Meiosis I (reductional division): homologous chromosome pairs separate -> daughter cells are haploid (each with 1 set of chromosomes). Sister chromatids remain attached. Meiosis II (equational division): sister chromatids separate -> produces 4 haploid gametes genetically distinct from each other and from the parent cell
Pairs of chromosomes with the same genes at the same loci (one from each parent). Humans have 23 pairs (22 autosome pairs + 1 sex chromosome pair). Homologs are similar in size and centromere position. They pair during prophase I of meiosis (synapsis), forming a tetrad
The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I. Creates new combinations of alleles on chromosomes. Increases genetic variation. The point of exchange is called a chiasma. The frequency of crossing over between genes can be used to create genetic maps
The principle that alleles for different genes segregate independently during gamete formation. Applies to genes on non-homologous chromosomes. During metaphase I, homologous chromosome pairs line up independently (random orientation) -> different combinations of maternal and paternal chromosomes in gametes. Increases genetic variation
Any sperm can fertilize any egg - adding another layer of genetic variation beyond meiosis. A human couple produces an enormous number of genetically unique zygotes (2n combinations from independent assortment alone). Increases offspring genetic diversity
During gamete formation, the two alleles for a gene separate so each gamete receives only one allele. Each parent contributes one allele to each offspring. Explains why offspring inherit one allele from each parent and why recessive traits can reappear in F2 generation. The physical basis is homologous chromosome separation during meiosis I
A cross between two individuals heterozygous for one gene (Aa × Aa). Produces a 3:1 phenotypic ratio in the F1 generation (if complete dominance) and 1:2:1 genotypic ratio. Punnett square: 4 equally likely outcomes. Demonstrates segregation of alleles. Test cross: cross with homozygous recessive to determine unknown genotype
A cross between individuals heterozygous for two genes (AaBb × AaBb). Produces a 9:3:3:1 phenotypic ratio (if complete dominance and independent assortment). Demonstrates both the law of segregation and law of independent assortment. 16 equally likely Punnett square outcomes
A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. Rows = one parent's gametes; columns = the other parent's gametes. Each cell = offspring genotype. Essential tool for predicting inheritance patterns for single and multiple genes
The alleles of two (or more) genes assort independently of each other during gamete formation. Only applies to genes on non-homologous chromosomes. Offspring can have new combinations of parental traits. The physical basis is the independent alignment of homologous chromosome pairs at metaphase I
Genes located on the same chromosome that tend to be inherited together. Do not follow independent assortment. Violates the 9:3:3:1 dihybrid ratio. Crossing over can separate linked genes (recombination frequency used to map gene distances). Morgan's Drosophila experiments first demonstrated linkage
Traits controlled by genes located on sex chromosomes (usually X). X-linked recessive: more common in males (only one X needed). Examples: color blindness, hemophilia, Duchenne muscular dystrophy. X-linked dominant: rarer; affects both sexes but more in females. No Y-linked traits except those on pseudoautosomal regions (e.g., SRY gene)
Both alleles are fully expressed in the heterozygote. Neither allele masks the other. Examples: ABO blood type (IAIB -> AB), MN blood group, roan cattle (red + white hairs). F2 ratio: 1:2:1 still applies genotypically. Distinct from incomplete dominance where phenotype is blended
Neither allele is completely dominant; the heterozygote shows a blended intermediate phenotype. Example: red (RR) × white (WW) snapdragons -> pink (RW) F1. The F2 generation shows 1:2:1 ratio of red:pink:white. The alleles are both expressed (codominance is a stricter case)
One trait is controlled by multiple genes (each contributing a small additive effect). Produces continuous variation (bell-shaped curve phenotype distribution). Examples: human skin color (~6+ genes), height (~700+ genes), eye color. Punnett squares become multidimensional
Mitochondrial DNA is inherited exclusively from the mother (maternal inheritance). All offspring of an affected mother are affected; male offspring do not pass on mitochondria. Causes: Leber hereditary optic neuropathy (LHON), mitochondrial myopathies. Not explained by Mendelian genetics
More than two possible alleles exist for a gene in a population. An individual can only have two alleles (one from each parent). Example: ABO blood group has three alleles (IA, IB, i). The alleles show codominance (IA and IB) or complete dominance (IA/IB over i)
The ability of an organism to change its phenotype in response to environmental conditions without changing its genotype. Examples: plant leaf size (more sun -> smaller leaves), animal coloration (temperature-sensitive pigment in Himalayan cats), human muscle development. The same genotype produces different phenotypes in different environments
Thomas Hunt Morgan's work with Drosophila melanogaster established the chromosomal theory of inheritance. Key findings: white-eye mutation in males (X-linked), gene linkage (genes on same chromosome), recombination frequency to map genes. Won Nobel Prize 1933. His student Sturtevant created the first genetic map
Failure of homologous chromosomes or sister chromatids to separate properly during meiosis. Results in gametes with abnormal chromosome numbers (aneuploidy). Can occur in Meiosis I (homologs fail to separate) or Meiosis II (sister chromatids fail to separate). Causes: Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY)
Diploid (2n): cells containing two complete sets of chromosomes (one from each parent). Human somatic cells: 2n = 46. Haploid (n): cells containing one complete set of chromosomes. Human gametes: n = 23. Haploid cells cannot undergo meiosis further; they can fertilize to restore diploidy
The condition of having more than two complete sets of chromosomes. Common in plants (e.g., wheat = 6n, strawberries = 8n). Often arises from hybridization followed by chromosome doubling. Polyploid organisms are usually larger and may have evolutionary advantages. Rare in animals (often lethal)
