Biology

Inheritance

5,027 words, 10 sections

Inheritance is the one chapter in the Biology paper where arithmetic carries most of the marks. It is official syllabus unit Biology 10, examined 388 distinct times across 91 official papers between 2000 and 2026, within the 81 marks Biology contributes to the 180-mark paper. Within that total, Mendelian Genetics accounts for 231 distinct questions, Sex-Linked and Non-Mendelian Inheritance for 125, and Gene Linkage and Recombination for 32. A student who has mastered this chapter can take any cross, square it into a Punnett square, read off a genotypic and a phenotypic ratio, then repeat the arithmetic on a human pedigree or on a pair of gene map distances. Mean item difficulty on the bank scale is 1.74, and 1814 active practice questions sit on it.

Mendel worked one trait at a time on the garden pea, and his second experimental step was to cross two pure-breeding parents. Notate the dominant factor with a capital letter and the recessive with a small one, $R$ for roundness and $r$ for wrinkledness, so a true-breeding round plant is $RR$ and a true-breeding wrinkled plant is $rr$ . Crossing them gives an $F_1$ in which every plant is $Rr$ and every seed is round, and that single observation is the law of dominance: in a heterozygote one allele is expressed in full and the other is masked.

Segregation is a statement about gametes, not offspring. The two alleles of a pair separate during gamete formation, so an $Rr$ plant produces equal numbers of $R$ -bearing and $r$ -bearing gametes, fertilisation is random, and the full pair is restored in the zygote. Crossing two $F_1$ plants, $Rr \times Rr$ , therefore gives $\tfrac{1}{4}RR : \tfrac{1}{2}Rr : \tfrac{1}{4}rr$ genotypically, which is $1:2:1$ , and $3:1$ phenotypically because $RR$ and $Rr$ look alike. Behind the letters sits a chromosome: each pair of homologous chromosomes orients on the metaphase plate independently of every other pair, and with $n$ pairs an individual makes $2^n$ genetically different gametes.

The vocabulary is examined harder than students expect. A locus is the position of a gene on a chromosome. Genotype is the genetic makeup carried for a trait, phenotype is the form in which the trait appears, and karyotype is the particular array of chromosomes an individual possesses. Both alleles the same means homozygous; alleles differing means heterozygous.

Worked example

Two black horses, black dominant and white recessive, produce a foal with a white coat. What is the probability that the next foal is black? The first coat colour is not decoration, it is information. A white foal is $rr$ , so each parent contributed an $r$ . Both parents are black, so neither is $rr$ , which means each also carries an $R$ , and both must be $Rr$ . The cross is therefore $Rr \times Rr$ , giving $\tfrac{1}{4}RR : \tfrac{1}{2}Rr : \tfrac{1}{4}rr$ , of which the first two classes are black, so $P(\text{black}) = \tfrac{1}{4} + \tfrac{1}{2} = \tfrac{3}{4}$ . Without a white foal the genotypes would be unknowable, because a black horse can be $RR$ or $Rr$ and the answer differs. The second error is treating successive foals as dependent, giving $\tfrac{3}{4} \times \tfrac{3}{4} = \tfrac{9}{16}$ , when each foal is an independent draw.

8 more sections in this chapter

Sign up free to read the full chapter notes, plus a bank of 83,000+ active MDCAT questions, an AI tutor, and spaced repetition flashcards.

Introduction

1. Dominance, segregation and the monohybrid crossMendel worked one trait at a time on the garden pea, and his second experimental step was to cross two pure-breeding parents. Notate the dominant factor with a