Genetics and Chromosomes: Study Guide for General Biology

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Concept 10.1: Heredity, Variation, and Chromosomes

Heredity and Variation

Heredity is the transmission of genetic traits from parents to offspring, while variation refers to the differences in traits among individuals of the same species.

Heredity: The passing of traits from parents to offspring through genes.
Variation: The differences in physical appearance or genetic makeup among individuals.
Example: Eye color, height, and blood type are traits that can vary due to heredity and genetic variation.

Genes and Their Origin

Genes are segments of DNA that code for specific proteins and determine traits. Individuals inherit genes from both biological parents.

Gene: A unit of heredity made up of DNA.
Inheritance: Each individual receives half of their genes from their mother and half from their father.

Types of Cells and Chromosomes

Organisms have two main types of cells: somatic cells and gametes. Somatic cells are body cells, while gametes are reproductive cells (sperm and egg).

Somatic cells: Contain two sets of chromosomes (diploid).
Gametes: Contain one set of chromosomes (haploid).
Chromosome: A structure made of DNA and proteins that contains genetic information.
Example: In humans, somatic cells have 46 chromosomes (23 pairs), while gametes have 23 chromosomes.

Chromosome Structure

Chromosomes are thread-like structures found in the nucleus of eukaryotic cells. Each chromosome has a centromere and two arms.

Label: Centromere (center), chromatids (arms), telomeres (ends).

Types of Reproduction

Reproduction can be asexual or sexual, affecting the genetic similarity of offspring to parents.

Asexual reproduction: Offspring are genetically identical to the parent (clones).
Sexual reproduction: Offspring are genetically unique due to the combination of genes from two parents.

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes that are similar in size and shape but may carry different versions of genes (alleles).

Homologous chromosomes: One from each parent, same genes but possibly different alleles.
Example: Human cells have 23 pairs of homologous chromosomes.

Diploid vs. Haploid Cells

Cells can be diploid (2n) or haploid (n), depending on the number of chromosome sets they contain.

Diploid (2n): Two sets of chromosomes (somatic cells).
Haploid (n): One set of chromosomes (gametes).
Formula: (humans), (humans).

Autosomes and Sex Chromosomes

Human chromosomes are classified as autosomes or sex chromosomes.

Autosomes: Non-sex chromosomes (22 pairs in humans).
Sex chromosomes: X and Y chromosomes (1 pair in humans).

Concept 10.3: Meiosis and Genetic Variation

Stages of Meiosis

Meiosis is a type of cell division that reduces the chromosome number by half, producing four haploid cells. It consists of two sequential divisions: meiosis I and meiosis II.

Meiosis I: Homologous chromosomes separate.
Meiosis II: Sister chromatids separate.
Key events: Formation of tetrads, crossing over, separation of chromosomes.

Key Events in Meiosis

Tetrads form: Homologous chromosomes pair up during prophase I.
Crossing over: Exchange of genetic material between homologous chromosomes, increasing genetic variation.
Cells are diploid/haploid: Cells start diploid, become haploid after meiosis I.
Separation of homologous chromosomes: Occurs in anaphase I.
Separation of sister chromatids: Occurs in anaphase II.

Comparison Table: Mitosis vs. Meiosis I vs. Meiosis II

Characteristic	Mitosis	Meiosis I	Meiosis II
At the start of this process, the cell is:	Diploid / Haploid	Diploid / Haploid	Haploid
At the end of this process, the cell is:	Diploid / Haploid	Haploid	Haploid
DNA replication occurs beforehand:	Yes	Yes	No
Crossing over (or synapsis of homologous chromosomes) occurs:	No	Yes	No
Number of daughter cells that result:	2	2	4 (total after both divisions)

Concept 11.1: Mendelian Genetics

Mendel's Experiments and Laws

Gregor Mendel's experiments with pea plants established the basic principles of heredity, including the concepts of dominant and recessive traits, segregation, and independent assortment.

Dominant trait: Expressed when at least one dominant allele is present.
Recessive trait: Expressed only when two recessive alleles are present.
Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.
Punnett Square: A tool to predict the probability of offspring genotypes and phenotypes.

Genetic Terms

Homozygote: An individual with two identical alleles for a gene (e.g., AA or aa).
Heterozygote: An individual with two different alleles for a gene (e.g., Aa).
Genotype: The genetic makeup of an organism (e.g., AA, Aa, or aa).
Phenotype: The observable traits of an organism (e.g., purple or white flowers).
Allele: Different forms of a gene found at the same locus on homologous chromosomes.

Types of Dominance

Complete dominance: One allele completely masks the effect of another.
Incomplete dominance: The heterozygote has an intermediate phenotype.
Codominance: Both alleles are fully expressed in the heterozygote.
Example: Incomplete dominance in snapdragon flowers (red x white = pink).

Concept 11.4: Human Genetics and Pedigrees

Widow's Peak and PTC Tasting

Human traits such as widow's peak and the ability to taste PTC are often used to illustrate Mendelian inheritance in humans.

Widow's peak: A dominant trait; individuals with at least one dominant allele (W) have a widow's peak.
PTC tasting: The ability to taste PTC is dominant; non-tasters are homozygous recessive.
Pedigree analysis: Used to determine inheritance patterns in families.

Carriers and Genetic Disorders

Carrier: An individual who has one copy of a recessive allele for a disorder but does not show symptoms.
Dominant vs. Recessive Disorders: Carriers are only found in recessive disorders.

Concept 12.2: Sex Chromosomes and Sex-Linked Inheritance

Sex Determination in Humans

Sex is determined by the combination of X and Y chromosomes. Females are XX, males are XY.

X chromosome: Inherited from both parents (mothers always pass an X; fathers pass X to daughters, Y to sons).
Y chromosome: Inherited only from the father.

Sex vs. Gender

Sex: Biological classification based on chromosomes and reproductive anatomy.
Gender: Social and cultural roles, behaviors, and identities.

X-Linked and Y-Linked Disorders

X-linked disorders: Caused by mutations on the X chromosome; more common in males.
Y-linked disorders: Rare, as the Y chromosome has fewer genes.
Carrier mothers: Can pass X-linked recessive disorders to sons.
For a female to be affected by an X-linked recessive disorder: She must inherit two copies of the mutant allele (one from each parent).