Genetics
8th Grade Science · 6 lessons
Start lesson 1
Lessons
- 1Meiosis Halves the Number and Shuffles the DeckMeiosis makes gametes carrying 23 chromosomes from cells carrying 46, but halving is only half of what it does. Homologous pairs line up and exchange segments in crossing over, and each pair orients independently at the metaphase plate, so a single person can produce more than eight million chromosomally distinct gametes before crossing over is even counted. Fertilisation then combines two of those at random, which is why siblings from the same parents differ.Experiment
- 2Punnett Squares Predict Ratios, Not IndividualsCrossing two heterozygotes, Bb × Bb, predicts a 3:1 ratio of dominant to recessive phenotype — but that is a probability applying to each offspring independently, not a quota the family must fill. All four children of such a cross can show the recessive trait, because each birth is a fresh 1-in-4. Ratios only emerge reliably over large numbers, which is precisely why Mendel counted thousands of pea plants rather than a handful.Lesson
- 3Genotype, Phenotype and the EnvironmentThe genotype is the pair of alleles an organism carries; the phenotype is the characteristic that actually appears, and the two are not the same thing. A hydrangea with one unchanged genotype flowers pink in alkaline soil and blue in acid soil. Identical twins share a genotype and still differ in height if one is better fed. Genes set the range of what is possible; the environment settles where within that range a particular individual ends up.Experiment
- 4The Double Helix Explains How DNA Copies ItselfDNA is two strands running in opposite directions, held together by base pairs in which adenine always pairs with thymine and cytosine with guanine. That pairing rule is not decoration, it is the mechanism: separate the two strands and each one specifies its partner exactly, so one molecule can produce two identical copies. Watson and Crick published the structure in 1953 and pointed out the copying mechanism in the same paper, because the structure made it obvious.Experiment
- 5From Gene to Protein to TraitA gene is a length of DNA whose bases are read in threes, and each triplet specifies one amino acid in a protein chain. Change a single base and you may change one amino acid, which may change the folded shape of the protein and therefore what it can do — the one substitution behind sickle-cell anaemia works exactly this way. Traits are not stored in DNA as such; proteins are specified, and traits follow from what those proteins do in the body.Lesson
- 6Sex-Linked Inheritance Follows Chromosome SizeThe X chromosome carries many genes that the much shorter Y chromosome does not, so a male carrying one recessive allele on his X has no second copy to mask it, while a female would need two. Red-green colour blindness and haemophilia therefore appear far more often in males, and are typically inherited from a carrier mother rather than from an affected father. The pattern is a direct consequence of chromosome size, not of anything about the trait itself.Lesson
🖨️
View in ShopGet all 6 Genetics lessons as PDF
Printable worksheets & answer keys — $4.99