Meiosis I (reductional division): homologous chromosome pairs separate -> daughter cells are haploid (each with 1 set of chromosomes). Sister chromatids remain attached. Meiosis II (equational division): sister chromatids separate -> produces 4 haploid gametes genetically distinct from each other and from the parent cell
Pairs of chromosomes with the same genes at the same loci (one from each parent). Humans have 23 pairs (22 autosome pairs + 1 sex chromosome pair). Homologs are similar in size and centromere position. They pair during prophase I of meiosis (synapsis), forming a tetrad
The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I. Creates new combinations of alleles on chromosomes. Increases genetic variation. The point of exchange is called a chiasma. The frequency of crossing over between genes can be used to create genetic maps
The principle that alleles for different genes segregate independently during gamete formation. Applies to genes on non-homologous chromosomes. During metaphase I, homologous chromosome pairs line up independently (random orientation) -> different combinations of maternal and paternal chromosomes in gametes. Increases genetic variation
Any sperm can fertilize any egg - adding another layer of genetic variation beyond meiosis. A human couple produces an enormous number of genetically unique zygotes (2n combinations from independent assortment alone). Increases offspring genetic diversity
During gamete formation, the two alleles for a gene separate so each gamete receives only one allele. Each parent contributes one allele to each offspring. Explains why offspring inherit one allele from each parent and why recessive traits can reappear in F2 generation. The physical basis is homologous chromosome separation during meiosis I
A cross between two individuals heterozygous for one gene (Aa × Aa). Produces a 3:1 phenotypic ratio in the F1 generation (if complete dominance) and 1:2:1 genotypic ratio. Punnett square: 4 equally likely outcomes. Demonstrates segregation of alleles. Test cross: cross with homozygous recessive to determine unknown genotype
A cross between individuals heterozygous for two genes (AaBb × AaBb). Produces a 9:3:3:1 phenotypic ratio (if complete dominance and independent assortment). Demonstrates both the law of segregation and law of independent assortment. 16 equally likely Punnett square outcomes
A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. Rows = one parent's gametes; columns = the other parent's gametes. Each cell = offspring genotype. Essential tool for predicting inheritance patterns for single and multiple genes
The alleles of two (or more) genes assort independently of each other during gamete formation. Only applies to genes on non-homologous chromosomes. Offspring can have new combinations of parental traits. The physical basis is the independent alignment of homologous chromosome pairs at metaphase I
Genes located on the same chromosome that tend to be inherited together. Do not follow independent assortment. Violates the 9:3:3:1 dihybrid ratio. Crossing over can separate linked genes (recombination frequency used to map gene distances). Morgan's Drosophila experiments first demonstrated linkage
Traits controlled by genes located on sex chromosomes (usually X). X-linked recessive: more common in males (only one X needed). Examples: color blindness, hemophilia, Duchenne muscular dystrophy. X-linked dominant: rarer; affects both sexes but more in females. No Y-linked traits except those on pseudoautosomal regions (e.g., SRY gene)
Both alleles are fully expressed in the heterozygote. Neither allele masks the other. Examples: ABO blood type (IAIB -> AB), MN blood group, roan cattle (red + white hairs). F2 ratio: 1:2:1 still applies genotypically. Distinct from incomplete dominance where phenotype is blended
Neither allele is completely dominant; the heterozygote shows a blended intermediate phenotype. Example: red (RR) × white (WW) snapdragons -> pink (RW) F1. The F2 generation shows 1:2:1 ratio of red:pink:white. The alleles are both expressed (codominance is a stricter case)
One trait is controlled by multiple genes (each contributing a small additive effect). Produces continuous variation (bell-shaped curve phenotype distribution). Examples: human skin color (~6+ genes), height (~700+ genes), eye color. Punnett squares become multidimensional
Mitochondrial DNA is inherited exclusively from the mother (maternal inheritance). All offspring of an affected mother are affected; male offspring do not pass on mitochondria. Causes: Leber hereditary optic neuropathy (LHON), mitochondrial myopathies. Not explained by Mendelian genetics
More than two possible alleles exist for a gene in a population. An individual can only have two alleles (one from each parent). Example: ABO blood group has three alleles (IA, IB, i). The alleles show codominance (IA and IB) or complete dominance (IA/IB over i)
The ability of an organism to change its phenotype in response to environmental conditions without changing its genotype. Examples: plant leaf size (more sun -> smaller leaves), animal coloration (temperature-sensitive pigment in Himalayan cats), human muscle development. The same genotype produces different phenotypes in different environments
Thomas Hunt Morgan's work with Drosophila melanogaster established the chromosomal theory of inheritance. Key findings: white-eye mutation in males (X-linked), gene linkage (genes on same chromosome), recombination frequency to map genes. Won Nobel Prize 1933. His student Sturtevant created the first genetic map
Failure of homologous chromosomes or sister chromatids to separate properly during meiosis. Results in gametes with abnormal chromosome numbers (aneuploidy). Can occur in Meiosis I (homologs fail to separate) or Meiosis II (sister chromatids fail to separate). Causes: Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY)
Diploid (2n): cells containing two complete sets of chromosomes (one from each parent). Human somatic cells: 2n = 46. Haploid (n): cells containing one complete set of chromosomes. Human gametes: n = 23. Haploid cells cannot undergo meiosis further; they can fertilize to restore diploidy
The condition of having more than two complete sets of chromosomes. Common in plants (e.g., wheat = 6n, strawberries = 8n). Often arises from hybridization followed by chromosome doubling. Polyploid organisms are usually larger and may have evolutionary advantages. Rare in animals (often lethal)