	0.28	0.72	0.08	0.40	0.52
No widow’s peak	ww	17	0.28	0.72	0.08	0.40	0.52
Dimples	DD, Dd	4	0.06	0.94	0.004	0.113	0.88
No dimples	dd	29	0.06	0.94	0.004	0.113	0.88

Additional Practice Problems

Trait	Genotype(s)	Class Data (n)	p	q	p2	2pq	q2
Darwin tubercle	DD, Dd	17	0.30	0.70	0.09	0.42	0.48
No Darwin tubercle	dd	16	0.30	0.70	0.09	0.42	0.48
PTC Tasting	TT, Tt	23	0.45	0.55	0.20	0.50	0.30
PTC Non-tasting	tt	10	0.45	0.55	0.20	0.50	0.30
Cleft chin	CC, Cc	1	0.02	0.98	0.0004	0.039	0.97
Smooth chin	cc	32	0.02	0.98	0.0004	0.039	0.97

Example Calculation

• Suppose 10 out of 100 students have attached earlobes (ee). The frequency of the homozygous recessive genotype (q2) is 0.10.

• Calculate q: $q = \sqrt{0.10} = 0.316$

• Calculate p: $p = 1 - 0.316 = 0.684$

• Genotype frequencies:

  • Homozygous dominant: $p^2 = (0.684)^2 = 0.468$

  • Heterozygous: $2pq = 2 \times 0.684 \times 0.316 = 0.432$

  • Homozygous recessive: $q^2 = 0.10$

• Check: $0.468 + 0.432 + 0.10 = 1.00$

Additional info:

• Understanding the Hardy-Weinberg principle is crucial for distinguishing between populations in equilibrium and those evolving due to selection, drift, migration, or mutation.

• Pedigree analysis is a key skill for identifying inheritance patterns in human genetics, especially for medical genetics and counseling.