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Plant Biology - Genetics

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    Study Guide Plant Biology – Genetics 1. Intricacies of Inheritance 1.1 Intricacies of Inheritance — Made Simple As scientists continued breeding experiments and improved microscopes, they learned that inheritance is more complex than Mendel first described. Mendel’s laws still matter, but many traits do not follow his simple patterns. Let’s break down these important exceptions in a clear, student - friendly way. 1.2 When Mendel’s Ratios Don’t Work: Gene Linkage After scientists accepted Mendel’s Law of Independent Assortment , they noticed something odd. Some crosses did not produce the expected 9:3:3:1 ratio in the F ₂ generation. What was happening? • The parental (original) traits appeared very often • New combinations of traits appeared rarely To explain this, scientists used parsimony , which means choosing the simplest explanation that fits the facts . The answer was linkage . Linkage explained • Some genes are located close together on the same chromosome • Because they are close, they tend to be inherited together • This breaks Mendel’s assumption that genes always assort independently 1.3 Linkage and Crossing Over Linkage explains why parental traits stay together — but what about the new combinations that appear sometimes?

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    Study Guide The answer: crossing over • During Prophase I of meiosis , homologous chromosomes pair up • Non - sister chromatids can exchange pieces of DNA • This exchange is called crossing over • The visible X - shaped point is called a chiasma (plural: chiasmata ) Why gene distance matters • Genes close together → crossing over is less likely • Genes far apart → crossing over is more likely Scientists use crossing - over frequency to make linkage maps , which show the approximate positions of genes on chromosomes. 1.4 Incomplete Dominance Sometimes, neither allele fully hides the other.

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    Study Guide 1.5 What is incomplete dominance? • The heterozygous phenotype is a blend • Example: o Red snapdragon × White snapdragon → Pink snapdragon o Pink × Pink → o 1 red : 2 pink : 1 white Important detail • The alleles themselves do not change • They still separate normally during meiosis • This can be proven with a backcross 1.6 Mutations: Changes in DNA A mutation is any change in an organism’s DNA. These changes can affect a single base or entire chromosomes.

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    Study Guide 1.7 Gene (point) mutations • Affect one base pair • Example: base substitution Chromosomal mutations • Affect large DNA segments or whole chromosomes • Include: o Deletion – DNA is lost o Duplication – DNA is repeated o Inversion – DNA flips direction o Translocation – DNA moves to another chromosome 1.8 Polyploidy: Extra Sets of Chromosomes Normally, organisms are diploid (2n) . But sometimes, cells have more than two sets of chromosomes.

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