Inheritance and variation Notes

How Are Traits Passed From Parents to Offspring?

1. Inheritance is the process by which genetic information is passed from parents to offspring.
2. Every child receives a unique combination of alleles, which shapes their characteristics.
3. These differences between individuals form the basis of variation in a species.

Why Do Siblings Look Alike but Not Identical?

1. Siblings resemble each other because they inherit genes from the same parents.
2. They are not identical because each sibling receives a different mix of alleles.
3. This happens because:
   1. Meiosis produces gametes with different combinations of chromosomes.
   2. Fertilization is random, any sperm can fuse with any egg.
   3. Each child ends up with a unique allele combination.

What Are Genes, Alleles, Genotypes, and Phenotypes?

1. A gene is a section of DNA that influences a trait.
2. An allele is a different version of the same gene.
3. A genotype is the combination of alleles an individual has.
4. A phenotype is the observable trait.

How Did Mendel Discover the Idea of Genes?

1. Gregor Mendel studied pea plants and noticed predictable patterns in how traits appeared.
2. He discovered that some traits are dominant and some are recessive.
3. His crosses produced consistent ratios, which he used to infer the existence of “factors” (now called genes).
4. Why Mendel succeeded:
   1. He studied one trait at a time
   2. Controlled pollination
   3. Used large sample sizes
   4. Counted and analysed ratios

What Makes Mendel’s Hybrids All the Same?

1. Cross: RR × rr
2. The “pure breeding” parents produce only one type of gamete each:
   1. Parent 1 → R
   2. Parent 2 → r
3. All offspring receive Rr.
4. So:
   1. Genotype (F₁): all Rr
   2. Phenotype: all show the dominant trait

Why Does Mendel’s 3:1 Ratio Occur?

1. The 3:1 ratio appears in the F₂ generation when heterozygous hybrids (Rr) self-fertilize.
2. Why it exists:
   1. F₁ hybrids produce two types of gametes: R and r.
   2. These combine randomly at fertilization.
   3. The resulting combinations follow predictable probabilities.

Do Mendel’s Rules Apply to All Species?

1. Mendel’s rules apply widely because the process, alleles separating in meiosis and recombining at fertilization, is universal.
2. Mendel’s rules apply well to:
   1. Seed shape in pea plants
   2. Simple dominant traits in animals
   3. Some human conditions inherited by a single gene
3. Inheritance becomes more complex when:
   1. Polygenic traits (height, skin colour)
   2. Incomplete dominance
   3. Codominance (AB blood group)
   4. Environmental effects
   5. Linked genes

How Does Meiosis Generate Variation?

1. Meiosis creates variation by producing gametes with different combinations of alleles.
2. How variation arises in meiosis:
   1. Chromosome shuffling
   2. Separation of allele pairs
   3. Random assortment
   4. Crossing-over (at GCSE/MYP level: chromosomes exchange segments)