Heredity full review

What this reading is about

Heredity is one of the four MCAS reporting categories. It covers everything from the shape of dnaThe molecule that carries the genetic instructions for life. Shaped like a twisted ladder. to the inheritance patterns you see in a pedigreeA family-tree diagram showing which family members have a trait. Squares = males, circles = females, filled-in = has the trait.. Blocks 1–4 covered these pieces one at a time. This reading puts them together so you can see how one piece connects to the next.

The big chain that runs through all of heredity:

DNA → mRNA → protein → trait → inheritance

Every heredity question on MCAS lives somewhere in that chain. If you know where in the chain a question is asking, you can find the answer.

Link 1: DNA holds the information

dnaThe molecule that carries the genetic instructions for life. Shaped like a twisted ladder. is a double-helixThe twisted-ladder shape of DNA. Two strands wound around each other. — a twisted ladder. The sides are sugar-phosphate backbone (not important for information). The rungs are pairs of nitrogenous-baseThe "letter" part of a nucleotide. The four bases in DNA are A, T, C, and G.s: A pairs with T, C pairs with G. The order of the bases is the genetic code. That sequence is what holds all the information.

When a cell needs to copy its DNA (before dividing), it uses replicationThe process where one DNA molecule is copied to make two identical DNA molecules.: the helix unzips, and each strand acts as a template for a new partner. The result is two identical copies of the original DNA.

Link 2: DNA → mRNA → protein

This is the central-dogmaThe flow of genetic information in a cell: DNA → RNA → protein.: DNA → RNA → protein.

• transcriptionThe first step of protein synthesis: a DNA gene is copied into a strand of mRNA. (in the nucleus): a geneA section of DNA that holds the instructions to build one protein. is copied into mrnaMessenger RNA — a single-stranded copy of a gene that carries the message from DNA to the ribosome.. The rule changes slightly — RNA uses U instead of T. So A in DNA pairs with U in mRNA.
• translationThe second step of protein synthesis: mRNA is read at the ribosome and used to build a protein. (at a ribosomeThe cell structure where proteins are built. The ribosome reads mRNA and links amino acids together.): the mRNA is read in groups of three bases called codonA group of three mRNA bases that codes for one amino acid.s. Each codon codes for one amino-acidThe building block of proteins. Amino acids link together in chains to form proteins.. The chain of amino acids folds into a protein.

Codon math: count the mRNA bases, divide by 3. That is how many codons — and how many amino acids — you get. An mRNA with 12 bases has 4 codons = 4 amino acids.

Link 3: Mutations change the DNA

A mutationA change in the DNA sequence that can create a new allele or alter a protein. is any change in the DNA base sequence. A base can be inserted, deleted, or swapped for a different one. Because DNA is the starting point of the whole chain, a mutation can ripple forward:

Changed DNA → changed mRNA → changed protein → changed trait.

But not every mutation changes the trait. Some changes to DNA don't alter the protein, and some protein changes don't affect how the organism looks or functions.

Where the mutation happens matters:

• Body cell — affects only that individual. Not inherited.
• Gamete (sex cell) — can be passed to offspring. This is how new alleleA version of a gene. The gene for eye color has alleles for blue, brown, and other colors.s enter a population.

Link 4: Cell division passes the DNA along

Two kinds of cell division, two different purposes:

	Mitosis	Meiosis
Purpose	Growth and repair	Making gametes (eggs/sperm)
Result	2 identical diploidA cell with the full set of chromosomes (2n). Body cells are diploid. cells	4 unique haploidA cell with half the chromosomes (n). Eggs and sperm are haploid. cells
Chromosome number	Same as parent (2n)	Half of parent (n)
Genetic variation?	No — copies are identical	Yes — crossing-overWhen paired chromosomes swap pieces during meiosis, increasing variation. + random assortment

Diploid/haploid math: if a body cell has 28 chromosomes (2n = 28), then a gamete has 14 (n = 14). Fertilization brings two gametes together: 14 + 14 = 28 again.

crossing-overWhen paired chromosomes swap pieces during meiosis, increasing variation. happens during meiosis when homologous chromosomes swap pieces. This creates new combinations of alleles on each chromosome, which is why siblings from the same parents are not identical.

Link 5: Inheritance — how traits pass from parent to offspring

Each parent passes one alleleA version of a gene. The gene for eye color has alleles for blue, brown, and other colors. per gene to each offspring. The combination of alleles is the genotypeThe two alleles a person has for a trait, written like Bb, BB, or bb.. The visible trait is the phenotypeThe trait you can see — like brown eyes, blue eyes, pink flowers, or type-A blood..

Four inheritance patterns to know:

• Simple dominance: one allele masks the other. Bb looks the same as BB.
• codominanceInheritance where BOTH alleles are visible at once, not blended. Example: a roan cow has red AND white hairs visible.: both alleles are visible at the same time. A red-and-white roan cow shows both colors side by side.
• incomplete-dominanceInheritance where the two alleles blend into a new in-between trait. Example: red flower × white flower → pink flowers.: alleles blend. Red × white = pink.
• sex-linkedA trait carried on the X chromosome. Recessive sex-linked traits show up more in males because males have only one X.: the gene is on the X chromosome. Recessive traits are more common in males because males have only one X.

A punnett-squareA grid that shows all the possible offspring genotypes when two parents are crossed. predicts offspring ratios. A Bb × Bb cross always gives a 3:1 phenotype ratio (3 dominant : 1 recessive). A pedigreeA family-tree diagram showing which family members have a trait. Squares = males, circles = females, filled-in = has the trait. tracks traits through a family: squares are male, circles are female, filled-in means the person has the trait.

The whole chain in one sentence

DNA is copied into mRNA, which is read by a ribosome to make a protein that determines a trait, which is inherited through gametes made by meiosis — and mutations are the only way new alleles are born.