Punnett Square Terms

• True-breeding organisms consistently produce offspring with the same traits as the parents.
• The original, true-breeding parents are called the P generation.
• The first generation of offspring is called the F1, or filial, generation.

Punnett Square Terms

• P generation: Parental generation in a genetic cross.
• F1 generation: First filial generation; offspring of the P generation.
• F2 generation: Second filial generation; offspring of the F1 generation.
• TT: Homozygous dominant genotype.
• Tt: Heterozygous genotype.
• tt: Homozygous recessive genotype.
• A monohybrid cross between two heterozygous organisms will result in a phenotypic ratio of 3:1.
• Alternate versions of a gene are called alleles.
• A dominant allele masks the expression of a recessive allele when both are present in an organism.
• A recessive allele must be in the presence of another recessive allele to express its phenotype.
• When an organism has two identical alleles, it is termed homozygous.
• When an organism has two different alleles at a gene locus, it is heterozygous.
• Phenotype refers to the physical appearance of an individual.
• Genotype refers to the alleles an individual receives at fertilization.
• The expected phenotypic ratio of a dihybrid cross is always 9:3:3:1.
• If an individual is homozygous dominant, all offspring will carry the dominant trait if the other parent contributes a dominant allele
• To determine if a fly with long wings and a gray body is homozygous dominant, cross it with a fly with short wings and a black body; if all offspring have long wings and gray bodies, it can be concluded.
• Crossing two heterozygous plants (Tt) will yield offspring with TT, Tt, and tt genotypes.
• An autosome is any chromosome other than a sex chromosome (X or Y).
• In a pedigree chart, when parents are carriers, it means the disease is an autosomal recessive disorder, and the parents are genotypically heterozygous.
• Pattern II in a pedigree chart is possible if the disease is an autosomal recessive disorder and the parents are heterozygous for the trait.
• If a disease is autosomal recessive, the 'aa' genotype will result in a person having the disease.
• The inheritance of the ABO blood group in humans demonstrates codominance, where both I^A and I^B alleles are fully expressed.
• Incomplete dominance is when a heterozygote has an intermediate phenotype between that of either homozygote.
• The flower image shown demonstrates incomplete dominance
• Pleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits.
• Polygenic inheritance occurs when a trait is governed by two or more sets of alleles.
• Multifactorial traits are controlled by polygenes and are subject to environmental influences.

Sex Chromosomes

• XX: Female
• XY: Male
• The SRY gene on the Y chromosome determines maleness.
• X-linked genes are genes that have nothing to do with gender yet are carried on the X chromosome.
• Sex-linked recessive traits appear much more frequently in males than in females.
• The alleles for an X-linked trait are shown as an allele attached to the X chromosome.
• Male flies have only two possible genotypes for eye color because eye color is X-linked, and the Y chromosome carries no eye color gene.
• Males are considered hemizygous for X-linked traits because they only have one X chromosome.

Operon

• Regulator gene: Codes for a repressor protein; located outside the operon.
• Repressor: A DNA-binding protein that can block transcription.
• Promoter: A DNA sequence where RNA polymerase attaches to begin transcription.
• Operator: A DNA sequence that controls the transcription of structural genes.
• Structural genes: Code for enzymes and proteins involved in the metabolic pathway of the operon.
• Different cell types have different genes turned on, despite having the same complete set of genes.
• Histones play an important role in the compaction of DNA.
• Active genes in eukaryotic cells are associated with more loosely packed chromatin called euchromatin.
• More tightly packed DNA, which is inactive or less active, is called heterochromatin.
• If an inherited allele is highly methylated, the gene is not expressed.
• With a methyl group attached, transcription cannot occur.
• Epigenetics refers to heritable changes in gene expression without changing the DNA sequence.
• Epigenetic changes are reversible.
• Posttranslational control begins once a protein has been synthesized and has become active.
• A gene mutation is a permanent change in the sequence of bases in DNA.
• UV light is easily absorbed by pyrimidines in DNA.
• UV radiation can cause thymine dimers to form when two thymine molecules are next to each other.
• If repair enzymes fail, it can cause cancer.
• The p53 tumor suppressor gene is frequently mutated in human cancers.
• p53 promotes apoptosis.
• An example of a regulatory sequence is the TATA box, which is found in eukaryotes.
• The operator is like an on and off switch.
• The lac operon exhibits positive control.
• The trp operon exhibits negative control.
• Humans have transcription factors instead of operons.

Learn about true-breeding organisms and the P, F1, and F2 generations. Understand genotypes like homozygous dominant (TT), heterozygous (Tt), and homozygous recessive (tt). Explore monohybrid crosses and the 3:1 phenotypic ratio.