Understanding DNA and Genetic Inheritance Concepts

DNA

Deoxyribonucleic acid

Structure of DNA

Double helix made of nucleotides (sugar, phosphate, base).

Location of DNA in the cell

In the nucleus (and mitochondria in small amounts).

Genome

The complete set of DNA in an organism.

Gene

A segment of DNA that codes for a protein.

Homologous chromosomes

Chromosome pairs with same genes in the same order. Features: Same length, centromere position, and banding pattern.

Autosomes and sex chromosomes

Autosomes are chromosomes 1-22; sex chromosomes are X and Y. Sex determination: XX = female, XY = male.

Differences between X and Y chromosomes

X is larger with more genes; Y is smaller and determines maleness.

m

m

Epigenetic factor

Environmental influences that affect gene expression without changing DNA.

Genotype

The genetic makeup (e.g. HH, Hh, hh).

Allele

A version of a gene.

Phenotype

The observable trait (e.g. eye color).

Factors affecting phenotype

Genotype + environment.

Heterozygous

Two different alleles (e.g. Hh).

Homozygous

Two same alleles (e.g. HH or hh).

Autosomal genotype example using H/h

Hh = heterozygous; HH or hh = homozygous.

Chromosome for autosomal genes

Autosomes (chromosomes 1-22).

Dominant phenotype

A trait shown when at least one dominant allele is present.

Dominant/recessive example using B/b

BB or Bb = dominant; bb = recessive.

Dominant human traits and their recessive forms

Widow's peak (dominant) vs straight hairline (recessive); Freckles (dominant) vs no freckles (recessive).

Lethal alleles

Alleles that cause death if both are inherited. Example: Homozygous dominant for some genetic diseases.

Monohybrid cross

A genetic cross involving one gene with two alleles.

Autosomal dominant and recessive traits

Dominant: trait appears if at least one allele is dominant. Recessive: trait appears only if both alleles are recessive.

Test cross

A cross to determine an unknown genotype using a homozygous recessive.

P, F1, and F2 generation

P = parents, F1 = first generation, F2 = second generation.

Punnett square

A diagram to predict offspring genotypes and phenotypes.

Typical genotype and phenotype ratios for a monohybrid cross

Genotype: 1:2:1 (e.g. BB:Bb:bb) Phenotype: 3:1 (dominant:recessive)

Symbols used in a pedigree chart

Circle = female, square = male, shaded = affected, half-shaded = carrier.

Pedigree chart

Tracking inheritance of traits across generations.

Determining if a trait is autosomal or sex-linked

Sex-linked traits often show more in males; autosomal affects both sexes similarly.

Determining if a trait is dominant or recessive from a pedigree

Dominant traits appear in every generation; recessive can skip generations.